title: Metastability of (d+n)-dimensional elastic manifolds

abstract:
  We investigate the depinning of a massive elastic manifold with $d$ internal
dimensions, embedded in a $(d+n)$-dimensional space, and subject to an
isotropic pinning potential $V({\bf u})=V(|{\bf u}|).$ The tunneling process is
driven by a small external force ${\bf F}.$ We find the zero temperature and
high temperature instantons and show that for the case $1\le d\le 6$ the
problem exhibits a sharp transition from quantum to classical behavior: At low
temperatures $T<T_{c}$ the Euclidean action is constant up to exponentially
small corrections, while for $T> T_{c},$ ${S_{\rm Eucl}(d,T)}/{\hbar} =
{U(d)}/{T}.$ The results are universal and do not depend on the detailed shape
of the trapping potential $V({\bf u})$. Possible applications of the problem to
the depinning of vortices in high-$T_{c}$ superconductors and nucleation in
$d$-dimensional phase transitions are discussed. In addition, we determine the
high-temperature asymptotics of the preexponential factor for the
$(1+1)$-dimensional problem.


title: Microwave spectroscopy on a quantum-dot molecule

abstract:
  Quantum dots are small conductive regions in a semiconductor, containing a
variable number of electrons (N=1 to 1000) that occupy well defined discrete
quantum states. They are often referred to as artificial atoms with the unique
property that they can be connected to current and voltage contacts. This
allows one to use transport measurements to probe the discrete energy spectra.
To continue the analogy with atoms, two quantum dots can be connected to form
an 'artificial molecule'. Depending on the strength of the inter-dot coupling,
the two dots can have an ionic binding (i.e. electrons are localized on the
individual dots) or a covalent binding (i.e. electrons are delocalized over
both dots). The covalent binding leads to a bonding and an anti-bonding state
with an energy splitting proportional to the tunnel coupling. In the dc current
response to microwave excitation we observe a transition from an ionic bonding
to a covalent bonding, when we vary the inter-dot coupling strength. This
demonstrates controllable quantum coherence in single electron devices.


title:
Analysis of a Population Genetics Model with Mutation, Selection, and
  Pleiotropy

abstract:
  We investigate the behavior of a population genetics model introduced by
Waxman and Peck incorporating mutation, selection, and pleiotropy. The
population is infinite and continuous variation of genotype is allowed.
Nonetheless, Waxman and Peck showed that if the degree of pleiotropy is large
enough,in this model a nonzero fraction of the population can have identical
alleles. This `condensed mode' behavior appears in the limit of infinite times.
  This paper explores the time-dependence of the distribution of alleles in
this model. First, the model is analyzed using a recursion technique which
enables the distribution of alleles to be calculated at finite times as well as
in Waxman and Peck's infinite-time limit. Second, both Waxman and Peck's
original model and a related model in which mutations occur continuously are
mapped onto problems in quantum mechanics. In both cases, the long-time
analysis for the biological model is equivalent to finding the nature of the
eigenstates of the quantum problem. The condensed mode appears if and only if
there is no bound state in the quantum problem.
  We compare the behavior of the discrete- and continuous-time versions of the
model. The results for the two cases are qualitatively similar, though there
are some quantitative differences. We also discuss our attempts to correlate
the statistics of DNA sequence variations with the degree of pleiotropy of
various genes.


title:
Rigorous Proof of a Liquid-Vapor Phase Transition in a Continuum
  Particle System

abstract:
  We consider particles in ${\Bbb R}^d, d \geq 2$, interacting via attractive
pair and repulsive four-body potentials of the Kac type. Perturbing about mean
field theory, valid when the interaction range becomes infinite, we prove
rigorously the existence of a liquid-gas phase transition when the interaction
range is finite but long compared to the interparticle spacing.


title:
Liquid-Vapor Phase Transitions for Systems with Finite Range
  Interactions

abstract:
  We consider particles in $\R^d, d \geq 2$ interacting via attractive pair and
repulsive four-body potentials of the Kac type. Perturbing about mean field
theory, valid when the interaction range becomes infinite, we prove rigorously
the existence of a liquid-gas phase transition, when the interaction range is
finite but long compared to the interparticle spacing for a range of
temperature.


title:
Exact exchange-correlation potential for a time-dependent two electron
  system

abstract:
  We obtain an exact solution of the time-dependent Schroedinger equation for a
two-electron system confined to a plane by an isotropic parabolic potential
whose curvature is periodically modulated in time. From this solution we
compute the exact time-dependent exchange correlation potential v_xc which
enters the Kohn-Sham equation of time-dependent density functional theory. Our
exact result provides a benchmark against which various approximate forms for
v_xc can be compared. Finally v_xc is separated in an adiabatic and a pure
dynamical part and it is shown that, for the particular system studied, the
dynamical part is negligible.


title:
Level spacings at the metal-insulator transition in the Anderson
  Hamiltonians and multifractal random matrix ensembles

abstract:
  We consider orthogonal, unitary, and symplectic ensembles of random matrices
with (1/a)(ln x)^2 potentials, which obey spectral statistics different from
the Wigner-Dyson and are argued to have multifractal eigenstates. If the
coefficient $a$ is small, spectral correlations in the bulk are universally
governed by a translationally invariant, one-parameter generalization of the
sine kernel. We provide analytic expressions for the level spacing distribution
functions of this kernel, which are hybrids of the Wigner-Dyson and Poisson
distributions. By tuning the single parameter, our results can be excellently
fitted to the numerical data for three symmetry classes of the
three-dimensional Anderson Hamiltonians at the metal-insulator transition,
previously measured by several groups using exact diagonalization.


title:
Generalized stacking fault energetics and dislocation properties:
  compact vs. spread unit dislocation structures in TiAl and CuAu

abstract:
  We present a general scheme for analyzing the structure and mobility of
dislocations based on solutions of the Peierls-Nabarro model with a two
component displacement field and restoring forces determined from the ab-initio
generalized stacking fault energetics (ie., the so-called $\gamma$-surface).
The approach is used to investigate dislocations in L1$_{0}$ TiAl and CuAu;
predicted differences in the unit dislocation properties are explicitly related
with features of the $\gamma$-surface geometry. A unified description of
compact, spread and split dislocation cores is provided with an important
characteristic "dissociation path" revealed by this highly tractable scheme.


title: Nonequilibrium Dynamic Phase Diagram for Vortex Lattices

abstract:
  The new dynamic phase diagram for driven vortices with varying lattice
softness we present here indicates that, at high driving currents, at least two
distinct dynamic phases of flux flow appear depending on the vortex-vortex
interaction strength. When the flux lattice is soft, the vortices flow in
independently moving channels with smectic structure. For stiff flux lattices,
adjacent channels become locked together, producing crystalline-like order in a
coupled channel phase. At the crossover lattice softness between these phases,
the system produces a maximum amount of voltage noise. Our results relate
spatial order with transport and are in agreement with experiments.


title:
Finite-temperature hole dynamics in the t-J model: Exact results for
  high dimensions

abstract:
  We discuss the dynamics of a single hole in the t-J model at finite
temperature, in the limit of large spatial dimensions. The problem is shown to
yield a simple and physically transparent solution, that exemplifies the
continuous thermal evolution of the underlying string picture from the T=0
string-pinned limit through to the paramagnetic phase.


title:
Circular-like Maps: Sensitivity to the Initial Conditions,
  Multifractality and Nonextensivity

abstract:
  We generalize herein the usual circular map by considering inflexions of
arbitrary power $z$, and verify that the scaling law which has been recently
proposed [Lyra and Tsallis, Phys.Rev.Lett. 80 (1998) 53] holds for a large
range of $z$. Since, for this family of maps, the Hausdorff dimension $d_f$
equals unity for all $z$ values in contrast with the nonextensivity parameter
$q$ which does depend on $z$, it becomes clear that $d_f$ plays no major role
in the sensitivity to the initial conditions.


title: Electron-fluctuation interaction in a non-Fermi superconductor

abstract:
  We studied the influence of the amplitude fluctuations of a non-Fermi
superconductor on the energy spectrum of the 2D Anderson non-Fermi system. The
classical fluctuations give a temperature dependence in the pseudogap induced
in the fermionic excitations.


title: Ground state energy of the $f=1$ spinor Bose-Einstein condensates

abstract:
  We calculate, in the standard Bogoliubov approximation, the ground state
energy of the spinor BEC with hyperfine spin $f=1$ where the two-body repulsive
hard-core and spin exchange interactions are both included. The coupling
constants characterized these two competing interactions are expressed in terms
of the corresponding s-wave scattering lengths using second-order perturbation
methods. We show that the ultraviolet divergence arising in the ground state
energy corrections can be exactly eliminated.


title: Magnon splitting induced by charge ordering in NaV_2O_5

abstract:
  We consider the effects of charge ordering in NaV_2O_5 (below T_SP) on the
exchange constants and on the magnon dispersion. We show that the
experimentally observed splitting of the magnon branches along the a direction
is induced by charge ordering. We find that one can distinguish between the
proposed 'zig-zag' and 'in-line' patterns of charge ordering. Only the zig-zag
ordering is consistent with the experimental results regarding (i) the unusual
intensity modulation observed in magnetic neutron scattering, (ii) the
reduction in the intra-ladder exchange constant below T_SP, and (iii) the
magnon dispersion along a. We estimate the inter-ladder exchange constant to be
1.01meV=11.7K for T>T_SP.


title: Van der Waals loops and the melting transition in two dimensions

abstract:
  Evidence for the existence of van der Waals loops in pressure p versus volume
v plots has for some time supported the belief that melting in two dimensions
is a first order phase transition. We report rather accurate equilibrium p(v)
curves for systems of hard disks obtained from long Monte Carlo simulations.
These curves, obtained in the constant volume ensemble, using periodic boundary
conditions, exhibit well defined van der Waals loops. We illustrate their
existence for finite systems that are known to undergo a continuous transition
in the thermodynamic limit. To this end, we obtain magnetization m versus
applied field curves from Monte Carlo simulations of the 2D Ising model, in the
constant m ensemble, at the critical point. Whether van der Waals loops for
disk systems behave in the thermodynamic limit as they do for the 2D Ising
model at the critical point cannot be ruled out. Thus, the often made claim
that melting in 2D is a first order phase transition, based on the evidence
that van der Waals loops exist, is not sound.


title:
On the Asymptotics of the Finite-Perimeter Partition Function of
  Two-Dimensional Lattice Vesicles

abstract:
  We derive the dominant asymptotic form and the order of the correction terms
of the finite-perimeter partition function of self-avoiding polygons on the
square lattice, which are weighted according to their area A as q^A, in the
inflated regime, q>1. The approach q->1^+ of the asymptotic form is examined.

