title: The Little-Parks effect in an inhomogeneous superconducting ring

abstract:
  An inhomogeneous superconducting ring placed in a magnetic field is
considered. It is shown that the superconducting transition of the section with
lowest critical temperature may be a first order phase transition if the
magnetic flux contained within the ring is not divisible by the flux quantum.
In the visinity of this transition, thermal fluctuations can induce the direct
voltage in the ring with rather small sizes. The voltage value depends in a
periodic manner on the magnetic flux contained within the ring. According to
this result, the entropy value can be decreased in the considered quantum
system. The chaotic heat energy can be transformed in the electric energy of
the direct current.


title: A new broken U(1)-symmetry in extreme type-II superconductors

abstract:
  A phase transition within the molten phase of the Abrikosov vortex system
without disorder in extreme type-II superconductors is found via large-scale
Monte-Carlo simulations. It involves breaking a U(1)-symmetry, and has a
zero-field counterpart, unlike vortex lattice melting. Its hallmark is the loss
of number-conservation of connected vortex paths threading the entire system
{\it in any direction}, driving the vortex line tension to zero. This tension
plays the role of a generalized ``stiffness'' of the vortex liquid, and serves
as a probe of the loss of order at the transition, where a weak specific heat
anomaly is found.


title: Depletion induced demixing in polydisperse mixtures of hard spheres

abstract:
  Polydisperse mixtures are those in which components with a whole range of
sizes are present. It is shown that the fluid phase of polydisperse hard
spheres is thermodynamically unstable unless the density of large spheres
decreases at least exponentially as their size increases. The instability is
with respect to the large spheres crystallising out into multiple solid phases.


title:
Proximity-induced transport in hybrid mesoscopic normal-superconducting
  metal structures

abstract:
  Using an approach based on quasiclassical Green's functions we present a
theoretical study of transport in mesoscopic S/N structures in the diffusive
limit. The subgap conductance in S/N structures with barriers (zero bias and
finite bias anomalies) are discused. We also analyse the temperature dependence
of the conductance variation $\delta S(T)$ for a Andreev interferometer. We
show that besides the well know low temperature maximum a second maximum near
$T_c$ may appear. We present the results of studies on the Josephson effect in
4 terminal S/N/S contacts and on the possible sign reversal of the Josephson
critical current.


title: Ab initio theory of the interlayer exchange coupling

abstract:
  Ab initio formulations of the interlayer exchange coupling (IEC) between two,
in general non-collinearly aligned magnetic slabs embedded in a non-magnetic
spacer are reviewed whereby both the spacer and the magnetic slabs as well as
their interfaces may be either ideal or random. These formulations are based on
the spin-polarized surface Green function technique within the tight-binding
linear muffin-tin orbital method, the Lloyd formulation of the IEC, and the
coherent potential approximation using the vertex-cancellation theorem. We also
present an effective method for the study of the temperature dependence of the
IEC. The periods, amplitudes, and phases are studied in terms of discrete
Fourier transformations, the asymptotic behavior of the IEC is briefly
discussed within the stationary-phase method. Numerical results illustrating
the theory are presented.


title: Bounds for the time to failure of hierarchical systems of fracture

abstract:
  For years limited Monte Carlo simulations have led to the suspicion that the
time to failure of hierarchically organized load-transfer models of fracture is
non-zero for sets of infinite size. This fact could have a profound
significance in engineering practice and also in geophysics. Here, we develop
an exact algebraic iterative method to compute the successive time intervals
for individual breaking in systems of height $n$ in terms of the information
calculated in the previous height $n-1$. As a byproduct of this method,
rigorous lower and higher bounds for the time to failure of very large systems
are easily obtained. The asymptotic behavior of the resulting lower bound leads
to the evidence that the above mentioned suspicion is actually true.


title: Self-consistent theory of dynamic melting of a vortex lattice

abstract:
  The dynamic melting of vortex lattices in type II superconductors is
considered. A field-theoretic formulation of the pinning problem allows the
average over the quenched disorder to be performed exactly. A self-consistent
theory is constructed using a functional method for the effective action,
allowing a determination of the pinning force and the vortex fluctuations. The
phase diagram for the dynamic melting transition is determined numerically. In
contrast to perturbation theory, the self-consistent theory is in quantitative
agreement with the prediction of a recent phenomenological theory and
simulations and experimental data.


title: Novel Circuit Theory of Andreev Reflection

abstract:
  We review here a novel circuit theory of superconductivity. The existed
circuit theory of Andreev reflection has been revised to account for
decoherence between electrons and holes and twofold nature of the distribution
function. The description of arbitrary connectors has been elaborated. In this
way one can cope with the most of the factors that limited applicability of the
old circuit theory. We give a simple example and discuss numerical
implementation of the theory.


title: Simulation of thermal conductivity and heat transport in solids

abstract:
  Using molecular dynamics (MD) with classical interaction potentials we
present calculations of thermal conductivity and heat transport in crystals and
glasses. Inducing shock waves and heat pulses into the systems we study the
spreading of energy and temperature over the configurations. Phonon decay is
investigated by exciting single modes in the structures and monitoring the time
evolution of the amplitude using MD in a microcanonical ensemble. As examples,
crystalline and amorphous modifications of Selenium and $\rm{SiO_2}$ are
considered.


title:
Density of Zeros on the Lee-Yang circle obtained from magnetization data
  of a two-dimensional Ising ferromagnet

abstract:
  In order to provide an experimental access to the statistical theory of Lee
and Yang (1952) the density function g(theta) of zeros on the Lee-Yang circle
has been determined for the first time by analyzing isothermal magnetization
data, m(H), of the Ising ferromagnet FeCl2 in axial magnetic fields, H, at
temperatures 34 < T < 99K. The validity of our approach is demonstrated by the
perfect agreement of magnetic specific heat data as calculated from g(theta)
and m(H) via Maxwells relation, respectively. Moreover, the correct in-plane
exchange constant of FeCl2 emerges from the series expansion of m(H) involved
in the analysis.


title:
Superconducting Material Diagnostics using a Scanning Near-Field
  Microwave Microscope

abstract:
  We have developed scanning near-field microwave microscopes which can image
electrodynamic properties of superconducting materials on length scales down to
about 2 $\mu$m. The microscopes are capable of quantitative imaging of sheet
resistance of thin films, and surface topography. We demonstrate the utility of
the microscopes through images of the sheet resistance of a YBa2Cu3O7-d thin
film wafer, images of bulk Nb surfaces, and spatially resolved measurements of
Tc of a YBa2Cu3O7-d thin film. We also discuss some of the limitations of the
microscope and conclude with a summary of its present capabilities.


title: Damage spreading and Lyapunov exponents in cellular automata

abstract:
  Using the concept of the Boolean derivative we study damage spreading for one
dimensional elementary cellular automata and define their maximal Lyapunov
exponent. A random matrix approximation describes quite well the behavior of
``chaotic'' rules and predicts a directed percolation-type phase transition.
After the introduction of a small noise elementary cellular automata reveal the
same type of transition.


title: Growth of surfaces generated by a probabilistic cellular automaton

abstract:
  A one-dimensional cellular automaton with a probabilistic evolution rule can
generate stochastic surface growth in $(1 + 1)$ dimensions. Two such discrete
models of surface growth are constructed from a probabilistic cellular
automaton which is known to show a transition from a active phase to a
absorbing phase at a critical probability associated with two particular
components of the evolution rule. In one of these models, called model $A$ in
this paper, the surface growth is defined in terms of the evolving front of the
cellular automaton on the space-time plane. In the other model, called model
$B$, surface growth takes place by a solid-on-solid deposition process
controlled by the cellular automaton configurations that appear in successive
time-steps. Both the models show a depinning transition at the critical point
of the generating cellular automaton. In addition, model $B$ shows a kinetic
roughening transition at this point. The characteristics of the surface width
in these models are derived by scaling arguments from the critical properties
of the generating cellular automaton and by Monte Carlo simulations.


title: Observation of metastable states in spinor Bose-Einstein condensates

abstract:
  Bose-Einstein condensates have been prepared in long-lived metastable excited
states. Two complementary types of metastable states were observed. The first
is due to the immiscibility of multiple components in the condensate, and the
second to local suppression of spin-relaxation collisions. Relaxation via
re-condensation of non-condensed atoms, spin relaxation, and quantum tunneling
was observed. These experiments were done with F=1 spinor Bose-Einstein
condensates of sodium confined in an optical dipole trap.


title:
Competition, efficiency and collective behavior in the "El Farol" bar
  model

abstract:
  The El Farol bar model, proposed to study the dynamics of competition of
agents in a variety of contexts (W. B. Arthur, Amer. Econ. Assoc. Pap. and
Proc. 84, 406 (1994)) is studied. We characterize in detail the three regions
of the phase diagram (efficient, inefficient and better than random) of the
simplest version of the model (D. Challet and Y.-C. Zhang, Physica A, 246, 407
(1997)). The efficient region is shown to have a rich structure, which is
investigated in some detail. Changes in the payoff function enhance further the
tendency of the model towards a wasteful distribution of resources.


title:
Statistical Mechanics Analysis of the Continuous Number Partitioning
  Problem

abstract:
  The number partitioning problem consists of partitioning a sequence of
positive numbers ${a_1,a_2,..., a_N}$ into two disjoint sets, ${\cal A}$ and
${\cal B}$, such that the absolute value of the difference of the sums of $a_j$
over the two sets is minimized. We use statistical mechanics tools to study
analytically the Linear Programming relaxation of this NP-complete integer
programming. In particular, we calculate the probability distribution of the
difference between the cardinalities of ${\cal A}$ and ${\cal B}$ and show that
this difference is not self-averaging.


