title: Hot Spots on the Fermi Surface of Bi2212: Stripes versus Superstructure

abstract:
  In a recent paper Saini et al. have reported evidence for a pseudogap around
(pi,0) at room temperature in the optimally doped superconductor Bi2212. This
result is in contradiction with previous ARPES measurements. Furthermore they
observed at certain points on the Fermi surface hot spots of high spectral
intensity which they relate to the existence of stripes in the CuO planes. They
also claim to have identified a new electronic band along Gamma-M1 whose one
dimensional character provides further evidence for stripes. We demonstrate in
this Comment that all the measured features can be simply understood by
correctly considering the superstructure (umklapp) and shadow bands which occur
in Bi2212.


title:
Dynamical Transition from Triplets to Spinon Excitations: A Series
  Expansion Study of the $J_1-J_2-\delta$ spin-half chain

abstract:
  We study the spin-half Heisenberg chain with alternating nearest neighbor
interactions $J_1(1+\delta)$ and $J_1(1-\delta)$ and a uniform second neighbor
interaction $J_2=y (1-\delta)$ by series expansions around the limit of
decoupled dimers ($\delta=1$). By extrapolating to $\delta=0$ and tuning $y$,
we study the critical point separating the power-law and spontaneously
dimerized phases of the spin-half antiferromagnet. We then focus on the
disorder line $y=0.5$, $0\le \delta\le 1$, where the ground states are known
exactly. We calculate the triplet excitation spectrum, their spectral weights
and wavevector dependent static susceptibility along this line. It is well
known that as $\delta \to 0$, the spin-gap is still non-zero but the triplets
are replaced by spinons as the elementary excitations. We study this dynamical
transition by analyzing the series for the spectral weight and the static
susceptibility. In particular, we show that the spectral weight for the
triplets vanishes and the static spin-susceptibility changes from a simple pole
at imaginary wavevectors to a branch cut at the transition.


title:
Josephson-Junction Qubits and the Readout Process by Single-Electron
  Transistors

abstract:
  Several physical realizations of quantum bits have been proposed. Of those,
nano-electronic devices appear most suitable for large-scale integration and
potential applications. We suggest to use low-capacitance Josephson junctions,
exploiting the coherence of tunneling in the superconducting state combined
with the possibility to control individual charges by Coulomb blockade effects
(cond-mat/9706016,cond-mat/9808067). These systems constitute quantum bits,
with logical states differing by one Cooper-pair charge. Single- and two-bit
operations can be performed by applying a sequence of gate voltages. The phase
coherence time is sufficiently long to allow a series of these steps. In
addition to the manipulation of qubits the resulting quantum state has to be
read out. This can be accomplished by coupling a single-electron transistor
capacitively to the qubit (cond-mat/9801125). To describe this quantum
measurement process we study the time evolution of the density matrix of the
coupled system. Only when a transport voltage is turned on, the transistor
destroys the phase coherence of the qubit; in this case within a short time.
The measurement is accomplished after a longer time scale, when the signal
resolves the different quantum states. At still longer times the measurement
process itself destroys the information about the initial state. We present a
suitable set of system parameters, which can be realized by present-day
technology.


title:
Various series expansions for a Heisenberg antiferromagnet model for
  SrCu$_2$(BO$_3$)$_2$

abstract:
  We use a variety of series expansion methods at both zero and finite
temperature to study an antiferromagnetic Heisenberg spin model proposed
recently by Miyahara and Ueda for the quasi two-dimensional material
SrCu$_2$(BO$_3$)$_2$. We confirm that this model exhibits a first-order quantum
phase transition at T=0 between a gapped dimer phase and a gapless N\'eel phase
when the ratio $x=J'/J$ of nearest and next-nearest neighbour interactions is
varied, and locate the transition at $x_c=0.691(6)$. Using longer series we are
able to give more accurate estimates of the model parameters by fitting to the
high temperature susceptibility data.


title:
Properties of low-lying states in a diffusive quantum dot and Fock-space
  localization

abstract:
  Motivated by an experiment by Sivan et al. (Europhys. Lett. 25, 605 (1994))
and by subsequent theoretical work on localization in Fock space, we study
numerically a hierarchical model for a finite many-body system of Fermions
moving in a disordered potential and coupled by a two-body interaction. We
focus attention on the low-lying states close to the Fermi energy. Both the
spreading width and the participation number depend smoothly on excitation
energy. This behavior is in keeping with naive expectations and does not
display Anderson localization. We show that the model reproduces essential
features of the experiment by Sivan et al.


title: Dynamics of defect formation

abstract:
  A dynamic symmetry-breaking transition with noise and inertia is analyzed.
Exact solution of the linearized equation that describes the critical region
allows precise calculation (exponent and prefactor) of the number of defects
produced as a function of the rate of increase of the critical parameter. The
procedure is valid in both the overdamped and underdamped limits. In one space
dimension, we perform quantitative comparison with numerical simulations of the
nonlinear nonautonomous stochastic partial differential equation and report on
signatures of underdamped dynamics.


title:
Time scale separation and heterogeneous off-equilibrium dynamics in spin
  models over random graphs

abstract:
  We study analytically and numerically the statics and the off-equilibrium
dynamics of spin models over finitely connected random graphs. We identify a
threshold value for the connectivity beyond which the loop structure of the
graph becomes thermodynamically relevant. Glauber dynamics simulations show
that this loop structure is responsible for the onset of dynamical features of
a local character (dynamical heterogeneities and spontaneous time scale
separation), consistently with previous (experimental and numerical) studies of
glasses and spin glasses in their approach to the low temperature phase.


title: Stochastic Dynamics in Quenched-in Disorder and Hysteresis

abstract:
  The conditions under which relaxation dynamics in the presence of quenched-in
disorder lead to rate-independent hysteresis are discussed. The calculation of
average hysteresis branches is reduced to the solution of the level-crossing
problem for the stochastic field describing quenched-in disorder. Closed
analytical solutions are derived for the case where the disorder is
characterized by Wiener-Levy statistics. This case is shown to be equivalent to
the Preisach model and the associated Preisach distribution is explicitly
derived, as a function of the parameters describing the original dynamic
problem.


title:
Quantum conductivity corrections in two dimensional long-range
  disordered systems with strong spin-orbit splitting of electron spectrum

abstract:
  We study quantum corrections to conductivity in a 2D system with a smooth
random potential and strong spin-orbit splitting of the spectrum. We show that
the interference correction is positive and down to the very low temperature
can exceed the negative correction related to electron-electron interactions.
We discuss this result in the context of the problem of the metal-insulator
transition in Si-MOSFET structures.


title: Fluctuations of addition spectra of independent quantum systems

abstract:
  Motivated by recent experiments on large quantum dots, we consider the energy
spectrum in a system consisting of $N$ particles distributed among $K<N$
independent sub-systems, such that the energy of each sub-system is a quadratic
function of the number of particles residing on it. On a large scale, the
ground state energy E(N) of such a system grows quadratically with $N$, but in
general there is no simple relation such as $E(N)=a N +b N^{2}$. The deviation
of E(N) from exact quadratic behavior implies that its second difference (the
inverse compressibility) $\chi_{N} \equiv E(N+1)-2 E(N)+E(N-1)$ is a
fluctuating quantity. Regarding the numbers $\chi_{N}$ as values assumed by a
certain random variable $\chi$, we obtain a closed-form expression for its
distribution $F(\chi)$. Its main feature is that the corresponding density
$P(\chi)=\frac{dF(\chi)} {d\chi}$ has a maximum at the point $\chi=0$. As $K
\to \infty$ the density is Poissonian, namely, $P(\chi) \to e^{-\chi}$.


title: Superfluidity of indirect excitons in a quantum dot

abstract:
  The superfluidity and Bose-Einstein condensation of indirect excitons in
two-dimensional quantum dot are studied by path-integral Monte Carlo
simulations. The temperature dependence of superfluid and bose-condensed
fraction are calculated at different strengths of interaction. Using the
Kosterlitz-Thouless recursion relations, we also predict behavior of superfluid
fraction in macroscopic large systems.


title: Slave Boson Approach to The Neutron Scattering in YBCO Superconductors

abstract:
  The evolution of the so-called `41meV resonance' in the magnetic response of
YBCO cuprates is studied with slave-boson theory for the t-t'-J-model. The
resonance appears as a collective spin fluctuation in the d-wave
superconducting (SC) state. It is undamped at optimal doping due to a threshold
in the excitation energies of particle--hole pairs with relative wave vector
(pi,pi). When hole filling is reduced, the resonance moves to lower energies
and broadens. Below the resonance energy we find a crossover to an
incommensurate response in agreement with a recent experiment on
YBa_2CU_3O_6.6. We show that dynamic nesting in the d-wave SC state causes this
effect.


title: Slow Forcing in the Projective Dynamics Method

abstract:
  We provide a proof that when there is no forcing the recently introduced
projective dynamics Monte Carlo algorithm gives the exact lifetime of the
metastable state, within statistical uncertainties. We also show numerical
evidence illustrating that for slow forcing the approach to the zero-forcing
limit is rather rapid. The model studied numerically is the 3-dimensional
3-state Potts ferromagnet.


title:
Polarized-neutron study of longitudinal Haldane-gap excitations in
  Nd$_2$BaNiO$_5$

abstract:
  Polarized and unpolarized inelastic neutron scattering is used to study
Haldane-gap excitations in the mixed-spin linear-chain antiferromagnet
Nd2BaNiO5. The longitudinal mode, polarized along the direction of ordered
moments, is observed for the first time. The model of isolated Haldane chains
in a static staggered exchange field, that is known to work very well for
static properties and transverse spin excitations in R2BaNiO5 compounds, fails
to explain new results for the longitudinal spin gap.


title:
Microwave Spectroscopy of Thermally Excited Quasiparticles in
  YBa_2Cu_3O_{6.99}

abstract:
  We present here the microwave surface impedance of a high purity crystal of
$YBa_2Cu_3O_{6.99}$ measured at 5 frequencies between 1 and 75 GHz. This data
set reveals the main features of the conductivity spectrum of the thermally
excited quasiparticles in the superconducting state. Below 20 K there is a
regime of extremely long quasiparticle lifetimes, due to both the collapse of
inelastic scattering below $T_c$ and the very weak impurity scattering in the
high purity $BaZrO_3$-grown crystal used in this study. Above 20 K, the
scattering increases dramatically, initially at least as fast as $T^4$.