categories: cond-mat.stat-mech math-ph math.CO math.MP


title:
Quantum Phase Transitions in Capacitively Coupled Two-Dimensional
  Josephson-Junction Arrays

abstract:
  Quantum-phase transitions in two layers of ultrasmall Josephson junctions,
coupled capacitively with each other, are investigated. As the interlayer
capacitance is increased, the system at zero temperature is found to exhibit an
insulator-to-superconductor transition. It is shown that, unlike
one-dimensional arrays with a similar coupling configuration, the transition
cannot be accounted for exclusively by particle-hole pairs.


title: Improved Peierls Argument for High Dimensional Ising Models

abstract:
  We consider the low temperature expansion for the Ising model on $\Z^d$, $d
\ge 2$, with ferromagnetic nearest neighbor interactions in terms of Peierls
contours. We prove that the expansion converges for all temperatures smaller
than $C d (\log d)^{-1}$, which is the correct order in $d$.


title:
Electronic Raman scattering in Tl2Ba2CuO6+x: symmetry of the order
  parameter, oxygen doping effects, and normal state scattering

abstract:
  Single crystals of the optimally doped, moderately and strongly overdoped
high temperature superconductor Tl2Ba2CuO6+x (Tl-2201) with Tc=80, 56 and 30K,
respectively, have been investigated by polarized Raman scattering. By taking
the peak position of the B_1g component of electronic Raman scattering as
2Delta_0 we found that the reduced gap value (2Delta_0/k_BT_c) strongly
decreases with increasing doping. The behavior of the low frequency scattering
for the B_1g and B_2g scattering components is similar for optimally doped and
overdoped crystals and can be described by a w^3 - and w -law, respectively,
which is consistent with a d-wave symmetry of the order parameter. In contrast
to the optimally doped Tl-2201 in both, moderately and strongly overdoped
Tl-2201, the relative (compared to the B_1g) intensity of the A_1g scattering
component is suppressed. We suggest that the van Hove singularity is
responsible for the observed changes of Raman intensity and reduced gap value
with doping. Electronic Raman scattering in the normal state is discussed in
the context of the scattering from impurities and compared to the existing
infrared data. The scattering rate evaluated from the Raman measurements is
smaller for the overdoped samples, compared to the moderately overdoped
samples.


title: Edge tunneling in fractional quantum Hall regime

abstract:
  We address the issue of an apparent disagreement between theory and
experiment on the I-V characteristic of electron tunneling from a metal to the
edge of a two-dimensional electron gas in the fractional quantum Hall regime. A
part of our result is a theory for the edge of the half-filled Landau level.


title: Unusual Coarsening During Phase Separation in Polymer Systems

abstract:
  We introduce a kinetic model of coarsening of transient polymer networks
during the intermediate stages of phase separation in polymer systems. The
model explicitly takes into account the effect of the connectivity of polymer
chains. We show that during the intermediate stages the size of the droplet
grows according to an unusual coarsening law $R \sim t^{1/(d + 3)}$. When the
network structure breaks up, the coarsening law may cross over to $R \sim \ln
t^{-1}$, until only at very late stages the Lifshitz-Slyozov coarsening law $R
\sim t^{1/3}$ is recovered.


title: Introduction to the Bethe ansatz I

abstract:
  The Bethe ansatz for the one-dimensional s=1/2 Heisenberg ferromagnet is
introduced at an elementary level. The presentation follows Bethe's original
work very closely. A detailed description and a complete classification of all
two-magnon scattering states and two-magnon bound states are given for finite
and infinite chains. The paper is designed as a tutorial for beginning graduate
students. It includes 10 problems for further study.


title: Introduction to the Bethe ansatz II

abstract:
  Building on the fundamentals introduced in part I, we employ the Bethe ansatz
to study some ground-state properties (energy, magnetization, susceptibility)
of the one-dimensional s=1/2 Heisenberg antiferromagnet in zero and nonzero
magnetic field. The 2-spinon triplet and singlet excitations from the
zero-field ground state are discussed in detail, and their energies are
calculated for finite and infinite chains. Procedures for the numerical
calculation of real and complex solutions of the Bethe ansatz equations are
discussed and applied. The paper is designed as a tutorial for beginning
graduate students. It includes 10 problems for further study.


title: A Self-Assembled Microlensing Rotational Probe

abstract:
  A technique to measure microscopic rotational motion is presented.
  When a small fluorescent polystyrene microsphere is attached to a larger
polystyrene microsphere, the larger sphere acts as a lens for the smaller
microsphere and provides an optical signal that is a strong function of the
azimuthal angle. We demonstrate the technique by measuring the rotational
diffusion constant of the microsphere in solutions of varying viscosity and
discuss the feasibility of using this probe to measure rotational motion of
biological systems.


title: Structure of vortex liquid phase in irradiated BSCCO(2212) crystals

abstract:
  The c-axis resistivity in irradiated and in pristine BSCCO(2212) crystals is
measured as a function of the in-plane magnetic field component at fixed
out-of-plane component B_\perp in the vortex liquid phase at T=67 K. From this
data we extract the dependence of the phase difference correlation length
inside layers on B_\perp and estimate the average length of pieces of vortex
lines confined inside columnar defects as a function of the filling factor
f=B_\perp / B_\phi. The maximum length, about 15 interlayer distances, is
reached near f=0.35.


title:
Novel non-equilibrium critical behavior in unidirectionally coupled
  stochastic processes

abstract:
  Phase transitions from an active into an absorbing, inactive state are
generically described by the critical exponents of directed percolation (DP),
with upper critical dimension d_c = 4. In the framework of single-species
reaction-diffusion systems, this universality class is realized by the combined
processes A -> A + A, A + A -> A, and A -> \emptyset. We study a hierarchy of
such DP processes for particle species A, B,..., unidirectionally coupled via
the reactions A -> B, ... (with rates \mu_{AB}, ...). When the DP critical
points at all levels coincide, multicritical behavior emerges, with density
exponents \beta_i which are markedly reduced at each hierarchy level i >= 2.
This scenario can be understood on the basis of the mean-field rate equations,
which yield \beta_i = 1/2^{i-1} at the multicritical point. We then include
fluctuations by using field-theoretic renormalization group techniques in d =
4-\epsilon dimensions. In the active phase, we calculate the fluctuation
correction to the density exponent for the second hierarchy level, \beta_2 =
1/2 - \epsilon/8 + O(\epsilon^2). Monte Carlo simulations are then employed to
determine the values for the new scaling exponents in dimensions d<= 3,
including the critical initial slip exponent. Our theory is connected to
certain classes of growth processes and to certain cellular automata, as well
as to unidirectionally coupled pair annihilation processes. We also discuss
some technical and conceptual problems of the loop expansion and their possible
interpretation.


title:
Mechanism of Formation of Monodispersed Colloids by Aggregation of
  Nanosize Precursors

abstract:
  It has been experimentally established in numerous cases that precipitation
of monodispersed colloids from homogeneous solutions is a complex process.
Specifically, it was found that in many systems nuclei, produced rapidly in a
supersaturated solution, grow to nanosize primary particles (singlets), which
then coagulate to form much larger final colloids in a process dominated by
irreversible capture of these singlets. This paper describes a kinetic model
that explains the formation of dispersions of narrow size distribution in such
systems. Numerical simulations of the kinetic equations, with experimental
model parameter values, are reported. The model was tested for a system
involving formation of uniform spherical gold particles by reduction of auric
chloride in aqueous solutions. The calculated average size, the width of the
particle size distribution, and the time scale of the process, agreed
reasonably well with the experimental values.


title: A Model Study of the Low-Energy Charge Dynamics of NaV_2O_5

abstract:
  An exact-diagonalization technique on small clusters is used to calculate the
dynamical density correlation functions of the dimerized t-J chain and coupled
anisotropic t-J ladders (trellis lattice) at quarter filling, i.e., the systems
regarded as a network of pairs (dimers or rungs) of sites coupled weakly via
the hopping and exchange interactions. We thereby demonstrate that the
intersite Coulomb repulsions between the pairs induce a low-energy collective
mode in the charge excitations of the systems where the internal charge degrees
of freedom of the pairs play an essential role. Implications to the electronic
states of NaV_2O_5, i.e., fluctuations of the valence state of V ions and phase
transition as a charge ordering, are discussed.


title: Excitation Spectrum of the Holstein Model

abstract:
  In this paper the polaron problem for the Holstein model is studied in the
weak coupling limit. We use second order perturbation theory to construct
renormalized electron and phonons. Eigenstates of the Hamiltonian are labelled
and the excitation spectrum is constructed.


title: Spin-glass model with partially annealed asymmetric bonds

abstract:
  We have considered the two-spin interaction spherical spin-glass model with
asymmetric bonds (coupling constants). Besides the usual interactions between
spins and bonds and between the spins and a thermostat with temperature
$T_{\sigma}$ there is also an additional factor: the bonds are not assumed
random {\it a priori} but interact with some other thermostat at the
temperature $T_{J}$. We show that when the bonds are frozen with respect to the
spins a first order phase transition to a spin-glass phase occurs, and the
temperature of this transition tends to zero if $T_J$ is large. Our analytical
results show that a spin-glass phase can exist in mean-field models with
nonrelaxational dynamics.


title:
Hartree-Fock based diagonalization: an efficient method for simulating
  disordered interacting electrons

abstract:
  We present an efficient numerical method for simulating the low-energy
properties of disordered many-particle systems. The method which is based on
the quantum-chemical configuration interaction approach consists in
diagonalizing the Hamiltonian in an energetically truncated basis build of the
low-energy states of the corresponding Hartree-Fock Hamiltonian. As an example
we investigate the quantum Coulomb glass, a model of spinless electrons in a
random potential interacting via long-range Coulomb interaction. We find that
the Coulomb interaction increases the conductance of strongly disordered
systems but reduces the conductance of weakly disordered systems.


title: Energy dependence of mass distributions in fragmentation

abstract:
  We study fragmentation numerically using a simple model in which an object is
taken to be a set of particles that interact pairwisely via a Lennard-Jones
potential while the effect of the fragmentation-induced forces is represented
by some initial velocities assigned to the particles. The motion of the
particles, which is given by Newton's laws, is followed by molecular dynamics
calculations. As time evolves, the particles form clusters which are identified
as fragments. The steady-state cumulative distribution of the fragment masses
is studied and found to have an effective power-law region. The power-law
exponent increases with the energy given to the particles by the
fragmentation-induced forces. This result is confirmed by experiments.