title: Metallic Surface Reconstruction Driven by Frustrated Antiferromagnetism

abstract:
  A magnetic origin for the honeycomb reconstruction of metallic surfaces with
three-fold symmetry like Pb/Ge (111) is proposed. Assuming that the groundstate
is an antiferromagnetic insulator over the triangular lattice of adatom sites
(Pb), we demonstrate that the former is simultaneously unstable to canting and
to a structural distortion if the surface is soft enough. We therefore predict
a net magnetization over the reconstructed surface at sufficiently low
temperature.


title: The integrable open XXZ chain with broken Z_2 symmetry

abstract:
  The hamiltonian of an asymmetric diffusion process with injection and
ejection of particles at the ends of a chain of finite length is known to be
relevant to that of the spin-1/2 XXZ chain with integrable boundary terms.
However, the inclusion of boundary sources and sinks of particles breaks the
arrow-reversal symmetry necessary for solution via the usual Bethe Ansatz
approach. Developments in solving the model in the absence of arrow-reversal
symmetry are discussed.


title:
On the correct strong-coupling limit in the evolution from BCS
  superconductivity to Bose-Einstein condensation

abstract:
  We consider the problem of the crossover from BCS superconductivity to
Bose-Einstein condensation in three dimensions for a system of fermions with an
attractive interaction, for which we adopt the simplifying assumption of a
suitably regularized point-contact interaction. We examine in a critical way
the fermionic (self-consistent) T-matrix approximation which has been widely
utilized in the literature to describe this crossover above the superconducting
critical temperature, and show that it fails to yield the correct behaviour of
the system in the strong-coupling limit, where composite bosons form as tightly
bound fermion pairs. We then set up the correct approximation for a ``dilute''
system of composite bosons and show that an entire new class of diagrams has to
be considered in the place of the fermionic T-matrix approximation for the
self-energy. This new class of diagrams correctly describes both the weak- and
strong-coupling limits, and consequently results into an improved interpolation
scheme for the intermediate (crossover) region. In this context, we provide
also a systematic mapping between the corresponding diagrammatic theories for
the composite bosons and the constituent fermions. As a preliminary result to
demonstrate the numerical effect of our new class of diagrams on physical
quantities, we calculate the value of the scattering length for composite
bosons in the strong-coupling limit and show that it is considerably modified
with respect to the result obtained within the self-consistent fermionic
T-matrix approximation.


title:
Critical dynamics at incommensurate phase transitions and NMR relaxation
  experiments

abstract:
  We study the critical dynamics of crystals which undergo a second-order phase
transition from a high-temperature normal phase to a structurally
incommensurate (IC) modulated phase. We give a comprehensive description of the
critical dynamics of such systems, e.g. valid for crystals of the A_2BX_4
family. Using an extended renormalization scheme, we present a framework in
which we analyze the phases above and below the critical temperature T_I. Above
T_I, the crossover from the critical behavior to the mean-field regime is
studied. Specifically, the resulting width of the critical region is
investigated. In the IC modulated phase, we consider explicitly the coupling of
the order parameter modes to one-loop order. Here the Goldstone anomalies and
their effect on measurable quantities are investigated. We show their relation
with the postulated phason gap. While the theory can be applied to a variety of
experiments, we concentrate on quadrupole-perturbed nuclear magnetic resonance
(NMR) experiments. We find excellent agreement with these dynamical
measurements and provide answers for some questions that arose from recent
results.


title:
Transport Phenomena at a Critical Point -- Thermal Conduction in the
  Creutz Cellular Automaton --

abstract:
  Nature of energy transport around a critical point is studied in the Creutz
cellular automaton. Fourier heat law is confirmed to hold in this model by a
direct measurement of heat flow under a temperature gradient. The thermal
conductivity is carefully investigated near the phase transition by the use of
the Kubo formula. As the result, the thermal conductivity is found to take a
finite value at the critical point contrary to some previous works. Equal-time
correlation of the heat flow is also analyzed by a mean-field type
approximation to investigate the temperature dependence of thermal
conductivity. A variant of the Creutz cellular automaton called the Q2R is also
investigated and similar results are obtained.


title: Calculations of the A_1 phonon frequency in photoexcited Tellurium

abstract:
  Calculations of the A_1 phonon frequency in photoexcited tellurium are
presented. The phonon frequency as a function of photoexcited carrier density
and phonon amplitude is determined. Recent pump probe experiments are
interpreted in the light of these calculatons. It is proposed that, in
conjunction with measurements of the phonon period in ultra-fast pump-probe
reflectivity experiments, the calculated frequency shifts can be used to infer
the evolution of the density of photoexcited carriers on a sub-picosecond
time-scale.


title:
The Density Matrix Renormalization Group, Quantum Groups and Conformal
  Field Theory

abstract:
  We present an overview of the Density Matrix Renormalization Group and its
connections to Quantum Groups, Matrix Products and Conformal Field Theory. We
emphasize some common formal structures in all these theories. We also propose
two-dimensional extensions of the variational matrix product ansatzs.


title: Stress Propagation through Frictionless Granular Material

abstract:
  We examine the network of forces to be expected in a static assembly of hard,
frictionless spherical beads of random sizes, such as a colloidal glass. Such
an assembly is minimally connected: the ratio of constraint equations to
contact forces approaches unity for a large assembly. However, the bead
positions in a finite subregion of the assembly are underdetermined. Thus to
maintain equilibrium, half of the exterior contact forces are determined by the
other half. We argue that the transmission of force may be regarded as
unidirectional, in contrast to the transmission of force in an elastic
material. Specializing to sequentially deposited beads, we show that forces on
a given buried bead can be uniquely specified in terms of forces involving more
recently added beads. We derive equations for the transmission of stress
averaged over scales much larger than a single bead. This derivation requires
the Ansatz that statistical fluctuations of the forces are independent of
fluctuations of the contact geometry. Under this Ansatz, the
$d(d+1)/2$-component stress field can be expressed in terms of a d-component
vector field. The procedure may be generalized to non-sequential packings. In
two dimensions, the stress propagates according to a wave equation, as
postulated in recent work elsewhere. We demonstrate similar wave-like
propagation in higher dimensions, assuming that the packing geometry has
uniaxial symmetry. In macroscopic granular materials we argue that our approach
may be useful even though grains have friction and are not packed
sequentially.=17


title: Strong coupling resistivity in the Kondo model

abstract:
  By applying methods of integrable quantum field theory to the Kondo problem,
we develop a systematic perturbation expansion near the IR (strong coupling)
fixed point. This requires the knowledge of an infinity of irrelevant operators
and their couplings, which we all determine exactly. A low temperature
expansion (ie all the corrections to Fermi liquid theory) of the resistivity
then follows, extending for instance the well known Nozieres $T^2$ result in
the exactly screened case to arbitrary order. The example of the ordinary Kondo
model is worked out in details: we determine $\rho$ up to order $T^6$, and
compare the result with available numerical data.


title: Negative Magnetoresistance of Granular Metals in a Strong Magnetic Field

abstract:
  The magnetoresistance of a granular superconductor in a strong magnetic field
destroying the gap in each grain is considered. It is assumed that the
tunneling between grains is sufficiently large such that all conventional
effects of localization can be neglected. A non-trivial sensitivity to the
magnetic field comes from superconducting fluctuations leading to the formation
of virtual Cooper pairs and reducing the density of states. At low temperature,
the pairs do not contribute to the macroscopic transport but their existence
can drastically reduce the conductivity. Growing the magnetic field one
destroys the fluctuations, which improves the metallic properties and leads to
the negative magnetoresistance.


title: Metal-Insulator Transition in Randomly Interacting Systems

abstract:
  We discuss a metal-insulator transition caused by random couplings of
magnetic moments in itinerant systems. An analytic solution for the single
particle Green function is derived from dynamical self consistency equations,
the corresponding density of states is characterized by the opening of a gap.
The scaling behavior of observables is analyzed in the framework of a scaling
theory and different crossover lines are identified. A fluctuation expansion
around the mean field solution accounts for both interaction and localization
effects in a consistent manner and is argued to be relevant for the description
of the recently discovered metal-insulator transition in 2d electronic systems.


title: Quantum Theory of Atomic Four-Wave Mixing in Bose-Einstein Condensates

abstract:
  We present an exact quantum mechanical analysis of collinear four-wave mixing
in a multicomponent Bose-Einstein condensate consisting of sodium atoms in the
F=1 ground state. Technically, this is achieved by taking advantage of the
conservation laws of the system to represent its Hamiltonian in terms of
angular momentum operators. We discuss explicitly the build-up of matter-wave
side-modes from noise, as well as the correlations between these modes. We show
the appearance of a strong quantum entanglement between hyperfine states. We
also demonstrate that for finite atomic numbers, the system exhibits periodic
collapses and revivals in the exchange of atoms between different spin states.


title:
Phonon relaxation of subgap levels in superconducting quantum point
  contacts

abstract:
  Superconducting quantum point contacts are known to possess two subgap states
per each propagating mode. In this note we compute the low-temperature
relaxation rate of the upper subgap state into the lower one with the emission
of an acoustic phonon. If the reflection in the contact is small, the
relaxation time may become much longer than the characteristic lifetime of a
bulk quasiparticle.