title:
Dynamic behavior of driven interfaces in models with two absorbing
  states

abstract:
  We study the dynamics of an interface (active domain) between different
absorbing regions in models with two absorbing states in one dimension;
probabilistic cellular automata models and interacting monomer-dimer models.
These models exhibit a continuous transition from an active phase into an
absorbing phase, which belongs to the directed Ising (DI) universality class.
In the active phase, the interface spreads ballistically into the absorbing
regions and the interface width diverges linearly in time. Approaching the
critical point, the spreading velocity of the interface vanishes algebraically
with a DI critical exponent. Introducing a symmetry-breaking field $h$ that
prefers one absorbing state over the other drives the interface to move
asymmetrically toward the unpreferred absorbing region. In Monte Carlo
simulations, we find that the spreading velocity of this driven interface shows
a discontinuous jump at criticality. We explain that this unusual behavior is
due to a finite relaxation time in the absorbing phase. The crossover behavior
from the symmetric case (DI class) to the asymmetric case (directed percolation
class) is also studied. We find the scaling dimension of the symmetry-breaking
field $y_h = 1.21(5)$.


title:
A two-channel Kondo impurity in the spin-1/2 chain: Consequences for
  Knight shift experiments

abstract:
  A magnetic impurity in the spin-1/2 chain is a simple realization of the
two-channel Kondo problem since the field theoretical descriptions in the
spin-sector are identical. The correlation functions near the impurity can be
calculated. Using a modified version of the numerical transfer matrix DMRG, we
are able to accurately determine local properties close to the impurity in the
thermodynamic limit. The local susceptibilities (Knight-shifts) show an
interesting behavior in a large range around the impurities. We are able to
make quantitative experimental predictions which would allow to observe
two-channel Kondo physics for the first time directly by doping of spin-1/2
chain compounds.


title:
Failure Probabilities and Tough-Brittle Crossover of Heterogeneous
  Materials with Continuous Disorder

abstract:
  The failure probabilities or the strength distributions of heterogeneous 1D
systems with continuous local strength distribution and local load sharing have
been studied using a simple, exact, recursive method. The fracture behavior
depends on the local bond-strength distribution, the system size, and the
applied stress, and crossovers occur as system size or stress changes. In the
brittle region, systems with continuous disorders have a failure probability of
the modified-Gumbel form, similar to that for systems with percolation
disorder. The modified-Gumbel form is of special significance in weak-stress
situations. This new recursive method has also been generalized to calculate
exactly the failure probabilities under various boundary conditions, thereby
illustrating the important effect of surfaces in the fracture process.


title: Myopic Bosonization

abstract:
  As the title suggests, this is an attempt at bosonizing fermions in any
number of dimensions without paying attention to the fact that the Fermi
surface is an extended object. One is tempted to introduce the density
fluctuation and its conjugate and recast the interacting problem in terms of
these canonical Bose fields. However, we find that the attempt is short-sighted
figuratively as well for the same reason.
  But surprisingly, this flaw, which manifests itself as an inconsistency
between Menikoff-Sharp's construction of the kinetic energy operator in terms
of currents and densities, and our ansatz for this operator, is nevertheless
able to reproduce(although reluctantly) many salient features of the free
theory.
  Buoyed by this success, we solve the interacting problem and compute the full
propagator.


title: Size Effects in Carbon Nanotubes

abstract:
  The inter-shell spacing of multi-walled carbon nanotubes was determined by
analyzing the high resolution transmission electron microscopy images of these
nanotubes. For the nanotubes that were studied, the inter-shell spacing
${\hat{d}_{002}}$ is found to range from 0.34 to 0.39 nm, increasing with
decreasing tube diameter. A model based on the results from real space image
analysis is used to explain the variation in inter-shell spacings obtained from
reciprocal space periodicity analysis. The increase in inter-shell spacing with
decreased nanotube diameter is attributed to the high curvature, resulting in
an increased repulsive force, associated with the decreased diameter of the
nanotube shells.


title: Spin Bose Glass Phase in Bilayer Quantum Hall Systems at $\nu=2$

abstract:
  We develop an effective spin theory to describe magnetic properties of the
$\nu=2$ Quantum Hall bilayer systems. In the absence of disorder this theory
gives quantitative agreement with the results of microscopic Hartree-Fock
calculations, and for finite disorder it predicts the existence of a novel spin
Bose glass phase. The Bose glass is characterized by the presence of domains of
canted antiferromagnetic phase with zero average antiferromagnetic order and
short range mean antiferromagnetic correlations. It has infinite
antiferromagnetic transverse susceptibility, finite longitudinal spin
susceptibility and specific heat linear in temperature. Transition from the
canted antiferromagnet phase to the spin Bose glass phase is characterized by a
universal value of the longitudinal spin conductance.


title:
Integrable Kondo impurities in the one-dimensional supersymmetric
  extended Hubbard model

abstract:
  An integrable Kondo problem in the one-dimensional supersymmetric extended
Hubbard model is studied by means of the boundary graded quantum inverse
scattering method. The boundary $K$ matrices depending on the local moments of
the impurities are presented as a nontrivial realization of the graded
reflection equation algebras in a two-dimensional impurity Hilbert space.
Further,the model is solved by using the algebraic Bethe ansatz method and the
Bethe ansatz equations are obtained.


abstract:
  Integrable Kondo impurities in the one-dimensional supersymmetric U model of
strongly correlated electrons are studied by means of the boundary graded
quantum inverse scattering method. The boundary K matrices depending on the
local magnetic moments of the impurities are presented as nontrivial
realizations of the reflection equation algebras in an impurity Hilbert space.
Furthermore, the model Hamiltonian is diagonalized and the Bethe ansatz
equations are derived. It is interesting to note that our model exhibits a free
parameter in the bulk Hamiltonian but no free parameter exists on the
boundaries. This is in sharp contrast to the impurity models arising from the
supersymmetric t-J and extended Hubbard models where there is no free parameter
in the bulk but there is a free parameter on each boundary.


title: The correction-to-scaling exponent in dilute systems

abstract:
  The leading correction-to-scaling exponent $\omega$ for the three-dimensional
dilute Ising model is calculated in the framework of the field theoretic
renormalization group approach. Both in the minimal subtraction scheme as well
as in the massive field theory (resummed four loop expansion) excellent
agreement with recent Monte Carlo calculations [Ballesteros H G, et al Phys.
Rev. B 58, 2740 (1998)] is achieved. The expression of $\omega$ as series in a
$\sqrt{\epsilon}$-expansion up to ${\cal O}(\epsilon^2)$ does not allow a
reliable estimate for $d=3$.


title: The vortex lattice melting theory as example of science fiction

abstract:
  The concept of the transition of type II superconductor into the Abrikosov
state as the appearance of long-range phase coherence is considered. It is
shown that this transition should be first order in ideal (without disorder)
superconductor. Such transition is indeed observed in bulk superconductors with
weak disorder at a field Hc4 < Hc2. No experimental evidence of the vortex
lattice melting exists now. It is shown that this popular concept has appeared
on a consequence of an incorrect interpretation of the Abrikosov state and
incorrect definition of the phase coherence. The absence of a sharp transition
in thin films with weak disorder is interpreted as absence of long-range phase
coherence down to very low magnetic field. In order to explain the observed
smooth phase coherence appearance in superconductors with strong disorder it is
proposed to return to the Mendelssohn model.


title: Correlations and Freezing in Steadily Settling Suspensions

abstract:
  We study the liquid-solid transition in a collection of interacting particles
moving through a dissipative medium under the action of a constant, spatially
uniform external force, e.g. a charge-stabilized suspension in a fluidized bed
or a flux-point lattice moving through a thin, current-carrying slab of type II
superconductor. The mobility of a given region in these systems is in general a
function of the local concentration. We show that the structure factor peak is
suppressed in an anisotropic manner as a result of this effect, resulting in a
shift of the crystal-liquid phase boundary towards the crystal side. A
nonequilibrium phase diagram is presented.


title: Simulation of Spin Glass with a Variable-system-size Ensemble

abstract:
  In this paper, we introduce a dynamical Monte Carlo algorithm for spin models
in which the number of the spins fluctuates from zero to a given number by
addition and deletion of spins with a probabilistic rule. Such simulations are
realized with a variable-system-size ensemble, a mixture of canonical ensembles
each of which corresponds to a system with different size. The weight of each
component of the mixture is controlled by a penalty term and systematically
tuned in a preliminary run in a way similar to the multicanonical algorithm. In
a measurement run, the system grows and shrinks without violating the detailed
balance condition and we can obtain the correct canonical averages if physical
quantities is measured only when its size is equal to the prescribed maximum
size. The mixing of Markov chain is facilitated by the fast relaxation at small
system sizes. The algorithm is implemented for the SK model of spin glass and
shows better performance than that of a conventional heat bath algorithm.


title:
Short grafted chains: Monte Carlo simulations of a model for monolayers
  of amphiphiles

abstract:
  We present Monte Carlo simulations of a coarse-grained model for Langmuir
monolayers of amphiphile molecules on a polar substrate. The molecules are
modelled as chains of Lennard-Jones beads, with one slightly larger end bead
confined in a planar surface. They are simulated in continuous space under
conditions of constant pressure, using a simulation box of variable size and
shape. The model exhibits a disordered phase (corresponding to the liquid
expanded phase), and various ordered phases (corresponding to the condensed
phases) with different types of tilt. We calculate the phase diagrams and
characterize the different phases and phase transitions. The effect of varying
the chain stiffness is also discussed.


title:
Polaron Crystallization and Melting: Effects of the Long-Range Coulomb
  Forces

abstract:
  On examining the stability of a Wigner crystal in an ionic dielectric, two
competitive effects due to the polaron formation are found to be important: (i)
the screening of the Coulomb force, which destabilizes the crystal, compensated
by (ii) the increase of the carrier mass (polaron mass). The competition
between the two effects is carefully studied, and the quantum melting of the
polaronic Wigner crystal is examined by varying the density at zero
temperature. By calculating the quantum fluctuations of both the electron and
the polarization, we show that there is a competition between the dissociation
of the polarons at the insulator-to-metal transition (IMT), and a melting
towards a polaron liquid. We find that at strong coupling, a liquid state of
dielectric polarons cannot exist, and the IMT is driven by the polaron
dissociation. Next, taking into account the dipolar interactions between
localized carriers, we show that these are responsible for an instability of
the transverse vibrational modes of the polaronic Wigner crystal as the density
increases. This provides a new mechanism for the IMT in doped dielectrics,
which yields interesting dielectric properties below and beyond the transition.
An optical signature of such a mechanism for the IMT is provided.