title:
Distribution of complex eigenvalues for symplectic ensembles of
  non-Hermitian matrices

abstract:
  Symplectic ensemble of disordered non-Hermitian Hamiltonians is studied.
Starting from a model with an imaginary magnetic field, we derive a proper
supermatrix $\sigma $-model. The zero-dimensional version of this model
corresponds to a symplectic ensemble of weakly non-Hermitian matrices. We
derive analytically an explicit expression for the density of complex
eigenvalues. This function proves to differ qualitatively from those known for
the unitary and orthogonal ensembles. In contrast to these cases, a {\it
depletion} of eigenvalues near the real axis occurs. The result about the
depletion is in agreement with a previous numerical study performed for QCD
models.


title: Surface spin-flop and discommensuration transitions in antiferromagnets

abstract:
  Phase diagrams as a function of anisotropy $D$ and magnetic field $H$ are
obtained for discommensurations and surface states for an antiferromagnet in
which $H$ is parallel to the easy axis, by modeling it using the ground states
of a one-dimensional chain of classical XY spins. A surface spin-flop phase
exists for all $D$, but the interval in $H$ over which it is stable becomes
extremely small as $D$ goes to zero. First-order transitions, separating
different surface states and ending in critical points, exist inside the
surface spin-flop region. They accumulate at a field $H'$ (depending on $D$)
significantly less than the value $H_{SF}$ for a bulk spin-flop transition. For
$H' < H < H_{SF}$ there is no surface spin-flop phase in the strict sense;
instead, the surface restructures by, in effect, producing a discommensuration
infinitely far away in the bulk. The results are used to explain in detail the
phase transitions occurring in systems consisting of a finite, even number of
layers.


title: Self-Diffusion in Simple Models: Systems with Long-Range Jumps

abstract:
  We review some exact results for the motion of a tagged particle in simple
models. Then, we study the density dependence of the self diffusion
coefficient, $D_N(\rho)$, in lattice systems with simple symmetric exclusion in
which the particles can jump, with equal rates, to a set of $N$ neighboring
sites. We obtain positive upper and lower bounds on
$F_N(\rho)=N((1-\r)-[D_N(\rho)/D_N(0)])/(\rho(1-\rho))$ for $\rho\in [0,1]$.
Computer simulations for the square, triangular and one dimensional lattice
suggest that $F_N$ becomes effectively independent of $N$ for $N\ge 20$.


title: Calculation of the Band Gap Energy of Ionic Crystals

abstract:
  The band gap of alkali halides, alkaline-earth oxides, Al2O3 and SiO2
crystals has been calculated using the Perturbed-Ion model supplemented with
some assumptions for the treatment of excited states. The gap is calculated in
several ways: as a difference between one-electron energy eigenvalues and as a
difference between the total energies of appropriate electronic states of the
crystal, both at the HF level and with inclusion of Coulomb correlation
effects. The results compare well with experimental band gap energies and with
other theoretical calculations, suggesting that the picture of bonding and
excitation given by the model can be useful in ionic materials.


title:
N-tree approximation for the largest Lyapunov exponent of a coupled-map
  lattice

abstract:
  The N-tree approximation scheme, introduced in the context of random directed
polymers, is here applied to the computation of the maximum Lyapunov exponent
in a coupled map lattice. We discuss both an exact implementation for small
tree-depth $n$ and a numerical implementation for larger $n$s. We find that the
phase-transition predicted by the mean field approach shifts towards larger
values of the coupling parameter when the depth $n$ is increased. We conjecture
that the transition eventually disappears.


title:
Spin correlations in an isotropic spin-5/2 two-dimensional
  antiferromagnet

abstract:
  We report a neutron scattering study of the spin correlations for the spin
5/2, two-dimensional antiferromagnet Rb_2MnF_4 in an external magnetic field.
Choosing fields near the system's bicritical point, we tune the effective
anisotropy in the spin interaction to zero, constructing an ideal S=5/2
Heisenberg system. The correlation length and structure factor amplitude are
closely described by the semiclassical theory of Cuccoli et al. over a broad
temperature range but show no indication of approaching the low-temperature
renormalized classical regime of the quantum non-linear sigma model.


title: Defect-unbinding transitions and inherent structures in two dimensions

abstract:
  We present a large-scale (36000-particle) computational study of the
"inherent structures" (IS) associated with equilibrium, two-dimensional,
one-component Lennard-Jones systems. Our results provide strong support both
for the inherent-structures theory of classical fluids, and for the KTHNY
theory of two-stage melting in two dimensions. This support comes from the
observation of three qualitatively distinct "phases" of inherent structures: a
crystal, a "hexatic glass", and a "liquid glass". We also directly observe, in
the IS, analogs of the two defect-unbinding transitions (respectively, of
dislocations, and disclinations) believed to mediate the two equilibrium phase
transitions. Each transition shows up in the inherent structures---although the
free disclinations in the "liquid glass" are embedded in a percolating network
of grain boundaries. The bond-orientational correlation functions of the
inherent structures show the same progressive loss of order as do the three
equilibrium phases: long-range to quasi-long-range to short-range.


title:
The breakdown of adiabatic polaron theory in one, two, and three
  dimensions and the reformation of the large polaron concept

abstract:
  We prove variationally that at weak coupling in one, two, and three
dimensions there exist correlated electron-phonon states below the approximate
ground states characteristically found by adiabatic polaron theory. Besides
differing non-trivially in quantitative aspects such as the value of the ground
state energy, these improved ground states are found to differ significantly in
qualitative aspects such as correlation structure and scaling behavior. These
differences are sufficiently severe as to require a reevaluation of the
physical meaning attached to such widely used terms as "large polaron" and
"self-trapping transition".


title: Transition Matrix Monte Carlo Reweighting and Dynamics

abstract:
  We study an induced dynamics in the space of energy of single-spin-flip Monte
Carlo algorithm. The method gives an efficient reweighting technique. This
dynamics is shown to have relaxation times proportional to the specific heat.
Thus, it is plausible for a logarithmic factor in the correlation time of the
standard 2D Ising local dynamics.


title: The Gradient Expansion for the Free-Energy of a Clean Superconductor

abstract:
  We describe a novel method for obtaining the gradient expansion for the free
energy of a clean BCS superconductor. We present explicit results up to fourth
order in the gradients of the order parameter.


title: Self-Similar Exponential Approximants

abstract:
  An approach is suggested defining effective sums of divergent series in the
form of self-similar exponential approximants. The procedure of constructing
these approximants from divergent series with arbitrary noninteger powers is
developed. The basis of this construction is the self-similar approximation
theory. Control functions governing the convergence of exponentially
renormalized series are defined from stability and fixed-point conditions and
from additional asymptotic conditions when the latter are available. The
stability of the calculational procedure is checked by analyzing cascade
multipliers. A number of physical examples for different statistical systems
illustrate the generality and high accuracy of the approach.


title: Comment on ``Nonuniversal Exponents in Interface Growth''

abstract:
  Recently, Newman and Swift[T. J. Newman and M. R. Swift, Phys. Rev. Lett.
{\bf 79}, 2261 (1997)] made an interesting suggestion that the strong-coupling
exponents of the Kardar-Parisi-Zhang (KPZ) equation may not be universal, but
rather depend on the precise form of the noise distribution. We show here that
the decrease of surface roughness exponents they observed can be attributed to
a percolative effect.


title: Lubricating Bacteria Model for Branching growth of Bacterial Colonies

abstract:
  Various bacterial strains (e.g. strains belonging to the genera Bacillus,
Paenibacillus, Serratia and Salmonella) exhibit colonial branching patterns
during growth on poor semi-solid substrates. These patterns reflect the
bacterial cooperative self-organization. Central part of the cooperation is the
collective formation of lubricant on top of the agar which enables the bacteria
to swim. Hence it provides the colony means to advance towards the food. One
method of modeling the colonial development is via coupled reaction-diffusion
equations which describe the time evolution of the bacterial density and the
concentrations of the relevant chemical fields. This idea has been pursued by a
number of groups. Here we present an additional model which specifically
includes an evolution equation for the lubricant excreted by the bacteria. We
show that when the diffusion of the fluid is governed by nonlinear diffusion
coefficient branching patterns evolves. We study the effect of the rates of
emission and decomposition of the lubricant fluid on the observed patterns. The
results are compared with experimental observations. We also include fields of
chemotactic agents and food chemotaxis and conclude that these features are
needed in order to explain the observations.

categories: cond-mat nlin.PS patt-sol physics.bio-ph q-bio


title: Fluctuations of entropy production in driven glasses

abstract:
  We study by Montecarlo simulation the non-equilibrium stationary behavior of
a three-dimensional stochastic lattice gas with reversible kinetic constraints
and in diffusive contact with two particle reservoirs at different chemical
potential. When one of the boundaries is placed in the glassy phase, the
statistics of entropy production fluctuations driven by the chemical potential
gradient satisfy the Gallavotti-Cohen fluctuation theorem provided that an
effective entropy production is introduced.


title: d-wave superconductivity in a generalized Hubbard model

abstract:
  We consider an extended Hubbard model with nearest-neighbor correlated
hopping and next nearest-neighbor hopping t' obtained as an effective model for
cuprate superconductors. Using a generalized Hartree-Fock BCS approximation, we
find that for high enough t' and doping, antiferromagnetism is destroyed and
the system exhibits d-wave superconductivity. Near optimal doping we consider
the effect of antiferromagnetic spin fluctuations on the normal self-energy
using a phenomenological susceptibility. The resulting superconducting critical
temperature as a function of doping is in good agreement with experiment.


title:
Interaction induced delocalization of two particles: large system size
  calculations and dependence on interaction strength

abstract:
  The localization length $L_2$ of two interacting particles in a
one-dimensional disordered system is studied for very large system sizes by two
efficient and accurate variants of the Green function method. The numerical
results (at the band center) can be well described by the functional form
$L_2=L_1[0.5+c(U) L_1]$ where $L_1$ is the one-particle localization length and
the coefficient $c(U)\approx 0.074 |U|/(1+|U|)$ depends on the strength $U$ of
the on-site Hubbard interaction. The Breit-Wigner width or equivalently the
(inverse) life time of non-interacting pair states is analytically calculated
for small disorder and taking into account the energy dependence of the
one-particle localization length. This provides a consistent theoretical
explanation of the numerically found $U$-dependence of $c(U)$.


title: Mesoscopic mechanism of adiabatic charge transport

abstract:
  We consider adiabatic charge transport through mesoscopic metallic samples
caused by a periodically changing external potential. We find that both the
amplitude and the sign of the charge transferred through a sample per period
are random sample specific quantities. The characteristic magnitude of the
charge is determined by the quantum interference.