title:
Comment on "Evidence for nontrivial ground-state structure of 3d +/- J
  spin glasses"

abstract:
  In a recent Letter [Europhys. Lett. 40, 429 (1997)], Hartmann presented
results for the structure of the degenerate ground states of the
three-dimensional +/- J spin glass model obtained using a genetic algorithm. In
this Comment, I argue that the method does not produce the correct
thermodynamic distribution of ground states and therefore gives erroneous
results for the overlap distribution. I present results of simulated annealing
calculations using different annealing rates for cubic lattices with
N=4*4*4spins. The disorder-averaged overlap distribution exhibits a significant
dependence on the annealing rate, even when the energy has converged. For fast
annealing, moments of the distribution are similar to those presented by
Hartmann. However, as the annealing rate is lowered, they approach the results
previously obtained using a multi-canonical Monte Carlo method. This shows
explicitly that care must be taken not only to reach states with the lowest
energy but also to ensure that they obey the correct thermodynamic
distribution, i.e., that the probability is the same for reaching any of the
ground states.


title:
A rapidly converging algorithm for solving the Kohn-Sham and related
  equations in electronic structure theory

abstract:
  We describe a rapidly converging algorithm for solving the Kohn--Sham
equations and equations of similar structure that appear frequently in
calculations of the structure of inhomogeneous electronic many--body systems.
The algorithm has its roots the Hohenberg-Kohn theorem and solves directly for
the electron density; single--particle wave functions are only used as
auxiliary quantities. The method has been implemented for symmetric ``slabs''
of jellium as well as for spherical jellium clusters. Starting from very rough
guesses for the initial electron density, convergence is reached within a few
iterations. The iterations are driven by the static electric susceptibility.


title: Spin-accumulation in small ferromagnetic double barrier junctions

abstract:
  The non-equilibrium spin accumulation in ferromagnetic double barrier
junctions is shown to govern the transport in small structures. Transport
properties of such systems are described by a generalization of the theory of
the Coulomb blockade. The spin accumulation enhances the magnetoresistance. The
transient non-linear transport properties are predicted to provide a unique
experimental evidence of the spin-accumulation in the form of a reversed
current on time scales of the order of the spin-flip relaxation time.


title: A Mesoscopic Quantum Eraser

abstract:
  Motivated by a recent experiment by Buks et al. [Nature 391, 871 (1998)] we
consider electron transport through an Aharonov-Bohm interferometer with a
quantum dot in one of its arms. The quantum dot is coupled to a quantum system
with a finite number of states acting as a which-path detector. The
Aharonov-Bohm interference is calculated using a two-particle scattering
approach for the joint transitions in detector and quantum dot. Tracing over
the detector yields dephasing and a reduction of the interference amplitude. We
show that the interference can be restored by a suitable measurement on the
detector and propose a mesoscopic quantum eraser based on this principle.


title: The Thermopower of Quantum Chaos

abstract:
  The thermovoltage of a chaotic quantum dot is measured using a current
heating technique. The fluctuations in the thermopower as a function of
magnetic field and dot shape display a non-Gaussian distribution, in agreement
with simulations using Random Matrix Theory. We observe no contributions from
weak localization or short trajectories in the thermopower.


title: Quadratic electronic response of a two-dimensional electron gas

abstract:
  The electronic response of a two-dimensional (2D) electron system represents
a key quantity in discussing one-electron properties of electrons in
semiconductor heterojunctions, on the surface of liquid helium and in
copper-oxide planes of high-temperature superconductors. We here report an
evaluation of the wave-vector and frequency dependent dynamical quadratic
density-response function of a 2D electron gas (2DEG), within a self-consistent
field approximation. We use this result to find the $Z_1^3$ correction to the
stopping power of a 2DEG for charged particles moving at a fixed distance from
the plane of the 2D sheet, $Z_1$ being the projectile charge. We reproduce, in
the high-density limit, previous full nonlinear calculations of the stopping
power of a 2DEG for slow antiprotons, and we go further to calculate the
$Z_1^3$ correction to the stopping power of a 2DEG for a wide range of
projectile velocities. Our results indicate that linear response calculations
are, for all projectile velocities, less reliable in two dimensions than in
three dimensions.


title: Flux Confinement in Mesoscopic Superconductors

abstract:
  We report on flux confinement effects in superconducting submicron line, loop
and dot structures. The main idea of our study was to vary the boundary
conditions for confinement of the superconducting condensate by taking samples
of different topology and, through that, modifying the lowest Landau level
E_{LLL}(H). Since the critical temperature versus applied magnetic field
T_{c}(H) is, in fact, E_{LLL}(H) measured in temperature units, it is varied as
well when the sample topology is changed. We demonstrate that in all studied
submicron structures the shape of the T_{c}(H) phase boundary is determined by
the confinement topology in a unique way.


title:
Global and Local Measures of the Intrinsic Josephson Coupling in
  Tl2Ba2CuO6

abstract:
  The Intlerlayer Josephson coupling between the planes of Tl2Ba2CuO6 was
determined using infrared spectroscopy and magnetic flux vortex imaging. These
methods give a consistent value of $\omega_J$= 28 cm$^{-1}$ which, when
combined with the condensation energy produces a discrepancy of at least an
order of magnitude with deductions based on the interlayer tunneling model.


title:
Reply to a Comment on "Quantum Decoherence in Disordered Mesoscopic
  Systems" (cond-mat/9808078, cond-mat/9808053)

abstract:
  We reply to the critique of our results raised by Aleiner, Altshuler and
Gershenzon (AAG) in cond-mat/9808078 and cond-mat/9808053. We demonstrate that
our path integral analysis fully reproduces the results of AAG if analyzed on a
perturbative level. This should settle the issue of "missing diagrams" in our
calculation. We, furthermore, demonstrate that our approach allows us to
proceed beyond the Golden-rule-type perturbation theory employed by AAG. This
explains the difference between our and AAG's results. We also comment on the
comparison of our results with the experiments presented by AAG.


title:
Energy level statistics at the metal-insulator transition in the
  Anderson model of localization with anisotropic hopping

abstract:
  Recently, a metal-insulator transition (MIT) was found in the anisotropic
Anderson model of localization by transfer-matrix methods (TMM). This MIT has
been also investigated by multifractal analysis (MFA) and the same critical
disorders $W_c$ have been obtained within the accuracy of the data. We now
employ energy level statistics (ELS) to further characterize the MIT. We find a
crossover of the nearest-neighbor level spacing distribution $P(s)$ from GOE
statistics at small disorder indicating metallic behavior to the Poisson
distribution at large disorder characteristic for localized states. An analysis
of the system size dependence of the spectral rigidity $\Delta_3(L)$ confirms
the values of $W_c$ from TMM and MFA.


title:
Lower critical field H_c1 and barriers for vortex entry in
  Bi_2Sr_2CaCu_2O_{8+delta} crystals

abstract:
  The penetration field H_p of Bi_2Sr_2CaCu_2O_{8+delta} crystals is determined
from magnetization curves for different field sweep rates dH/dt and
temperatures. The obtained results are consistent with theoretical reports in
the literature about vortex creep over surface and geometrical barriers. The
frequently observed low-temperature upturn of H_p is shown to be related to
metastable configurations due to barriers for vortex entry. Data of the true
lower critical field H_c1 are presented. The low-temperature dependence of H_c1
is consistent with a superconducting state with nodes in the gap function.
[PACS numbers: 74.25.Bt, 74.60.Ec, 74.60.Ge, 74.72.Hs]


title:
Comment on "Mesoscopic Superconducting-Normal Metal-Superconducting
  Transistor"

abstract: Comment withdrawn. To be superseded by a subseqent paper.


title:
Critical Properties in Photoemmision Spectra for One Dimensional
  Orbitally Degenerate Mott Insulator

abstract:
  Critical properties in photoemission spectra for the one-dimensional Mott
insulator with orbital degeneracy are studied by exploiting the integrable {\it
t-J} model, which is a supersymmetric generalization of the SU($n$) degenerate
spin model. We discuss the critical properties for the holon dispersion as well
as the spinon dispersions, by applying the conformal field theory analysis to
the exact finite-size energy spectrum. We study the effect of orbital-splitting
on the spectra by evaluating the momentum-dependent critical exponents.


title:
Ginzburg-Landau Theory for a p-Wave Sr_2RuO_4 Superconductor: Vortex
  Core Structure and Extended London Theory

abstract:
  Based on a two dimensional odd-parity superconducting order parameter for
Sr_2RuO_4 with p-wave symmetry, we investigate the single vortex and vortex
lattice structure of the mixed phase near H_{c1}. Ginzburg-Landau calculations
for a single vortex show a fourfold structure with an orientation depending on
the microscopic Fermi surface properties. The corresponding extended London
theory is developed to determine the vortex lattice structure and we find near
H_{c1} a centered rectangular vortex lattice. As the field is increased from
H_{c1} this lattice continuously deforms until a square vortex lattice is
achieved. In the centered rectangular phase the field distribution, as
measurable through \mu-SR experiments, exhibits a characteristic two peak
structure (similar to that predicted in high temperature and borocarbide
superconductors).


title: Spin polarons in triangular antiferromagnets

abstract:
  The motion of a single hole in a 2D triangular antiferromagnet is
investigated using the t-J model. The one-hole states are described by strings
of spin deviations around the hole. Using projection technique the one-hole
spectral function is calculated. For large J/t we find low-lying
quasiparticle-like bands which are well separated from an incoherent background
by a gap of order J. However, for small J/t this gap vanishes and the spectrum
becomes broad over an energy range of several t. The results are compared with
SCBA calculations and numerical data.