title: Quantum kinetic theory: The disordered electron problem

abstract:
  These are notes for lectures delivered at the NATO ASI on Dynamics in Leiden,
The Netherlands, in July 1998. The quantum kinetic theory for noninteracting
electrons in a disordered solid is introduced and discussed. We first use
many-body theory to derive the quantum Boltzmann equation that describes
transport and time correlation functions in this system. Particular attention
is paid to the calculation of the electrical conductivity and the density
response function. We then consider corrections to the Boltzmann equation due
to wave interference effects. The disorder expansion of the conductivity is
addressed, and the so-called weak localization or long-time tail contribution
to the conductivity is discussed. We conclude with a brief discussion of the
influence of electron-electron interactions on the properties of disordered
electronic systems.


title:
Reptation of star polymers in a network: Monte Carlo results of
  diffusion coefficients

abstract:
  We report on Monte Carlo results of diffusion coefficients of lattice star
polymers trapped inside a fixed network (de Gennes model). It is found that our
data are in agreement with the Helfand-Pearson exponential factor \alpha=0.29.
For the pre-exponential power law exponent we find \beta=2. In contrast to
existing theoretical predictions, we find that the number of arms f leads to a
pre-exponential factor of the form exp(-0.75 f).


title: Quantum phase transitions

abstract:
  These are notes for lectures delivered at the NATO ASI on Dynamics in Leiden,
The Netherlands, in July 1998. The main concepts relating to quantum phase
transitions are explained, using the paramagnet-to-ferromagnet transition of
itinerant electrons as the primary example. Some aspects of metal-insulator
transitions are also briefly discussed. The exposition is strictly pedagogical
in nature, with no ambitions with respect to completeness or going into
technical details. The goal of the lectures is to provide a bridge between
textbooks on classical critical phenomena and the current literature on quantum
phase transitions. Some familiarity with the concepts of classical phase
transitions is helpful, but not absolutely necessary.


title: Order parameter fluctuation effects in d-wave BCS superconductors

abstract:
  We study order parameter fluctuation effects in the superconducting state as
a possible precursor to the pseudogap phenomena.
  Using a low-energy effective theory in the d-wave BCS model, we
self-consistently calculate the single-particle properties. We find that the
fluctuations reduce the spectral gap, and the shape of the gap is deformed,
showing reduced slope of the gap at the nodes. We also show the
angle-dependence of the quasiparticle lifetime due to the fluctuations.


title:
Hole Localization in Underdoped Superconducting Cuprates Near 1/8th
  Doping

abstract:
  Measurements of thermal conductivity versus temperature over a broad range of
doping in YBa$_2$Cu$_3$O$_{6+x}$ and HgBa$_2$Ca$_{n-1}$Cu$_n$O$_{2n+2+\delta}$
($n$=1,2,3) suggest that small domains of localized holes develop for hole
concentrations near $p$=1/8. The data imply a mechanism for localization that
is intrinsic to the CuO$_2$-planes and is enhanced via pinning associated with
oxygen-vacancy clusters.


title: Fluctuation Effects on Quadratic Autocatalysis Fronts

abstract:
  A Markov chain model for spatially distributed autocatalytic systems with a
quadratic reaction rate is considered. An approximate solution for the local
probability distribution is obtained in the form of a perturbation expansion
for the regimes where diffusion is relatively fast. Using this approximate
distribution, properties of the chemical wave fronts found in these
autocatalytic systems are studied, and deviations of the minimum propagation
velocity and the concentration profile from deterministic predictions are
analyzed. A comparison with numerical results from lattice-gas automaton
simulations is also provided.


title: Phonons from neutron powder diffraction

abstract:
  The spherically averaged structure function $\soq$ obtained from pulsed
neutron powder diffraction contains both elastic and inelastic scattering via
an integral over energy. The Fourier transformation of $\soq$ to real space, as
is done in the pair density function (PDF) analysis, regularizes the data, i.e.
it accentuates the diffuse scattering. We present a technique which enables the
extraction of off-center phonon information from powder diffraction experiments
by comparing the experimental PDF with theoretical calculations based on
standard interatomic potentials and the crystal symmetry. This procedure
(dynamics from powder diffraction(DPD)) has been successfully implemented for
two systems, a simple metal, fcc Ni, and an ionic crystal, CaF$_{2}$. Although
computationally intensive, this data analysis allows for a phonon based
modeling of the PDF, and additionally provides off-center phonon information
from powder neutron diffraction.


title:
Non-Linear Effects in Resonant Tunneling; Bistabilities and
  Self-Sustained Oscillating Currents

abstract:
  We study non-linear phenomena in double barrier heterostructures. Systems in
3D under the effect of an external magnetic field along the current and 1D
systems are analyzed. Non-linearities are reflected in the I-V characteristic
curve as bistabilities, instabilities and time dependent oscillations of the
currents. The nature of the non-linear behavior depends upon the parameters
that define the system.


title: Secondary Coulomb Blockade Gap in a Four-Island Tunnel-Junction Array

abstract:
  In the ring-shaped tunnel-junction array with four islands, the secondary
Coulomb blockade gap in a low bias-voltage range is observed in the I-V
characteristics. We attribute its appearance to the unique topology of the
array which induces up to two electrons to get trapped inside. We have analyzed
the formation and destruction of the gap in terms of detailed single-electron
tunneling processes. The negative differential resistance behavior when the
thermal and quantum fluctuations are present is also studied.


title: Resonant hyper-Raman scattering in spherical quantum dots

abstract:
  A theoretical model of resonant hyper-Raman scattering by an ensemble of
spherical semiconductor quantum dots has been developed. The electronic
intermediate states are described as Wannier-Mott excitons in the framework of
the envelope function approximation. The optical polar vibrational modes of the
nanocrystallites (vibrons) and their interaction with the electronic system are
analized with the help of a continuum model satisfying both the mechanical and
electrostatic matching conditions at the interface. An explicit expression for
the hyper-Raman scattering efficiency is derived, which is valid for incident
two-photon energy close to the exciton resonances. The dipole selection rules
for optical transitions and Fr\"ohlich-like exciton-lattice interaction are
derived: It is shown that only exciton states with total angular momentum
$L=0,1$ and vibrational modes with angular momentum $l_p=1$ contribute to the
hyper-Raman scattering process. The associated exciton energies, wavefunctions,
and vibron frequencies have been obtained for spherical CdSe zincblende-type
nanocrystals, and the corresponding hyper-Raman scattering spectrum and
resonance profile are calculated. Their dependence on the dot radius and the
influence of the size distribution on them are also discussed.


title: Modeling the Stock Market prior to large crashes

abstract:
  We propose that the minimal requirements for a model of stock market price
fluctuations should comprise time asymmetry, robustness with respect to
connectivity between agents, ``bounded rationality'' and a probabilistic
description. We also compare extensively two previously proposed models of
log-periodic behavior of the stock market index prior to a large crash. We find
that the model which follows the above requirements outperforms the other with
a high statistical significance.


title:
Effect of magnetic frustration on single-hole spectral function in the
  t-t'-t''-J model

abstract:
  We examine the effect of the magnetic frustration J' on the single-hole
spectral function in the t-t'-t''-J model. At zero temperature, the exact
diagonalization (ED) and the self-consistent Born approximation (SCBA) methods
are used. We find that the frustration suppresses the quasiparticle (QP) weight
at small momentum k, whereas the QP peak at k=(pi/2,pi/2) remains sharp. We
also show the temperature dependence of the single-hole spectral function by
using the ED method. It is found that the lineshapes at (pi/2,0) and
(pi/2,pi/2) show different temperature dependence. These findings are
consistent with the angle-resolved photoemission data on Sr2CuO2Cl2, and
indicate the importance of the magnetic frustration on the electronic states of
the insulating cuprates.


title:
Zero Temperature Phase Transition in Spin-ladders: Phase Diagram and
  Dynamical studies of Cu(Hp)Cl

abstract:
  In a magnetic field, spin-ladders undergo two zero-temperature phase
transitions at the critical fields Hc1 and Hc2. An experimental review of
static and dynamical properties of spin-ladders close to these critical points
is presented. The scaling functions, universal to all quantum critical points
in one-dimension, are extracted from (a) the thermodynamic quantities
(magnetization) and (b) the dynamical functions (NMR relaxation). A simple
mapping of strongly coupled spin ladders in a magnetic field on the exactly
solvable XXZ model enables to make detailed fits and gives an overall
understanding of a broad class of quantum magnets in their gapless phase
(between Hc1 and Hc2). In this phase, the low temperature divergence of the NMR
relaxation demonstrates its Luttinger liquid nature as well as the novel
quantum critical regime at higher temperature. The general behaviour close
these quantum critical points can be tied to known models of quantum magnetism.


title:
Microscopic analysis of shot-noise suppression in nondegenerate
  diffusive conductors

abstract:
  We present a theoretical investigation of shot-noise suppression due to
long-range Coulomb interaction in nondegenerate diffusive conductors.
Calculations make use of an ensemble Monte Carlo simulator self-consistently
coupled with a one-dimensional Poisson solver. We analyze the noise in a
lightly doped active region surrounded by two contacts acting as thermal
reservoirs. By taking the doping of the injecting contacts and the applied
voltage as variable parameters, the influence of elastic and inelastic
scattering in the active region is investigated. The transition from ballistic
to diffusive transport regimes under different contact injecting statistics is
analyzed and discussed. Provided significant space-charge effects take place
inside the active region, long-range Coulomb interaction is found to play an
essential role in suppressing the shot noise at $qU \gg k_BT$. In the elastic
diffusive regime, momentum space dimensionality is found to modify the
suppression factor $\gamma$, which within numerical uncertainty takes values
respectively of about 1/3, 1/2 and 0.7 in the 3D, 2D and 1D cases. In the
inelastic diffusive regime, shot noise is suppressed to the thermal value.


title: Systems involving surfactants

abstract:
  Computer simulations of amphiphilic systems are reviewed. Research areas
cover a wide range of length and time scales, and a whole hierarchy of models
and methods has been developed to address them all. They range from
atomistically realistic models, idealized chain models, lattice spin models, to
phenomenological models such as Ginzburg-Landau models and random interface
models. Selected applications are discussed in order to illustrate the use of
the models and the insights they can offer.


