The Department of Physics at the University of Illinois at Urbana-Champaign has strong research programs in many branches of physics. Condensed matter research, both experimental and theoretical, has flourished for more than 40 years, during much of that period benefiting from interdisciplinary contacts stimulated by the Materials Research Laboratory (MRL). MRL has extraordinary central facilities for the preparation and microcharacterization of materials. Innovative programs in complex and non-equilibrium systems and nonlinear dynamics have been launched with the cooperation of the new Beckman Institute for Advanced Science and Technology. Other departmental programs profiting from extensive interdisciplinary contact include astrophysical theory, bio- and biomedical physics, and semiconductor physics.
Two large and successful experimental groups involving more than a dozen faculty are engaged in high-energy (``elementary particle'') and nuclear physics research and collaborate with colleagues at Fermilab, SLAC, MIT-Bates, Cornell, and other major laboratories in the U.S. and abroad. Theoretical seminars, often presented by visiting scientists, record the rapid developments in fields ranging from lattice gauge theory to biomolecular dynamics.
In addition to the sources of support noted on the individual project descriptions, the department continues to receive fellowship and other support from the National Science Foundation, U.S. Department of Education, IBM, AT&T Bell Laboratories, Exxon, Shell Foundation, Kodak, Petroleum Research Foundation, Tektronix, Silicon Graphics, Whirlpool, Xerox, and from numerous alumni and friends.
Optical Monitors for Vascular Insufficiency in Peripheral Tissue
E. Gratton,* W. W. Mantulin, M. A. Franceschini,
National Institutes of Health, 5 R01 RR10966
Peripheral vascular disease (PVD), a chronic disease, afflicts
diabetics and others with vascular pathologies. The level of tissue
oxygenation in extremities is an important parameter for diagnosis of
PVD. We have developed a new technology based on near-IR frequency
domain spectroscopy that provides quantitative information on the
level of tissue oxygenation. The optical signal is derived from
penetration of photon density waves in tissue. We have designed and
built noninvasive, portable, tissue oxygen saturation monitors.
Preliminary tests show that the optical oxygen monitor can be
clinically useful by providing the clinician with a quantitative
physiological parameter which is a meaningful index for the early
detection and treatment of PVD.
Laboratory for Fluorescence Dynamics
E. Gratton,* W. W. Mantulin,* T. Hazlett, S. Sanchez,
National Institutes of Health, P41-RR-03155
The Laboratory for Fluorescence Dynamics (LFD), a national biomedical
resource, has a dual and equal commitment to foster fluorescence
research and to provide service in a user-oriented facility.
Fluorescence Research and Development
The research goal of the LFD is to develop new fluorescence
instrumentation, design new theoretical formulations of fluorescence
phenomena, and compile appropriate software, with the aim of advancing
basic research and biomedical applications. Examples of current
projects include: instrumentation (frequency domain fluorometer with
lifetime and spectral resolution, laser heterodyning, lifetime
fluorescence microscopy pump probe stimulated emission spectroscopy),
software (global analysis of multifrequency data sets), optical
imaging (near-infrared images of tissue), and applications (two-photon
fluorescence correlation spectroscopy). These advances in fluorescence
technology are transferred to the user fluorescence laboratory.
Fluorescence Laboratory
The laboratory serves both the campus research community and visiting
scientists. To date, core and collaborative research has stressed
macromolecular assembly and dynamics, membrane structure/function
relationships, and fluorescence microscopy of cells. The LFD houses a
spectropolarimeter for circular dichroism measurements. Fluorescence
equipment includes high-sensitivity, photon-counting, scanning
fluorometers (with polarization accessory), three laser-based variable
multifrequency phase/modulation fluorometers with different excitation
wavelength and modulation frequency options, stopped flow and high
pressure accessories. Dedicated personal computers assist in data
collection and analysis. Ancillary support for biomedical research is
housed in a general biochemistry laboratory, which is equipped for
biological sample manipulation.
Fluorescence Microscopy Development Laboratory (FMDL)
FMDL is a technology development
laboratory for multiphotonic fluorescence microscopy. It conducts core
and collaborative research on a variety of cellular components and
systems (membranes, receptors, antibodies, etc.). The instrumentation
includes Ti:sapphire lasers, upright and inverted fluorescence
microscopes, and correlation systems for photon counting. The
multiphotonic techniques under development include: fluctuation
correlation spectroscopy, fluorescence lifetime imaging, pump-prove
stimulated emission, particle tracking, and single molecular studies.
Optical Imaging of Thick Tissues
E. Gratton,* S. Fantini, J. Maier, S. Walker, M. Filiaci,
National Institutes of Health, 1RO1 CA57032
This project explores the use of frequency-domain methods to obtain
near-infrared optical images of thick tissues. The use of near-
infrared radiation has been proposed as an attractive alternative to
obtain information about the oxygenation state of tissues due to the
difference in optical spectra of the oxy- and deoxy- form of
hemoglobin. Our frequency-domain approach uses the propagation of
high-frequency AM light. In the frequency-domain, propagation of the
AM intensity wave in a highly scattering medium is analogous with wave
optics. An object immersed in the medium produces deformation of the
propagation wavefront of the amplitude modulated wave and results in
an easy identification of absorbing and scattering objects such as
blood vessels or bone. Computer algorithms display in real-time the
wavefront of the AM wave after traversing the tissue.
Solution and Interfacial Self-association of Dimeric Phospholipases
A2
T. L. Hazlett*
American Heart Assn., Illinois Affiliate 96-GS-10
The phospholipase A (PLA2) project focuses on understanding the
physical state of the PLA2 enzymes in solution, the physical state of
these enzymes on the surface of a membrane, the correlation of the
enzymes' physical states, and the subsequent hydrolytic activity. Of
utmost importance in this research is to develop and apply techniques
that are capable of examining protein structure under native
conditions and in the presence of an interface. The long-range benefit
of this research in the health field is to further our understanding
of PLA2's function which, being part of the arachidonic acid pathway,
plays a critical role in clotting and upkeep of the cardiovascular
system.
Characterization of the Oligomeric State of Phospholipase A2
T. Hazlett,* Y. Mahalakshmi, S. Sanchez
American Heart Association, Illinois Affiliate
Phospholipase A2 (PLA2), an enzyme isolated from Crotalus atrox venom,
may function as a dimer on the membrane surface. This event gives rise
to the interfacial activation and the lag phase events observed in
substrate hydrolysis kinetics. Using time-resolved fluorescence
techniques and fluorescence energy transfer, the enzyme oligomeric
behavior and the enzyme dynamics in solution and on the surface of
lipid vesicles are examined. We are labeling the PLA2 monomer subunits
with a variety of fluorophores and monitoring the interactions between
subunits using subunit exchange protocols. Once the solution behavior
is well characterized, the influence of lipid membrane surface on
enzyme dimerization will be explored. Correlations between the
protein's kinetic events and the physical state.
Noninvasive Near-Infrared Neonatal Brain Hemoximetry
J. S. Maier*
National Institutes of Health, Predoctoral NRSA M.D./Ph.D. Fellowship,
NIH 1F30MH11432-01
Our primary focus is near-infrared tissue spectrometry for clinical
use based on the physical understanding of how light travels in
tissues. This neonatal study investigates two problems with near-
infrared spectroscopy of the brain: (1) the curved surface of the
skull and (2) light piping by the cerebrospinal fluid. Neonatal
brain/hemoximetry is interesting because of the correlation of long-
term pathology including cerebral palsy, attention deficit disorder,
and mental retardation, with ischemic and hemodynamic insults to
neonates. The accuracy of a simple model for curved surfaces through
in vitro laboratory experiments will be investigated using tissue
simulating phantoms. Using Monte Carlo modeling and experimental
studies on in vitro laboratory samples, we will explore the CSF's
effect current measurement protocols.
Single-Molecule Studies of Protein Dynamics
G. U. Nienhaus,* E. Gratton, W. W. Mantulin, J. Maller,
National Science Foundation, PHY 95-13217
Our goal is to study protein dynamics over a wide range of times at
the level of single molecules, using a fluorescence microscope based
on two-photon excitation. To obtain the high spatio-temporal density
of photons necessary for a high probability of two-photon absorption,
fluorescence excitation will be achieved by focusing the 150 fs wide
pulses of a femtosecond Ti:sapphire laser to a diffraction limited
spot in an epi-illuminated microscope. The method promises an
extremely good signal-to-noise ratio. Processes that will be studied
include rotational and translational diffusion, internal
conformational relaxations of proteins, and molecular interactions,
for example, protein aggregation and binding of small ligands, such as
fluorescent antigens binding to antibodies.
Theory and Simulation of Biopolymer Aggregates
K. Schulten,* R. Skeel* (Comput. Sci.), L. Kale* (Comput. Sci.), T.
Martinez* (Chemistry), X. Hu (Beckman),
University of Illinois
Our research focuses on the structure, dynamics, and function of
biopolymer aggregates, e.g., lipids and water forming membrane
bilayers, proteins complexing with DNA and regulating gene expression,
and proteins involved in complexes with other proteins. The studies
require very-large-scale computer simulations and have become possible
through the development of statistical mechanical theory, efficient
algorithms, graphics tools, a simulation program, and the group's
network of powerful workstations which function as a high-performance
parallel computer.
Quantum Chaos and Integrability
D. K. Campbell,* P. Phillips,* B. Bunker, R. T. Clay,
University of Illinois
The amount of controversy concerning the concept of "quantum
chaos" is exceeded only by the amount of interest in potential
applications, which range from prediction of the conductivity and
response of mesoscopic quantum nanostructures to modeling the excited
state spectra of complex nuclei. We are currently focusing on one
aspect of quantum chaos which, although not yet fully understood, is
very intriguing--the difference in statistics of energy levels for
chaotic and integrable systems--with the dual aims of understanding
the relationships among chaos, integrability, and statistics and of
using this understanding to interpret and predict transport properties
of actual mesoscopic electronic devices.
Piecewise Linear Approximations to Nonlinear Maps
D. K. Campbell,* D. Horton
National Science Foundation, PHY 93-22320
Iterated nonlinear maps of the unit interval form perhaps the
simplest, most instructive, and most extensively studied class on
chaotic dynamical systems, as well as an important set of simplified
models for applications such as population dynamics. The sequence of
sudden changes--bifurcations--in the behavior of a map as the strength
of the nonlinearity is varied is one of the most interesting aspects
of the dynamics. Starting from the quadratic "logistic map,"
we have studied the extent to which this sequence can be reproduced by
piecewise linear approximations to the nonlinear map.
Studies of Nonlinear Dynamics
S.-J. Chang,* Y. Oono*
University of Illinois
The group does various research projects in nonlinear dynamics. The
research projects include (1) the formation and dynamics of spatial
patterns, (2) Hamiltonian systems with few degrees of freedom, (3)
dynamic cell models, and (4) almost periodic and quasiperiodic
systems.
Classical and Quantum Chaos
S.-J. Chang,* M. Stuller, A. Yurchenko
University of Illinois
I work on research problems related to classical and quantum chaos. In
particular, I wish to understand the transition from a quantum system
to a classical system where the classical system is chaotic. The
project includes the studies of (1) quantum dynamics around KAM tori,
(2) semiclassical approximation and trace formulas, (3)
renormalization group transformations for a classical field theory,
and (4) zero modes and the breakup of KAM tori.
Center for Complex Systems Research
S.-J. Chang,* A. Hubler,* E. A. Jackson,* J. Mittenthal (Biology), P.
Newton (Mathematics), A. Scheeline (Chemistry), D. Goldberg (General
Engr.)
University of Illinois
The Center for Complex Systems Research is an interdisciplinary group
of faculty and students involved in research on complex dynamic
processes in a variety of scientific fields. Current studies include:
adaptive controls of time-varying systems; dynamics of amplitude
equations and weak turbulence; turbulence experiments; quantum
relationships with classical chaos; constructing equations of motion
from data; forecasting high-dimensional chaotic systems; principles of
organization and morphogenesis in organisms; chemical oscillations and
chaotic dynamics; coupled cellular systems and neural networks;
measurements of evolutionary activity.