title:
Interplay of the CE-type charge ordering and the A-type spin ordering in
  a half-doped bilayer manganite La{1}Sr{2}Mn{2}O{7}

abstract:
  We demonstrate that the half-doped bilayer manganite La_{1}Sr_{2}Mn_{2}O_{7}
exhibits CE-type charge-ordered and spin-ordered states below $T_{N, CO}^A =
210$ K and below $T_{N}^{CE} \sim 145$ K, respectively. However, the volume
fraction of the CE-type ordering is relatively small, and the system is
dominated by the A-type spin ordering. The coexistence of the two types of
ordering is essential to understand its transport properties, and we argue that
it can be viewed as an effective phase separation between the metallic
$d(x^{2}-y^{2})$ orbital ordering and the charge-localized
$d(3x^{2}-r^{2})/d(3y^{2}-r^{2})$ orbital ordering.


title:
Interstitials, Vacancies and Dislocations in Flux-Line Lattices: A
  Theory of Vortex Crystals, Supersolids and Liquids

abstract:
  We study a three dimensional Abrikosov vortex lattice in the presence of an
equilibrium concentration of vacancy, interstitial and dislocation loops.
Vacancies and interstitials renormalize the long-wavelength bulk and tilt
elastic moduli. Dislocation loops lead to the vanishing of the long-wavelength
shear modulus. The coupling to vacancies and interstitials - which are always
present in the liquid state - allows dislocations to relax stresses by climbing
out of their glide plane. Surprisingly, this mechanism does not yield any
further independent renormalization of the tilt and compressional moduli at
long wavelengths. The long wavelength properties of the resulting state are
formally identical to that of the ``flux-line hexatic'' that is a candidate
``normal'' hexatically ordered vortex liquid state.


title: Mesoscopic Josephson Effect

abstract:
  In the classical Josephson effect the phase difference across the junction is
well defined, and the supercurrent is reduced only weakly by phase diffusion.
For mesoscopic junctions with small capacitance the phase undergoes large
quantum fluctuations, and the current is also decreased by Coulomb blockade
effects. We discuss the behavior of the current-voltage characteristics in a
large range of parameters comprising the phase diffusion regime with coherent
Josephson current as well as the supercurrent peak due to incoherent Cooper
pair tunneling in the Coulomb blockade regime.


title: Multiple Andreev reflections as a transport problem in energy space

abstract:
  We present an approach for analyzing the dc current in voltage biased quantum
superconducting junctions. By separating terms from different $n$-particle
processes, we find that the $n$-particle current can be mapped on the problem
of wave transport through a potential structure with $n$ barriers. We discuss
the relation between resonances in such structures and the subgap structures in
the current-voltage characteristics. At zero temperature we find, exactly, that
only processes creating real excitations contribute to the current. Our results
are valid for a general SXS-junction, where the X-region is an arbitrary
non-superconducting region described by an energy-dependent transfer matrix.


title: Persistence in systems with algebraic interaction

abstract:
  Persistence in coarsening 1D spin systems with a power law interaction
$r^{-1-\sigma}$ is considered. Numerical studies indicate that for sufficiently
large values of the interaction exponent $\sigma$ ($\sigma\geq 1/2$ in our
simulations), persistence decays as an algebraic function of the length scale
$L$, $P(L)\sim L^{-\theta}$. The Persistence exponent $\theta$ is found to be
independent on the force exponent $\sigma$ and close to its value for the
extremal ($\sigma \to \infty$) model, $\bar\theta=0.17507588...$. For smaller
values of the force exponent ($\sigma< 1/2$), finite size effects prevent the
system from reaching the asymptotic regime. Scaling arguments suggest that in
order to avoid significant boundary effects for small $\sigma$, the system size
should grow as ${[{\cal O}(1/\sigma)]}^{1/\sigma}$.


title: Gauge Physics of Finance: simple introduction

abstract:
  In this paper we state the fundamental principles of the gauge approach to
financial economics and demonstrate the ways of its application. In particular,
modelling of realistic price processes is considered for an example of S&P500
market index. Derivative pricing and portfolio theory are also briefly
discussed.

categories: cond-mat.stat-mech adap-org nlin.AO physics.soc-ph q-fin.PR quant-ph


title: Electro-Mechanical Fredericks Effects in Nematic Gels

abstract:
  The solid nematic equivalent of the Fredericks transition is found to depend
on a critical field rather than a critical voltage as in the classical case.
This arises because director anchoring is principally to the solid rubbery
matrix of the nematic gel rather than to the sample surfaces. Moreover, above
the threshold field, we find a competition between quartic (soft) and
conventional harmonic elasticity which dictates the director response. By
including a small degree of initial director misorientation, the calculated
field variation of optical anisotropy agrees well with the conoscopy
measurements of Chang et al (Phys.Rev.E56, 595, 1997) of the electro-optical
response of nematic gels.


title: Theoretical Atomic Volumes of fcc Thorium and Plutonium

abstract:
  The zero-presssure zero-temperature equilibrium volume and bulk modulus are
calculated for fcc Th and Pu by two independent all-electron, full-potential,
electronic-structure methods: the full-potential linear augmented-plane-wave
(FLAPW) method and the linear combinations of Gaussian type orbitals-fitting
function (LCGTO-FF) method. The results produced by these two distinctly
different electronic-structure techniques are in excellent agreement with each
other, but differ significantly from a previous calculation using the
full-potential linear muffin-tin-orbital (FP-LMTO) method. The theoretically
calculated equilibrium volumes are nearly 10% larger than the previous FP-LMTO
calculations. While the new theoretical volume for Th is in good agreement with
experiment when the generalized gradient approximation is used, the calculated
equilibrium volume for fcc Pu remains substantially smaller than experiment in
qualitative agreement with previous calculations.


title:
Mesoscopic superconductors under irradiation: Microwave spectroscopy of
  Andreev states

abstract:
  We show that irradiation of a voltage-biased superconducting quantum point
contact at frequencies of the order of the gap energy can remove the
suppression of subgap dc transport through Andreev levels. Quantum interference
among resonant scattering events involving photon absorption is furthermore
shown to make microwave spectroscopy of the Andreev levels feasible. We also
discuss how the same interference effect can be applied for detecting weak
electromagnetic signals up to the gap frequency, and how it is affected by
dephasing and relaxation.


title:
Dynamical transitions and sliding friction in the two-dimensional
  Frenkel-Kontorova model

abstract:
  The nonlinear response of an adsorbed layer on a periodic substrate to an
external force is studied via a two dimensional uniaxial Frenkel-Kontorova
model. The nonequlibrium properties of the model are simulated by Brownian
molecular dynamics. Dynamical phase transitions between pinned solid, sliding
commensurate and incommensurate solids and hysteresis effects are found that
are qualitatively similar to the results for a Lennard-Jones model, thus
demonstrating the universal nature of these features.


title:
Properties of layer-by-layer vector stochastic models of force
  fluctuations in granular materials

abstract:
  We attempt to describe the stress distributions of granular packings using
lattice-based layer-by-layer stochastic models that satisfy the constraints of
force and torque balance and non-tensile forces at each site. The inherent
asymmetry in the layer-by-layer approach appears to lead to an asymmetric force
distribution, in disagreement with both experiments and general symmetry
considerations. The vertical force component probability distribution is robust
and in agreement with predictions of the scalar q model while the distribution
of horizontal force components is qualitatively different and depends on the
details of implementation.


title:
Critical Behavior for 2d Uniform and Disordered Ferromagnets at
  Self-Dual Points

abstract:
  We consider certain two-dimensional systems with self--dual points including
uniform and disordered $q$-state Potts models. For systems with continuous
energy density (such as the disordered versions) it is established that the
self--dual point exhibits critical behavior: Infinite susceptibility, vanishing
magnetization and power law bounds for the decay of correlations.

categories: cond-mat.stat-mech cond-mat.dis-nn math-ph math.MP


title:
Inter- and Intra-Chain Attractions in Solutions of Flexible
  Polyelectrolytes at Nonzero Concentration

abstract:
  Constant temperature molecular dynamics simulations were used to study
solutions of flexible polyelectrolyte chains at nonzero concentrations with
explicit counterions and unscreened coulombic interactions. Counterion
condensation, measured via the self-diffusion coefficient of the counterions,
is found to increase with polymer concentration, but contrary to the prediction
of Manning theory, the renormalized charge fraction on the chains decreases
with increasing Bjerrum length without showing any saturation. Scaling analysis
of the radius of gyration shows that the chains are extended at low polymer
concentrations and small Bjerrum lengths, while at sufficiently large Bjerrum
lengths, the chains shrink to produce compact structures with exponents smaller
than a gaussian chain, suggesting the presence of attractive intrachain
interactions. A careful study of the radial distribution function of the
center-of-mass of the polyelectrolyte chains shows clear evidence that
effective interchain attractive interactions also exist in solutions of
flexible polyelectrolytes, similar to what has been found for rodlike
polyelectrolytes. Our results suggest that the broad maximum observed in
scattering experiments is due to clustering of chains.


title: Observation of Geometry-Limited Mobility in Porous Silicon

abstract:
  We report photocarrier time-of-flight measurements in diode structures made
of highly porous crystalline silicon. The corresponding electron and hole drift
mobilities are very small compared to homogeneous crystalline silicon. They
show two additional signatures of semiclassical transport that are directly
limited by a porous geometry: the mobilities are dispersive (i.e. dependent on
time), and have very little temperature-dependence. A pure, geometry-limited
transport is surprising in the context of nanometer-scale models for electronic
states such as quantum percolation or nanocrystal aggregation. In such models,
confinement-induced energy shifts and localization are the dominant effects.