title: Recoil growth: an efficient simulation method for multi-polymer systems

abstract:
  We present a new Monte Carlo scheme for the efficient simulation of
multi-polymer systems. The method permits chains to be inserted into the system
using a biased growth technique. The growth proceeds via the use of a
retractable feeler, which probes possible pathways ahead of the growing chain.
By recoiling from traps and excessively dense regions, the growth process
yields high success rates for both chain construction and acceptance. Extensive
tests of the method using self-avoiding walks on a cubic lattice show that for
long chains and at high densities it is considerably more efficient than
Configurational Bias Monte Carlo, of which it may be considered a
generalisation.


title: Generalized Dielectric Breakdown Model

abstract:
  We propose a generalized version of the Dielectric Breakdown Model (DBM) for
generic breakdown processes. It interpolates between the standard DBM and its
analog with quenched disorder, as a temperature like parameter is varied. The
physics of other well known fractal growth phenomena as Invasion Percolation
and the Eden model are also recovered for some particular parameter values. The
competition between different growing mechanisms leads to new non-trivial
effects and allows us to better describe real growth phenomena.
  Detailed numerical and theoretical analysis are performed to study the
interplay between the elementary mechanisms. In particular, we observe a
continuously changing fractal dimension as temperature is varied, and report an
evidence of a novel phase transition at zero temperature in absence of an
external driving field; the temperature acts as a relevant parameter for the
``self-organized'' invasion percolation fixed point. This permits us to obtain
new insight into the connections between self-organization and standard phase
transitions.


title:
Observation of total omnidirectional reflection from a one-dimensional
  dielectric lattice

abstract:
  We show that under certain conditions one-dimensional dielectric lattice
possesses total omnidirectional reflection of incident light. The predictions
are verified experimentally using Na3AlF6/ZnSe multilayer structure developed
by means of standard optical technology. The structure was found to exhibit
reflection coefficient more then 99% in the range of incident angles 0-86
(degree) at the wavelength of 632.8 nm for s-polarization. The results are
believed to stimulate new experiments on photonic crystals and controlled
spontaneous emission.


title:
Fermi and non-Fermi liquid behavior in quantum impurity systems:
  Conserving slave boson theory

abstract:
  The question of Fermi liquid vs. non-Fermi liquid behavior induced by strong
correlations is one of the prominent problems in metallic local moment systems.
As standard models for such systems, the SU(N)xSU(M) Anderson impurity models
exhibit both Fermi liquid and non-Fermi liquid behavior, depending on their
symmetry. Taking the Anderson model as an example, these lectures first give an
introduction to the auxiliary boson method to describe correlated systems
governed by a strong, short-range electronic repulsion. It is then shown how to
include the relevant low-lying excitations (coherent spin flip and charge
fluctuation processes), while preserving the local gauge symmetry of the model.
This amounts to a conserving T-matrix approximation (CTMA). We prove a
cancellation theorem showing that the CTMA incorporates all leading and
subleading infrared singularities at any given order in a self-consistent loop
expansion of the free energy. As a result, the CTMA recovers the correct
infrared behavior of the auxiliary particle propagators, indicating that it
correctly describes both the Fermi and the non-Fermi regimes of the Anderson
model.


title: Classical frustration and quantum disorder in spin-orbital models

abstract:
  The most elementary of all physical spin-orbital models is the Kugel-Khomskii
model describing a S=1/2, $e_g$ degenerate Mott-insulator. Recent theoretical
work is reviewed revealing that the classical limit is characterized by a point
of perfect dynamical frustration. It is suggested that this might give rise to
a quantum disordered ground state.


title:
Dynamics of spherical particles on a surface: About collision induced
  sliding and other effects

abstract:
  We present a model for the motion of hard spherical particles on a two
dimensional surface. The model includes both the interaction between the
particles via collisions, as well as the interaction of the particles with the
substrate. We analyze in details the effects of sliding and rolling friction,
that are usually overlooked. It is found that the properties of this
particulate system are influenced significantly by the substrate-particle
interactions. In particular, sliding of the particles relative to the substrate
after a collision leads to considerable energy loss for common experimental
conditions. The presented results provide a basis, that can be used to
realistically model the dynamical properties of the system, and provide further
insight into density fluctuations and related phenomena of clustering,
structure formations, and inelastic collapse.


title: Scaling properties of discrete fractals

abstract:
  An important problem in the analysis of experimental data showing fractal
properties, is that such samples are composed by a set of points limited by an
upper and a lower cut off. We study how finite size effect due to the
discreteness of the sets may influence self similar properties even far from
these cut-offs. Estimations of these effects are provided on the basis of the
characteristics of the samples. In particular we present an estimate of the
length scale above which information about average quantities is reliable, by
explicitly computing discreteness effects in number counting . The results have
particular importance in the statistical analysis of the distribution of
galaxies.


title:
Quantum phase transitions in cuprates: stripes and antiferromagnetic
  supersolids

abstract:
  An elementary phenomenological consideration is presented regarding the long
wavelength quantum dynamics in cuprate superconductors. The emphasis is on the
interplay of the charge- and spin order associated with the stripe phenomenon
on the one hand, and superconductivity on the other hand.


title:
Test of the Kolmogorov-Johnson-Mehl-Avrami picture of metastable decay
  in a model with microscopic dynamics

abstract:
  The Kolmogorov-Johnson-Mehl-Avrami (KJMA) theory for the time evolution of
the order parameter in systems undergoing first-order phase transformations has
been extended by Sekimoto to the level of two-point correlation functions.
Here, this extended KJMA theory is applied to a kinetic Ising lattice-gas
model, in which the elementary kinetic processes act on microscopic length and
time scales. The theoretical framework is used to analyze data from extensive
Monte Carlo simulations. The theory is inherently a mesoscopic continuum
picture, and in principle it requires a large separation between the
microscopic scales and the mesoscopic scales characteristic of the evolving
two-phase structure. Nevertheless, we find excellent quantitative agreement
with the simulations in a large parameter regime, extending remarkably far
towards strong fields (large supersaturations) and correspondingly small
nucleation barriers. The original KJMA theory permits direct measurement of the
order parameter in the metastable phase, and using the extension to correlation
functions one can also perform separate measurements of the nucleation rate and
the average velocity of the convoluted interface between the metastable and
stable phase regions. The values obtained for all three quantities are verified
by other theoretical and computational methods. As these quantities are often
difficult to measure directly during a process of phase transformation, data
analysis using the extended KJMA theory may provide a useful experimental
alternative.

categories: cond-mat.mtrl-sci chao-dyn cond-mat.soft cond-mat.stat-mech nlin.CD physics.chem-ph


title:
Comment on "General Method to Determine Replica Symmetry Breaking
  Transitions"

abstract:
  In a recent letter Marinari et al [Phys. Rev. Lett. 81, 1698 (1998)]
introduced a new method to study spin glass transitions and argued that by
probing replica symmetry (RS) as opposed to time reversal symmetry (TRS), their
method unambiguously shows that replica symmetry breaking (RSB) occurs in
short-range spin glasses. In this comment we show that while the new method is
indeed useful for studying transitions in systems where TRS is absent (such as
the p-spin model studied by them), the conclusion that it shows the existence
of RSB in short-range spin glasses is wrong.


title: Absence of local thermal equilibrium in two models of heat conduction

abstract:
  A crucial assumption in the conventional description of thermal conduction is
the existence of local thermal equilibrium. We test this assumption in two
simple models of heat conduction. Our first model is a linear chain of planar
spins with nearest neighbour couplings, and the second model is that of a
Lorentz gas. We look at the steady state of the system when the two ends are
connected to heat baths at temperatures T1 and T2. If T1=T2, the system reaches
thermal equilibrium. If T1 is not equal to T2, there is a heat current through
the system, but there is no local thermal equilibrium. This is true even in the
limit of large system size, when the heat current goes to zero. We argue that
this is due to the existence of an infinity of local conservation laws in their
dynamics.


title:
Frustrated kinetic energy, the optical sum rule, and the mechanism of
  superconductivity

abstract:
  The theory that the change of the electronic kinetic energy in a direction
perpendicular to the CuO-planes in high-temperature superconductors is a
substantial fraction of the condensation energy is examined. It is argued that
the consequences of this theory based on a rigorous $c$-axis conductivity sum
rule are consistent with recent optical and penetration depth measurements.


title:
Intermediate temperature dynamics of one-dimensional Heisenberg
  antiferromagnets

abstract:
  We present a general theory for the intermediate temperature (T) properties
of Heisenberg antiferromagnets of spin-S ions on p-leg ladders, valid for 2Sp
even or odd. Following an earlier proposal for 2Sp even (Damle and Sachdev,
cond-mat/9711014), we argue that an integrable, classical, continuum model of a
fixed-length, 3-vector applies over an intermediate temperature range; this
range becomes very wide for moderate and large values of 2Sp. The coupling
constants of the effective model are known exactly in terms of the energy gap
above the ground state (for 2Sp even) or a crossover scale (for 2Sp odd).
Analytic and numeric results for dynamic and transport properties are obtained,
including some exact results for the spin-wave damping. Numerous quantitative
predictions for neutron scattering and NMR experiments are made. A general
discussion on the nature of T>0 transport in integrable systems is also
presented: an exact solution of a toy model proves that diffusion can exist in
integrable systems, provided proper care is taken in approaching the
thermodynamic limit.


title:
Bond-versus-site doping models for off-chain-doped Haldane-gap system
  Y$_2$ Ba Ni O$_5$

abstract:
  Using the density matrix renormalization-group technique, we calculate the
impurity energy levels for two different effective models of off-chain doping
for quasi-one-dimensional Heisenberg chain compound Y$_2$ Ba Ni O$_5$:
ferromagnetic bond doping and antiferromagnetic site spin-1/2 doping.
Thresholds of the impurity strength for the appearance of localized states are
found for both models. However, the ground-state and low-energy excitations for
weak impurity strength are different for these two models and the difference
can be detected by experiments.


title:
Transition from the compact to the dense phase of two-dimensional
  polymers

abstract:
  We present a unifying picture of the compact, dense and dilute phases of
two-dimensional polymers. The lattice dependence of the scaling exponents for
compact polymers is reconciled with their universality in the dense and dilute
case. In particular, we show that violations of the fully-packing constraint in
the compact phase can be interpreted as magnetic screening in the associated
Coulomb gas, which induces a flow to either the dense or the dilute phase. When
more than one flavour of polymers is present the flow away from the compact
phase leads to a decoupling of the flavours, and the central charge decreases
by an integer. If charge asymmetry develops the polymer flavours may
independently flow to either of the two non-compact phases.