title: The Nishimori line and Bayesian Statistics

abstract:
  ``Nishimori line'' is a line or hypersurface in the parameter space of
systems with quenched disorder, where simple expressions of the averages of
physical quantities over the quenched random variables are obtained. It has
been playing an important role in the theoretical studies of the random
frustrated systems since its discovery around 1980. In this paper, a novel
interpretation of the Nishimori line from the viewpoint of statistical
information processing is presented. Our main aim is the reconstruction of the
whole theory of the Nishimori line from the viewpoint of Bayesian statistics,
or, almost equivalently, from the viewpoint of the theory of error-correcting
codes. As a byproduct of our interpretation, counterparts of the Nishimori line
in models without gauge invariance are given. We also discussed the issues on
the ``finite temperature decoding'' of error-correcting codes in connection
with our theme and clarify the role of gauge invariance in this topic.


title:
Attraction between like-charged colloidal particles induced by a surface
  a density - functional analysis

abstract:
  We show that the first non-linear correction to the linearised
Poisson-Boltzman n (or DLVO) theory of effective pair interactions between
charge-stabilised, co lloidal particles near a charged wall leads to an
attractive component of entro pic origin. The position and depth of the
potential compare favourably with rec ent experimental measurements


title: Interacting electrons in a 2D quantum dot

abstract:
  The exact numerical diagonalization of the Hamiltonian of a 2D circular
quantum dot is performed for 2, 3, and 4 electrons.The results are compared
with those of the perturbation theory.Our numerical results agree reasonably
well for small values of the dimensionles coupling constant \lambda=a\over a_B
where a is the dot radius and a_B is the effective Bohr radius.Exact
diagonalization results are compared with the classical predictions, and they
are found to be almost coincident for large \lambda values. PACS Numbers:
73.20.Dx, 73.61.-r


title:
A multi-species asymmetric exclusion process;steady state and
  correlation functions on a periodic lattice

abstract:
  By generalizing the algebra of operators of the Asymmetric Simple Exclusion
Process (ASEP), a multi-species ASEP in which particles can overtake each
other,is defined on both open and closed one dimensional chains. On the ring
the steady state and the correlation functions are obtained exactly. The
relation to particle hopping models of traffic and the possibility of shock
waves in open systems is discussed. The effect of the boundary condition on the
steady state properties of the bulk is studied.


title: Granular cooling of hard needles

abstract:
  We have developed a kinetic theory of hard needles undergoing binary
collisions with loss of energy due to normal and tangential restitution. In
addition, we have simulated many particle systems of granular hard needles. The
theory, based on the assumption of a homogeneous cooling state, predicts that
granular cooling of the needles proceeds in two stages: An exponential decay of
the initial configuration to a state where translational and rotational
energies take on a time independent ratio (not necessarily unity), followed by
an algebraic decay of the total kinetic energy $\sim t^{-2}$. The simulations
support the theory very well for low and moderate densities. For higher
densities, we have observed the onset of the formation of clusters and shear
bands.


title:
Binary Bose-Einstein Condensate Mixtures in Weakly and Strongly
  Segregated Phases

abstract:
  We perform a mean-field study of the binary Bose-Einstein condensate mixtures
as a function of the mutual repulsive interaction strength. In the phase
segregated regime, we find that there are two distinct phases: the weakly
segregated phase characterized by a `penetration depth' and the strongly
segregated phase characterized by a healing length. In the weakly segregated
phase the symmetry of the shape of each condensate will not take that of the
trap because of the finite surface tension, but its total density profile still
does. In the strongly segregated phase even the total density profile takes a
different symmetry from that of the trap because of the mutual exclusion of the
condensates. The lower critical condensate-atom number to observe the complete
phase segregation is discussed. A comparison to recent experimental data
suggests that the weakly segregated phase has been observed.


title: The kinesin walk: a dynamic model with elastically coupled heads

abstract:
  Recently individual two-headed kinesin molecules have been studied in in
vitro motility assays revealing a number of their peculiar transport
properties. In this paper we propose a simple and robust model for the kinesin
stepping process with elastically coupled Brownian heads showing all of these
properties. The analytic and numerical treatment of our model results in a very
good fit to the experimental data and practically has no free parameters.
Changing the values of the parameters in the restricted range allowed by the
related experimental estimates has almost no effect on the shape of the curves
and results mainly in a variation of the zero load velocity which can be
directly fitted to the measured data. In addition, the model is consistent with
the measured pathway of the kinesin ATPase.


title: High dimensional behavior of the Kardar-Parisi-Zhang growth dynamics

abstract:
  We investigate analytically the large dimensional behavior of the
Kardar-Parisi-Zhang (KPZ) dynamics of surface growth using a recently proposed
non-perturbative renormalization for self-affine surface dynamics. Within this
framework, we show that the roughness exponent $\alpha$ decays not faster than
$\alpha\sim 1/d$ for large $d$. This implies the absence of a finite upper
critical dimension.


title: Chiral patterns arising from electrostatic growth models

abstract:
  Recently, unusual and strikingly beautiful seahorse-like growth patterns have
been observed under conditions of quasi-two-dimensional growth. These
`S'-shaped patterns strongly break two-dimensional inversion symmetry; however
such broken symmetry occurs only at the level of overall morphology, as the
clusters are formed from achiral molecules with an achiral unit cell. Here we
describe a mechanism which gives rise to chiral growth morphologies without
invoking microscopic chirality. This mechanism involves trapped electrostatic
charge on the growing cluster, and the enhancement of growth in regions of
large electric field. We illustrate the mechanism with a tree growth model,
with a continuum model for the motion of the one-dimensional boundary, and with
microscopic Monte Carlo simulations. Our most dramatic results are found using
the continuum model, which strongly exhibits spontaneous chiral symmetry
breaking, and in particular finned `S' shapes like those seen in the
experiments.


title: Quantum Field Theory of Treasury Bonds

abstract:
  The Heath-Jarrow-Morton (HJM) formulation of treasury bonds in terms of
forward rates is recast as a problem in path integration. The HJM-model is
generalized to the case where all the forward rates are allowed to fluctuate
independently. The resulting theory is shown to be a two-dimensional Gaussian
quantum field theory. The no arbitrage condition is obtained and a functional
integral derivation is given for the price of a futures and an options
contract.


title: Phase Transitions in Condensed Matter and Relativistic QFT

abstract:
  Kibble and Zurek have provided a unifying picture for the onset of phase
transitions in relativistic QFT and condensed matter systems respectively,
strongly supported by agreement with condensed matter experiments in He3. The
failure of a recent experiment on He4 to agree with the predictions has
prompted a reappraisal of this picture. We provide an alternative picture that
explains the experimental evidence.


title: A Thermometer for the 2D Electron Gas using 1D Thermopower

abstract:
  We measure the temperature of a 2D electron gas in GaAs from the thermopower
of a one-dimensional ballistic constriction, using the Mott relation to confirm
the calibration from the electrical conductance. Under hot electron conditions,
this technique shows that the power loss by the electrons follows a $T^5$
dependence in the Gruneisen-Bloch regime, as predicted for acoustic phonon
emission with a screened piezoelectric interaction. An independent measurement
using conventional thermometry based on Shubnikov-de Haas oscillations gives a
$T^3$ loss rate; we discuss reasons for this discrepancy.


title:
Solving the Poisson-Boltzmann Equation to Obtain Interaction Energies
  Between Confined, Like-charged Cylinders

abstract:
  We numerically solve the non-linear Poisson-Boltzmann equation for two
cylinders confined by two parallel charged plates. The repulsive electrical
double layer component of the cylinder pair potential is substantially reduced
by confinement between like-charged plates. While the effective cylinder
surface charge is increased by the confinement, the effective interaction
screening length is reduced, this effect being dominant so that the repulsive
confined cylinder-cylinder interaction potential is reduced.


title:
Stationary states of the Gross-Pitaevskii equation with linear
  counterpart

abstract:
  We study the stationary solutions of the Gross-Pitaevskii equation that
reduce, in the limit of vanishing non-linearity, to the eigenfunctions of the
associated Schr\"odinger equation. By providing analytical and numerical
support, we conjecture an existence condition for these solutions in terms of
the ratio between their proper frequency (chemical potential) and the
corresponding linear eigenvalue. We also give approximate expressions for the
stationary solutions which become exact in the opposite limit of strong
non-linearity. For one-dimensional systems these solutions have the form of a
chain of dark or bright solitons depending on the sign of the non-linearity. We
demonstrate that in the case of negative non-linearity (attractive interaction)
the norm of the solutions is always bounded for dimensions greater than one.

categories: cond-mat math-ph math.MP nlin.PS patt-sol quant-ph


title:
Duality in two capacitively coupled layered arrays of ultrasmall
  Josephson junctions

abstract:
  We consider the problem of two capacitively coupled Josephson junction arrays
made of ultrasmall junctions. Each one of the arrays can be in the
semiclassical or quantum regimes, depending on their physical parameter values.
The former case is dominated by a Cooper-pair superfluid while the quantum one
is dominated by dynamic vortices leading to an insulating behavior. We first
consider the limit when both arrays are in the semiclassical limit, and next
the case when one array is quantum and the other semiclassical. We present WKB
and Mean Field theory results for the critical temperature of each array when
both are in the semiclassical limit. When one array is in the semiclassical
regime and the other one in the quantum fluctuations dominated regimes, we
derive a duality transformation between the charged and vortex dominated arrays
that involve a gauge vector field, which is proportional to the site coupling
capacitance between the arrays. The system considered here has been fabricated
and we make some predictions as to possible experimentally measurable
quantities that could be compared with theory.


title:
Stability of Texture and Shape of Circular Domains of Langmuir
  Monolayers

abstract:
  Finite domains of a Langmuir monolayer in a phase with tilted molecules can
be modeled by a simple elastic free energy of an XY order parameter with
isotropic and anisotropic line tension terms. The domains can and often do
contain nontrivial textures, which in turn influence the shape of the domains.
Herein we investigate the properties of a simplified isotropic model with a
single elastic constant. For circular domains a first-order phase transition is
found between two distinct textures: an exterior defect (or ``virtual boojum'')
texture, and an interior defect texture. Starting with a circular domain and
either of these two textures as a ground state, we find shape instabilities
develop that depend on the elastic constants and line tensions in the
simplified model. In both cases a necessary but not sufficient condition for
the onset of shape instabilities is the possibility for a local negative
effective line tension to develop from the anisotropic line tension term.


title:
Photoelectron Escape Depth and Inelastic Secondaries in High Temperature
  Superconductors

abstract:
  We calculate the photoelectron escape depth in the high temperature
superconductor Bi2212 by use of electron energy-loss spectroscopy data. We find
that the escape depth is only 3 Ang. for photon energies typically used in
angle resolved photoemission measurements. We then use this to estimate the
number of inelastic secondaries, and find this to be quite small near the Fermi
energy. This implies that the large background seen near the Fermi energy in
photoemission measurements is of some other origin.