Reconant and Transferal Interactions with Complex Systems
E. A. Jackson*
University of Illinois; Beckman Institute for Advanced Science and
Technology
Complex systems often have a large number of dynamic attractors with
very different behaviors. The problem of obtaining a mathematical
model that can describe the dynamics of such systems is a fundamental
challenge in science. A new method of open-plus-closed-loop
interactions on general systems of ordinary differential equations
developed by Jackson and Grosu is being used experimentally to
transfer systems among any of its attractors. It will also be used in
a resonant-modeling technique to explore the most accurate global
dynamic model of a system.
The Changing Bases, Methods, and Unifying Objectives of Science
E. A. Jackson*
University of Illinois; Santa Fe Institute; Beckman Institute for
Advanced Science and Technology
Since 1890 there have been basic changes in the sources of information
in science, based on mathematical discoveries, and new digital
computer and experimental opportunities. These sources of information
continue to greatly enrich the historic scientific modes of
understanding (knowledge), which are based on the discovery of new
analytic, recursive, and holistic relationships between physical
observables or "conceptuals" (e.g., fields, wave functions,
etc.). This research seeks to understand the discoveries of
transdisciplinary dynamic concepts, common to various fields of
science, and the dynamics that are unique to the complexity of
particular areas.
Neural Network Dynamics
E. A. Jackson*
University of Illinois; Beckman Institute for Advanced Science and
Technology
The dynamic-attractor characteristics of small neural networks
(modules) are being investigated for neural dynamics with realistic
features. These features include absolute and relative refractory
periods, arbitrary excitatory and inhibitory connections satisfying
Dale's law, variable thresholds, and noisy connectivity influences.
The response of these networks to selected input signals and how the
multiple-attractor dynamics might relate to information processing and
short-term memory are being investigated.
Search for Quantum Chaos
M. H. Nayfeh,* H. Thompson
University of Illinois
We are studying the question of the existence of chaotic behavior in
quantum mechanical systems whose classical analogs are known to be
nonintegrable and exhibit chaotic behavior. The system that we use is
the interaction of low-frequency, high-power microwave radiation with
one-dimensional hydrogen atoms. These atoms are prepared by laser
excitation of atomic hydrogen in the presence of strong dc electric
fields.
Semiconductor Surfaces and Interfaces
T.-C. Chiang,* T. Miller, P. Reese, D. Luh, T. Kidd
U.S. Department of Energy, DE-FG02-96ER45439
(In cooperation with the Materials Research Laboratory)
Photoemission and scanning tunneling microscopy techniques are
employed to determine the electronic properties and the atomic
structure of surfaces and thin films. The behavior of crystal growth
on surfaces by molecular beam epitaxy and chemical vapor deposition is
also investigated. Key issues of interest include the chemical
reactions and atomic interactions leading to the deposition of
materials, the nature of atomic bonding, and the atomic processes and
the resulting morphology of film formation. This investigation
includes high-resolution core-level spectroscopy work carried out at
synchrotron radiation facilities, and much effort has been directed
toward the development of photoelectron holography techniques for
site-selected 3-D imaging of atomic bonding configurations.
Determination of Surface Atomic Structure and Holography
T.-C. Chiang,* E. Hanson
Petroleum Research Fund, American Chemical Society
This research project is a study of the properties of surfaces,
adsorbates, and overlayers using scanning tunneling
microscopy/spectroscopy and synchrotron photoemission. We will
determine the adsorption site geometry and the overlayer growth mode,
and study the modifications to the surface electronic states and the
evolution of the interface/overlayer properties during adsorption and
overlayer formation. Effects of surface defects, impurities, and
atomic steps will be investigated.
X-Ray Diffraction Studies of Surfaces and Interfaces
T.-C. Chiang,* Z. Wu
U.S. Department of Energy, DE-FG02-96ER45439
X-ray diffraction studies of the atomic structure of surfaces,
interfaces, and thin films are being carried out at the National
Synchrotron Light Source (Brookhaven National Laboratory) and the
Advanced Photon Source (Argonne National Laboratory). Current emphasis
of this research is on the chemical reactions and atomic
rearrangements near an interface when a thin film deposited on a
substrate is subjected to high-temperature annealing. The atomic
coordinates are determined by Patterson analyses of the in-plane
diffraction intensity distributions, by truncation rod scans, and by
reflectivity measurements.
Metallic and Magnetic Quantum Structures
T.-C. Chiang,* T. Miller, E. D. Hansen
National Science Foundation, DMR 95-31809
This project is a study of the structure, growth behavior, and
properties of selected metal surfaces, interfaces, quantum wells, and
superlattices. The experimental techniques include angle-resolved
synchrotron photoemission, Auger spectroscopy, electron diffraction,
and scanning tunneling microscopy. The work will include
investigations of (1) the bulk and surface band structure, coherence
and scattering lengths, and spatial distributions of electronic
states, (2) the physics and chemistry of film growth, (3) the
structure-property relationships for various quantum configurations,
(4) electron scattering effects in two-dimensional surface alloys, and
(5) spin effects in systems containing magnetic materials.
In Situ Investigations of Nonequilibrium Synthesis of Superhard
Materials Using Energetic Ablation Beams
T.-C. Chiang,* J. Abelson, H. Chen, S. M. Gorbatkin,
U.S. Army Research Office, 36120-MS-RIP
Superhard materials made of light elements such as boron, nitrogen,
carbon, oxygen, and various metallic elements will be synthesized by
pulsed-laser-ablation deposition. The time dependence of the pulsed
deposition process and the resulting film structure and properties
will be examined by in situ x-ray diffraction and ellipsometric
measurements. This work will be carried out at the Advanced Photon
Source using an undulator beamline. The hardness of the resulting film
will be measured using a nanoindentation instrument which is capable
of detecting micro-Newton forces and nanometer displacements.
Acquisition and Development of a High-Resolution Photoelectron
Analyzer for Studies of Surface, Films, and Multilayers of Metals,
Semiconductors, and Magnetic Materials
T.-C. Chiang,* T. Miller, J. Paggel
National Science Foundation, DMR 95-31582
A new high-resolution photoelectron analyzer will be purchased to
support research based on photoelectron spectroscopy, and a spin
detector will be constructed to analyze the spin states of the
photoelectrons. The research subjects include: (1) atomic structure of
surfaces, which will be studied using the techniques of photoelectron
diffraction and holography, and (2) electronic effects associated with
quantum confinement, coupling, interface scattering, and crystal
potential modulation in artificially layered systems. This analyzer
and the spin detector will be used in conjunction with a new undulator
source that is under construction at the Synchrotron Radiation Center.
Optical Properties of High-Tc Superconductors
S. L. Cooper,* M. A. Karlow, P. Nyhus
NSF Science and Technology Center for Superconductivity
We are studying the optical properties of the high-Tc cuprates in
order to better understand both the strongly correlated normal state
and the nature of quasi-particle pairing in the superconducting state.
We are also interested in the unconventional charge dynamics
perpendicular to the highly conducting layers in these materials, and
are using optical techniques to study the nature of interlayer
coupling and c-axis charge transport in the cuprates.
Spectroscopic Studies of Low Carrier Density Magnetic Systems
S. L. Cooper,* C. Snow
National Science Foundation, DMR 97-00716
We are interested in a number of low carrier density magnetic systems
with rich phase diagrams as a function of doping, including
insulating, ferromagnetic metal as well as antiferromagnetic ground
states. The diverse phase diagrams of these materials derive largely
from the competition between strong Coulomb correlations, electron-
phonon coupling, and spin interactions. We are using various optical
techniques, including reflectance and light-scattering spectroscopies,
to characterize the excitation spectra of these materials, and to
elucidate the mechanisms driving the different phase transitions.
Spectroscopic Studies of the Magnetic Oxides
S. L. Cooper,* S. Yoon, H.-L. Liu, P. Dua
U.S. Department of Energy, DE-FG02-96ER45439
The magnetic oxides exhibit a wide variety of exotic phenomena,
including paramagnetic insulating-to-ferromagnetic metal transitions
and "colossal magnetoresistance" behavior at intermediate
doping, as well as ordered charge and spin structures at high doping.
We are attempting to elucidate the physics governing these interesting
phase regions by using reflectance, Raman, and Brillouin scattering
spectroscopies to study the interactions between the lattice, charge,
and spin degrees of freedom in these materials.
Coherent Properties of Single-Crystal Metallic Superlattices
C. P. Flynn,* M. B. Salamon,* K. Ritley, M. Conover,
National Science Foundation, DMR 94-24339
Metallic superlattices with a high degree of crystalline perfection
are produced by molecular beam epitaxy. A rich variety of new
properties has been found in Dy/Lu superlattices, which are distinctly
different from those of the Dy/Y and Er/Y systems. Recent work has
also addressed the behavior of single magnetic layers grown
epitaxially and of Nd/Y superlattices. A key element is the effect of
strains induced by epitaxy on the magnetic phase diagram. It has
proven possible to detect the induced polarization of nonmagnetic
constituents of rare-earth alloys using resonant x-ray scattering.
Magnetic susceptibility, neutron scattering, and x-ray diffraction
methods are used.
Quantum Circuits at High Frequencies
R. Giannetta,* I. Adesida (Elect. & Comput. Engr.),
UIUC Critical Research Initiative
In high-mobility semiconductor nanostructures, novel forms of
electrical transport such as quantized conductance and Coulomb
blockade have been firmly established. New, nonclassical phenomena are
also predicted to occur in the time domain. These include quantum
inductance and photon-assisted tunneling. Research into this regime of
very high-frequency response is the focus of this effort. Our
experiments require a combination of semiconductor nanofabrication,
low-temperature transport measurements, and modern electrooptic
techniques in the terahertz domain.
RF and Microwave Electrodynamics in High-Temperature Superconductors
R. Giannetta*
NSF Science and Technology Center for Superconductivity
A number of different experiments indicate that high-temperature
superconductors have an unconventional, d-wave type of pairing
symmetry. Our goal is to understand how the electromagnetic response
of these materials is altered by d-wave pairing. High-sensitivity
radio-frequency oscillator measurements are being used to look for
changes in the superconducting penetration depth indicative of new
pairing states. In addition, we are using penetration depth
measurements to study a wide variety of new superconducting vortex
phenomena. These include vortex lattice melting, Josephson vortex
viscosity, and connections of vortex dynamics to the pairing symmetry.
Thermoelectricity in Microcircuits
R. Giannetta,* I. Adesida (Elect. & Computer Engr.)
U.S. Army Research Office, ARO 1-5-20650
We are studying thermoelectric transport phenomena in quantum dots,
wires, and point contacts. Measurements are performed in the
temperature range 0.05 to 4.2 Kelvin. The transport coefficients in
these quantum-scale devices depend strongly upon the conduction of
charge and heat through single electron channels. We are interested in
both the basic heat transfer processes in submicron devices as well as
possible applications for ultrasensitive microwave detection.
Atomistic Studies of Silicon Oxidation
J. M. Gibson,* X. Chen
Semiconductor Research Corp.
We use in situ transmission electron microscopy to examine the
fundamental mechanisms of silicon oxidation. In particular, the
origins of interfacial roughness, which has serious impact on
semiconductor device reliability, are studied using electron diffusion
and imaging.
Studies of Amorphous Materials with Electron Fluctuation Microscopy
J. M. Gibson,* P. M. Voyles
National Science Foundation, DMR 97-03906
Using statistical measurement of fluctuations in higher solution
transmission electron microscopy of amorphous thin films, we are
examining medium-range ordering.
In Situ Studies of Materials Growth
J. M. Gibson,* J. C. Yang, M. Yeadon, W. Henstrom,
U.S. Department of Energy, DE-FG02-96ER45439
This is a study of surface and interface structure using quantitative
transmission electron microscopy. TEM studies are made of surface
reactions and in situ epitaxial growth using image formation using
surface-related diffracted intensities. Quantitative atomic resolution
microscopy is being applied to interface structure and chemistry.