title: Dynamic transitions and hysteresis

abstract:
  When an interacting many-body system, such as a magnet, is driven in time by
an external perturbation, such as a magnetic field,the system cannot respond
instantaneously due to relaxational delay. The response of such a system under
a time-dependent field leads to many novel physical phenomena with intriguing
physics and important technological applications. For oscillating fields, one
obtains hysteresis that would not occur under quasistatic conditions in the
presence of thermal fluctuations. Under some extreme conditions of the driving
field, one can also obtain a non-zero average value of the variable undergoing
such dynamic hysteresis. This non-zero value indicates a breaking of symmetry
of the hysteresis loop, around the origin. Such a transition to the
spontaneously broken symmetric phase occurs dynamically when the driving
frequency of the field increases beyond its threshold value which depends on
the field amplitude and the temperature. Similar dynamic transitions also occur
for pulsed and stochastically varying fields. We present an overview of the
ongoing researches in this not-so-old field of dynamic hysteresis and
transitions.


title:
A new perturbation treatment applied to the transport through a quantum
  dot

abstract:
  Resonant tunnelling through an Anderson impurity is investigated by employing
a new perturbation scheme at nonequilibrium. This new approach gives the
correct weak and strong coupling limit in $U$ by introducing adjustable
parameters in the self-energy and imposing self-consistency of the occupation
number of the impurity. We have found that the zero-temperature linear response
conductance agrees well with that obtained from the exact sum rule. At finite
temperature the conductance shows a nonzero minimum at the Kondo valley, as
shown in recent experiments. The effects of an applied bias voltage on the
single-particle density of states and on the differential conductances are
discussed for Kondo and non-Kondo systems.


title: Role of Impurities in Core Contribution to Vortex Friction

abstract:
  We present a microscopic calculation of core states contribution to vortex
friction in a type II superconductor at zero temperature. In weak impurity
limit, a perturbative calculation leads to a vortex friction proportional to
the normal state resistivity. With the aid of random matrix theory, vortex
friction due to core states is determined only by the Fermi energy and the
energy gap in dirty limit, similar to the phenomenological results of Bardeen
and Stephen, but disagrees with the results obtained under the relaxation time
approximation. In addition, an explicit demonstration is given on the the
insensitivity of transverse force to impurities.


title:
Lax pair formulation for a small-polaron chain with integrable
  boundaries

abstract:
  Using a fermionic version of the Lax pair formulation, we construct an
integrable small-polaron model with general open boundary conditions. The Lax
pair and the boundary supermatrices $K_{\pm}$ for the model are obtained. This
provides a direct proof of the integrability of the model.


title: Quasicondensation in 2D Interacting Bose Gas: Quantum Monte Carlo Study

abstract:
  We present a detailed Monte Carlo study of correlations in an interacting
two-dimensional Bose gas. The data for one-particle density matrix in
coordinate representation are compared to the results for the local
many-particle density correlators, which are responsible for the recombination
rate in real experiments. We found that the appearance and growth of
quasicondensate fluctuations changes local correlations between the particles
well before the Kosterlitz-Thouless transition point. The amplitude of the
$1/m!$-effect is very sensitive even to a rather moderate interaction, and is
considerably smaller than its limiting value.


title: Impurity mediated nucleation in hexadecane-in-water emulsions

abstract:
  We report detailed nucleation studies on the liquid-to-solid transition of
hexadecane using nearly monodisperse hexadecane-in-water emulsions. A careful
consideration of the kinetics of isothermal and nonisothermal freezing show
deviations from predictions of classical nucleation theory, if one assumes that
the emulsion droplet population is homogeneous. Similar deviations have been
observed previously. As an explanation, we propose a novel argument based on
the dynamic generation of droplet heterogeneity mediated by mobile impurities.
This proposal is in excellent agreement with existing data.


title:
Low Temperature Behavior of the Thermopower in Disordered Systems near
  the Anderson Transition

abstract:
  We investigate the behavior of the thermoelectric power [S] in disordered
systems close to the Anderson-type metal-insulator transition [MIT] at low
temperatures. In the literature, we find contradictory results for S. It is
either argued to diverge or to remain a constant as the MIT is approached. To
resolve this dilemma, we calculate the number density of electrons at the MIT
in disordered systems using an averaged density of states obtained by
diagonalizing the three-dimensional Anderson model of localization. From the
number density we obtain the temperature dependence of the chemical potential
necessary to solve for S. Without any additional approximation, we use the
Chester-Thellung-Kubo-Greenwood formulation and numerically obtain the behavior
of S at low T as the Anderson transition is approached from the metallic side.
We show that indeed S does not diverge.


title: Formation and manipulation of a metallic wire of single gold atoms

abstract:
  The continuing miniaturization of microelectronics raises the prospect of
nanometre-scale devices with mechanical and electrical properties that are
qualitatively different from those at larger dimensions. The investigation of
these properties, and particularly the increasing influence of quantum effects
on electron transport, has therefore attracted much interest. Quantum
properties of the conductance can be observed when `breaking' a metallic
contact: as two metal electrodes in contact with each other are slowly
retracted, the contact area undergoes structural rearrangements until it
consists in its final stages of only a few bridging atoms. Just before the
abrubt transition to tunneling occurs, the electrical conductance through a
monovalent metal contact is always close to a value of 2e^2/h, where e is the
charge on an electron and h is Plack's constant. This value corresponds to one
quantum unit of conductance, thus indicating that the `neck' of the contact
consists of a single atom. In contrast to previous observations of only
single-atom necks, here we describe the breaking of atomic-scale gold contacts,
which leads to the formation of gold chains one atom thick and at least four
atoms long. Once we start to pull out a chain, the conductance never exceeds
2e^2/h, confirming that it acts as a one-dimensional quantized nanowire. Given
their high stability and the ability to support ballistic electron transport,
these structures seem well suited for the investigation of atomic-scale
electronics.


title: The Dynamics of Money

abstract:
  We present a dynamical many-body theory of money in which the value of money
is a time dependent ``strategic variable'' that is chosen by the individual
agents. The value of money in equilibrium is not fixed by the equations, and
thus represents a continuous symmetry. The dynamics breaks this continuous
symmetry by fixating the value of money at a level which depends on initial
conditions. The fluctuations around the equilibrium, for instance in the
presence of noise, are governed by the ``Goldstone modes'' associated with the
broken symmetry. The idea is illustrated by a simple network model of
monopolistic vendors and buyers.

categories: cond-mat.stat-mech adap-org nlin.AO q-fin.GN


title:
Combining dynamical mean-field theory and realistic bandstructure of
  V2O3

abstract:
  Recent neutron scattering experiments on V2O3 show that the magnetic
fluctuations on the metallic side of the antiferromagnetic metal-insulator
transition are not related to the spin structure of the insulator, but rather
to the bandstructure-driven spin-density wave phase of the doped system
V(2-y)O3. We calculate these magnetic fluctuations starting from a
Slater-Koster bandstructure and incorporating the correlation effects through
the dynamical mean-field theory (DMFT). Our results demonstrate that the
magnetic properties of the paramagnetic metallic phase are dominated by the
Fermi surface topology. On the other hand, the electron-electron interaction
drives the paramagnetic metal-insulator transition in V2O3. The transition to
the antiferromagnetic insulator by virtue of orbital ordering is discussed in
the framework of the DMFT.


title:
Application of Renormalization Group Techniques to a Homogeneous Bose
  Gas at Finite Temperature

abstract:
  A homogeneous Bose gas is investigated at finite temperature using
renormalization group techniques. A non--perturbative flow equation for the
effective potential is derived using sharp and smooth cutoff functions.
Numerical solutions of these equations show that the system undergoes a second
order phase transition in accordance with universality arguments. We obtain the
critical exponent $\nu =0.73$ to leading order in the derivative expansion.


title: SU(N) Quantum Hall Skyrmions

abstract:
  We have investigated skyrmions in N-component quantum Hall systems. We find
that SU(N) skyrmions are the lowest energy charged excitations for filling
factors \nu = 1,2,...,N-1 for small enough symmetry breaking terms. N>2
skyrmions can be realized in Si QH systems based on the (110) or (111)
interfaces of Si, or perhaps in Si (100) systems, where the spin and valley
isospin together provide an SU(4)-symmetry, or in multilayer QH systems. We
also present Hartree-Fock results for a phenomenological easy-axis
SU(2)-breaking model appropriate to valley degeneracy.


title: Structure and oxidation kinetics of the Si(100)-SiO2 interface

abstract:
  We present first-principles calculations of the structural and electronic
properties of Si(001)-SiO2 interfaces. We first arrive at reasonable structures
for the c-Si/a-SiO2 interface via a Monte-Carlo simulated annealing applied to
an empirical interatomic potential, and then relax these structures using
first-principles calculations within the framework of density-functional
theory. We find a transition region at the interface, having a thickness on the
order of 20\AA, in which there is some oxygen deficiency and a corresponding
presence of sub-oxide Si species (mostly Si^+2 and Si^+3). Distributions of
bond lengths and bond angles, and the nature of the electronic states at the
interface, are investigated and discussed. The behavior of atomic oxygen in
a-SiO2 is also investigated. The peroxyl linkage configuration is found to be
lower in energy than interstitial or threefold configurations. Based on these
results, we suggest a possible mechanism for oxygen diffusion in a-SiO2 that
may be relevant to the oxidation process.


title:
Diagonal charge stripes giving a suppression of spectral weight at
  selected spots on the Fermi surface

abstract:
  Here we report an asymmetric suppresion of spectral weight at the Fermi
surface around the M points using angle resolved photoemission spectroscopy.
The results provide direct evidence for diagonal stripes in the Bi2212
superconductors.


title: Magnetic and Charge Fluctuations in High-$T_c$ Superconductors

abstract:
  Neutron scattering has been used to study the spin fluctuations in the
YBa$_2$Cu$_3$O$_{7-x}$ and Bi$_2$Sr$_2$CaCu$_2$O$_8$ materials. Evidence is
found for both incommensurate fluctuations and a commensurate resonance
excitation. Measurements on the lattice dynamics for YBa$_2$Cu$_3$O$_{6.6}$
show incommensurate structure that appears to stem from charge fluctuations
that are associated with the spin fluctuations.


title: Theory of angular magnetoresistance oscillations in Tl2Ba2CuO6

abstract:
  Using the calculated electron energy band structure of Tl2Ba2CuO6, we compute
the dependence of the c axis magnetoresistance on the orientation of the
magnetic field for different magnitudes of the magnetic field. We explain the
known experimental results for the in-plane rotation of the magnetic field and
predict the shape of the magnetoresistance oscillations for the out-of-plane
rotations of the magnetic field. We show how the latter oscillations can be
utilized to reconstruct the shape of the Fermi surface and to study the
coherence of inter-plane electron tunneling.