title:
Small-$q$ Anomaly in the Dielectric Function and High-Temperature
  Oscillations of Screening Potential in 2D Electron Gas with Spin-Orbit
  Coupling

abstract:
  We study the static dielectric function $\epsilon(q)$ of 2D electron system
with spin-orbit coupling in the frame of the random phase approximation. We
demonstrate that, in addition to the well-known $2k_F$-Kohn anomaly, spin-orbit
coupling gives rise to the novel anomaly in the dielectric function at small
$q=q_0\ll k_F$, where $q_0$ is the distance between two Fermi surfaces. As a
result of this anomaly a large-distance behavior of the potential from a point
charge exhibits (in addition to the conventional Friedel oscillations) novel
oscillations with a period $2\pi/q_0$. The remarkable feature of these
oscillations is that they are not smeared out by the temperature. We show that
the small-$q$ anomaly also modifies the indirect exchange interaction of
localized magnetic moments (RKKY interaction). In the presense of spin-orbit
coupling this interaction acquires a high-temperature component.


title: Irreversibility from a Reversible Equation

abstract:
  After a discussion on the state of local equilibrium with temperature
inhomogeneity, comparing mixture state reprsentation in statistical mechanics
and pure state representation in thermo field dynamics, a simple model is
solved to show that a reversible equation of motion with the initial condition
having inhomogeneous temperature can lead to irreversible, viz. diffusive,
behaviour. Yet, the solution is time symmetric exhibiting diffusion both
towards future and past.


title:
Finite-size scaling of the error threshold transition in finite
  population

abstract:
  The error threshold transition in a stochastic (i.e. finite population)
version of the quasispecies model of molecular evolution is studied using
finite-size scaling. For the single-sharp-peak replication landscape, the
deterministic model exhibits a first-order transition at $Q=Q_c=1/a$, where $%
Q$ is the probability of exact replication of a molecule of length $L \to
\infty$, and $a$ is the selective advantage of the master string. For
sufficiently large population size, $N$, we show that in the critical region
the characteristic time for the vanishing of the master strings from the
population is described very well by the scaling assumption $\tau = N^{1/2} f_a
\left [ \left (Q - Q_c) N^{1/2} \right ] $, where $f_a$ is an $a$-dependent
scaling function.


title: Bethe Approximation for a Semi-flexible Polymer Chain

abstract:
  We present a Bethe approximation to study lattice models of linear polymers.
The approach is variational in nature and based on the cluster variation method
(CVM). We focus on a model with $(i)$ a nearest neighbor attractive energy
$\epsilon_v$ between pair of non--bonded monomers, $(ii)$ a bending energy
$\epsilon_h$ for each pair of successive chain segments which are not
collinear. We determine the phase diagram of the system as a function of the
reduced temperature $t=\frac{T}{\epsilon_v}$ and of the parameter
$x=\frac{\epsilon_h}{\epsilon_v}$. We find two different qualitative behaviors,
on varying $t$. For small values of $x$ the system undergoes a $\theta$
collapse from an extended coil to a compact globule; subsequently, on
decreasing further $t$, there is a first order transition to an anisotropic
phase, characterized by global orientational order. For sufficiently large
values of $x$, instead, there is directly a first order transition from the
coil to the orientational ordered phase. Our results are in good agreement with
previous Monte Carlo simulations and contradict in some aspects mean--field
theory. In the limit of Hamiltonian walks, our approximation recovers results
of the Flory-Huggins theory for polymer melting.


title: A map from 1d Quantum Field Theory to Quantum Chaos on a 2d Torus

abstract:
  Dynamics of a class of quantum field models on 1d lattice in Heisenberg
picture is mapped into a class of `quantum chaotic' one-body systems on
configurational 2d torus (or 2d lattice) in Schr\" odinger picture. Continuum
field limit of the former corresponds to quasi-classical limit of the latter.


title:
The moment sum rule and its consequences for ferromagnetism in the
  Hubbard model

abstract:
  The sum rule for the moments of the spectral density is discussed for the
single-band Hubbard model. It is shown that respecting the sum rule up to the
order m=3 is conceptually important for a qualitatively correct description of
the quasi-particle band structure in the strong-correlation regime. Different
analytical approximations for the self-energy are analyzed with respect to
their compatibility with the moment sum rule. To estimate the practical
usefulness of the sum rule, correlation functions and dynamical quantities are
determined. The results obtained within the various approximation schemes of
different complexity are compared with each other and also with essentially
exact results available for infinite-dimensional lattices. It turns out that
the m=3 moment is rather unimportant for the paramagnetic phase on the
hyper-cubic lattice. Contrary, it decisively influences the magnetic phase
boundary as well as the critical temperature for the ferromagnetic phase on an
fcc-type lattice.


title:
Solid friction at high sliding velocities: an explicit 3D dynamical SPH
  approach

abstract:
  We present realistic 3D numerical simulations of elastic bodies sliding on
top of each other in a regime of velocities ranging from meters to tens of
meters per second using the so-called Smoothed Particle Hydrodynamics (SPH)
method. Our investigations are restricted to regimes of pressure and roughness
where only elastic deformations occur between asperities at the contact surface
between the slider block and the substrate. In this regime, solid friction is
due to the generation of vibrational radiations which are subsequently damped
out. We study periodic commensurate and incommensurate asperities and various
types of disordered surfaces. We report the evidence of a transition from zero
(or non-measurable $\mu < 0.001$) friction to a finite friction as the normal
pressure increases above about $10^6~Pa$. For larger normal pressures (up to
$10^9~Pa$), we find a remarkably universal value for the friction coefficient
$\mu \approx 0.06$, which is independent of the internal dissipation strength
over three order of magnitudes, and independent of the detailled nature of the
slider block-substrate interactions. We find that disorder may either decrease
or increase $\mu$ due to the competition between two effects: disorder detunes
the coherent vibrations of the asperties that occur in the periodic case,
leading to weaker acoustic radiation and thus weaker damping. On the other
hand, large disorder leads to stronger vibration amplitudes at local asperities
and thus stronger damping. Our simulations have confirmed the existence of
jumps over steps or asperities of the slider blocks occurring at the largest
velocities studied ($10~m/s$). These jumps lead to chaotic motions similar to
the bouncing-ball problem. We find a velocity strengthening with a doubling of
the friction coefficient as the velocity increases from $1~m/s$ to $10~m/s$.


title: Inertial Effects on Fluid Flow through Disordered Porous Media

abstract:
  We study the fluid flow through disordered porous media by numerically
solving the complete set of the Navier-Stokes equations in a two dimensional
lattice with a spatially random distribution of solid obstacles (plaquettes).
We simulate viscous and non-viscous flow through these idealized pore spaces to
determine the origin of the deviations from the classical Darcy's law behavior.
Due to the non-linear contribution of inertia to the transport of momentum at
the pore scale, we observe a typical departure from Darcy's law at sufficiently
high Reynolds numbers. Moreover, we show that the classical Forchheimer
equation provides a valid phenomenological model to correlate the variations of
the friction factor of the porous media over a wide range of Reynolds
conditions.


title:
Charge glass in two-dimensional arrays of capacitively coupled grains
  with random offset charges

abstract:
  We study the effect of random offset charges in the insulator to conductor
transition in systems of capacitively coupled grains, as realized in
two-dimensional arrays of ultrasmall Josephson junctions. In presence of
disorder, the conductive transition and charge ordering at nonzero gate
voltages are both destroyed for any degree of disorder $\sigma$ at finite
temperatures $T$, in the thermodynamic limit, but crossover effects will
dominate at length scales smaller than $\xi_{\sigma, T}=T^{-\nu} f(\sigma
T^{-\nu})$, where $\nu $ is the thermal critical exponent of the
zero-temperature charge glass transition. The conductance is linear and
thermally activated but nonlinear behavior sets in at a crossover voltage which
decreases as temperature decreases. For large disorder, the results are
supported by Monte Carlo dynamics simulations of a Coulomb gas with offset
charges and are consistent with the thermally activated behavior found in
recent experiments.


title:
Frequency-dependent magnetotransport and particle dynamics in magnetic
  modulation systems

abstract:
  We analyze the dynamics of a charged particle moving in the presence of
spatially-modulated magnetic fields. From Poincare surfaces of section and
Liapunov exponents for characteristic trajectories we find that the fraction of
pinned and runaway quasiperiodic orbits {\em vs}. chaotic orbits depends
strongly on the ratio of cyclotron radius to the structure parameters, as well
as on the amplitude of the modulated field. We present a complete
characterization of the dynamical behavior of such structures, and investigate
the contribution to the magnetoconductivity from all different orbits using a
classical Kubo formula. Although the DC conductivity of the system depends
strongly on the pinned and runaway trajectories, the frequency response
reflects the topology of all different orbits, and even their unusual temporal
behavior.


title: Acceptor binding energies in GaN and AlN

abstract:
  We employ effective mass theory for degenerate hole-bands to calculate the
acceptor binding energies for Be, Mg, Zn, Ca, C and Si substitutional acceptors
in GaN and AlN. The calculations are performed through the 6$\times $6
Rashba-Sheka-Pikus and the Luttinger-Kohn matrix Hamiltonians for wurtzite (WZ)
and zincblende (ZB) crystal phases, respectively. An analytic representation
for the acceptor pseudopotential is used to introduce the specific nature of
the impurity atoms. The energy shift due to polaron effects is also considered
in this approach. The ionization energy estimates are in very good agreement
with those reported experimentally in WZ-GaN. The binding energies for ZB-GaN
acceptors are all predicted to be shallower than the corresponding impurities
in the WZ phase. The binding energy dependence upon the crystal field splitting
in WZ-GaN is analyzed. Ionization levels in AlN are found to have similar
`shallow' values to those in GaN, but with some important differences, which
depend on the band structure parameterizations, especially the value of crystal
field splitting used.


title:
Correlation and symmetry effects in transport through an artificial
  molecule

abstract:
  Spectral weights and current-voltage characteristics of an artificial
diatomic molecule are calculated, considering cases where the dots connected in
series are in general different. The spectral weights allow us to understand
the effects of correlations, their connection with selection rules for
transport, and the role of excited states in the experimental conductance
spectra of these coupled double dot systems (DDS). An extended Hubbard
Hamiltonian with varying interdot tunneling strength is used as a model,
incorporating quantum confinement in the DDS, interdot tunneling as well as
intra- and interdot Coulomb interactions. We find that interdot tunneling
values determine to a great extent the resulting eigenstates and corresponding
spectral weights. Details of the state correlations strongly suppress most of
the possible conduction channels, giving rise to effective selection rules for
conductance through the molecule. Most states are found to make insignificant
contributions to the total current for finite biases. We find also that the
symmetry of the structure is reflected in the I-V characteristics, and is in
qualitative agreement with experiment.