title:
Magnetic irreversibility and relaxation in assembly of ferromagnetic
  nanoparticles

abstract:
  Measurements of the magnetic irreversibility line and time-logarithmic decay
of the magnetization are described for three $Fe_{2}O_{3}$ samples composed of
regular amorphous, acicular amorphous and crystalline nanoparticles. The
relaxation rate is the largest and the irreversibility temperature is the
lowest for the regular amorphous nanoparticles. The crystalline material
exhibits the lowest relaxation rate and the largest irreversibility
temperature. We develop a phenomenological model to explain the details of the
experimental results. The main new aspect of the model is the dependence of the
barrier for magnetic relaxation on the instantaneous magnetization and
therefore on time. The time dependent barrier yields a natural explanation to
the time-logarithmic decay of the magnetization. Interactions between particles
as well as shape and crystalline magnetic anisotropies define a new energy
scale that controls the magnetic irreversibility. Introducing this energy scale
yields a self-consistent explanation of the experimental data.


title:
Phase Fluctuations and Single Fermion Spectral Density in 2D Systems
  with Attraction

abstract:
  The effect of static fluctuations in the phase of the order parameter on the
normal and superconducting properties of a 2D system with attractive
four-fermion interaction is studied. Analytic expressions for the fermion
Green's function, its spectral density, and the density of states are derived
in the approximation where the coupling between the spin and charge degrees of
freedom is neglected. The resulting single-particle Green's function clearly
demonstrates a non-Fermi liquid behavior. The results show that as the
temperature increases through the 2D critical temperature, the width of the
quasiparticle peaks broadens significantly.


title: Spatio-selection in Expanding Bacterial Colonies

abstract:
  Segregation of populations is a key question in evolution theory. One
important aspect is the relation between spatial organization and the
population's composition. Here we study a specific example -- sectors in
expanding bacterial colonies. Such sectors are spatially segregated
sub-populations of mutants. The sectors can be seen both in disk-shaped
colonies and in branching colonies. We study the sectors using two models we
have used in the past to study bacterial colonies -- a continuous
reaction-diffusion model with non-linear diffusion and a discrete
``Communicating Walkers'' model. We find that in expanding colonies, and
especially in branching colonies, segregation processes are more likely than in
a spatially static population. One such process is the establishment of stable
sub- population having neutral mutation. Another example is the maintenance of
wild-type population along side with sub-population of advantageous mutants.
Understanding such processes in bacterial colonies is an important subject by
itself, as well as a model system for similar processes in other spreading
populations.

categories: cond-mat comp-gas nlin.CG nlin.PS patt-sol physics.bio-ph q-bio.PE


title:
The Kinetic Theory of Dilute Solutions of Flexible Polymers:
  Hydrodynamic Interaction

abstract:
  The development of a coherent conceptual basis for the treatment of
non-linear microscopic phenomena, such as, hydrodynamic interaction, finite
extensibility, excluded volume and internal viscosity, in molecular theories of
dilute polymer solutions, is discussed. In particular, recent advances in the
treatment of hydrodynamic interaction are reviewed, and the successive
refinements which have ultimately led to the prediction of universal
viscometric functions in theta solvents are highlighted.


title:
Non-universal dynamics of staggered non-equilibrium particle systems and
  Ising chains

abstract:
  Non-universal dynamics is shown to occur in a one-dimensional non-equilibrium
system of hard-core particles. The stochastic processes included are pair
creation and annihilation (with rates e and e') and symmetric hopping rates
which alternate from one bond to the next (Pa, Pb). A dynamical scaling
relation between the relaxation time and the correlation length in the steady
state is derived in a simple way for the case e' > Pa >> Pb >> e. We find that
the dynamical exponent takes the non-universal value z = 2 ln(e'/e)/ln[(Pb
e')/(Pa e)]. For the special condition e + e'= Pa + Pb, where the stochastic
system is in principle soluble by reduction to a free fermion system, the model
is mapped to the Glauber dynamics of an Ising chain with alternating
ferromagnetic bonds of values J1 and J2, in contact with a quantum thermal
bath. The full time dependence of the space-dependent magnetization and of the
equal time spin-spin correlation function are studied by writing the master
equation for this system in the quantum Hamiltonian formalism. In particular we
obtain the dispersion relations and rigorously confirm the results obtained for
the correlation length and for the dynamical exponent.


title:
A nonlinear evolution equation for sand ripples based on geometry and
  conservation

abstract:
  From geometry and conservation we derive two nonlinear evolution equations
for sand ripples. In the case of a strong wind leading to a net erosion of the
sand bed, ripples obey the Benney equation. This leads either to order or
disorder depending on whether dispersion is strong or weak. In the most
frequent case where erosion is counterbalanced by deposition, we derive a new
one-parameter nonlinear equation. It reveals ripple structures which then
undergo a coarsening process at long times, a process which then slows down
dramatically with the growth of the ripple wavelength.


title: Resonant tunneling through Andreev levels

abstract:
  We use a semiclassical approach for analysing the tunneling transport through
a normal conductor in contact with superconducting mirrors. Our analysis of the
electron-hole propagation along semiclassical trajectories shows that resonant
transmission through Andreev levels is possible resulting in an excess,
low-energy quasiparticle contribution to the conductance. The excess
conductance oscillates with the phase difference between the superconductors
having maxima at odd multiples of $\pi$ for temperatures much below the
Thouless temperature


title:
Metal-Insulator Transition in a Generalized Hubbard Model with
  Correlated Hopping at Half-Filling

abstract:
  In the present paper metal-insulator transition is studied in a generalized
Hubbard model with correlated hopping at half-filling and zero temperature.
Single-particle Green function and energy spectrum of electron system are
calculated. The expressions for energy gap width and the concentration of polar
states (holes or doublons) are obtained. The conditions for metallic and
insulating states are found.


title: The Elasticity of an Electron Liquid

abstract:
  The zero temperature response of an interacting electron liquid to a
time-dependent vector potential of wave vector q and frequency w such that q <<
q_F, qv_F<< w << E_F/hbar (where q_F, v_F and E_F are the Fermi wavevector,
velocity and energy respectively) is equivalent to that of a continuous elastic
medium with nonvanishing shear modulus mu, bulk modulus K, and viscosity
coefficients eta and zeta. We establish the relationship between the
visco-elastic coefficients and the long-wavelength limit of the "dynamical
local field factors" G_{L(T)}(q,w), which are widely used to describe
exchange-correlation effects in electron liquids. We present several exact
results for mu, including its expression in terms of Landau parameters, and
practical approximate formulas for mu, eta and zeta as functions of density.
These are used to discuss the possibility of a transverse collective mode in
the electron liquid at sufficiently low density. Finally, we consider impurity
scattering and/or quasiparticle collisions at non zero temperature. Treating
these effects in the relaxation time (tau) approximation, explicit expressions
are derived for mu and eta as functions of frequency. These formulas exhibit a
crossover from the collisional regime (w tau << 1), where mu \sim 0 and eta
\sim n E_F tau, to the collisionless regime (w tau >> 1), where mu \sim nE_F
and eta \sim 0.


title:
High frequency vortex dynamics and magnetoconductivity of high
  temperature superconductors

abstract:
  The vortex lattice with the superconducting and normal state charge carriers
fractions may be regarded as three independent subsystems mutually connected by
interactions. The equation of motion for these three subsystems must be solved
simultaneously. In this way a new consistent theory of the vortex dynamics is
obtained and the magnetoconductivity calculated.


title:
Anomalous transport in normal-superconducting and
  ferromagnetic-superconducting nanostructures

abstract:
  We have calculated the temperature dependence of the conductance variation
($\delta S(T)$) of mesoscopic superconductor normal metal(S/N) structures, in
the diffusive regime, analysing both weak and strong proximity effects. We show
that in the case of a weak proximity effect there are two peaks in the
dependence of $\delta S(T)$ on temperature. One of them (known from previous
studies) corresponds to a temperature $T_1$ of order of the Thouless energy
($\epsilon_{Th}$), and another, newly predicted maximum, occurs at a
temperature $T_2$ where the energy gap in the superconductor $\Delta(T_2)$ is
of order $\epsilon_{Th}$. In the limit $L_{\phi}<L$ the temperature $T_1$ is
determined by $D \hbar /L^2_{\phi}$ ($L_{\phi}$ is the phase breaking length),
and not $\epsilon_{Th}$. We have also calculated the voltage dependence $
\delta S(V)$ for a S/F structure (F is a ferromagnet) and predict non-monotonic
behaviour at voltages of order the Zeeman splitting.


title: Properties of superconductor - Luttinger liquid hybrid systems

abstract:
  In this paper we review some recent results concerning the physics of
superconductor - Luttinger liquid proximity systems. We discuss both
equilibrium (the pair amplitude, Josephson current, and the local density of
states) and nonequilibrium (the subgap current) properties.


title:
Anomalous Frequency-Dependent Conductivity near the Quantum Hall
  Transition

abstract:
  The dynamical transport properties near the integer quantum Hall transition
are investigated at zero temperature by means of the Dirac fermion approach.
These properties have been studied experimentally at low frequency omega and
low temperature near the nu=1 filling factor Hall transition, with the
observation of an anusual broadening and an overall increase of the
longitudinal conductivity Re sigma_{xx} as a function of omega. We find in our
approach that, unlike for normal metals, the longitudinal conductivity
increases as the frequency increases, whilst the width Delta B (or Delta nu) of
the conductivity peak near the Hall transition increases. These findings are in
reasonable quantitative agreement with recent experiments by Engel et al. as
well as with recent numerical work by Avishai and Luck.