Growth and Characterization of Epitaxial GaN Using Energetic Ion
Beams
J. M. Gibson,* H. Morkoc (Elect. & Comput. Engr.),
U.S. Office of Naval Research, N00014-95-1-0324
GaN is important for blue light-emitting devices. This research
focuses on understanding the initial stages of GaN growth and the
enhancement of growth with energetic nitrogen beam deposition. In situ
experiments with TEM and LEEM are part of this program.
High-Temperature Superconductivity
D. M. Ginsberg,* A. I. Schegolev
University of Illinois
We produce and characterize high-temperature superconducting compounds
and measure their properties. Our characterization methods include
magnetic susceptibility, optical microscopy, and x-ray diffraction. We
determine the effect of replacing copper atoms at specific
crystallographic sites by other atoms. This provides an important test
of the quantum-mechanical theories of these fascinating materials, and
helps interpret various data that we and others are obtaining.
Single-Crystal High-Temperature Superconductors
D. M. Ginsberg,* J. T. Manson
National Science Foundation, DMR 93-18740
We grow and characterize single crystals of superconducting materials
and investigate their properties. We want to understand why they are
superconducting at such high temperatures, why the critical magnetic
field of these materials is highly anisotropic, and how this
anisotropy can be related to fundamental parameters of these
materials. We determine the superconducting transition temperature and
critical magnetic fields. We measure resistance and Hall effect vs.
temperature to obtain fundamental information about thermodynamic
fluctuations near this second-order transition. Our collaborators
measure Raman scattering, spin resonance, and nuclear resonance.
Superconducting Materials Development
D. M. Ginsberg,* J. Giapintzakis, J.-T. Kim
NSF Science and Technology Center for Superconductivity
We develop new methods of making high-temperature superconductors to
provide single crystals of very homogeneous materials with sharp
superconducting transitions and good surface properties. The materials
are characterized by x-ray diffraction as well as by measurements of
their superconducting properties. The samples produced are used in
fundamental experiments by our group and by the other groups in the
Science and Technology Center for Superconductivity. The aim of these
experiments is to help determine the microscopic phenomena responsible
for macroscopic properties. We also do electron microscopy to explore
the role of lattice defects in the pinning of quantized magnetic
vortices.
The Effect of Viscosity on Dislocation Tunneling
A. V. Granato,* T. Kosugi, D. McKay
National Science Foundation, DMR 93-19773
Dislocation tunneling in superconductors provides an example of
mesoscopic tunneling with different damping rates. The effect of
viscosity on dislocation tunneling through impurity pinning points is
determined by comparing the temperature dependence of the
ultrasonically found microyield stress in the normal and
superconducting state. No tunneling is found in dilute aluminum
alloys, but tunneling in pure aluminum in the superconducting state is
suppressed in the normal state.
Properties of Simple Liquids and Glasses
A. V. Granato,* A. B. Lebedev, C. Gordon, W. Baint
National Science Foundation, DMR 97-05750
Critical tests of the interstitialcy theory of condensed matter states
are being made by measuring the temperature dependence of the elastic
constants of crystals just below the melting temperature in the
crystalline state and just above the glass temperature in the
supercooled liquid state. Observation of predicted results would
confirm a simple, quantitative, easily visualized model according to
which simple liquids and amorphous materials are crystals containing a
few percent of self-interstitials.
Superconductive Tunneling Spectroscopy and Electronic Transport in
Pure and Doped
YBa2Cu3O7
Thin Films
L. H. Greene,* E. Paraoanu, M. Dittrich
National Science Foundation, DMR 94-21957
Reliable film growth, electronic transport, magnetization
measurements, and superconductive tunneling provide the foundation for
our investigations into the electronic properties of high-termperature
superconductors. Thin films of pure and doped
YBa2Cu3O7 are grown by
sputter deposition. These thin films are also grown in various
crystallographic orientations, allowing charge transport measurements
along different lattice directions in this highly anisotropic
material. Information on the interface properties is being provided
through these measurements. Furthermore, tunneling provides a powerful
spectroscopy of the superconducting state, which will help elucidate
the mechanism of high-temperature superconductivity.
Reliable Planar Tunnel Junction Fabrication with Self-assembled
Monolayers
L. H. Greene,* E. A. Pugel, in collaboration
NSF Science and Technology Center for Superconductivity
To date, the most reliable method of tunnel junction fabrication on
high-temperature superconductors has been by evaporation of Pb counter
electrodes directly on the YBa2Cu3O7
surface. A chemical interaction between these materials causes a
reproducible, insulating tunnel barrier, but also a ~30 Å thick
damage layer. To avoid such surface degradation, we are investigating
self-assembled monolayers as the tunneling barrier material. The surface
of YBa2Cu3O7 thin films
are chemically modified with, for example, an alkylamines insulating
layer. Resulting tunnel junctions are reliable and have provided
intriguing new spectroscopic data indicating the existence of
nonconventional order parameters.
Novel Spectroscopic Studies of High-Temperature Superconductors: Using
Their Unconventional Nature to Study Normal-State and Superconducting
Properties
L. H. Greene,* E. A. Pugel
NSF Science and Technology Center for Superconductivity
We take advantage of the unconventional nature of high-temperature
superconductors to probe details of the superconducting and normal-
state properties. Tunneling spectroscopic studies of the surface-
induced Andrew bound state (ABS) are performed. This (ABS) is a bound-
state of quasi-electrons and quasi-holes that form near to the
interface of an unconventional superconductor, such as the d-wave
superconductor, YBa2Cu3O7.
We will investigate this ABS as a function
of several physical parameters. In particular, other high-temperature
or unconventional superconductors and the effects of high-magnetic
field will be studied.
Charge Transport across Superconductor/Semiconductor and
Superconductor/Normal-Metal Interfaces
L. H. Greene,* A. C. Abeyta, I. V. Roshchin, T. Tanzer (Chemistry), D.
Maier (Chemistry), W. L. Feldman, in collaboration with the research
groups of D. J. Van Harlingen, P. M. Goldbart, P. W. Bohn (Chemistry),
U.S. Department of Energy, DE-FG02-91ER45439
This research program is a coordinated experimental and theoretical
study of the static and dynamic properties of hybrid superconductor-
semiconductor structures. Electronic transport, superconductive
tunneling, magnetization, and light-scattering measurements are
conducted on planar, microfabricated structures of high-quality Nb and
NbNx thin films grown directly on III-V semiconductor
heterostructures. Details of the superconducting proximity effect,
Andreev reflection, and tunneling are investigated. We have performed
the first optical detection of the superconducting proximity effect:
Raman spectroscopy is the optical probe of an InAs interface in good
electrical contact with a superconductor.
Understanding Transport across Superconducting Interfaces
L. H. Greene,* K. Krajnak, S. Baily, K. Xiu, W. L. Feldmann
U.S. Office of Naval Research, N00014-97-1-0682
We aim to measure and understand electronic transport across
interfaces to the unconventional (i.e., d-wave) high-temperature
superconductors. Signatures of the unconventional nature of some high-
temperature superconductors will be mimicked in conventional
superconducting and multilayered thin-film structures. We grow
structures with a conventional superconductor layered with normal
metals, superconductors above their superconducting transition
temperature, insulators and ferromagnetic metals and compare our
results with those obtained on d-wave high-temperature
superconductors. We also use resistivity vs. temperature and quasi-
particle tunneling. Our primary goal is to elucidate the physics and
practical ramifications of the unconventional nature of d-wave
superconductivity.
Excitations in Strongly Correlated Systems Studied by Inelastic
Scattering of Visible and X-Ray Photons
M. V. Klein,* H-L. Liu, P. Abbamonte
National Science Foundation, DMR 9705131; Lucent Technologies; Bell
Laboratories
Using optical absorption and Raman scattering measurements, we have
identified excitations in the 1.5-3 eV energy range that are
associated with strong correlations in the cuprate high-temperature
superconductors and their insulating parent compounds. We continue
this work in close coordination with studies of these phenomena using
inelastic x-ray scattering spectroscopy and resonant inelastic x-ray
scattering spectroscopy. These spectroscopies are coming on-line at
third generation synchrotron sources such as the Advanced Photon
Source at Argonne National Laboratory, and they offer the possibility
of determining the mass of these excitations, namely how they disperse
or propagate.
Raman Scattering from High-Temperature Superconductors
M. V. Klein,* G. Blumberg
NSF Science and Technology Center for Superconductivity
Systematics of the effect of changes in doping and stoichiometry on
the Raman spectra of high-temperature superconductors (HTS) are
investigated. Phonon features, the two-magnon spectrum, the unusual
electronic continuum, and superconducting gap excitations are of
particular interest. The gap excitations of superconductors with lower
transition temperatures are studied in a magnetic field. Emphasis has
also been placed on the two-magnon spectrum in doped, superconducting
materials. Resonance Raman scattering is used to identify the
fundamental charge-transfer excitation in these materials.
In Situ X-Ray Diffraction Studies of Surfaces and Interfaces
I. K. Robinson,* O. Robach
U.S. Department of Energy, DE-FG02-96ER45439
We are operating a surface x-ray diffraction station connected to the
same vacuum system as a number of MBE growth chambers in the EpiCenter
central facility. The equipment has full three-dimensional
capabilities and is optimally designed for structure determination at
surfaces and interfaces of samples transferred directly from one of
the growth chambers. X-ray diffraction data allow precise atomic-level
structural information to be obtained. The analysis is nondestructive,
so growth experiments may be continued after analysis. We are
currently using this for determining segregation profiles in
semiconductor and metal heterostructures, such as Sn/Ge(100) and
Ge/Si(100).
Kinetics of Two- and Three-dimensional Growth at Surfaces
I. K. Robinson,* A. Ghosh, D. Walko
National Science Foundation, DMR 93-15691; U.S. Department of Energy,
DE-AC02-76CH00016; National Synchrotron Light Source
X-ray diffraction experiments are carried out in ultrahigh vacuum on a
custom-built diffractometer operating on the X16A beamline at NSLS. We
prepare the samples on-line and measure the time dependence of their
diffraction patterns during deposition. The lineshape tells us about
the shape and size distribution of structures that form. The
properties of these structures depend on growth rate, coverage, and
temperature. When new stable or metastable structures are discovered,
we can perform crystallographic determinations. At present we are
examining the O/Cu(115), Pt/Pt(110), and Au/Si(100) systems.
Solid-Liquid Interface Studies by X-Ray Diffraction
I. K. Robinson,* A. A. Gewirth
U.S. Department of Energy, DE-FG02-96ER45439, DE-AC02-76CH00016;
National Synchrotron Light Source
We have constructed a high-speed diffractometer, employing the
"kappa" geometry, for use on the X16C beamline at NSLS,
Brookhaven National Laboratory. The beamline has a focused beam of
1010 photons per second in a submillimeter spot on the sample.
There we use a teflon environmental cell with a thin mylar window to hold
samples inside liquids under full electrochemical control. The thin-
layer geometry allows the transmission of the x-ray beam to the sample
and out again. As part of a campuswide effort, we are studying metal
and ionic adsorbed species on copper surfaces, to try to gain an
understanding of copper corrosion.
Coherent X-Ray Diffraction
I. K. Robinson,* J. A. Pitney, I. Vartaniants
National Science Foundation, DMR 93-15691, DMR 97-24294
In these experiments, we prepare beams of x-rays that are so narrow
that they are coherent across their width. A diffraction measurement
with such a beam is representative of the entire object under
illumination. An image of the object can therefore be derived by
inversion of its diffraction pattern, using computer algorithms.
X-Ray Diffraction Studies of Perovskite Ferroelectric Materials
I. K. Robinson,* D. M. Fanning, M. B. Weissman,
U.S. Department of Energy, DE-FG02-96ER45439
We are exploring the structural properties of "relaxor"
ferroelectric materials, such as lead magnesium niobate (PMN) using x-
ray diffraction. We are growing crystals of PMN by including various
metallic dopants that cause changes in their structural properties. In
this way we are learning about the structural origins of the unusual
dielectric properties of the materials.