title: Zeeman splitting of shallow donors in GaN

abstract:
  The Zeeman splitting of the donor spectra in cubic- and hexagonal-GaN are
studied using an effective mass theory approach. Soft-core pseudopotentials
were used to describe the chemical shift of the different substitutional
dopants. The donor ground states calculated range from 29.5 to 33.7 meV, with
typically 1 meV higher binding in the hexagonal phase. Carbon is found to
produce the largest donor binding energy. The ionization levels and excited
states are in excellent agreement with Hall and optical measurements, and
suggest the presence of residual C in recent experiments.


title: Anderson transition of three dimensional phonon modes

abstract:
  Anderson transition of the phonon modes is studied numerically. The critical
exponent for the divergence of the localization length is estimated using the
transfer matrix method, and the statistics of the modes is analyzed. The latter
is shown to be in excellent agreement with the energy level statistics of the
disrodered electron system belonging to the orthogonal universality class.


title:
Universal conductance distribution in three dimensional systems in high
  magnetic fields

abstract:
  The nature of the critical point of the Anderson transition in high magnetic
fields is discussed with an emphasis on scale invariance and universality of
the critical exponent. Special attention is paid to the distribution function
of the conductance which becomes size and model independent at the critical
point. The fractal properties of the wave function which are related to scale
invariance are also discussed.


title: Interface modes of two-dimensional composite structures

abstract:
  The surface modes of a composite consisting of aligned metallic wires with
square cross sections are investigated, on the basis of photonic band structure
calculations. The effective long-wavelength dielectric response function is
computed, as a function of the filling fraction. The dependence of the optical
absorption on the shape of the wires and the polarization of light is
discussed, and the effect of sharp corners analyzed. The effect of the
interaction between the wires on the localization of surface plasmons is also
addressed.


title: Nucleation model for the description of glass formation

abstract:
  We present in this letter a model of glass formation using energy barriers
and a crystal nucleation process. We then analyze the corresponding dynamical
equation in the vicinity of the stationary solutions. The occurence of a pure
amorphous solution (i.e. glass) is due to the contribution of a ratio $\Lambda$
depending on the cooling rate and the crystal nucleation frequency. We also
construct time-temperature-transformation (TTT) curves in order to relate the
model with the kinetic treatment of glass formation.


title:
Finite Temperature DMRG Investigation of the Spin-Peierls Transition in
  CuGeO$_3$

abstract:
  We present a numerical study of thermodynamical properties of dimerized
frustrated Heisenberg chains down to extremely low temperatures with
applications to CuGeO$_3$. A variant of the finite temperature density matrix
renormalization group (DMRG) allows the study of the dimerized phase previously
unaccessible to ab initio calculations. We investigate static dimerized systems
as well as the instability of the quantum chain towards lattice dimerization.
The crossover from a quadratic response in the free energy to the distortion
field at finite temperature to nonanalytic behavior at zero temperature is
studied quantitatively. Various physical quantities are derived and compared
with experimental data for CuGeO$_3$ such as magnetic dimerization, critical
temperature, susceptibility and entropy.


title:
The effect of phase fluctuations on the single-particle properties of
  the underdoped cuprates

abstract:
  We study the effect of order parameter phase fluctuations on the
single-particle properties of fermions in the underdoped cuprate
superconductors using a phenomenological low-energy theory. We identify the
fermion-phase field coupling as the Doppler-shift of the quasiparticle spectrum
induced by the fluctuating superfluid velocity and we calculate the effect of
these fluctuations on the fermion self-energy. We show that the vortex pair
unbinding near the superconducting transition causes a significant broadening
in the fermion spectral function, producing a pseudogap-like feature. We also
discuss the specific heat and show that the phase fluctuation effect is visible
due to the short coherence length.


title: Transport in two dimensional periodic magnetic fields

abstract:
  Ballistic transport properties in a two dimensional electron gas are studied
numerically, where magnetic fields are perpendicular to the plane of two
dimensional electron systemsand periodically modulated both in $x$ and $y$
directions. We show that there are three types of trajectories of classical
electron motions in this system; chaotic, pinned and runaway trajectories. It
is found that the runaway trajectories can explain the peaks of
magnetoresistance as a function of external magnetic fields, which is believed
to be related to the commensurability effect between the classical cyclotron
diameter and the period of magnetic modulation. The similarity with and
difference from the results in the antidot lattice are discussed.


title:
Electron interactions, classical integrability, and level statistics in
  quantum dots

abstract:
  The role of electronic interactions in the level structure of semiconductor
quantum dots is analyzed in terms of the correspondence to the integrability of
a classical system that models these structures. We find that an otherwise
simple system is made strongly non-integrable in the classical regime by the
introduction of particle interactions. In particular we present a two-particle
classical system contained in a $d$-dimensional billiard with hard walls.
Similarly, a corresponding two-dimensional quantum dot problem with three
particles is shown to have interesting spectral properties as function of the
interaction strength and applied magnetic fields.


title: Spectral Gaps of Quantum Hall Systems with Interactions

abstract:
  A two-dimensional quantum Hall system without disorder for a wide class of
interactions including any two-body interaction with finite range is studied by
using the Lieb-Schultz-Mattis method [{\it Ann. Phys. (N.Y.)} {\bf 16}: 407
(1961)]. The model is defined on an infinitely long strip with a fixed large
width, and the Hilbert space is restricted to the lowest $(n_{\rm max}+1)$
Landau levels with a large integer $n_{\rm max}$. We proved that, for a
non-integer filling $\nu$ of the Landau levels, either (i) there is a symmetry
breaking at zero temperature or (ii) there is only one infinite-volume ground
state with a gapless excitation. We also proved the following two theorems: (a)
If a pure infinite-volume ground state has a non-zero excitation gap for a
non-integer filling $\nu$, then a translational symmetry breaking occurs at
zero temperature. (b) Suppose that there is no non-translationally invariant
infinite-volume ground state. Then, if a pure infinite-volume ground state has
a non-zero excitation gap, the filling factor $\nu$ must be equal to a rational
number. Here the ground state is allowed to have a periodic structure which is
a consequence of the translational symmetry breaking. We also discuss the
relation between our results and the quantized Hall conductance, and
phenomenologically explain why odd denominators of filling fractions $\nu$
giving the quantized Hall conductance, are favored exclusively.


title: The specific heat of amorphous silica within the harmonic approximation

abstract:
  We investigate to what extent the specific heat of amorphous silica can be
calculated within the harmonic approximation. For this we use molecular
dynamics computer simulations to calculate, for a simple silica model (the BKS
potential), the velocity autocorrelation function and hence an effective
density of states g(nu). We find that the harmonic approximation is valid for
temperatures below 300K but starts to break down at higher temperatures. We
show that in order to get a reliable description of the low frequency part of
g(nu), i.e. where the boson peak is observed, it is essential to use large
systems for the simulations and small cooling rates with which the samples are
quenched. We find that the calculated specific heat is, at low temperatures
(below 50K), about a factor of two smaller than the experimental one. In the
temperature range 200K <= T <= T_g, where T_g=1450K is the glass transition
temperature, we find a very good agreement between the theoretical specific
heat and the experimental one.


title:
Viscometric Functions for a Dilute Solution of Polymers in a Good
  Solvent

abstract:
  A dilute polymer solution is modeled as a suspension of non-interacting
Hookean dumbbells and the effect of excluded volume is taken into account by
incorporating a narrow Gaussian repulsive potential between the beads of each
dumbbell. The narrow Gaussian potential is a means of regularising a
delta-function potential---it tends to the delta-function potential in the
limit of the width parameter going to zero. Exact predictions of viscometric
functions in simple shear flow are obtained with the help of a retarded motion
expansion and by Brownian dynamics simulations. It is shown that for relatively
small non-zero values of the width parameter, the presence of excluded volume
causes a swelling of the dumbbell at equilibrium, and shear thinning in simple
shear flow. On the other hand, a delta function excluded volume potential does
not lead to either swelling or to shear thinning. Approximate viscometric
functions, obtained by assuming that the bead-connector vector is described by
a Gaussian non-equilibrium distribution function, are found to be accurate
above a threshold value of the width parameter, for a given value of the
strength of excluded volume interaction. A first order perturbation expansion
reveals that the Gaussian approximation is exact to first order in the strength
of excluded volume interaction. The predictions of an alternative quadratic
excluded volume potential suggested earlier by Fixman (J. Chem. Phys., 1966,
45, 785; 793) are also compared with those of the narrow Gaussian potential.


title: Dynamics of dense Polyelectrolyte Solutions

abstract:
  We investigate a system of dense polyelectrolytes in solution. The Langevin
dynamics of the system with linearized hydrodynamics is formulated in the
functional integral formalism and a transformation made to collective
coordinates. Within a dynamical Random Phase Approximation (RPA) integration
over the counter- and salt ions produces the Debye-H\"uckel-like screening of
the Coulomb interactions with dependence on the frequency only as part of a
more complicated coupling structure. We investigate the dynamics of the
structure factor as well as the collective diffusion coefficient and comment
upon the viscosity of the whole system of polymers with counterions and fluid
in the simplest approximation. The coupling of the various components of the
system produces nontrivial diffusive behavior. We draw conclusions about the
relationship of the three length scales in the present system, i.e. the static
screening length, the hydrodynamic screening length and the Debye length.


title: Multi-step Bose-Einstein Condensation of Trapped Ideal Bose Gases

abstract:
  The Phenomenon of multi-step Bose-Einstein condensation (BEC) of a finite
number of non-interacting bosons in anisotropic traps has been demonstrated by
studing the populations on eight subsets of states. The cusp in the specific
heat is found to be associated with the crossover between subsets of states
involving Bose functions $g_{n}(z)$ of different classes, as specified by their
behaviour at $z=1$. {PACS: 03.75.Fi, 05.30.-d, 64.60.-i}


title: Density-matrix renormalization using three classes of block states

abstract:
  An extension of the the density matrix renormalization group (DMRG) method is
presented. Besides the two groups or classes of block states considered in
White's formulation, the retained $m$ states and the neglected ones, we
introduce an intermediate group of block states having the following $p$
largest eigenvalues $\lambda_i$ of the reduced density matrix: $\lambda_1 \ge
>... \lambda_m \ge \lambda_{m+1}\ge ... \ge \lambda_{m+p}$. These states are
taken into account when they contribute to intrablock transitions but are
neglected when they participate in more delocalized interblock fluctuations.
Applications to one-dimensional models (Heisenberg, Hubbard and dimerized
tight-binding) show that in this way the involved computer resources can be
reduced without significant loss of accuracy. The efficiency and accuracy of
the method is analyzed by varying $m$ and $p$ and by comparison with standard
DMRG calculations. A Hamiltonian-independent scheme for choosing $m$ and $p$
and for extrapolating to the limit where $m$ and $p$ are infinite is provided.
Finally, an extension of the 3-classes approach is outlined, which incorporates
the fluctuations between the $p$ states of different blocks as a
self-consistent dressing of the block interactions among the retained $m$
states.