title:
Decay of the Two-Point Function in One-Dimensional O(N) Spin Models with
  Long Range Interactions

abstract:
  Using Griffiths and Lieb-Simon type inequalities, it is shown that the
two-point function of ferromagnetic spin models with N components in one
dimension decays like the interaction provided that N=1,2,3,4 and T > T_c.


title:
A Gallavotti-Cohen Type Symmetry in the Large Deviation Functional for
  Stochastic Dynamics

abstract:
  We extend the work of Kurchan on the Gallavotti-Cohen fluctuation theorem,
which yields a symmetry property of the large deviation function, to general
Markov processes. These include jump processes describing the evolution of
stochastic lattice gases driven in the bulk or through particle reservoirs,
general diffusive processes in physical and/or velocity space, as well as
Hamiltonian systems with stochastic boundary conditions. For dynamics
satisfying local detailed balance we establish a link between the action
functional of the fluctuation theorem and the entropy production. This gives,
in the linear regime, an alternative derivation of the Green-Kubo formula and
the Onsager reciprocity relations. In the nonlinear regime consequences of the
new symmetry are harder to come by and the large derivation functional
difficult to compute. For the asymmetric simple exclusion process the latter is
determined explicitly using the Bethe ansatz in the limit of large N.


title: Metal-Insulator Transition in Mn Perovskite Compounds

abstract:
  We discuss Mott insulating and metallic phases of a model with $e_g$ orbital
degeneracy to understand physics of Mn perovskite compounds. Quantum Monte
Carlo and Lanczos diagonalization results are discussed in this model. To
reproduce experimental results on charge gap and Jahn-Teller distortions, we
show that a synergy between the strong correlation effects and the Jahn-Teller
coupling is important. The incoherent charge dynamics and strong charge
fluctuations are characteristic of the metallic phase accompanied with critical
enhancement of short-ranged orbital correlation near the insulator.


title: Hall voltage sign reversal in type II superconductors

abstract:
  The Hall voltage sign reversal is consistently explained by the model in
which vortices with the superconducting and normal state charge carriers are
regarded as three subsystems mutually connected by interactions. The equations
of motion for these three subsystems are solved simultaneously and a new
formula for the Hall resistivity is obtained. It is shown that it is possible
to explain qualitatively experimental data by this model.


title: Dynamical Anomalies and Intermittency in Burgers Turbulence

abstract:
  We analyze the field theory of fully developed Burgers turbulence. Its key
elements are shock fields, which characterize the singularity statistics of the
velocity field. The shock fields enter an operator product expansion describing
intermittency. The latter is found to be constrained by dynamical anomalies
expressing finite dissipation in the inviscid limit. The link between dynamical
anomalies and intermittency is argued to be important in a wider context of
turbulence.


title: Spin relaxation in quantum Hall systems

abstract:
  We study the spin relaxation in an interacting two--dimensional electron gas
in a strong magnetic field for the case that the electron density is close to
filling just one Landau sub--level of one spin projection, i.e., for filling
factor near one. Assuming the relaxation to be caused by scattering with
phonons, we derive the kinetic equations for the electron's spin--density which
replace the Bloch equations in our case. These equations are non--linear and
their solution depends crucially on the filling factor and on the temperature
of the phonon bath. In the limit of zero temperature and for filling factor 1,
the solution relaxes asymptotically with a power law inversely proportional to
time, instead of following the conventional exponential behavior.


title: Optimizing Smith-Waterman alignments

abstract:
  Mutual correlation between segments of DNA or protein sequences can be
detected by Smith-Waterman local alignments. We present a statistical analysis
of alignment of such sequences, based on a recent scaling theory. A new
fidelity measure is introduced and shown to capture the significance of the
local alignment, i.e., the extent to which the correlated subsequences are
correctly identified. It is demonstrated how the fidelity may be optimized in
the space of penalty parameters using only the alignment score data of a single
sequence pair.


title: Superconductivity in a magnetically ordered background

abstract:
  Borocarbide compounds with the formula RNi2B2C show interesting
superconducting and magnetic properties and the coexistence of the two
phenomena. BCS theory is extended to systems with underlying commensurate
magnetic order. In the case of helical phases the technique may be extended to
any Q-vector and there exists a well defined limit for incommensurate values.
The way magnetic order influences superconductivity depends crucially on the
details of both the magnetic structure and the electron bands, but some
qualitative criteria may be given. As an example we give a brief analysis of
the compound HoNi2B2C.


title: Crowd effects and volatility in a competitive market

abstract:
  We present analytic and numerical results for two models, namely the minority
model and the bar-attendance model, which offer simple paradigms for a
competitive marketplace. Both models feature heterogeneous agents with bounded
rationality who act using inductive reasoning. We find that the effects of
crowding are crucial to the understanding of the macroscopic fluctuations, or
`volatility', in the resulting dynamics of these systems.


title: Monolayer Solid Helium-4 Clusters on Graphite

abstract:
  In order to resolve the controversy about the low density region of the phase
diagram of the 4He monolayer on graphite, we have undertaken a path integral
Monte Carlo study of the system. We provide direct evidence that the low
density monolayer possesses solid clusters and a low density vapor as opposed
to the most recent proposal that the system is in a superfluid phase. We
further establish that the rounded heat capacity peaks observed at low
densities are caused by melting of such solid clusters and are not associated
with the suggested superfluid transition.


title:
Director Field Configurations around a Spherical Particle in a Nematic
  Liquid Crystal

abstract:
  We study the director field around a spherical particle immersed in a
uniformly aligned nematic liquid crystal and assume that the molecules prefer a
homeotropic orientation at the surface of the particle. Three structures are
possible: a dipole, a Saturn-ring, and a surface-ring configuration, which we
investigate by numerically minimizing the Frank free energy supplemented by a
magnetic-field and a surface term. In the dipole configuration, which is the
absolutely stable structure for micron-size particles and sufficiently strong
surface anchoring, a twist transition is found and analyzed. We show that a
transition from the dipole to the Saturn ring configuration is induced by
either decreasing the particle size or by applying a magnetic field. The effect
of metastability and the occurence of hysteresis in connection with a magnetic
field are discussed. The surface-ring configuration appears when the
surface-anchoring strength W is reduced. It is also favored by a large
saddle-splay constant K_24. A comparison with recent experiments by Poulin et
al. gives a lower bound for W, i.e., W > 0.6 erg/cm^2 for the interface of
water and pentylcyanobiphenyl (5CB) in the presence of the surfactant sodium
dodecyl sulfate.


title: Boson-Fermion Model of High-T_C Superconductivity ----A Progress Report

abstract:
  This is an invited lecture at the XXII International School of Theoretical
Physics, Ustron, Poland, 9/10-9/15, 1998. The boson-fermion model of high T_C
superconductivity is reviewed. Its applications to the pseudo-gap and DC
transport coefficients are discussed.


title: On-Line Learning with Restricted Training Sets: An Exactly Solvable Case

abstract:
  We solve the dynamics of on-line Hebbian learning in large perceptrons
exactly, for the regime where the size of the training set scales linearly with
the number of inputs. We consider both noiseless and noisy teachers. Our
calculation cannot be extended to non-Hebbian rules, but the solution provides
a convenient and welcome benchmark with which to test more general and advanced
theories for solving the dynamics of learning with restricted training sets.


title: Quantum squeezing micron-sized cantilevers

abstract:
  We show that substantial quantum squeezing of mechanical motion can be
achieved for micron-sized cantilever devices fabricated using available
techniques. A method is also described for measuring the cantilever fluctuation
magnitudes in the squeezing regime.


title: Finite-Temperature Phase Diagram of the Hubbard Model

abstract:
  The finite-temperature phase diagram of the Hubbard model in $d=3$ is
obtained from renormalization-group analysis. It exhibits, around half filling,
an antiferromagnetic phase and, between 30%--40% electron or hole doping from
half filling, a new $\tau $ phase in which the electron hopping strength $t$
asymptotically becomes infinite under repeated rescalings. Next to the $\tau $
phase, a first-order phase boundary with very narrow phase separation (less
than 2% jump in electron density) occurs. At temperatures above the $\tau $
phase, an incommensurate spin modulation phase is indicated. In $d=2$, we find
that the Hubbard model has no phase transition at finite temperature.


title:
Anti-Symmetrically Fused Model and Non-Linear Integral Equations in the
  Three-State Uimin-Sutherland Model

abstract:
  We derive the non-linear integral equations determining the free energy of
the three-state pure bosonic Uimin-Sutherland model. In order to find a
complete set of auxiliary functions, the anti-symmetric fusion procedure is
utilized. We solve the non-linear integral equations numerically and see that
the low-temperature behavior coincides with that predicted by conformal field
theory. The magnetization and magnetic susceptibility are also calculated by
means of the non-linear integral equation.


title: Strongly Asymmetric Tricriticality of Quenched Random-Field Systems

abstract:
  In view of the recently seen dramatic effect of quenched random bonds on
tricritical systems, we have conducted a renormalization-group study on the
effect of quenched random fields on the tricritical phase diagram of the spin-1
Ising model in $d=3$. We find that random fields convert first-order phase
transitions into second-order, in fact more effectively than random bonds. The
coexistence region is extremely flat, attesting to an unusually small
tricritical exponent $\beta_u$; moreover, an extreme asymmetry of the phase
diagram is very striking. To accomodate this asymmetry, the second-order
boundary exhibits reentrance.