title: Field Theoretic Approach to Long Range Reactions

abstract:
  We analyze bimolecular reactions that proceed by a long-ranged reactive
interaction, using a field theoretic approach that takes into account
fluctuations.
  We consider both the one-species, $A+A \to \emptyset$ reaction and the
two-species, $A+B \to \emptyset$ reaction. We consider both mobile and immobile
reactants, both in the presence and in the absence of adsorption.


title: Quantum Field Theory of the Pinned Density Wave

abstract:
  A model is discussed in which an electric field induces quantum nucleation of
soliton-antisoliton pairs in a pinned charge or spin density wave. Coulomb
blockade prevents pair creation until the electric field exceeds a sharp
threshold value, which can be much smaller than the classical depinning field.
We calculate the vacuum state energy and expectation value of the phase, which
is treated as a quantum scalar field. We find that the phase can also be much
smaller, below threshold, than predicted by classical ``sliding'' density wave
models.


title:
Normal-state resistivity anisotropy in underdoped RBa_2Cu_3O_{6+x}
  crystals

abstract:
  We have revealed new features in the out-of-plane resistivity rho_c of
heavily underdoped RBa_2Cu_3O_{6+x} (R=Tm,Lu) single crystals, which give
evidence for two distinct mechanisms contributing the c-axis transport. We have
observed a crossover towards "metal-like" (d rho_c/d T > 0) behavior at the
temperature T_m which quickly increases with decreasing doping. The
"metal-like" conductivity contribution dominates at T < T_m and provides a
saturation of the resistivity anisotropy, rho_c / rho_{ab}. The
antiferromagnetic ordering is found to block this "metal-like" part of the
c-axis conductivity and complete decoupling of CuO_2 planes, which may be the
reason of superconductivity disappearance.


title: Smectic Liquid Crystals in Random Environments

abstract:
  We study smectic liquid crystals in random environments, e.g., aerogel. A low
temperature analysis reveals that even arbitrarily weak quenched disorder
(i.e., arbitrarily low aerogel density) destroys translational (smectic) order.
A harmonic approximation to the elastic energy suggests that there is no
``smectic Bragg glass'' phase in this system: even at zero temperature, it is
riddled with dislocation loops induced by the quenched disorder. This result
implies the destruction of orientational (nematic) order as well, and that the
thermodynamically sharp Nematic-Smectic-A transition is destroyed by disorder,
in agreement with recent experimental results. We also show that the anharmonic
elastic terms neglected in the above treatment are important (i.e., are
``relevant'' in the renormalization group sense); whether they alter the above
conclusions about the smectic Bragg glass, orientational disorder, and the
existence of sharp transitions, remains an open question. However, they do not
alter our conclusion that translational (smectic) order is always destroyed. In
contrast, we expect that weak annealed disorder should have no qualitative
effects on the smectic order.


title:
Scaling of the Hall Resistivity in the Solid and Liquid Vortex Phases in
  Twinned Single Crystal YBCO

abstract:
  Longitudinal and Hall voltages are measured in a clean twinned YBCO single
crystal in the liquid and solid vortex phases. For magnetic fields tilted away
from the c-axis more than about 2 deg, a scaling law |rho_xy|=A (rho_xx)^beta
with beta approx.=1.4 is observed, which is unaffected by the vortex-lattice
melting transition. The vortex-solid Hall conductivity is non-linear and
diverges to negative values at low temperature. When the magnetic field is
aligned to the c-axis, the twin-boundary correlated disorder modifies the
scaling law, and beta approx.=2. The scaling law is unaffected by the
Bose-glass transition. We discuss the scaling behaviour in terms of the
dimension-dependent theory for percolation in metallic conductors.


title: Formation of disclination lines near a free nematic interface

abstract:
  We have studied the nucleation and the physical properties of a -1/2 wedge
disclination line near the free surface of a confined nematic liquid crystal.
The position of the disclination line has been related to the material
parameters (elastic constants, anchoring energy and favored anchoring angle of
the molecules at the free surface). The use of a planar model for the structure
of the director field (whose predictions have been contrasted to those of a
fully three-dimensional model) has allowed us to relate the experimentally
observed position of the disclination line to the relevant properties of the
liquid crystals. In particular, we have been able to observe the collapse of
the disclination line due to a temperature-induced anchoring angle transition,
which has allowed us to rule out the presence of a real disclination line near
the nematic/isotropic front in directional growth experiments.
  61.30.Jf,61.30.Gd


title: Electronic transport in a randomly amplifying and absorbing chain

abstract:
  We study localization properties of a one-dimensional disordered system
characterized by a random non-hermitean hamiltonian where both the randomness
and the non-hermiticity arises in the local site-potential; its real part being
ordered (fixed), and a random imaginary part implying the presence of either a
random absorption or amplification at each site. The transmittance (forward
scattering) decays exponentially in either case. In contrast to the disorder in
the real part of the potential (Anderson localization), the transmittance with
the disordered imaginary part may decay slower than that in the case of ordered
imaginary part.


title:
Long-term properties of time series generated by a perceptron with
  various transfer functions

abstract:
  We study the effect of various transfer functions on the properties of a time
series generated by a continuous-valued feed-forward network in which the next
input vector is determined from past output values. The parameter space for
monotonic and non-monotonic transfer functions is analyzed in the unstable
regions with the following main finding; non-monotonic functions can produce
robust chaos whereas monotonic functions generate fragile chaos only. In the
case of non-monotonic functions, the number of positive Lyapunov exponents
increases as a function of one of the free parameters in the model, hence, high
dimensional chaotic attractors can be generated. We extend the analysis to a
combination of monotonic and non-monotonic functions.


title:
High temperature relaxational dynamics in low-dimensional quantum field
  theories

abstract:
  This paper presents a unified perspective on the results of two recent works
(C. Buragohain and S. Sachdev cond-mat/9811083 and S. Sachdev cond-mat/9810399)
along with additional background. We describe the low frequency, non-zero
temperature, order parameter relaxational dynamics of a number of systems in
the vicinity of a quantum critical point. The dynamical correlations are
properties of the high temperature limit of renormalizable quantum field
theories in spatial dimensions d=1,2. We study, as a function of d and the
number of order parameter components, n, the crossover from the finite
frequency, "amplitude fluctuation", gapped quasiparticle mode in the quantum
paramagnet (or Mott insulator), to the zero frequency "phase" (n>=2) or "domain
wall" (n=1) relaxation mode near the ordered phase. Implications for dynamic
measurements on the high temperature superconductors and antiferromagnetic spin
chains are noted.


title: Projective Construction of Non-Abelian Quantum Hall Liquids

abstract:
  Using projective construction, a generalized parton construction, we
construct many non-Abelian quantum Hall (QH) states, which include the Pfaffian
state at filling fraction $\nu=1/2$. The projective construction allows us to
calculate the bulk and the edge effective theory for the constructed QH state.
We illustrate how to use the bulk effective theory to calculate the ground
state degeneracy of non-Abelian QH liquids on torus. We point out that the full
description of the effective theory requires both the effective Lagrangian and
the definition of electron operators. The latter generates all physical states
and defines the gauge structure of the theory.


title: Insensitivity of Quantized Hall Conductance to Disorder and Interactions

abstract:
  A two-dimensional quantum Hall system is studied for a wide class of
potentials including single-body random potentials and repulsive
electron-electron interactions. We assume that there exists a non-zero
excitation gap above the ground state(s), and then the conductance is derived
from the linear perturbation theory with a sufficiently weak electric field.
Under these two assumptions, we proved that the Hall conductance $\sigma_{xy}$
and the diagonal conductance $\sigma_{yy}$ satisfy
$|\sigma_{xy}+e^2\nu/h|\le{\rm const.}L^{-1/12}$ and $|\sigma_{yy}|\le{\rm
const.}L^{-1/12}$. Here $e^2/h$ is the universal conductance with the charge
$-e$ of electron and the Planck constant $h$; $\nu$ is the filling factor of
the Landau level, and $L$ is the linear dimension of the system. In the
thermodymanic limit, our results show $\sigma_{xy}=-e^2\nu/h$ and
$\sigma_{yy}=0$. The former implies that integral and fractional filling
factors $\nu$ with a gap lead to, respectively, integral and fractional
quantizations of the Hall conductance.


title: Coherent current states in mesoscopic four-terminal Josephson junction

abstract:
  A theory is offered for the ballistic 4-terminal Josephson junction. The
studied system consists of a mesoscopic two-dimensional normal rectangular
layer which is attached in each side to the bulk superconducting banks
(terminals). The relation between the currents through the different terminals,
which is valid for arbitrary temperatures and junction sizes, is obtained. The
nonlocal coupling of the supercurrents leads to a new effect, specific for the
mesoscopic weak link between two superconducting rings; an applied magnetic
flux through one of the rings produces a magnetic flux in the other ring even
in the absence of an external flux through the other one. The phase dependent
distributions of the local density of Andreev states, of the supercurrents and
of the induced order parameter are obtained. The "interference pattern" for the
anomalous average inside the two dimensional region can be regulated by the
applied magnetic fluxes or the transport currents. For some values of the phase
differences between the terminals, the current vortex state and the two
dimensional phase slip center are appeared.


title: Reaction-Diffusion-Branching Models of Stock Price Fluctuations

abstract:
  Several models of stock trading [P. Bak et al, Physica A {\bf 246}, 430
(1997)] are analyzed in analogy with one-dimensional, two-species
reaction-diffusion-branching processes. Using heuristic and scaling arguments,
we show that the short-time market price variation is subdiffusive with a Hurst
exponent $H=1/4$. Biased diffusion towards the market price and blind-eyed
copying lead to crossovers to the empirically observed random-walk behavior
($H=1/2$) at long times. The calculated crossover forms and diffusion constants
are shown to agree well with simulation data.


title: Exact Thermodynamics of Disordered Impurities in Quantum Spin Chains

abstract:
  Exact results for the thermodynamic properties of ensembles of magnetic
impurities with randomly distributed host-impurity couplings in the quantum
antiferromagnetic Heisenberg model are presented. Exact calculations are done
for arbitrary values of temperature and external magnetic field. We have shown
that for strong disorder the quenching of the impurity moments is absent. For
weak disorder the screening persists, but with the critical non-Fermi-liquid
behaviors of the magnetic susceptibility and specific heat. A comparison with
the disordered Kondo effect experiments in dirty metallic alloys is performed.