Magnetic Behavior of Oxides and Nanophase Materials
M. B. Salamon,* M. Jaime, P. Kennedy, S.-H. Chun
U.S. Department of Energy, DE-FG02-96ER45439
Certain manganese oxides, when doped, exhibit remarkable changes in
electrical resistance at the ferromagnetic transition temperature.
These changes are sensitive to magnetic fields, causing colossal
magnetoresistance. We have demonstrated that conductivity above the
transition temperature occurs via the activated hopping of small
lattice polarons. Recent work focuses on the ferromagnetic state and
the nature of the transition to it. We have found that the resistivity
of single-crystal samples is independent of temperature at the lowest
temperatures, followed by the sudden onset of temperature dependence
arising from electron-spin wave scattering processes above 20 K. This
questions assumptions about the applicability of the so-called double
exchange model in its simplest form.
Phase Transitions in High-Temperature Superconductors
M. B. Salamon,* M. Hubbard, M. Jaime
NSF Science and Technology Center for Superconductivity
Several aspects of high-temperature superconductivity are under study.
The primary area of interest is the thermal conductivity of untwinned
samples of YBa2Cu3O7-x, of
YBa2Cu4O8, and of Tl-based superconductors,
especially the effect of magnetic fields. A novel anisotropy in the
thermal conductivity has been discovered in
YBa2Cu3O7-x crystals that
is induced by magnetic fields applied in the CuO planes and gives
evidence of nodes in the superconducting gap along certain directions.
Evidence has been found for a second transition at low temperatures in
doped materials that suggests the breaking of this reverse symmetry.
Contribution of the University of Illinois to the Magnetic Random
Access Memory Project
M. B. Salamon,* M. B. Weissman, E. Nowak, S.-H. Chun
U.S. Army/IBM 2040
We are subcontracted by IBM to explore the basic properties of
magnetic multilayer structures that might be useful for magnetic
random access memory (MRAM) modules. Specifically, we are studying the
noise characteristics of trilayer junctions that exhibit spin-
dependent tunneling and examining the distribution of magnetic
material in the structures by means of neutron reflectometry. The
latter work is being performed at the Missouri University Research
Reactor. Future work will involve detailed examination of tunneling
characteristics, perhaps using superconductors in place of the
magnetic top layer.
Nuclear Magnetic Resonance in Solids
C. P. Slichter,* N. Curro, C. Milling, K. Sakaie, R. Stern,
U.S. Department of Energy, DE-FG02-96ER45439
We probe magnetic and electric fields at the atomic level by NMR to
study many-body effects, phase transitions, magnetism, solids
possessing unusual properties, and electronic and structural aspects
of surface atoms and absorbed molecules (including catalysis).
Examples: Solids (1) High-temperature superconductors, for which NMR
provides detailed information about both the normal and
superconducting states. (2) Charge density waves (NMR of NbSe3)
including study of the motion under applied electric fields. Surfaces
(1) Electronic properties of the surface layer of atoms of Pt
particles, by 195Pt NMR. (2) Quantum effects arising from the
small size of the metal particles. (3) Bonding and structure of molecules
(e.g., CO, C2H2) adsorbed on Pt, by 13C NMR. (4) Special
methods: 1H, 13C double resonance to monitor
breaking of the C-H bond.
NMR Studies of High-Temperature Superconductors
C. P. Slichter,* N. Curro, C. Milling, R. Stern, I. Haase
NSF Science and Technology Center for Superconductivity
NMR has proved to be an important tool to study superconductivity. We
are investigating the normal and superconducting states of high-
temperature superconductors such as YBa2Cu3O7-d or La2-xSrxCuO4, to
learn how to describe the normal state, what mechanism leads to
superconductivity, and why the transition temperatures are so high.
The resonances of 63,65Cu, 17O, 89Y,
135,137Ba permit NMR to probe
specific atomic sites (e.g., Cu nuclei in the CuO2 planes).
Properties of Crystalline Condensed Gases
R. O. Simmons,* D. A. Arms, R. Shah
U.S. Department of Energy, DE-FG02-96ER45439
Atomic motions and mobilities in condensed matter influence many
important properties of materials. Condensed atomic and molecular
gases form excellent prototype systems, since they exhibit phenomena
in an enhanced form and can be studied under extreme conditions of
density, temperature, and quantum effects. Synchrotron x-ray and
neutron scattering is used to measure phase transformations, dynamics,
such as momentum distributions, in liquid and solid He, H2, Ne,
Ar, selected mixtures, C2F
Studies of Solid Helium-Three by Inelastic X-Ray Spectroscopy
R. O. Simmons,* D. A. Arms; A. T. Macrander
U.S. Department of Energy, DE-FG02-96ER45439, W-31-109-ENG-38
At the Advanced Photon Source at Argonne National Laboratory, a
special high-resolution x-ray spectrometer has been built to be used
for measurements of phonons. It is planned to study solid helium-
three, the most extreme quantum solid, at low temperatures and high
pressures. This quantum solid cannot readily be studied by
conventional neutron scattering methods because of its enormous
neutron absorption.
Excitations in Solids by Inelastic X-Ray Scattering
R. O. Simmons;* E. Burkel,* D. A. Arms, C. Seyfert, and H. Sinn (Univ.
of Rostock)
U.S. Department of Energy, DE-FG02-96ER45439; German Federal Ministry
of Research and Technology
Excitations in condensed matter systems have characteristic properties
that are summarized in the dynamic structure factor (S(Q,E) where
Q and E are the momentum and energy transfers, respectively, in a
scattering process. Insertion devices at synchrotron sources now
provide enough x-ray flux to measure S(Q,E) from very weakly
scattering systems, such as solid helium. Electronic excitations have
been studied in hcp 4He and phonons in both hcp 3He and hcp
4He using spectrometers at the Hamburg HASYLAB and at the
Grenoble ESRF, respectively.
Experimental Determination of the Pairing State of the Heavy Fermion
and Organic Superconductors
D. J. Van Harlingen,* B. Yanoff
National Science Foundation, DMR 97-05695
Experiments are underway to determine the symmetry of the
superconducting pairing state of two exotic superconducting materials-
-the heavy fermion superconductors and the organic superconductors.
Both of these are suspected to have unconventional pairing mechanisms
that lead to anisotropic energy gap structure. By measuring the
magnetic response of single Josephson junctions and dc SQUIDs
fabricated between single crystals of the exotic superconductor (the
heavy fermion superconductor UPt3, or the organic system
k-ET2Cu[N(CN)2]Br) and conventional superconducting
thin films, we can determine the relative phase of the order parameter in
different directions and thus can distinguish proposed anisotropic pairing
states.
Vortex Configurations and Dynamics in Superconductor Arrays
D. J. Van Harlingen,* M. S. Wistrom
National Science Foundation, DMR 97-05695
Magnetic vortices dominate the thermodynamics and transport properties
of microfabricated superconductor arrays and two-dimensional
superconducting films. We are studying the motion, pinning, and
interactions of vortices by a combination of experimental techniques
and computer simulations. The standard approach is to measure
magnetotransport properties that reflect the averaged vortex dynamics.
In addition, we have developed a scanning SQUID microscope system that
allows us to image directly the spatial arrangement and monitor the
dynamics of discrete vortices. Images of the configuration of vortices
in large arrays and clusters have been obtained and analyzed in terms
of vortex diffusion and trapping models.
Experimental Determination of the Pairing State of the High-
Temperature Superconductors
D. J. Van Harlingen,* J. Hilliard
NSF Science and Technology Center for Superconductivity
We are carrying out a series of experiments designed to determine the
symmetry of the superconducting pairing state of the high-temperature
superconductors. The work involves measurements of the magnetic
response of single Josephson junctions and dc SQUIDs fabricated
between single crystals of YBCO and conventional superconducting thin
films. The experiment is sensitive to the relative phase of the order
parameter in different directions and thus can distinguish proposed
anisotropic pairing states from the conventional isotropic pairing.
Our results give strong evidence for d-wave pairing in YBCO as has
been predicted for electron coupling via magnetic spin fluctuations.
Work is underway to extend this to other materials and configurations.
Magnetic Imaging of Vortices in High-Temperature Superconductor Films
and Devices
D. J. Van Harlingen,* B. Plourde
NSF Science and Technology Center for Superconductivity
We are developing a novel structural and magnetic imaging scanning
instrument for studying vortices in high-temperature superconductors.
The scanning junction microscope uses a modified STM to scan a single
Josephson junction over the surface of a superconducting sample to
detect the local magnetic field distribution with submicron
resolution. Our goal is to image simultaneously the surface topography
and the location of magnetic vortices in the sample. With this
capability, we can determine the effect of grain boundaries and
defects on the motion and pinning of vortices. We also hope to probe
the energy level spectra of quasi-particle bound states in the core of
vortices in the cuprates.
Phase Coherence and Bound Quasi-Particle States in Mesoscopic
Superconducting Structures
D. J. Van Harlingen,* K. D. Osborn
U.S. Department of Energy, DE-FG02-96ER45439
We use scanning tunneling microscopy (STM) to investigate confined
quasi-particle states in inhomogeneous superconductor structures. STM
is a powerful tool that can simultaneously determine the surface
structure and give information on the local electronic density of
states. We have observed quasi-particle states bound in a normal metal
island on top of a superconductor--the STM detects the spatial
variation and energy dependence of the tunneling conductance. These
experiments give information about the interface between
superconductors and normal metals and the dynamics of quasi-particles
and the proximity effect. These measurements will be extended to study
charge transport in semiconductor-superconductor interfaces.
Superconducting Vortex Dynamics
M. B. Weissman,* M. Rabin
NSF Science and Technology Center for Superconductivity
The pinning and depinning of magnetic vortices determines whether a
superconductor remains superconducting in a magnetic field. Individual
vortices usually do not pin well enough to maintain good
superconductivity, but collective vortex pinning can be very strong.
We are using new noise techniques to study how the collective effects
develop by which vortices strongly pin.
Noise Investigations in Condensed Matter Systems
M. B. Weissman,* R. Merrithew, J. Petta
National Science Foundation, DMR 96-23478
Most conducting materials exhibit conductivity fluctuations with a
spectral density approximately inversely proportional to frequency
over a wide range. This simple, universal appearance hides a multitude
of different mechanisms that can provide information on dynamical
properties of many condensed materials, especially ones with
significant disorder. We are using this noise to study the dynamics of
magnetic vortices in superconductors, domain dynamics in magnets, and
basic problems in the formation of glasses and spin glasses.
Mesoscopic Noise Studies in Condensed Materials
M. B. Weissman,* H. T. Hardner, K. O'Brien
University of Illinois
Many of the dynamical properties of amorphous materials are not
accessible to ordinary measurements in large samples, because most of
the detailed behavior of individual sites is lost in an average over
many differing sites. Ideally, one wants a method to study individual
sites at which, for example, atoms move or spins flip. We are now
using the statistics of conductance noise to study the fundamental
mechanism of the "colossal magnetoresistance" effect. We
have just developed a new dielectric noise technique, which we are
applying to understand a class of unusual dielectrics, called relaxor
ferroelectrics, with puzzling and potentially useful properties.
Quantum Statistics of Excitons in Semiconductors
J. P. Wolfe,* K. O'Hara, J. Warren, J. Guillingsrud
National Science Foundation, DMR 88-22761
Excitons, or bound electron-hole pairs, are produced by optical
excitation of semiconductors at low temperatures. The excitons in Cu
2O behave as an ideal gas of particles within the crystal. The
volume of this gas is determined by the excitonic lifetime (nanoseconds to
microseconds) and diffusion rate. As the laser excitation level is
increased, the excitonic gas displays quantum statistics. We are
examining the kinetics and thermodynamics of this unique gas and the
occurance of Bose-Einstein statistics. The techniques used include
time- and space-resolved photoluminescence following laser pulses of
nanosecond and picosecond duration.