title: Dual Order Parameter for the Nodal Liquid

abstract:
  The guiding conception of vortex-condensation-driven Mott insulating behavior
is central to the theory of the nodal liquid. We amplify our earlier
description of this idea and show how vortex condensation in 2D electronic
systems is a natural extension of 1D Mott insulating and 2D bosonic Mott
insulating behavior. For vortices in an underlying superconducting pair field,
there is an important distinction between the condensation of flux hc/2e and
flux hc/e vortices. The former case leads to spin-charge confinement,
exemplified by the band insulator and the charge-density-wave. In the latter
case, spin and charge are liberated leading directly to a 2D Mott insulator
exhibiting *spin-charge separation*. Possible upshots include not only the
nodal liquid, but also a novel undoped antiferromagnetic insulating phase with
gapped excitations exhibiting spin-charge separation.


title: Binary data corruption due to a Brownian agent

abstract:
  We introduce a model of binary data corruption induced by a Brownian agent
(active random walker) on a d-dimensional lattice. A continuum formulation
allows the exact calculation of several quantities related to the density of
corrupted bits \rho; for example the mean of \rho, and the density-density
correlation function. Excellent agreement is found with the results from
numerical simulations. We also calculate the probability distribution of \rho
in d=1, which is found to be log-normal, indicating that the system is governed
by extreme fluctuations.


title:
Corrections to the RPA Dielectric Function Induced by Fluctuations in
  the Momentum Distribution

abstract:
  In this article, we introduce a new dielectric function that is different
even from the generalised RPA, first introduced in cond-mat/9810043, in that it
pays attention to fluctuations in the momentum distribution. We argued that
this is important when the momentum distribution is very nonideal. This new
dielectric function reduces to the generalised RPA when the number-fluctuations
are small, and the generalised RPA in turn reduces to the traditional RPA
dielectric function when the momentum distribution is close to ideal.


title:
Effects of magnetic field and disorder on electronic properties of
  Carbon Nanotubes

abstract:
  Electronic properties of metallic and semiconducting carbon nanotubes are
investigated in presence of magnetic field perpendicular to the CN-axis, and
disorder introduced through energy site randomness. The magnetic field field is
shown to induce a metal-insulator transition (MIT) in absence of disorder, and
surprisingly disorder does not affect significantly the MIT. These results may
find confirmation through tunneling experiments


title: Disorder-induced neutral solitons in degenerate ground state polymers

abstract:
  In this letter, we study the effects of weak off-diagonal disorder on
conjugated polymers with a doubly degenerate ground-state. We find that
disorder induces a finite density of neutral solitons in the lattice
dimerization of a polymer chain. Interchain interactions result in a linear
potential between the solitons and, if sufficiently strong, bind them into
pairs resulting in an exponential suppression of the soliton density. As
neutral solitons carry spin 1/2, they contribute to the polymer's magnetic
properties. We calculate the magnetic susceptibility and suggest measurements
of the magnetic susceptibility in {\it trans}-polyacetylene at low
temperatures.


title: Highly site-specific H2 adsorption on vicinal Si(001) surfaces

abstract:
  Experimental and theoretical results for the dissociative adsorption of H_2
on vicinal Si(001) surfaces are presented. Using optical second-harmonic
generation, sticking probabilities at the step sites are found to exceed those
on the terraces by up to six orders of magnitude. Density functional theory
calculations indicate the presence of direct adsorption pathways for
monohydride formation but with a dramatically lowered barrier for step
adsorption due to an efficient rehybridization of dangling orbitals.


title:
Density-functional theory calculations of hopping rates of surface
  diffusion

abstract:
  Using density-functional theory we compute the energy barriers and attempt
frequencies for surface diffusion of Ag on Ag(111) with different lattice
constants, and on an Ag adsorbate monolayer on Pt(111). We find that the
attempt frequency is of the order of 1 THz for all the systems studied. This is
in contrast to the so-called compensation effect, and to recent experimental
studies. Our analysis suggests that the applicability of simple (commonly used)
scaling laws for the determination of diffusion and growth parameters is often
not valid.


abstract:
  The beta function of the multichannel Kondo model is calculated exactly in
the limit of large spin N and channel number M=gamma*N, with constant gamma.
There are no corrections in any finite order of 1/N. One zero is found at a
finite coupling strength, showing directly the Non--Fermi liquid behavior of
the model. This renormalization group flow allows to introduce a variational
principle for the entropy, to obtain the low temperature thermodynamics. Such
in particular the low temperature thermodynamics of the non--crossing
approximation to the Kondo model becomes accessible.


title: A replica approach to the state-dependent synapses neural network

abstract:
  The replica method is applied to a neural network model with state-dependent
synapses built from those patterns having a correlation with the state of the
system greater than a certain threshold. Replica-symmetric and first-step
replica-symmetry-breaking results are presented for the storage capacity at
zero temperature as a function of this threshold value. A comparison is made
with existing results based upon mean-field equations obtained by using a
statistical method.


title:
Bose-Einstein statistics in thermalization and photoluminescence of
  quantum well excitons

abstract:
  Quasi-equilibrium relaxational thermodynamics is developed to understand
LA-phonon-assisted thermalization of Bose-Einstein distributed excitons in
quantum wells. We study the quantum-statistical effects in the relaxational
dynamics of the effective temperature of excitons $T = T(t)$. When $T$ is less
than the degeneracy temperature $T_0$, well-developed Bose-Einstein statistics
of quantum well excitons leads to nonexponential and density-dependent
thermalization. At low bath temperatures $T_b \to 0$ the thermalization of
quantum-statistically degenerate excitons effectively slows down and $T(t)
\propto 1 / \ln t$. We also analyze the optical decay of Bose-Einstein
distributed excitons in perfect quantum wells and show how nonclassical
statistics influences the effective lifetime $\tau_{opt}$. In particular,
$\tau_{opt}$ of a strongly degenerate gas of excitons is given by $2 \tau_R$,
where $\tau_R$ is the intrinsic radiative lifetime of quasi-two-dimensional
excitons. Kinetics of resonant photoluminescence of quantum well excitons
during their thermalization is studied within the thermodynamic approach and
taking into account Bose-Einstein statistics. We find density-dependent
photoluminescence dynamics of statistically degenerate excitons. Numerical
modeling of the thermalization and photoluminescence kinetics of
quasi-two-dimensional excitons are given for GaAs/AlGaAs quantum wells.


title:
Protein structures and optimal folding emerging from a geometrical
  variational principle

abstract:
  Novel numerical techniques, validated by an analysis of barnase and
chymotrypsin inhibitor, are used to elucidate the paramount role played by the
geometry of the protein backbone in steering the folding to the correct native
state. It is found that, irrespective of the sequence, the native state of a
protein has exceedingly large number of conformations with a given amount of
structural overlap compared to other compact artificial backbones; moreover the
conformational entropies of unrelated proteins of the same length are nearly
equal at any given stage of folding. These results are suggestive of an
extremality principle underlying protein evolution, which, in turn, is shown to
be associated with the emergence of secondary structures.


title: Hydrodynamics of bacterial motion

abstract:
  In this paper we present a hydrodynamic approach to describe the motion of
migrating bacteria as a special class of self-propelled systems. Analytical and
numerical calculations has been performed to study the behavior of our model in
the turbulent-like regime and to show that a phase transition occurs as a
function of noise strength. Our results can explain previous experimental
observations as well as results of numerical simulations.


title:
Numerical investigation of the influence of the history on the local
  structure of glasses

abstract:
  By means of molecular dynamics simulations and the Voronoi tessellation, we
study the influence of the history on the low temperature characteristics of
soft sphere and silica glasses. The quench from the liquid is interrupted at an
intermediate temperature $T_i$ for a given relaxation time, and then the
cooling process is continued down to 0K. The local structure at 0K depends on
the temperature $T_i$ and the effect is larger for $T_i$ close to the glass
transition temperature $T_g$. This dependence, coherent with recent results, is
expected in the strong glass former where the characteristics of a particular
state depend on its history. In the soft-sphere case, because of
crystallization effects, the dependence of the local structure of the glassy
samples on their history can only be detected in the supercooled liquid region.


title: Dual Quantum Monte Carlo Algorithm for Hardcore Bosons

abstract:
  We derive the exact dual representation of the bosonic Hubbard model which
takes the form of conserved current loops. The hardcore limit, which
corresponds to the quantum spin-${1\over 2}$ Heisenberg antiferromagnet, is
also obtained. In this limit, the dual partition function takes a particularly
simple form which is very amenable to numerical simulations. In addition to the
usual quantities that we can measure (energy, density-density correlation
function and superfluid density) we can with this new algorithm measure
efficiently the order parameter correlation function, $<a_i a_j^{\dagger}>,
|i-j|\ge 1$. We demonstrate this with numerical tests in one dimension.


title: The Anti-Jahn-Teller Polaron in LaMnO$_3$

abstract:
  Distortions of the oxygen sublattice couple to $e_g$ orbitals of Mn$^{3+}$
and drive a cooperative Jahn-Teller (orbital ordering) transition in LaMnO$_3$.
A simple model for this transition is studied. Without further adjustment, the
model predicts the shape and stability of small (anti-Jahn-Teller) polarons
which form when holes are doped into the material. This leads to a new
description of the lightly doped insulator, the anti-ferromagnetic to
ferromagnetic transition, and the metal-insulator transition.