title: Charge Conjugation and Pairing in a model Cu$_{5}$O$_{4}$ Cluster

abstract:
  Highly-symmetric three-band Hubbard Cu-O clusters have peculiar properties
when the hole number is such that they admit W=0 hole pairs. These are two-hole
eigenstates of the on-site Hubbard repulsion with eigenvalue 0, get bound by
correlation effects when dressed by the interaction with the background, and
cause superconducting flux quantization. We study the Cu$_{5}$O$_{4}$ cluster
by exact diagonalization and show that bound electron pairs of $^{1}$% B$_{2}$
symmetry are obtained at an appropriate filling, and quantize flux like the
hole pairs. The basic mechanism for pairing in this model is the second-order
exchange diagram, and an approximate charge conjugation symmetry holds between
electron and hole pairs. Further, the flux quantization property requires that
the W=0 pairs of $d$ symmetry have $s$ symmetry couterparts, still with W=0;
the former are due to a spin fluctuation, while the latter arise from a charge
fluctuation mechanism. The simultaneous existence of both is an essential
property of our model and is required for any model of superconducting $d$
pairs.


title:
Sub-gap conductance in ferromagnetic-superconducting mesoscopic
  structures

abstract:
  We study the sub-gap conductance of a ferromagnetic mesoscopic region
attached to a ferromagnetic and a superconducting electrode by means of tunnel
junctions. In the absence of the exchange field, the ratio $r= \gamma /
\epsilon_T$ of the two tunnel junction resistances determines the behaviour of
the sub-gap conductance which possesses a zero-bias peak for $r\gg 1$ and for
$r\ll 1$ a peak at finite voltage. We show that the inclusion of the exchange
field leads to a peak splitting for $r\ll 1$, while it shifts the zero-bias
anomaly to finite voltages for $r\gg 1$.


title: Structural evidence for the continuity of liquid and glassy water

abstract:
  An open question is whether the liquid and glassy phases of water are
thermodynamically distinct or continuous. Here we address this question using
molecular dynamics simulations in comparison with neutron scattering
experiments to study the effect of temperature and pressure on the local
structure of liquid water. From both simulations and experiments, we find that
the liquid structure at high pressure is nearly independent of temperature, and
remarkably similar to the known structure of the high-density amorphous ice
(HDA). Further at low pressure, the liquid structure appears to approach the
experimentally-measured structure of low-density amorphous ice (LDA) as
temperature decreases. These results are consistent with continuity between the
liquid and glassy phases of H$_2$O.


title:
A numerical study of wave-function and matrix-element statistics in the
  Anderson model of localization

abstract:
  We have calculated wave functions and matrix elements of the dipole operator
in the two- and three-dimensional Anderson model of localization and have
studied their statistical properties in the limit of weak disorder. In
particular, we have considered two cases. First, we have studied the
fluctuations as an external Aharonov-Bohm flux is varied. Second, we have
considered the influence of incipient localization. In both cases, the
statistical properties of the eigenfunctions are non-trivial, in that the joint
probability distribution function of eigenvalues and eigenvectors does no
longer factorize. We report on detailed comparisons with analytical results,
obtained within the non-linear sigma model and/or the semiclassical approach.


title: Fast and slow dynamics of hydrogen bonds in liquid water

abstract:
  We study hydrogen-bond dynamics in liquid water at low temperatures using
molecular dynamics simulations. We find that bond lifetime (``fast dynamics'')
has Arrhenius temperature dependence. We also calculate the bond correlation
function and find that the correlation time (``slow dynamics'') shows power-law
behavior. This power-law behavior, as well as the decay of the bond
correlations, is consistent with the predictions of the mode-coupling theory.
The correlation time at the lowest temperature studied shows deviation from
power-law behavior that suggests continuity of dynamic functions between the
liquid and glassy states of water at low pressure.


title: Bose Glass in Large N Commensurate Dirty Boson Model

abstract:
  The large N commensurate dirty boson model, in both the weakly and strongly
commensurate cases, is considered via a perturbative renormalization group
treatment. In the weakly commensurate case, there exists a fixed line under RG
flow, with varying amounts of disorder along the line. Including 1/N
corrections causes the system to flow to strong disorder, indicating that the
model does not have a phase transition perturbatively connected to the Mott
Insulator-Superfluid (MI-SF) transition. I discuss the qualitative effects of
instantons on the low energy density of excitations. In the strongly
commensurate case, a fixed point found previously is considered and results are
obtained for higher moments of the correlation functions. To lowest order,
correlation functions have a log-normal distribution. Finally, I prove two
interesting theorems for large N vector models with disorder, relevant to the
problem of replica symmetry breaking and frustration in such systems.


title: Shape-dependent universality in percolation

abstract:
  The shape-dependent universality of the excess percolation cluster number and
cross-configuration probability on a torus is discussed. Besides the aspect
ratio of the torus, the universality class depends upon the twist in the
periodic boundary conditions, which for example are generally introduced when
triangular lattices are used in simulations.


title:
Collective charge density fluctuations in superconducting layered
  systems with bilayer unit cells

abstract:
  Collective modes of bilayered superconducting superlattices (e.g., YBCO) are
investigated within the conserving gauge-invariant ladder diagram approximation
including both the nearest interlayer single electron tunneling and the
Josephson-type Cooper pair tunneling. By calculating the density-density
response function including Coulomb and pairing interactions, we examine the
two collective mode branches corresponding to the in-phase and out-of-phase
charge fluctuations between the two layers in the unit cell. The out-of-phase
collective mode develops a long wavelength plasmon gap whose magnitude depends
on the tunneling strength with the mode dispersions being insensitive to the
specific tunneling mechanism (i.e., single electron or Josephson). We also show
that in the presence of tunneling the oscillator strength of the out-of-phase
mode overwhelms that of the in-phase-mode at $k_{\|} = 0$ and finite $k_z$,
where $k_z$ and $k_{\|}$ are respectively the mode wave vectors perpendicular
and along the layer. We discuss the possible experimental observability of the
phase fluctuation modes in the context of our theoretical results for the mode
dispersion and spectral weight.


title: Subsurface charge accumulation imaging of a quantum Hall liquid

abstract:
  The unusual properties of two-dimensional electron systems that give rise to
the quantum Hall effect have prompted the development of new microscopic models
for electrical conduction. The bulk properties of the quantum Hall effect have
also been studied experimentally using a variety of probes including transport,
photoluminescence, magnetization, and capacitance measurements. However, the
fact that two-dimensional electron systems typically exist some distance (about
100 nm) beneath the surface of the host semiconductor has presented an
important obstacle to more direct measurements of microscopic electronic
structure in the quantum Hall regime. Here we introduce a cryogenic
scanning-probe technique-- subsurface charge accumulation imaging-- that
permits very high resolution examination of systems of mobile electrons inside
materials. We use it to image directly the nanometer-scale electronic
structures that exist in the quantum Hall regime.


title:
Numerical study of current distributions in finite planar Hall samples
  with disorder

abstract:
  A numerical study is made of current distributions in finite-width Hall bars
with disorder and some theoretical observations are verified. The equilibrium
current and the Hall current are substantially different in distribution. It is
observed that in the Hall-plateau regime the Hall current tends to concentrate
near the sample edges while it diminishes on average in the sample interior as
a consequence of localization. The edge states themselves scarcely affect both
the equilibrium and Hall currents in the plateau regime. The sample edge
combines with disorder to efficiently delocalize electrons near the edge while
the Hall field competes with disorder to delocalize electrons in the sample
interior. A possible mechanism is suggested for the breakdown of the quantum
Hall effect.


title: Coupling between electron-spin and ferromagnetic clusters

abstract:
  We analyze the adiabatic magnetization of ferromagnetic clusters in an
intermediate coupling regime, where the anisotropic potential is comparable to
other energy scales. We find a non-monotonic behavior of the magnetic
susceptibility as a function of coupling with a peak. Coriolis coupling effects
are calculated for the first time; they reduce the susceptibility somewhat.


title: Geometrically Reduced Number of Protein Ground State Candidates

abstract:
  Geometrical properties of protein ground states are studied using an
algebraic approach. It is shown that independent from inter-monomer
interactions, the collection of ground state candidates for any folded protein
is unexpectedly small: For the case of a two-parameter Hydrophobic-Polar
lattice model for $L$-mers, the number of these candidates grows only as $L^2$.
Moreover, the space of the interaction parameters of the model breaks up into
well-defined domains, each corresponding to one ground state candidate, which
are separated by sharp boundaries. In addition, by exact enumeration, we show
there are some sequences which have one absolute unique native state. These
absolute ground states have perfect stability against change of inter-monomer
interaction potential.


title:
Disclination Dipoles as the Basic Structural Elements of Dielectric
  Glasses

abstract:
  We show that the experimentally observed behavior of thermal conductivity of
dielectric glasses over a wide temperature range can be explained by a
combination of two scattering processes. The first one comes from the phonon
scattering due to biaxial dipoles of wedge disclinations while the second one
is the Rayleigh type scattering. The results obtained support the cluster
picture suggested earlier for glassy materials.


title:
Self-consistent effects of continuous wave output coupling of atoms from
  a Bose-Einstein condensate

abstract:
  We present a self-consistent mean field model of the extraction of atoms from
a Bose-Einstein condensate to form a CW atom laser. The model is based upon the
Hartree-Fock Bogoliubov equations within the Popov approximation, modified by
the inclusion of spatially dependent source and sink terms, which lead to
current flow within the condensate. The effects of this current flow are
investigated for traps containing Rubidium (repulsive effective interaction)
and Lithium (attractive interaction) atoms. The extra kinetic energy associated
with this flow is shown to be able to stabilise the condensate in the
attractive case against mechanical instability.


title:
Single-particle properties of a model for coexisting charge and spin
  quasi-critical fluctuations coupled to electrons

abstract:
  We study the single-particle spectral properties of a model for coexisting
AFM and ICDW critical fluctuations coupled to electrons, which naturally arises
in the context of the stripe-quantum-critical-point scenario for high-Tc
superconducting materials. Within a perturbative approach, we show that the
on-shell inverse scattering time deviates from the normal Fermi-liquid behavior
near the points of the Fermi surface connected by the characteristic
wave-vectors of the critical fluctuations (hot spots). The anomalous behavior
is stronger when the hot spots are located near singular points of the
electronic spectrum.
  The violations to the normal Fermi-liquid behavior are associated with the
transfer of spectral weight from the quasi-particle peak to incoherent shadow
peaks, which produces an enhancement of incoherent spectral weight near the
Fermi level.
  We use our results to discuss recent ARPES experiments on Bi2212 near optimal
doping.