Nonequilibrium Studies of Photoexcited Carriers and Phonons in
Semiconductors and Superconductors
J. P. Wolf,* J. Kim, J. Short, R. Vines
U.S. Department of Energy, DE-FG02-96ER45439
Photoexcitation of GaAs/AlGaAs quantum wells produces nonequilibrium
electron-hole pairs with a lifetime of about 1 nsec. These carriers
form excitons (at low densities) and electron-hole plasmas (at high
densities), which propagate in the confining plane of the GaAs quantum
well. We have concentrated on the quantum statistics of such particles
in two dimensions and discovered evidence for this phenomenon. On
another front, we are measuring nonequilibrium phonons in
superconductors such as Nb and Pb and have imaged their propagation
directly.
Nuclear Physics Research
A. M. Nathan,* R. J. Holt,* D. H. Beck, P. T. Debevec,
National Science Foundation, PHY 94-20787
The group doing experimental nuclear physics at Illinois pursues its
studies at a variety of accelerator facilities throughout the world,
using high-energy beams of electrons, photons, muons, and antiprotons.
Nuclear Physics Studies Using Beams of Photons
A series of experiments has begun that are aimed at obtaining an
understanding of the meson exchange/quark nature of the nucleon and
few-nucleon systems. The Illinois group is leading this effort at
Jefferson. These experiments include deuteron particles, integration,
prior photoproduction from nucleons and light nulei, and Compton
scattering from the nucleon.
Nuclear Physics Studies Using Beams of Electrons
There are two programs in which we are
heavily involved. First is a program to measure the contribution of
strange quarks to the vector current of the nucleon. These experiments
utilize the parity-nonconserving interference between electromagnetic
and weak neutral currents in the scattering of longitudinally
polarized electrons from the proton. One of these experiments, called
SAMPLE, will measure the strange quark contribution to the magnetic
moment of the proton and will take data at the MIT/Bates Linear
Accerator. A far more ambitious program is the G0 experiment, which
will take place at the new national electron facility, Jefferson
Laboratory. It involves the construction of a novel and dedicated
magnetic spectrometer. The entire effort is being spearheaded by the
Illinois group. Second is a series of experiments (HERMES), which will
measure the spin structure of the proton and neutron by scattering
longitudinally polarized electrons from polarized protons and
neutrons. This will allow one to determine how much each type of quark
contributes to the spin of the nucleon. An exciting new initiative is
an experiment to measure the contribution of the gluons to the spin of
the proton. This involves upgrading the HERMES experiment to detect
"charmed" mesons, an effort in which the Illinois group
plays a major role. The experiments are underway at the DESY facility
in Hamburg, Germany. One of the principal contributions of the
Illinois group to the collaboration is the design and construction of
laser-driven polarized hydrogen and deuterium targets to be used in
the experiments. This target is currently being tested at the Indiana
University Cyclotron Facility in Bloomington, Indiana. It will shortly
be used in an experimental trial to measure the spin structure of the
deuterium.
Physics Using Beams of Antiprotons
These experiments involve work at the Low Energy Antiproton
Ring (LEAR) at CERN, the multinational European accelerator complex
located in Geneva. With several other institutions, we are studying
two topics in antiproton physics that are of high present interest,
both involving the production of strange quarks. The first topic
involves hyperon production near threshold; the physics interest is to
understand the dynamics of strangeness production in the low-energy
regime. This is possible because the observables of the final state
hyperons, which exhibit strangeness, are thought to mirror the
behavior of the internal strange quark. The second topic features the
investigation of reactions that can proceed through channels
exhibiting large gluonic "exotic" matter (the so-called
JETSET experiment). A major Illinois contribution to these projects
was the construction of a novel electromagnetic calorimeter.
Precision Measurement of the Anomalous Magnetic Moment of the Muon
Measurements of the magnetic dipole moments of particles have played
an important role in understanding of the structure of matter.
Deviations from the expected characteristics of "point-like"
particles appear as so-called anomalous moments and are sensed by
observation of the precession rates of such particles in magnetic
fields. For protons and neutrons, anomalous magnetic moments are big,
as expected for these particles, which are each built from three
quarks. But for electrons and muons the anomaly is tiny, and so far is
in agreement with theoretical expectations to an extraordinary degree
of precision. We are participating in a new experimental effort to
measure the muon anomalous magnetic moment 20 times better than
previous work; this will result in a test of the relevant quantity
termed "(g-2)" to a level of 0.35 ppm. If achieved, the
result will test the contributions of the weak interaction to the muon
(g-2) factor--an essential component of the electroweak theory, which
has not yet been detected experimentally. Deviations from the theory
may occur only by invoking new physics phenomena. The experiment is
being mounted at the Brookhaven National Laboratory. Our Illinois
group is building the major detectors, constructing novel electronics
simulation systems, and developing a unique electron traceback system
from state-of-the-art particle-tracking devices.
Elementary Particle Experiment
L. Holloway,* J. J. Thaler,* B. Eisenstein, D. Errede,
U. S. Department of Energy, DE-FG02-91ER40677
The two main thrusts of high-energy physics research are to determine
the form and strength of the fundamental interactions in nature and to
determine the properties of the particles that enter into these
interactions. The two main thrusts of elementary particle physics
research are to determine the form and strength of the fundamental
interactions in nature and to determine the properties of the
particles that enter into these interactions. Our group presently
works on experiments at Fermilab and Cornell, and plans to work on one
at CERN. We participated in the discovery of the top quark and expect
to observe time reversal symmetry violation in B-meson decays. In the
future, we hope to observe the Higgs boson, thought to be responsible
for the existence of mass.
Collider Detector at Fermilab
L. Holloway,* D. Errede, S. Errede, M. Kasten, T. Liss,
The superconducting particle accelerator at Fermilab is used to store
beams of protons and antiprotons at 1000 GeV, the world's highest
energy. The CDF group has built a large detector to investigate the
nature of the interactions that occur when these beams collide head-
on. Precise measurements of the properties of the W boson, top quark,
and other elementary particles are being made.
Study of Heavy Flavors at the Cornell Electron Storage Ring (CESR)
T. Bergfeld, B. Eisenstein, J. Ernst, G. Gladding, G. Gollin, M.
Haney, E. Johnson, I. Karliner, S. Luo, M. Marsh,
We use the CLEO detector at CESR to study the properties of the lepton
and of particles containing the b and c quarks. These studies allow us
to perform stringent tests of the standard model of the fundamental
interactions. This is the modern equivalent of the atomic physics
experiments performed early this century to test quantum mechanics. We
are participating in a major upgrade of the CLEO detector, which will
effect dramatic improvements in the experiment's resolution and
statistical precision. One of our goals is to determine whether or not
the standard model can account for the small matter-antimatter
asymmetry present in our universe.
CLEO Experiment at CESR
J. J. Thaler,* B. I. Eisenstein, G. E. Gladding, G. D. Gollin, I.
Karliner, M. A. Selen, J. A. Ernst, M. J. Haney, R. Hans, M. A.
Palmer, T. J. Bergfeld, J. Buckley, C. Sedlack,
U.S. Department of Energy, DE-FG02-91ER40677
The CLEO experiment at the Cornell electron positron storage ring
(CESR) studies the properties of the bottom and charmed quarks and the
tau lepton. The primary goals of these studies are: (1) the
understanding of the origin of the Cabibbo-Kobayashi-Maskawa (CKM)
mixing matrix, for which no dynamical theory exists; (2) understanding
of time reversal symmetry violation, which is appears to be a
necessary prerequisite to the observed matter-antimatter asymmetry of
the universe; and (3) tests of the "standard model" of
particle physics, whose very precise predictions have been tested very
accurately, but which, nonetheless, is known not to be correct.
Deviations from these predictions will tell us where the flaw lies.
High-Energy Photoproduction
J. Wiss,* E. Gottschalk, K. S. Park, C. Caulfield, A. Rahimi, in
collaboration with physicists from other institutions
We study charm particles produced by the interaction of high-energy
photons on nuclear targets. We have reported on new measurements of
charm lifetimes, photoproduction dynamics, hadronic and semileptonic
decay, and excited state spectroscopy from our sample of 80,000 charm
decays collected in the 1990/1991 run of Fermilab E687. Our next
experiment, Focus, has collected over an order of magnitude more data
with a significantly upgraded spectrometer. The Illinois group has
made major contributions to the experimental software, hardware, and
data analysis.
Causes of Interannual Variability of Global Climate
P. Handler*
University of Illinois
Aerosols that result from large injection by volcanoes of millions of
tonnes of sulfur dioxide into the stratosphere have a large effect on
global climate. These stratospheric aerosols decrease the net
radiation reaching the earth. The loss of radiation produces a
coherent climate anomaly around the world. One of these global
anomalies is the El Niño/Southern Oscillation. Another anomaly
associated with the decreased radiation is the decrease in monsoon
rainfall. In other regions such as the Great Lakes the aerosols induce
additional rainfall that shows up in later years as an increase in the
levels of Lake Michigan and Lake Huron.
Fermilab, 1965-1990: A Case Study in the Emergence of Big Science
L. Hoddeson* (History), C. Westfall (Michigan State Univ.)
National Science Foundation Program in History and Philosophy of
Science, DIR 90-15473
This historical study explores critical features in the emergence of
large-scale research in America through a close examination of the
founding and first decades of operation of Fermi National Accelerator
Laboratory (Fermilab). Through research in documents, supplemented by
oral history interviews with leading participants, the project is
designed to examine, in a book-length study and articles, historical
problems of "big science," including: the effects on
research of increased size, scope, and cost of facilities and
experiments; changes in research resulting from much larger working
teams, budgets, and time scales; the role of theoretical models and
computer analysis in experimentation and accelerator building; and the
influences of geographical concentration, national and international
politics, and economic policy.
Bardeen Scientific Biography
L. Hoddeson* (History)
Richard Lounsbury Foundation; Dibner Fund; University of Illinois
This study is oriented toward writing a scientific biography of John
Bardeen set into the context of the history of solid-state physics
between 1930 and 1990. Special attention is given to the discovery of
the transistor in 1947 and the BCS Theory of Superconductivity in
1957.
Writing with Atoms
M. H. Nayfeh,* J. Hetrick, A. Archer
U.S. Office of Naval Research, N00014-87-K-0354
The project aims at achieving selective deposition of single atoms on
surfaces with very high resolution that may reach atomic dimensions.
Tunable lasers photodissociate molecules and highly excite the atomic
fragments in the field of the sharp needle of a scanning tunneling
microscope, which ionizes and guides the atoms to the surface.
Preparation and Characterization of Porous Silicon
M. H. Nayfeh,* N. Rigakis, L. A. Hassan, Z. Yamani
University of Illinois
The project focuses on the preparation and characterization of the
newly discovered optically active porous silicon. The studies include
topographical, compositional, structural, optical, electrical, and
chemical characterizations. These characterizations are correlated
with conditions of preparation and with stability under different
conditions.
Atomic Electronics
M. H. Nayfeh,* J. Hetrick, A. Archer
U.S. Office of Naval Research
Using scanning tunneling microscopes augmented by laser radiation,
this project aims to develop a new kind of electronics (atomic
electronics), one that relies on quantum mechanics and the movement of
single particles, with the purpose of one day producing devices many
times faster and smaller than anything available today. In the
project, atomic scale (nanometer scale)-fabricated structures will be
embedded in the gate area of micron scale Si/SiO2 metal-oxide-
semiconductor field effect transistors (FET) and GaAs/AlGaAs high-
electron-mobility transistors.
Theoretical Studies of X-Ray and Gamma-Ray Emission by Neutron Stars,
White Dwarfs, and Black Holes
F. K. Lamb,* C. J. Pethick, L. Zampieri, D. Psaltis
National Aeronautics and Space Administration, NAG 5-2925
This project involves theoretical research that directly supports
analysis and interpretation of data from recent and forthcoming NASA-
supported high-energy astrophysics missions. This research focuses on
six main topics: neutron star structure, dynamics and evolution;
accretion by magnetic and nonmagnetic neutron stars and black holes;
quasi-periodic x-ray brightness oscillations (QPOs), pulse frequency
changes in x-ray radio pulsars, and x-ray bursts; x-ray spectroscopy
of accretion-powered pulsars, accreting neutron stars in low-mass
binary systems, and solitary neutron stars; gamma-ray emission by
accreting neutron stars; and feeding of black holes in active galactic
nuclei.