title:
Light emission from a scanning tunneling microscope: Fully retarded
  calculation

abstract:
  The light emission rate from a scanning tunneling microscope (STM) scanning a
noble metal surface is calculated taking retardation effects into account. As
in our previous, non-retarded theory [Johansson, Monreal, and Apell, Phys. Rev.
B 42, 9210 (1990)], the STM tip is modeled by a sphere, and the dielectric
properties of tip and sample are described by experimentally measured
dielectric functions. The calculations are based on exact diffraction theory
through the vector equivalent of the Kirchoff integral. The present results are
qualitatively similar to those of the non-retarded calculations. The light
emission spectra have pronounced resonance peaks due to the formation of a
tip-induced plasmon mode localized to the cavity between the tip and the
sample. At a quantitative level, the effects of retardation are rather small as
long as the sample material is Au or Cu, and the tip consists of W or Ir.
However, for Ag samples, in which the resistive losses are smaller, the
inclusion of retardation effects in the calculation leads to larger changes:
the resonance energy decreases by 0.2-0.3 eV, and the resonance broadens. These
changes improve the agreement with experiment. For a Ag sample and an Ir tip,
the quantum efficiency is $\approx$ 10$^{-4}$ emitted photons in the visible
frequency range per tunneling electron. A study of the energy dissipation into
the tip and sample shows that in total about 1 % of the electrons undergo
inelastic processes while tunneling.


title: Topological Spectral Correlations in 2D Disordered Systems

abstract:
  It is shown that the tail in the two-level spectral correlation function R(s)
for particles on 2D closed disordered surfaces is determined entirely by
surface topology: $R(s)=-\chi/(6\pi^2\beta s^2)$, where $\beta$ = 1,2 or 4 for
the orthogonal, unitary and symplectic ensembles, and $\chi$ = 2(1-p) is the
Euler characteristics of the surface with p "handles" (holes). The result is
valid for g << s << g^2 for $\beta$=1,4 and for g << s << g^3 for $\beta$=2,
where g >> 1 is the dimensionless conductance.


title: Electronic properties of the three-band Hubbard model

abstract:
  We study the electronic band-structure and transport properties of a
CuO2-plane within the three-band Hubbard model. The Dynamical Mean-Field Theory
(DMFT) is used to solve the many particle problem. The calculations show that
the optical gap Delta_opt is given by excitations from the lower Hubbard band
into the so called Zhang-Rice singlet band. The optical gap Delta_opt turns out
to be considerably smaller than the charge transfer energy Delta (Delta=ep-ed)
for a typical set of parameters, which is in agreement with experiment. For the
two-dimensional CuO2-plane we investigated the dependency of the shape of the
Fermi surface on the different hopping parameters t_CuO and t_OO. A value
t_OO/t_CuO >0$ leads to a Fermi surface surrounding the M point. An additional
different static shift of the oxygen energies is also considered to calculate
the electronic response due to a displacement of the oxygen atoms given by a
frozen phonon. The density-density correlation for the oxygen orbitals is
linear in doping for both hole and electron doping but shows a different
temperature dependency in the two regimes. In the first case it is temperature
independent and increases upon doping, which leads to an increasing
electron-phonon coupling for the B_1g-mode in high-Tc superconductors.


title:
Magnetization Switching in Small Ferromagnetic Particles: Nucleation and
  Coherent Rotation

abstract:
  The mechanisms of thermally activated magnetization switching in small
ferromagnetic particles driven by an external magnetic field are investigated.
For low uniaxial anisotropy the spins rotate coherently while for sufficiently
large uniaxial anisotropy they behave Ising-like, i.e. the switching then is
due to nucleation. The crossover from coherent rotation to nucleation is
studied for the classical three-dimensional Heisenberg model with uniaxial
anisotropy by Monte Carlo simulations. From the temperature dependence of the
metastable lifetime the energy barrier of a switching process can be
determined. For the case of infinite anisotropy we compare numerical results
from simulations of the Ising model with theoretical results for energy
barriers for both, single-droplet and multi-droplet nucleation. The simulated
barriers are in agreement with the theoretical predictions.


title: Aspects of the Noisy Burgers Equation

abstract:
  The noisy Burgers equation describing for example the growth of an interface
subject to noise is one of the simplest model governing an intrinsically
nonequilibrium problem. In one dimension this equation is analyzed by means of
the Martin-Siggia-Rose technique. In a canonical formulation the morphology and
scaling behavior are accessed by a principle of least action in the weak noise
limit. The growth morphology is characterized by a dilute gas of nonlinear
soliton modes with gapless dispersion law with exponent z=3/2 and a superposed
gas of diffusive modes with a gap. The scaling exponents and a heuristic
expression for the scaling function follow from a spectral representation.


title:
Noise in Multiterminal Diffusive Conductors: Universality, Nonlocality
  and Exchange Effects

abstract:
  We study noise and transport in multiterminal diffusive conductors. Using a
Boltzmann-Langevin equation approach we reduce the calculation of shot-noise
correlators to the solution of diffusion equations. Within this approach we
prove the universality of shot-noise in multiterminal diffusive conductors of
arbitrary shape and dimension for purely elastic scattering as well as for hot
electrons. We show that shot-noise in multiterminal conductors is a non-local
quantity and that exchange effects can occur in the absence of quantum phase
coherence even at zero electron temperature. It is also shown that the exchange
effect measured in one contact is always negative -- in agreement with the
Pauli principle. We discuss a new phenomenon in which current noise is induced
by thermal transport. We propose a possible experiment to measure locally the
effective noise temperature. Concrete numbers for shot-noise are given that can
be tested experimentally.


title: Strict Detailed Balance is Unnecessary in Monte Carlo Simulation

abstract:
  Detailed balance is an overly strict condition to ensure a valid Monte Carlo
simulation. We show that, under fairly general assumptions, a Monte Carlo
simulation need satisfy only the weaker balance condition. Not only does our
proof show that sequential updating schemes are correct, but also it
establishes the correctness of a whole class of new methods that simply leave
the Boltzmann distribution invariant.


title: Persistence of Kardar-Parisi-Zhang Interfaces

abstract:
  The probabilities $P_\pm(t_0,t)$ that a growing Kardar-Parisi-Zhang interface
remains above or below the mean height in the time interval $(t_0, t)$ are
shown numerically to decay as $P_\pm \sim (t_0/t)^{\theta_\pm}$ with $\theta_+
= 1.18 \pm 0.08$ and $\theta_- = 1.64 \pm 0.08$. Bounds on $\theta_\pm$ are
derived from the height autocorrelation function under the assumption of
Gaussian statistics. The autocorrelation exponent $\bar \lambda$ for a
$d$--dimensional interface with roughness and dynamic exponents $\beta$ and $z$
is conjectured to be $\bar \lambda = \beta + d/z$. For a recently proposed
discretization of the KPZ equation we find oscillatory persistence
probabilities, indicating hidden temporal correlations.


title:
Persistent X-Ray Photoconductivity and Percolation of Metallic Clusters
  in Charge-Ordered Manganites

abstract:
  Charge-ordered manganites of composition $\rm
Pr_{1-x}(Ca_{1-y}Sr_{y})_{x}MnO_3$ exhibit persistent photoconductivity upon
exposure to x-rays. This is not always accompanied by a significant increase in
the {\it number} of conduction electrons as predicted by conventional models of
persistent photoconductivity. An analysis of the x-ray diffraction patterns and
current-voltage characteristics shows that x-ray illumination results in a
microscopically phase separated state in which charge-ordered insulating
regions provide barriers against charge transport between metallic clusters.
The dominant effect of x-ray illumination is to enhance the electron {\it
mobility} by lowering or removing these barriers. A mechanism based on magnetic
degrees of freedom is proposed.


title: Muon Spin Relaxation and Nonmagnetic Kondo State in PrInAg_2

abstract:
  Muon spin relaxation experiments have been carried out in the Kondo compound
PrInAg_2. The zero-field muon relaxation rate is found to be independent of
temperature between 0.1 and 10 K, which rules out a magnetic origin (spin
freezing or a conventional Kondo effect) for the previously-observed specific
heat anomaly at \sim0.5 K. The low-temperature muon relaxation is
quantitatively consistent with nuclear magnetism including hyperfine
enhancement of the ^{141}Pr nuclear moment. This is strong evidence against a
Pr^{3+} electronic magnetic moment, and for the \Gamma_3
crystalline-electric-field-split ground state required for a nonmagnetic route
to heavy-electron behavior. The data imply the existence of an exchange
interaction between neighboring Pr^{3+} ions of the order of 0.2 K in
temperature units, which should be taken into account in a complete theory of a
nonmagnetic Kondo effect in PrInAg_2.


title:
Renormalization Group for Large N Strongly Commensurate Dirty Boson
  Model

abstract:
  The large N sigma model, in D<4 space-time dimensions, with disorder a
function of d space dimensions, is analyzed via a renormalization group
treatment. Critical exponents for average quantities are calculated, first to
lowest order and then to all orders, in $\epsilon=D-2 - d/2$. In particular, it
is found that $\nu d =2$. When D=d+1, this model is equivalent to a large N
limit of the strongly commensurate dirty boson problem.


title: On the glass transition temperature in covalent glasses

abstract:
  We give a simple demonstration of the formula relating the glass transition
temperature, $T_g$, to the molar concentration $x$ of a modifier in two types
of glasses: binary glasses, whose composition can be denoted by
$X_nY_m+xM_pY_q$, with ^$X$ an element of III-rd or IV-th group (e.g. B, or Si,
Ge), while $M_pY_q$ is an alkali oxide or chalcogenide; next, the network
glasses of the type $A_xB_{1-x}$, e.g. $Ge_xSe_{1-x}$, $Si_xTe_{1-x}$, etc.
After comparison, this formula gives an exact expression of the parameter
$\beta$ of the modified Gibbs-Di Marzio equation.