title: Universality of electron correlations in conducting carbon nanotubes

abstract:
  Effective low-energy Hamiltonian of interacting electrons in conducting
single-wall carbon nanotubes with arbitrary chirality is derived from the
microscopic lattice model. The parameters of the Hamiltonian show very weak
dependence on the chiral angle, which makes the low energy properties of
conducting chiral nanotubes universal. The strongest Mott-like electron
instability at half filling is investigated within the self-consistent harmonic
approximation. The energy gaps occur in all modes of elementary excitations and
estimate at $0.01-0.1$ eV.


title:
The combined effect of temperature and disorder on interlayer exchange
  coupling in magnetic multilayers

abstract:
  We study the combined effect of temperature and disorder in the spacer on the
interlayer exchange coupling. The temperature dependence is treated on ab
initio level. We employ the spin-polarized surface Green function technique
within the tight-binding linear muffin-tin orbital method and the Lloyd
formulation of the IEC. The integrals involving the Fermi-Dirac distribution
are calculated using an efficient method based on representation of integrands
by a sum of complex exponentials. Application is made to
Co/Cu_{100-x}M_x/Co(001) trilayers (M=Zn, Au, and Ni) with varying thicknesses
of the spacer.


title: Properties of SAS Josephson junctions in SO(5) theory

abstract:
  We derive the qualitative behavior of superconductor-antiferromagnet-
superconductor (SAS) Josephson junctions described by Zhang's SO(5) theory. The
main differences between these junctions and conventional SIS junctions arise
from the non-sinusoidal current-phase relation derived by Demler et al. for
thin SAS junctions. Using a simple approximation to this non-sinusoidal
function, the current voltage relation, Shapiro steps, thermal fluctuation
effects and the diffraction pattern in a magnetic field are obtained.


title: Field Theory of the Random Flux Model

abstract:
  The long-range properties of the random flux model (lattice fermions hopping
under the influence of maximally random link disorder) are shown to be
described by a supersymmetric field theory of non-linear sigma model type,
where the group GL(n|n) is the global invariant manifold. An extension to
non-abelian generalizations of this model identifies connections to lattice
QCD, Dirac fermions in a random gauge potential, and stochastic non-Hermitian
operators.


title: Surface metal-insulator transition in the Hubbard model

abstract:
  The correlation-driven metal-insulator (Mott) transition at a solid surface
is studied within the Hubbard model for a semi-infinite lattice by means of the
dynamical mean-field theory. The transition takes place at a unique critical
strength of the interaction. Depending on the surface geometry, the interaction
strength and the wave vector, we find one-electron excitations in the coherent
part of the surface-projected metallic spectrum which are confined to two
dimensions.


title: Effective mass at the surface of a Fermi liquid

abstract:
  Using the dynamical mean-field theory, we calculate the effective electron
mass in the Hubbard model on a semi-infinite lattice. At the surface the
effective mass is strongly enhanced. Near half-filling this gives rise to a
correlation-driven one-electron surface mode.


title: Dynamical mean-field study of the Mott transition in thin films

abstract:
  The correlation-driven transition from a paramagnetic metal to a paramagnetic
Mott-Hubbard insulator is studied within the half-filled Hubbard model for a
thin-film geometry. We consider simple-cubic films with different low-index
surfaces and film thickness d ranging from d=1 (two-dimensional) up to d=8.
Using the dynamical mean-field theory, the lattice (film) problem is
self-consistently mapped onto a set of d single-impurity Anderson models which
are indirectly coupled via the respective baths of conduction electrons. The
impurity models are solved at zero temperature using the exact-diagonalization
algorithm. We investigate the layer and thickness dependence of the electronic
structure in the low-energy regime. Effects due to the finite film thickness
are found to be the more pronounced the lower is the film-surface coordination
number. For the comparatively open sc(111) geometry we find a strong layer
dependence of the quasi-particle weight while it is much less pronounced for
the sc(110) and the sc(100) film geometries. For a given geometry and thickness
d there is a unique critical interaction strength Uc2(d) at which all effective
masses diverge and there is a unique strength Uc1(d) where the insulating
solution disappears. Uc2(d) and Uc1(d) gradually increase with increasing
thickness eventually approaching their bulk values. A simple analytical
argument explains the complete geometry and thickness dependence of Uc2. Uc1 is
found to scale linearly with Uc2.


title: Dynamic and static properties of the invaded cluster algorithm

abstract:
  Simulations of the two-dimensional Ising and 3-state Potts models at their
critical points are performed using the invaded cluster (IC) algorithm. It is
argued that observables measured on a sub-lattice of size l should exhibit a
crossover to Swendsen-Wang (SW) behavior for l sufficiently less than the
lattice size L, and a scaling form is proposed to describe the crossover
phenomenon. It is found that the energy autocorrelation time tau(l,L) for an
l*l sub-lattice attains a maximum in the crossover region, and a dynamic
exponent z for the IC algorithm is defined according to tau_max ~ L^z.
Simulation results for the 3-state model yield z=.346(.002) which is smaller
than values of the dynamic exponent found for the SW and Wolff algorithms and
also less than the Li-Sokal bound. The results are less conclusive for the
Ising model, but it appears that z<.21 and possibly that tau_max ~ log L so
that z=0 -- similar to previous results for the SW and Wolff algorithms.


title:
A formula for the hopping expansion of 8-vertex models coupled to an
  external field

abstract:
  We study a generalized 8-vertex model where the vertices are coupled to a
locally varying field. We rewrite the partition function as an integral over
Grassmann variables. In this form it is possible to explicitly evaluate all
terms of the hopping expansion. Applications of the resulting formula, in
particular its relation to 2-D lattice field theories with fermions are
discussed.


title:
Imaging Microwave Electric Fields Using a Near-Field Scanning Microwave
  Microscope

abstract:
  By scanning a fine open-ended coaxial probe above an operating microwave
device, we image local electric fields generated by the device at microwave
frequencies. The probe is sensitive to the electric flux normal to the face of
its center conductor, allowing different components of the field to be imaged
by orienting the probe appropriately. Using a simple model of the microscope,
we are able to interpret the system's output and determine the magnitude of the
electric field at the probe tip. We show images of electric field components
above a copper microstrip transmission line driven at 8 GHz, with a spatial
resolution of approximately 200 $\mu$m.


title:
Frequency Following Imaging of Electric Fields from Resonant
  Superconducting Devices using a Scanning Near-Field Microwave Microscope

abstract:
  We have developed a scanning near-field microwave microscope that operates at
cryogenic temperatures. Our system uses an open-ended coaxial probe with a 200
mm inner conductor diameter and operates from 77 to 300 K in the 0.01-20 GHz
frequency range. In this paper, we present microwave images of the electric
field distribution above a Tl2Ba2CaCu2O8 microstrip resonator at 77 K, measured
at several heights. In addition, we describe the use of a frequency-following
circuit to study the influence of the probe on the resonant frequency of the
device.


title:
Correlations for the orthogonal-unitary and symplectic-unitary
  transitions at the hard and soft edges

abstract:
  For the orthogonal-unitary and symplectic-unitary transitions in random
matrix theory, the general parameter dependent distribution between two sets of
eigenvalues with two different parameter values can be expressed as a
quaternion determinant. For the parameter dependent Gaussian and Laguerre
ensembles the matrix elements of the determinant are expressed in terms of
corresponding skew-orthogonal polynomials, and their limiting value for
infinite matrix dimension are computed in the vicinity of the soft and hard
edges respectively. A connection formula relating the distributions at the hard
and soft edge is obtained, and a universal asymptotic behaviour of the two
point correlation is identified.


title:
Scanned Potential Microscopy of Edge and Bulk Currents in the Quantum
  Hall Regime

abstract:
  Using an atomic force microscope as a local voltmeter, we measure the Hall
voltage profile in a 2D electron gas in the quantum Hall (QH) regime. We
observe a linear profile in the bulk of the sample in the transition regions
between QH plateaus and a distinctly nonlinear profile on the plateaus. In
addition, localized voltage drops are observed at the sample edges in the
transition regions. We interpret these results in terms of theories of edge and
bulk currents in the QH regime.


title:
Dynamical Mean Field Theory of the Antiferromagnetic Metal to
  Antiferromagnetic Insulator Transition

abstract:
  We study the antiferromagnetic metal to antiferromagnetic insulator using
dynamical mean field theory and exact diagonalization methods. We find two
qualitatively different behaviors depending on the degree of magnetic
correlations. For strong correlations combined with magnetic frustration, the
transition can be described in terms of a renormalized slater theory, with a
continuous gap closure driven by the magnetism but strongly renormalized by
correlations. For weak magnetic correlations, the transition is weakly first
order.


title:
Growing spatial correlations of particle displacements in a simulated
  liquid on cooling toward the glass transition

abstract:
  We define a correlation function that quantifies the spatial correlation of
single-particle displacements in liquids and amorphous materials. We show for
an equilibrium liquid that this function is related to fluctuations in a bulk
dynamical variable. We evaluate this function using computer simulations of an
equilibrium glass-forming liquid, and show that long range spatial correlations
of displacements emerge and grow on cooling toward the mode coupling critical
temperature.


title: Preformed Pairs, SU(2) Slave-boson Theory, and High T_c Superconductors

abstract:
  A preformed-pair model was considered. The quantum disordered phase of the
d-wave superconducting state was obtained by turning on an on-site repulsion.
The low energy effective theory of the quantum disordered phase was derived,
which was found to be qualitatively equivalent to the low energy effective
theory of the SU(2) slave-boson theory in the staggered-flux phase. Many
physical properties of the disordered phase at low energies were obtained, such
as the Mott insulator property at zero doping, spin gap and low superfluid
density at low doping.


title: Instabilities of wave function monopoles in Bose-Einstein condensates

abstract:
  We present analytic and numerical results for a class of monopole solutions
to the two-component Gross-Pitaevski equation for a two-species Bose condensate
in an effectively two-dimensional trap. We exhibit dynamical instabilities
involving vortex production as one species pours through another, from which we
conclude that the sub-optical sharpness of potentials exerted by matter waves
makes condensates ideal tools for manipulating condensates. We also show that
there are two equally valid but drastically different hydrodynamic descriptions
of a two-component condensate, and illustrate how different phenomena may
appear simpler in each.