X-Ray Properties of Accreting and Isolated Neutron Stars
F. K. Lamb,* C. J. Pethick, L. Zampieri, D. Psaltis
National Aeronautics and Space Administration, NRA-95-02- SZ-069
Much of what we know about neutron stars has come from x-ray
observations. We are investigating disk accretion by magnetic and
nonmagnetic neutron stars; subcritical and supercritical radial flows
onto nonmagnetic and magnetic neutron stars; the x-ray spectra of disk
and radially accreting neutron stars; and neutron star structure,
cooling processes, and thermal evolution. The results are used to
improve understanding of spin-up and spin-down of accretion-powered
pulsars, the nature of the Z and atoll sources, quasi-periodic
brightness oscillations in neutron stars and black holes, and thermal
x-ray emission by isolated neutron stars. These studies directly
support NASA's high-energy astrophysics missions, including ROSAT, the
Compton Observatory, EVVE, RXTE, and AXAF.
Study of Kilohertz QPOs in Z Sources
F. K. Lamb,* L. Zampieri
National Aeronautics and Space Administration, RXTE-30040
This research project focuses on additional observations of the Z
sources using the Rossi X-Ray Timing Explorer and further development
of theoretical models. The project is measuring the photon energy
dependence of the kilohertz quasi-periodic x-ray brightness
oscillations (QPOs) discovered earlier by the team using the Rossi
Explorer, the dependence of QPO frequencies on accretion rate, and
microsecond time variability, and is comparing the results with gas
dynamical and radiation transport calculations. New data analysis
algorithms and advanced time series analysis techniques developed by
the team are being used.
A Comprehensive Survey of Atoll Sources
F. K. Lamb,* L. Zampieri
National Aeronautics and Space Administration, RXTE-30701
With the co-discovery by the UIUC team of quasi-periodic x-ray
brightness oscillations (QPOs) at kilohertz frequencies using the
Rossi X-Ray Timing Explorer, we appear to be on the threshold of a
breakthrough in measuring general relativistic effects in the strong-
field regime and in determining the masses and radii of neutron stars
and the equation of state of neutron star matter. The major survey of
the atoll sources and further theoretical research that this grant is
supporting are expected to provide much of the additional results
needed to achieve this breakthrough.
Rapid X-Ray Variability of Z Sources
F. K. Lamb,* L. Zampieri, D. Psaltis
National Aeronautics and Space Administration, RXTE-AO-1-10061
We are using NASA's Rossi X-ray Timing Explorer (RXTE) satellite to
study, for the first time, the submillisecond variability of the so-
called Z sources, which are among the brightest known x-ray stars. The
goals of this project include detailed analysis of the quasi-periodic
brightness oscillations (QPOs) discovered by the team in four sources,
comparison of the results with the detailed predictions of QPO
properties made previously by the theoretical group at Illinois, and
searches for predicted new, fast aperiodic variability and millisecond
periodic oscillations.
High-Time-Resolution X-Ray Spectroscopy of Z Sources
F. K. Lamb,* L. Zampieri, D. Psaltis
National Aeronautics and Space Administration, XTE-AO-1-10065
NASA's Rossi X-ray Timing Explorer satellite is being used to study,
for the first time, variations in the x-ray spectra of two Z sources
on timescales of milliseconds, which are the timescales of the quasi-
periodic oscillations and aperiodic flickering observed in these x-ray
stars. We are reconstructing x-ray spectra to determine how the x-ray
spectra of these sources vary on very short timescales. We are also
developing quantitative models of the x-ray spectrum and the
millisecond x-ray variability expected when matter falls onto the
compact object in these systems, which is thought to be a neutron
star. Successful comparison of the predictions of these models with
our analysis results will test the models and allow us to derive the
physical properties of the neutron stars and accretion disks in these
systems.
Luminosity Dependence of the X-Ray Spectra and Variability of Atoll
Sources
F. K. Lamb,* L. Zampieri, D. Psaltis
National Aeronautics and Space Administration, RXTE-10072
At low luminosities, the x-ray properties of the x-ray stars called
atoll sources are very similar to those of black hole candidates in
their low states. We are using NASA's Rossi X-ray Timing Explorer
satellite to observe a selected sample of atoll sources, which are
thought to be accreting neutron stars, in order to study the
luminosity dependence of their x-ray spectra and x-ray variability.
This research is aimed at understanding the respective roles of the
mass accretion rate and the magnetic field in determining the x-ray
spectrum and its variability and the reasons for the similarities
between the properties of atoll sources and accreting black holes.
Detailed theoretical modeling of x-ray spectral formation in LMXBs is
an integral part of the study.
Analysis of X-Ray Emission from the Bursting Pulsar
F. K. Lamb*
National Aeronautics and Space Administration, RXTE-20078
The Bursting Pulsar is a rotating neutron star that was discovered on
December 2, 1996, as it began a giant outburst that lasted six months.
This pulsar is unique among all known pulsars in producing both
periodic x-ray oscillations with a frequency of 2 Hz and powerful x-
ray bursts at intervals ranging from three minutes to ten hours. The
cause of the outburst and the mechanism that generates the x-ray
bursts are both unknown. This grant is supporting a year-long
monitoring campaign in which 10 ksec of data on the Bursting Pulsar
are collected approximately every two weeks using the Rossi X-Ray
Timing Explorer. Analysis of these data will be used to develop and
test models of the accretion torque on the neutron star and models of
the outburst and x-ray bursts.
Analysis of Unusual X-Ray Behavior of the Bursting Pulsar
F. K. Lamb*
National Aeronautics and Space Administration, RXTE-20077
This grant will support collection, analysis, and interpretation of x-
ray and gamma-ray data on the Bursting Pulsar taken with the Rossi X-
ray Timing Explorer when the pulsar displays unusual behavior, such as
a pronounced increase in its brightness between x-ray bursts, a
substantial increase in the rate or brightness of the bursts, or
unusual changes in its spin rate. These data will be used to develop
and test models of the accretion torque on the neutron star and models
of the outburst and x-ray bursts.
Further Studies of Rapid X-Ray Variability in Z Sources
F. K. Lamb,* L. Zampieri, D. Psaltis
National Aeronautics and Space Administration, RXTE-20053
The immediate goal of this project is to explore further the
relatively coherent pairs of quasi-periodic brightness oscillations
(QPOs) with frequencies in the kilohertz range that we discovered in
the x-ray stars Sco X-1 and GX 5-1, and the anomalous frequency
behavior of the horizontal-branch QPO that we discovered in the x-ray
star GX 172 using NASA's Rossi X-Ray Timing Explorer satellite. The
kilohertz QPOs we have discovered are the highest frequency coherent
behavior ever detected in neutron stars and have important
implications for their structure and the equation of state of dense
matter. We are modeling these phenomena using analytical methods as
well as radiation- and magnetohydrodynamic computer simulations.
Further Studies of Rapid X-Ray Variability in Atoll Sources
F. K. Lamb,* L. Zampieri, D. Psaltis
National Aeronautics and Space Administration, RXTE-20064
The immediate goal is to explore further the relatively coherent pairs
of quasi-periodic brightness oscillations (QPOs) with frequencies in
the kilohertz range that we discovered in four atoll sources and the
correlated x-ray spectral variations seen in these x-ray stars. The
kilohertz QPOs we have discovered are the highest frequency coherent
behavior ever detected in neutron stars and have important
implications for their structure and the equation of state of dense
matter. We are carrying out new observations using NASA's Rossi X-ray
Timing Explorer satellite and new modeling calculations using
analytical methods and radiation- and magnetohydrodynamic computer
simulations.
Studies in Theoretical Physics and Astrophysics
F. K. Lamb,* C. J. Pethick, S. L. Shapiro, D. Markovic
National Science Foundation, AST 96-18524
This broadly based and highly interdisciplinary research project is
addressing problems in condensed matter physics, nuclear physics, and
general relativity as well as neutrino astrophysics, stellar dynamics,
hydrodynamics, supernovae, neutron stars, black holes, and active
galactic nuclei. A common theme uniting the diverse research of the
group is understanding the physics of compact objects. The project is
investigating the properties of matter under extreme conditions and in
strong gravitational fields, phase transitions, accretion,
gravitational collapse, binary coalescence, relativistic accretion
disks, and the generation of electromagnetic, gravitational, and
neutrino radiation, using large-scale numerical computations as well
as analytical techniques.
Numerical Models of Colliding Galaxies and Global Properties of
Observed Systems
S. Lamb,* in collaboration with R. Gerber, Lawrence Berkeley Lab, and
D. Balsara, National Computational Science Alliance
University of Illinois
We are investigating galaxy interactions in which one galaxy passes
through the gas and stars of another using a 3-D combined N-
body/smooth particle hydrodynamics (SPH) code and initial stable
galaxy models. We have computed numerical simulations of collisions
between a gas-rich disk galaxy and a gas-free spherical galaxy with
the trajectory parallel to the spin axis of the disk galaxy and with
various impact parameters. By varying the ratio of the masses of the
two galaxies over an order of magnitude, we are demonstrating the
dependencies of the time scales for global star formation and other
relevant phenomena.
Global Star Formation in Impact-induced Starburst Galaxies
S. Lamb*
University of Illinois
The first impact of two colliding galaxies takes place on a time scale
of approximately 108 years, the dynamical time scale. Within this
period it is anticipated that much star formation will be triggered as
a result of density increases and shocks in the gas which are produced
by inflow to the nuclear regions. We are currently comparing our array
of simulations of galaxy collisions to observations of collisionally
produced starburst galaxies (both our own observations and those of
others) and investigating the resulting implications for both the
stellar and gaseous components.
Active Galactic Nuclei, Dense Stellar Systems, and Galactic
Environment
S. Lamb,* in collaboration with J. Perry, Cambridge, England, and
twelve others in the U.S. and Europe
University of Illinois
We are investigating a self-consistent model on a large range of
scales to understand the processes leading to nuclear activity in
galaxies. Current observations support the view that interactions
between galaxies may be crucial in triggering episodes of activity in
some active galactic nuclei. Interactions also trigger some
starbursts, and we are investigating the relationship between these
two phenomena. We employ numerical simulations of colliding galaxies
and analytical studies of the physics of the central regions of
galaxies to obtain a detailed model that can be compared to
observations of these systems.
Studies in Theoretical Astrophysics
S. L. Shapiro*
National Science Foundation, AST-91-19475
A gravitomagnetic field arises from moving matter just as an ordinary
magnetic field arises from moving charges. The upcoming Gravity Probe
B satellite will measure the rotating Earth's gravitomagnetic force.
Near a rapidly rotating black hole the gravitomagnetic force rivals
the static gravitational field in strength. A changing gravitomagnetic
force emanating from a rapidly rotating black hole can induce matter
currents inside, say, a neutron star spiraling in toward the hole.
This induced vorticity will influence the spin of the star, its
internal structure, and its orbital motion and will be discernible in
gravitational waves reaching future Earth-based detectors.
Theory of Diffuse Matter in Astrophysics
W. D. Watson,* D. Wiebe, S. Menshchikov, A. Sobolev,
National Science Foundation, AST 94-01348
Theoretical research is conducted to elucidate physical processes in
the diffuse astrophysical environment. Currently, the primary effort
is in understanding the transport of maser radiation in rotating disks
that occurs in the formation of stars from the interstellar gas.
Cosmic Microwave Background Anisotropies and Structure Formation
M. White*
University of Illinois
The tens of micro-Kelvin variations in the temperature of the cosmic
microwave background (CMB) radiation across the sky encode a wealth of
information about the universe. The full-sky, high-resolution maps of
the CMB that will be made in the next decade should determine
cosmological parameters to unprecedented precision and sharply test
inflation and other theories of the early universe. We are working on
the theoretical foundations of CMB anisotropy formation and the
interface between theory and experiment in this highly active field.