title: Extraction of the Spin Glass Correlation Length

abstract:
  The peak of the spin glass relaxation rate, S(t)=d{-M_{TRM}(t,t_w)}/H/{d ln
t}, is directly related to the typical value of the free energy barrier which
can be explored over experimental time scales. A change in magnetic field H
generates an energy E_z={N_s}{X_fc}{H^2} by which the barrier heights are
reduced, where X_{fc} is the field cooled susceptibility per spin, and N_s is
the number of correlated spins. The shift of the peak of S(t) gives E_z,
generating the correlation length, Ksi(t,T), for Cu:Mn 6at.% and
CdCr_{1.7}In_{0.3}S_4. Fits to power law dynamics, Ksi(t,T)\propto
{t}^{\alpha(T)} and activated dynamics Ksi(t,T) \propto {ln t}^{1/psi} compare
well with simulation fits, but possess too small a prefactor for activated
dynamics.


title:
Array-induced collective transport in the Brownian motion of coupled
  nonlinear oscillator systems

abstract:
  Brownian motion of an array of harmonically coupled particles subject to a
periodic substrate potential and driven by an external bias is investigated. In
the linear response limit (small bias), the coupling between particles may
enhance the diffusion process, depending on the competition between the
harmonic chain and the substrate potential. An analytical formula of the
diffusion rate for the single-particle case is also obtained. In the nonlinear
response regime, the moving kink may become phase-locked to its radiated phonon
waves, hence the mobility of the chain may decrease as one increases the
external force.


title:
Influence of boundary conditions on level statistics and eigenstates at
  the metal insulator transition

abstract:
  We investigate the influence of the boundary conditions on the scale
invariant critical level statistics at the metal insulator transition of
disordered three-dimensional orthogonal and two-dimensional unitary and
symplectic tight-binding models. The distribution of the spacings between
consecutive eigenvalues is calculated numerically and shown to be different for
periodic and Dirichlet boundary conditions whereas the critical disorder
remains unchanged. The peculiar correlations of the corresponding critical
eigenstates leading to anomalous diffusion seem not to be affected by the
change of the boundary conditions.


title:
Ab initio determination of exchange integrals and Neel temperature in
  the chain cuprates

abstract:
  We report ab initio quantum chemical cluster calculations of the chain (J_a)
and the largest interchain (J_b) Heisenberg exchange of the chain cuprates
Ca_2CuO_3 and Sr_2CuO_3. We find that J_a is comparable to the in-plane J in
layered cuprates and J_a/J_b ~250-400. Using recent theory we obtain close
agreement with experiment for the staggered moments and the critical
temperatures. This implies that T_N does not depend on the third parameter J_c
<< J_b, and cannot be calculated using spin-wave theory. We propose an
explanation for this interms of a 1D->2D cross-over.


title:
Microphase Separation and modulated phases in a Coulomb frustrated Ising
  ferromagnet

abstract:
  We study a 3-dimensional Ising model in which the tendency to order due to
short-range ferromagnetic interactions is frustrated by competing long-range
(Coulombic) interactions. Complete ferromagnetic ordering is impossible for any
nonzero value of the frustration parameter, but the system displays a variety
of phases characterized by periodically modulated structures. We have performed
extensive Monte-Carlo simulations which provide strong evidence that the
microphase separation transition between paramagnetic and modulated phases is a
fluctuation-induced first-order transition. Additional transitions to various
commensurate phases may also occur when further lowering the temperature.


title:
Hopping-disorder-induced effects upon the two-Magnon Raman Scattering
  spectrum in an Antiferromagnet

abstract:
  Two-magnon Raman scattering intensity is obtained for two-dimensional
spin-1/2 antiferromagnets in presence of hopping disorder. A consistent mode
assignment scheme is prescribed which essentially establishes a correspondence
between the hopping-disordered and the pure system. It is seen that a minute
amount of disorder in hopping leads to a good agreement of the Raman lineshape
experimentally obtained for the copper-oxide insulators such as ${\rm
{La_{2}CuO_{4}}}$. Also it is observed that a considerable asymmetry with
respect to the two-magnon peak appears and the spectral intensity persisting
much beyond $4J$ (where the joint magnon density of states peaks). This has
been argued earlier by us [S. Basu and A. Singh, Phys. Rev. B, {\bf {53}}, 6406
(1996)] to be arising due to highly asymmetric magnon-energy renormalization
because of a cooperative effect arising from local correlation in hopping
disorder.


title: Effects of Magnetic Field on Josephson Current in SNS System

abstract:
  The effect of a magnetic field on Josephson current has been studied for a
superconductor/normal-metal/superconductor (SNS) system, where N is a
two-dimensional electron gas in a confining potential. It is found that the
dependence of Josephson currents on the magnetic field are sensitive to the
width of the normal metal. If the normal metal is wide and contains many
channels (subbands), the current on a weak magnetic field shows a dependence
similar to a Fraunhofer-pattern in SIS system and, as the field gets strong, it
shows another type of oscillatory dependence on the field resulting from the
Aharonov-Bohm interference between the edge states. As the number of channels
decreases (i.e. normal metal gets narrower), however, the dependence in the
region of the weak field deviates from a clear Fraunhofer pattern and the
amplitude of the oscillatory dependence in the region of the strong field is
reduced.


title: Universal amplitudes in the FSS of three-dimensional spin models

abstract:
  In a MC study using a cluster update algorithm we investigate the finite-size
scaling (FSS) of the correlation lengths of several representatives of the
class of three-dimensional classical O(n) symmetric spin models on a column
geometry. For all considered models we find strong evidence for a linear
relation between FSS amplitudes and scaling dimensions when applying
antiperiodic instead of periodic boundary conditions across the torus. The
considered type of scaling relation can be proven analytically for systems on
two-dimensional strips with periodic bc using conformal field theory


title: Correlation amplitude for the XXZ spin chain in the disordered regime

abstract:
  We proposed an analytical expression for the amplitude defining the long
distance asymptotic of the correlation function <\sigma^z_k \sigma^z_{k+n}>.

categories: cond-mat.stat-mech hep-th nlin.SI solv-int


title: Conductance of carbon nanotubes with disorder: A numerical study

abstract:
  We study the conductance of carbon nanotube wires in the presence of
disorder, in the limit of phase coherent transport. For this purpose, we have
developed a simple numerical procedure to compute transmission through carbon
nanotubes and related structures. Two models of disorder are considered, weak
uniform disorder and isolated strong scatterers. In the case of weak uniform
disorder, our simulations show that the conductance is not significantly
affected by disorder when the Fermi energy is close to the band center.
Further, the transmission around the band center depends on the diameter of
these zero bandgap wires. We also find that the calculated small bias
conductance as a function of the Fermi energy exhibits a dip when the Fermi
energy is close to the second subband minima. In the presence of strong
isolated disorder, our calculations show a transmission gap at the band center,
and the corresponding conductance is very small.


title: Prediction of Ferromagnetic Ground State of NaCl-type FeN

abstract:
  Ab-initio results for structural and electronic properties of NaCl-type FeN
are presented in a framework of plane-wave and ultrasoft pseudopotentials.
Competition among different magnetic ordering is examined. We find the
ferromagnetic phase stable overall. Stabilization over the unpolarized phase is
obtained by splitting one flat t_2g-type band crossing the Fermi energy. A
comparison with CrN is considered. We find large differences in the properties
of the two systems that can be addressed to the smaller ionicity and
magnetization of FeN.


title: Optical Spin Vortex in Nonlinear Anisotropic Media

abstract:
  A theoretical study is given of a new type of optical vortex in nonlinear
anisotropic media. This is called an ``optical spin vortex'', which is realized
as a special solution of the two-component non-linear Schr\"{o}dinger equation.
The existence of spin vortex is inherent in the spin texture that is caused by
the anisotropy of dielectric tensor, where a role of spin is played by the
Stokes vector. By introducing the effective Lagrangian governing the non-linear
Schr\"{o}dinger equation, we derive the evolution equation for the motion of
spin vortex, which exhibits a clear distinction from the conventional singular
vortex inherent in the phase function that is described by the single component
complex field.


title: Magnetic Raman Scattering of Insulating Cuprates

abstract:
  We study the $B_{1g}$ and $A_{1g}$ Raman profiles of M$_{2}$Cu$O_{4}$ (with
M= La, Pr, Nd, Sm, Gd), Bi$_{2}$Sr$_{2}$Ca$_{0.5}$Y$_{0.5}$Cu$_{2}$O$_{8+y}$%,
YBa$_{2}$Cu$_{3}$O$_{6.2}$ and PrBa$_{2}$Cu$_{2.7}$Al$_{0.3}$O$_{7}$ insulating
cuprates within the Loudon-Fleury theory, in the framework of an extended
Hubbard model for moderate on-site Coulomb interaction $U$. We calculate the
non-resonant contribution to these Raman profiles by using exact
diagonalization techniques and analyze two types of contributing mechanisms to
the line shapes: 4-spin cyclic exchange and spin-phonon interactions. Although
these interactions contribute to different parts of the spectra, together, they
account for the enhanced linewidth and asymmetry of the $B_{1g}$ mode, as well
as the non-negligible intensity of the $A_{1g}$ Raman line observed in these
materials.


title:
Hyperelliptic curves for multi-channel quantum wires and the
  multi-channel Kondo problem

abstract:
  We study the current in a multi-channel quantum wire and the magnetization in
the multi-channel Kondo problem. We show that at zero temperature they can be
written simply in terms of contour integrals over a (two-dimensional)
hyperelliptic curve. This allows one to easily demonstrate the existence of
weak-coupling to strong-coupling dualities. In the Kondo problem, the curve is
the same for under- and over-screened cases; the only change is in the contour.

categories: cond-mat.stat-mech cond-mat.str-el hep-th math.QA


title: Electronic and Magnetic Properties of Nanographite Ribbons

abstract:
  Electronic and magnetic properties of ribbon-shaped nanographite systems with
zigzag and armchair edges in a magnetic field are investigated by using a tight
binding model. One of the most remarkable features of these systems is the
appearance of edge states, strongly localized near zigzag edges. The edge state
in magnetic field, generating a rational fraction of the magnetic flux (\phi=
p/q) in each hexagonal plaquette of the graphite plane, behaves like a
zero-field edge state with q internal degrees of freedom. The orbital
diamagnetic susceptibility strongly depends on the edge shapes. The reason is
found in the analysis of the ring currents, which are very sensitive to the
lattice topology near the edge. Moreover, the orbital diamagnetic
susceptibility is scaled as a function of the temperature, Fermi energy and
ribbon width. Because the edge states lead to a sharp peak in the density of
states at the Fermi level, the graphite ribbons with zigzag edges show
Curie-like temperature dependence of the Pauli paramagnetic susceptibility.
Hence, it is shown that the crossover from high-temperature diamagnetic to
low-temperature paramagnetic behavior of the magnetic susceptibility of
nanographite ribbons with zigzag edges.