Microscopic Many-Body Models for Novel Superconductors
D. K. Campbell,* P. Prelovsek,* J. Bonca, H.-Q. Lin
U.S.-Slovenia Joint Research Grant, JF-971
Considerable experimental evidence suggests a relation of high-
temperature (high Tc) superconductivity to antiferromagnetism
(AFM). To clarify this relationship, we have developed a novel approach in
which a staggered--i.e., alternating--external magnetic field is
applied to one-dimensional Hubbard and t-J Hamiltonians describing the
strongly correlated electrons. The staggered field models the AFM
background that exists in the real planar, high Tc materials. Applying
both numerical and analytic techniques, we find that even modest
staggered fields induce the formation of bound hole pairs--thus
enhancing the tendency to superconductivity--in the strongly coupled
limits of both the Hubbard and t-J models.
Many-Particle Tunneling Effects and Resonant Processes in Mesoscopic
Systems
D. K. Campbell,* G. R. Berman,* K. N. Alekseev,
NATO Linkage Grant, NANO.LG 931602
Remarkable recent advances in materials science permit the
construction of new "mesoscopic/nanoscale" materials with
structures on the scale of 10-100 nm. These "quantum dots,"
"wires," and "layers" exhibit many new physical
phenomena as nonlinear, quantum, and finite-size effects combine and
compete. We have initiated three theoretical studies in this area: (1)
correlated electron models for quantum dots and wires; (2) resonant
processes in weak and strong electromagnetic fields; and (3) ground
states and phase transitions in discrete quantum 1-D and 2-D systems,
including the role of many-particle tunneling effects, diffusion, and
quantum fluctuations. We will compare our results with experiments and
seek applications in the designs of novel electronic devices.
Electronic Structure of Condensed Matter
D. M. Ceperley,* R. M. Martin,* F. H. Zong, M. Dewing,
National Science Foundation, DMR 94-22496
The goals of our research are to develop computational methods for
condensed matter starting from the fundamental many-body equations.
The primary methods used are quantum Monte Carlo simulations, which
can find exact properties of many-body systems, and density functional
methods, which can be applied to diverse solids and liquids. We are
combining these approaches to create new methods and to test the
accuracy of calculations on materials. Current research includes
studies of silicon crystals, metal surfaces, metalization of hydrogen
at high pressure, rare gas layers, simulations of solids and liquids
as a function of temperature, atoms in strong magnetic fields, and the
fractional quantum Hall effect.
Optical Properties of Semiconductor Quantum Wires grown by the SILO
Process
Y.-C. Chang,* L. Li
National Science Foundation, ECS 96-17153
Band structures and optical matrix elements of strained multiple
quantum wires (QWRs) are investigated theoretically via the effective
bond-orbital model, which takes into account the effects of valence-
band anisotropy and band mixing. The Ga1-xInxAs QWRs
grown by strain-induced lateral ordering (SILO) are considered. Long
wavelength Ga1-xInxAs QWR lasers have been
fabricated via a single-step MBE technique which uses the SILO process.
Low threshold current and high optical anisotropy have been achieved.
Multiaxial strains for the QWR
(combinations of biaxial strains in the [001] and [011] planes) are
considered. Our calculated anisotropy in optical matrix elements is in
good agreement with the experimental results.
Exchange Coupling in Magnetic Multilayers
Y. C. Chang,* B. Lee, L. Tsetseris
University of Illinois
We performed theoretical studies of the interlayer exchange coupling
(IEC) in magnetic multilayer (Co/Cu and Fe/Cr) systems, taking into
account the realistic band structures. We also investigated the
magnetic layer thickness dependence of IEC in magnetic multilayers in
which the extremal points of the Fermi surfaces for the spacer and
magnetic material do not coincide. We showed that the oscillation
period is determined by a stationary condition that depends on the
mixed geometry of Fermi surfaces for both materials.
Electronic and Optical Properties of Surfaces and Heterostructures
Y.-C. Chang,* G. Li, L. Wei
U.S. Office of Naval Research, N00014-90-J-1267; National Science
Foundation, ECS 96-17153
This project concentrates on theoretical studies of electronic and
optical properties of semiconductor surfaces and heterostructures by
using a newly developed first-principle pseudopotential method in
planar-orbital basis (products of two-dimensional plane waves and one-
dimensional Gaussian functions). The method is efficient and accurate
and well suited for treating layered systems. In particular, we are
investigating the work functions, hydrogen passivation, and optical
responses of various semiconductor surfaces. Planar Wannier functions
can be constructed directly from Bloch states expressed in terms of
planar orbitals and they can be used for modeling of realistic
heterostructure devices.
A Chern-Simons Effective Field Theory for the Pfaffian Quantum Hall
State
E. Fradkin,* C. Nayak (UCSB), A. Tsvelik (Oxford Univ.),
National Science Foundation, DMR 94-24511
We present a low-energy effective field theory describing the
universality class of the Pfaffian quantum Hall state. To arrive at
this theory, we observe that the edge theory of the Pfaffian state of
bosons at v = 1/2 is an SU(2)2 Kac-Moody algebra. It follows
that the corresponding bulk effective field theory is an SU(2)2
Chern-Simons theory with coupling constant K = 2. The effective field
theories for other Pfaffian states, such as the fermionic one at v = 1/2
are obtained by a flux-attachment procedure. We discuss the non-Abelian
statistics of quasi-particles in the context of this effective field
theory.
Andreev Reflection in the Fractional Quantum Hall Effect
E. Fradkin,* N. P. Sandler, C. Chamon
National Science Foundation, DMR 94-24511
We study the reflection of electrons and quasi-particles on point-
contact interfaces between fractional quantum Hall (FQH) states and
normal metals (leads), as well as interfaces between two FQH states
with mismtached filling fractions. We classify the processes taking
place at the interface in the strong coupling limit. In this regime a
set of quasi-particles can decay into quasi-holes on the FQH side and
charge excitations on the other side of the junction. This process is
analogous to an Andreev reflection in normal-metal/superconductor
interfaces.
Applications of Field Theory to Condensed Matter Physics
E. Fradkin,* P. Goldbart, M. Stone, C. Chamon,
National Science Foundation, DMR 94-24511
This program is aimed at advancing the theoretical understanding of a
variety of condensed matter systems, each involving many strongly coupled
degrees of freedom. Attention is primarily focused on the following
areas: electronic liquid crystal phases in Mott insulators; the quantum
Hall effect; geometric phases and their condensed matter implications;
superfluids and superconductors, including vortex motion in dirty systems,
quantum critial behavior of magnetic impurities in D-wave
superconductors; vulcanized matter and the vulcanization
transition; structural glasses and network-forming systems, glassiness
of superfluid helium-three in aerogel, shapes adopted by large
biological macromolecules, and static and dynamic properties of
polysoap macromolecules.
Electronic Liquid Crystal Phases of a Doped Mott Insulator
E. Fradkin,* S. Kivelson (UCLA), V. Emery (Brookhaven)
National Science Foundation, DMR 94-24511
The character of the ground state of an antiferromagnetic insulator is
fundamentally altered upon addition of even a small amount of charge.
The added charge agglomerates along domain walls at which the spin
correlations suffer a π phase shift. In two dimensions, these domain
walls are "stripes" which are either insulating, or
conducting. Here it is shown that a transition to a charge density
wave (CDW) is eliminated if the zero-point energy of transverse stripe
fluctuations is sufficiently large in comparison to the CDW coupling
between stripes. As a consequence, there exist novel electronic
quantum liquid crystal phases which constitute new states of matter
and which can be either a high-temperature superconductor or two-
dimensional anisotropic "metallic" non-Fermi liquids.
Exact Calculation of Multifractal Exponents of the Critical Wave
Function of Dirac Fermions in a Random Magnetic Field
E. Fradkin,* H. Castillo, C. Chamon, P. M. Goldbart,
National Science Foundation, DMR 94-24511
The multifractal scaling exponents are calculated for the critical
wave function of a two-dimensional Dirac fermion in the presence of a
random magnetic field. It is shown that the problem of calculating the
multifractal spectrum maps into the thermodynamics of a static
particle in a random potential. The multifractal exponents are simply
given in terms of thermodynamic functions, such as free energy and
entropy, which are argued to be self-averaging in the thermodynamic
limit. These thermodynamic functions are shown to coincide exactly
with those of a generalized random energy model, in agreement with
previous results obtained using Gaussian field theories in an
ultrametric space.
Distinct Universal Conductances in Tunneling to Quantum Hall States:
The Role of Contacts
E. Fradkin,* C. Chamon
National Science Foundation, DMR 94-24511
We have shown that different universal values can be obtained for the
two-terminal conductance of a fractional quantum Hall state. We have
also shown that devices with different types of contacts between the
reservoir and the FQH state lead to distinct universal values of
saturation conductance which are rational multiples of e2/h. We have
demonstrated that the problem of tunneling between an electron gas and
a fractional quantum Hall state through an impurity is exactly
equivalent to the problem of tunneling between a chiral Fermi liquid
and a chiral Luttinger liquid. We are investigating in detail the case
of tunneling to a v = 1/3 FQH state which we show to be equivalent to
the problem of tunneling between two G = 1/2 chiral Luttinger liquids.
This system provides an experimental realization of this important
exactly solvable case.
Overscreening of Magnetic Impurities in
dx2-y2-wave Superconductors
E. Fradkin,* C. R. Cassanello
National Science Foundation, DMR 94-24511
We consider the screening of a magnetic impurity in a
dx2-y2 wave superconductor. The
properties of the dx2-y2
state lead to an unusual behavior in the impurity magnetic susceptibility,
ihe impurity specific heat, and in the quasi-particle phase shift which
can be used to diagnose the nature of the condensed state. We construct
an effective theory for this problem and show that there is a quantum
phase transition from an unscreened impurity state to an overscreened
Kondo state at a critical value Jc which varies with
Δ%epsilon;, the superconducting gap away from the nodes.
In the overscreened phase, the impurity Fermi level Ef and
the amplitude Δ of the ground state singlet vanish at Jc
like δ0\exp(-const/Δ) / Δ and J-Jc
respectively. We derive the scaling laws for the susceptibility and
specific heat in the overscreened phase.
Vulcanized Matter
P. M. Goldbart,* H. E. Castillo, W. Peng, K. Shakhnovich
National Science Foundation, DMR 94-24511
This project aims to develop a semimicroscopic theory of vulcanized
matter, the prototypical example of such matter being randomly cross-
linked macromolecular networks (e.g., rubber). The central challenges
are to construct theoretical approaches to static and dynamical
aspects of the structure of vulcanized matter, as well as its response
to external perturbations. A recurrent theme is the utility of
characterizing the matter via universal statistical distributions,
such as the distribution of localization lengths. This approach is
mandated by the essentially random character of the amorphous solid
state that emerges upon sufficient vulcanization. Applications to a
wide range of vulcanized media, including polymeric and low-molecular-
weight varieties, are being constructed.
Superconductivity and Antiferromagnetism--Excitations and Dissipation
P. M. Goldbart,* D. E. Sheehy
U.S. Department of Energy, DE-FG02-96ER45439
This project aims to explore certain phenomenological implications of
a recent approach to the physics of high-temperature superconducting
materials: Zhang's SO(5) theory. It has been found that a novel type
of excitation should arise in nearly superconducting antiferromagnets.
These excitations resemble antiferromagnetic "hedgehogs" at
large distances but are predominantly superconducting inside a core
region. Their structure and experimental implications are currently
being elucidated. It has also been found that Zhang's SO(5) theory
implies an unusual type of dissipation mechanism via which
supercurrent can decay. The relativity due to this mechanism, which
involves orientation rather than amplitude order-parameter
fluctuations, is being explored.
Bose-Einstein Condensed Alkali Gases--Current-carrying States and
Their Decay
P. M. Goldbart,* E. J. Mueller, Y. Lyanda-Geller
National Science Foundation, DMR 94-24511
The ability to support metastable current-carrying states in multiply-
connected settings is one of the prime signatures of superfluidity.
This project aims to investigate such states theoretically for the
case of trapped Bose condensed alkali gases, particularly with regard
to the rate at which they decay via thermal fluctuations. It has been
found that the lifetimes of metastable currents can be either longer
or shorter than experimental time scales, depending on the
experimental setting. Schemes for the experimental detection of
metastable stages are being analyzed.
Andreev Reflection and Spectral Geometry
P. M. Goldbart,* I. Adagideli, D. L. Maslov
U.S. Department of Energy, DE-FG02-96ER45439
When a low-energy electron quasi-particle encounters a superconducting
region it can be retroreflected from it and converted into a hole
quasi-particle: its velocity and charge are reversed. This project
aims to explore the physics of Andreev reflection in a selection of
novel settings. In Andreev billiards (i.e., normal metal regions
surrounded by a superconductor) quasi-particles are confined by a
Andreev reflection, and the connection between the geometry of region
of confinement and the spectrum of quantal energy levels is strikingly
sensitive to this confinement mechanism. A type of Andreev reflection
has also been found to occur from superconducting fluctuations in
quasi-one-dimensional conductors.
Theory of Polysoap Molecules
P. M. Goldbart,* A. Halperin
National Science Foundation, INT 96-03228
This project aims to develop a theory of systems containing flexible
macromolecules into which amphiphilic monomers have been covalently
incorporated. This combination leads to polysoaps, an interesting
class of materials central in many industrial settings, from oil
recovery to paper coating. Incorporating amphilphilic monomers induces
striking modifications of the spatial configurations of the polymers.
What were featureless random coils now exhibit a remarkable hierarchy
of self-organization: the surfactants incorporated into the polymers
aggregate into micelles, thus imparting organization to the polymer
itself. Because of this self-organization, essentially all paradigms
describing the behavior of polymers (above the chemical level) will
need to be significantly modified.
Equilibrium and Nonequilibrium Phenomena in Condensed Matter
N. Goldenfeld,* Y. Oono,* Q. Hou,
National Science Foundation, DMR 93-14938
We are modeling phase transition kinetics, developing large deviation
and fluctuations theories for nonequilibrium systems, and studying
dynamic scaling in high-temperature superconductors. Ongoing projects
include numerical renormalization group methods for PDEs and critical
scaling in black hole formation.
Adaptive Grid Methods for Phase Field Models of Microstructure
Development
N. Goldenfeld,* J. Dantzig (Mech. & Indus. Engr.),
National Aeronautics and Space Administration, NAG B-1249
We are developing adaptive grid methods for solving asymptotically
efficient phase field models of microstructure development. Present
applications include free dendritic growth, directional
solidification, and eutectic growth.
Prediction of Macroscopic Properties of Liquid Helium from Computer
Simulation
N. Goldenfeld,* D. Ceperley,* T. Chay,
National Aeronautics and Space Administration, NRA-94-OLMSA-05
We are studying phase separation kineics in helium-3/helium-4 mixtures
by using path integral Monte Carlo methods and cell dynamic models.
Our goal is to predict quantitatively the morphology generated during
phase ordering from quantum mechanics alone, with as little
experimental input as possible.
Experimentally Oriented Studies of Quantum Mechanics
A. J. Leggett*
John D. and Catherine T. MacArthur Foundation
We are studying the application of the quantum-mechanical formalism to
the description of various experiments that severely test one's
understanding of its meaning. In addition, we study possible
alternative explanations of ostensibly relevant experiments in the
literature.
Superfluidity and Phase Coherence in Very Degenerate Atomic Gases
A. J. Leggett,* C. Lobo, J. Hahm, A. G. K. Modawi,
National Science Foundation, DMR 96-14133
Studies are being made of the superfluid density of an arbitrary many-
body system, possible phase-coherence and interference experiments in
Bose-condensed atomic gases, superfluidity in very degenerate dilute
Fermi gases, and thermal transport in the ultralow-temperature regime
of superfluid 3He.
Computational Methods for Materials
R. M. Martin,* E. Koch, in collaboration with
University of Illinois
We have developed a new method for Monte Carlo simulations of
interacting electrons, and we have applied the method to study the
doped fullerines. The key feature is that the bands crossing the Fermi
surface are degenerate, which leads to new effects not present in one-
band models. Our work shows that the actual systems are near a metal-
insulator transition and are metallic only because of the degeneracy.
Theory of Solids, Surfaces, and Heterostructures
R. M. Martin,* E. Koch, I. Souza, B. Tuttle
U.S. Department of Energy, DE-FG02-96ER45439
We are developing theoretical methods to describe the electronic
structure of solids and applying them to the calculation of properties
of crystalline solids, surfaces, and interfaces. Recent work has
included Monte Carlo simulations of the many-body electron problem in
two-dimensional electron liquids and in the doped Fullerines. We are
using density functional methods to study materials and developing new
"linear scaling" algorithms for simulations of materials.
Local Pairing at U-Impurities in BCS Superconductor Liquid
P. Phillips,* I. Martin
National Science Foundation, DMR 95-10680; American Chemical Society
We analyze the role d-electrons on Anderson U-impurities play in
superconductivity in a metal alloy. We find that phonon coupling at
impurities counteracts the traditional effects that dominate
T-c suppression in the nonmagnetic limit. In some cases, we
find that nonmagnetic impurities can enhance T-c. Qualitative
agreement is found between the predicted increase and the experimental
data for VI-VI degenerate semiconductors doped with Tl or In. In the Kondo
limit, a Fermi liquid analysis reveals that it is the enhancement in the
density of states arising from the Kondo resonance that counteracts
pair-weakening.
Theory of the B = 0 Insulator-Metal Transition in d = 2 Field Effect
Transistors
P. Phillips,* Y. Wan, I. Martin, S. Knysh, D. Dalidovich
National Science Foundation, DMR 95-10680; American Chemical Society
We have been studying extensively the insulator-metal transition in Si
MOSFETs recently reported by Kravchenko and colleagues. These
experiments provide clear evidence for a conducting phase in a dilute
2-D gas. We analyzed this transition in the context of the frequency-
dependent dielectric function for a dilute 2-D electron gas. We showed
that for a wide range of moments and frequencies, the dielectric
function is negative, suggesting the existence of an electron
attraction. We showed that this attraction was sufficient to give rise
to superconducting pair fluctuations in the temperature regime
relevant to the experiments. In addition, we showed that
phenomenologically the experimental observations are consistent with
an insulator-superconductor transition.
Magnetoresistance in Quasi-1-D Organic Conductors
P. Phillips,* I. Martin
National Science Foundation, DMR 95-10680; American Chemical Society
We present simple qualitative models that explain the positive
magnetoresistance in TTT-2I-3 as well as in the
metallo-porphyrin conductor, M(pc)I. For materials such as
TTT-2I-3 which exhibit strongly localized transport
at low temperatures, we show that spin-flip scattering can explain the
origin of the (H/T)2 behavior of the magnetoresistance observed
experimentally, with H the magnetic field
and T the temperature. Whereas in M(pc)I, the positive
magnetoresistance at high temperatures arises from the cyclotron
motion of an electron executing diffusive motion in three dimensions.
Theoretical Nuclear Physics
G. A. Baym,* V. R. Pandharipande,* D. G. Ravenhall,*
National Science Foundation, PHY 94-21309
Projects include evolution of ultrarelativistic heavy-ion collisions,
studies of dense matter applied to the problem of neutron star
interiors, and other problems in astrophysics; correlations between
nucleons in nuclei and of subnuclear degrees of freedom, as seen in
high-energy lepton scattering; studies of nuclear vibrational modes
applied to scattering experiments on nuclear bound states, giant
resonances, the quasi-elastic region, and collective motion at finite
temperature.
Studies in Quantum Field Theory
S.-J. Chang,* J. B. Kogut*
National Science Foundation, PHY 92-00148
Quantum field theory is the union of quantum mechanics and relativity
theory. It provides a framework suitable for the study of the
fundamental interactions of nature--the strong, the electromagnetic,
and the weak interactions of the elementary particles. Research by the
group includes (1) predictions of quantum chromodynamics, the natural
generalization of electromagnetism for the structure of strongly
interacting particles, (2) semiclassical approach, (3) physics at
superhigh energies, and (4) other model field theories.
Elementary Particle Theory
A. X. El-Khadra,* R. G. Leigh,* S. S. Willenbrock,*
U.S. Department of Energy, DE-FG02-91ER04677
The high-energy theory group has a wide variety of research interests.
Topics include the top quark, electroweak symmetry breaking, quantum
chromodynamics and lattice field theory, standard-model phenomenology,
dynamical supersymmetry breaking, duality in supersymmetric field
theory and string theory, M theory, and grand unification.
Standard Model Phenomenology with Lattice QCD
A. X. El-Khadra,* J. N. Simone
U.S. Department of Energy, DE-FG02-91ER40677
Quantum chromodynamics (QCD), the theory of the strong interactions,
is amenable to perturbative calculations only at high energies. A
quantitative understanding of the low-energy behavior of QCD, like the
interactions of quarks inside hadrons, requires nonperturbative
methods. Lattice field theory offers a systematic approach to solving
OCD nonperturbatively. The space-time continuum is replaced by a
discrete lattice. Part of our research is concerned with improvements
in the formulation of lattice QCD. Other projects deal with
applications of lattice QCD to phenomenologically interesting
processes that yield insight into the standard model of particle
physics.
Lattice Field Theory
J. B. Kogut,* M. P. Lombardo, P. Dreher
National Science Foundation, PHY 92-00148
In the theory called quantum chromodynamics, observed particles such
as the proton are composed of quarks, held together by forces
transmitted by gluons, described by a nonabelian gauge field. To
calculate one uses simulation techniques on a space-time lattice. New
methods for dealing with fermions in lattice gauge theories have been
developed by our group and are now being extensively exploited to
study chiral symmetry restoration and quark deconfinement at finite
temperatures and to study particle spectra. Quantum electrodynamics
and fluctuating surfaces are also under investigation.
Superstring Theory
R. G. Leigh*
University of Illinois
Superstring theory is our only candidate for a consistent unification
of quantum field theory and gravity. It provides a framework in which
an understanding of the components of the standard model of particle
physics may be sought. Research here includes studies of the
nonperturbative aspects of string theory, including the special role
played by D-branes, which are multidimensional solitonic states.
Nonperturbative Aspects of Supersymmetric Quantum Field Theories
R. G. Leigh*
University of Illinois
The study of supersymmetric field theories is of great interest, since
it is possible to obtain exact nonperturbative information, which may
be of use in understanding the strong coupling regime of realistic
field theories. There is an important property, known as duality,
which connects the physical observables of one field theory to
another. The full consequences of duality in non-Abelian gauge
theories is only now being worked out.
Dynamical Mechanisms for Supersymmetry Breaking
R. G. Leigh*
U.S. Department of Energy, DE-FG02-91ER40677
Supersymmetry is thought to be a desirable property of microscopic
theories which subsume the Standard Model of particle physics. In
order that it be consistent with present day experiments,
supersymmetry must be a broken symmetry. Field theory models which
dynamically break supersymmetry are being studied with the hope of
understanding the mechanism more fully and applying it to realistic
situations.
Strong and Electroweak Interactions
S. Willenbrock,* T. Stelzer, M. Smith, Z. Sullivan
U.S. Department of Energy, DE-FG02-91ER40677 Task P
The top quark, discovered in 1995, is the most recently discovered
fundamental particle of nature. It is much heavier than the other five
known quarks and may therefore be exotic in some way. We perform
theoretical calculations related to measurements, which will be made
in the near future, to test the properties of the top quark. Hopefully
these measurements will point the way to understanding nature at a
deeper level. We are also studying the mechanism responsible for
breaking the electroweak symmetry, which ultimately generates the
masses of all elementary particles.