title: Scaling of the specific heat of superfluids confined in pores

abstract:
  We investigate the scaling properties of the specific heat of the XY model on
lattices H x H x L with L >> H (i.e. in a bar-like geometry) with respect to
the thickness H of the bar, using the Cluster Monte Carlo method. We study the
effect of the geometry and boundary conditions on the shape of the universal
scaling function of the specific heat by comparing the scaling functions
obtained for cubic, film, and bar-like geometry. In the presence of physical
boundary conditions applied along the sides of the bars we find good agreement
between our Monte Carlo results and the most recent experimental data for
superfluid helium confined in pores.


title: The role of the energy gap in protein folding dynamics

abstract:
  The dynamics of folding of proteins is studied by means of a phenomenological
master equation. The energy distribution is taken as a truncated exponential
for the misfolded states plus a native state sitting below the continuum. The
influence of the gap on the folding dynamics is studied, for various models of
the transition probabilities between the different states of the protein. We
show that for certain models, the relaxation to the native state is accelerated
by increasing the gap, whereas for others it is slowed down .


title: A model of semimetallic behavior in strongly correlated electron systems

abstract:
  Metals with values of the resistivity and the Hall coefficient much larger
than typical ones, e.g., of sodium, are called semimetals. We suggest a model
for semimetals which takes into account the strong Coulomb repulsion of the
charge carriers, especially important in transition-metal and rare-earth
compounds. For that purpose we extend the Hubbard model by coupling one
additional orbital per site via hybridization to the Hubbard orbitals. We
calculate the spectral function, resistivity and Hall coefficient of the model
using dynamical mean-field theory. Starting from the Mott-insulating state, we
find a transition to a metal with increasing hybridization strength
(``self-doping''). In the metallic regime near the transition line to the
insulator the model shows semimetallic behavior. We compare the calculated
temperature dependence of the resistivity and the Hall coefficient with the one
found experimentally for $\rm Yb_4As_3$. The comparison demonstrates that the
anomalies in the transport properties of $\rm Yb_4As_3$ possibly can be
assigned to Coulomb interaction effects of the charge carriers not captured by
standard band structure calculations.


title: Cumulant expansion for systems with large spins

abstract:
  A method is proposed for obtaining a systematic expansion of thermodynamic
functions of spin systems with large spin S in powers of 1/S. It uses the
cumulant technique and a coherent-state representation of the partition
function Z. The expansion of Z in terms of cumulants yields an effective
classical Hamiltonian with temperature-dependent quantum corrections. For the
Heisenberg quantum Hamiltonian, they have a non-Heisenberg form. The effective
Hamiltonian can be solved by methods familiar for classical systems.


title:
Theory of Percolative Conduction in Polycrystaline High-temperature
  Superconductors

abstract:
  Conduction in bulk polycrystalline high-T$_c$ superconductors with relatively
high critical currents has been shown to be percolative. This phenomenon is due
to weak links at grain boundaries. These weak links are the major limiting
factor for technological applications which require high current densities. We
formulate a model of these materials which can be reduced to a nonlinear
resistor network. The model is solved by analytical approximations and a new
numerical technique. The numerical technique is variational, which makes it
capable of solving a wide variety of nonlinear problems. The results show that
an unambiguous connection can be made between the I-V characteristics at the
microscopic level and the macroscopic electrical properties.


title: Yoctocalorimetry: phonon counting in nanostructures

abstract:
  It appears feasible with nanostructures to perform calorimetry at the level
of individual thermal phonons. Here I outline an approach employing
monocrystalline mesoscopic insulators, which can now be patterned from
semiconductor heterostructures into complex geometries with full, three-
dimensional relief. Successive application of these techniques also enables
definition of integrated nanoscale thermal transducers; coupling these to a dc
SQUID readout yields the requisite energy sensitivity and temporal resolution
with minimal back action. The prospect of phonon counting opens intriguing
experimental possibilities with analogies in quantum optics. These include
fluctuation-based phonon spectroscopy, phonon shot noise in the energy
relaxation of nanoscale systems, and quantum statistical phenomena such as
phonon bunching and anticorrelated electron-phonon exchange.


title: Elastic Properties of Single-Wall Nanotubes

abstract:
  We report results of theoretical studies on the elastic properties of
single-wall nanotubes of the following compositions: C, BN, ${BC}_3$,
${BC}_2{N}$ and ${C}_3{N}_4$. These studies have been carried out using a total
energy, non-orthogonal tight-binding parametrisation which is shown to provide
results in good agreement both with calculations using higher levels of theory
and the available experimental data. Our results predict that of all types of
nanotubes considered, carbon nanotubes have the highest Young's modulus. We
have considered tubes of different diameters, ranging from 0.5 to 2~nm, and
find that in the limit of large diameters the mechanical properties of
nanotubes approach those of the corresponding flat graphene-like sheets.


title: Smoothed universal correlations in the two-dimensional Anderson model

abstract:
  We report on calculations of smoothed spectral correlations in the
two-dimensional Anderson model for weak disorder. As pointed out in (M.
Wilkinson, J. Phys. A: Math. Gen. 21, 1173 (1988)), an analysis of the
smoothing dependence of the correlation functions provides a sensitive means of
establishing consistency with random matrix theory. We use a semiclassical
approach to describe these fluctuations and offer a detailed comparison between
numerical and analytical calculations for an exhaustive set of two-point
correlation functions. We consider parametric correlation functions with an
external Aharonov-Bohm flux as a parameter and discuss two cases, namely broken
time-reversal invariance and partial breaking of time-reversal invariance.
Three types of correlation functions are considered: density-of-states,
velocity and matrix element correlation functions. For the values of smoothing
parameter close to the mean level spacing the semiclassical expressions and the
numerical results agree quite well in the whole range of the magnetic flux.


title:
Unconventional clustering of discrete energy levels in an ultrasmall Au
  grain

abstract: This papaer has been withdrawn


title: Quantum Mechanics with Random Imaginary Scalar Potential

abstract:
  We study spectral properties of a non-Hermitian Hamiltonian describing a
quantum particle propagating in a random imaginary scalar potential. Cast in
the form of an effective field theory, we obtain an analytical expression for
the ensemble averaged one-particle Green function from which we obtain the
density of complex eigenvalues. Based on the connection between non-Hermitian
quantum mechanics and the statistical mechanics of polymer chains, we determine
the distribution function of a self-interacting polymer in dimensions $d>4$.


title:
Stripes, Electron-Like and Polaron-Like Carriers, and High-T_c in the
  Cuprates

abstract:
  Both "large-U" and "small-U" orbitals are used to study the electronic
structure of the high-T_c cuprates. A striped structure with three types of
carriers are induced, polaron-like "stripons" which carry charge,
"quasielectrons" which carry both charge and spin, and "svivons" which carry
spin and lattice distortion. Anomalous physical properties of the cuprates are
derived, and specifically the systematic behavior of the resistivity, Hall
constant, and thermoelectric power. Transitions between pair states of
quasielectrons and stripons drive high-temperature superconductivity.


title: Frictional Mechanics of Wet Granular Material

abstract:
  The mechanical response of a wet granular layer to imposed shear is studied
experimentally at low applied normal stress. The granular material is immersed
in water and the shear is applied by sliding a plate resting on the upper
surface of the layer. We monitor simultaneously the horizontal and the vertical
displacements of the plate to submicron accuracy with millisecond time
resolution. The relations between the plate displacement, the dilation of the
layer and the measured frictional force are analyzed in detail. When slip
begins, the dilation increases exponentially over a slip distance comparable to
the particle radius. We find that the total dilation and the steady state
frictional force do not depend on the driving velocity, but do depend linearly
on the applied normal stress. The frictional force also depends linearly on the
dilation rate (rather than the dilation itself), and reaches a maximum value
during the transient acceleration. We find that the layer can temporarily
sustain a shear stress that is in excess of the critical value that will
eventually lead to slip. We describe an empirical model that describes much of
what we observe. This model differs in some respects from those used previously
at stresses 10^6 times larger.


title:
Bandgap Change of Carbon Nanotubes: Effect of Small Tensile and
  Torsional Strain

abstract:
  We use a simple picture based on the $\pi$ electron approximation to study
the bandgap variation of carbon nanotubes with uniaxial and torsional strain.
We find (i) that the magnitude of slope of bandgap versus strain has an almost
universal behaviour that depends on the chiral angle, (ii) that the sign of
slope depends on the value of $(n-m) \bmod 3$ and (iii) a novel change in sign
of the slope of bandgap versus uniaxial strain arising from a change in the
value of the quantum number corresponding to the minimum bandgap. Four orbital
calculations are also presented to show that the $\pi$ orbital results are
valid.


title: Planar lattice gases with nearest-neighbour exclusion

abstract:
  We discuss the hard-hexagon and hard-square problems, as well as the
corresponding problem on the honeycomb lattice. The case when the activity is
unity is of interest to combinatorialists, being the problem of counting binary
matrices with no two adjacent 1's. For this case we use the powerful corner
transfer matrix method to numerically evaluate the partition function per site,
density and some near-neighbour correlations to high accuracy. In particular
for the square lattice we obtain the partition function per site to 43 decimal
places.


title: Controlled Fabrication of Metallic Electrodes with Atomic Separation

abstract:
  We report a new technique for fabricating metallic electrodes on insulating
substrates with separations on the 1 nm scale. The fabrication technique, which
combines lithographic and electrochemical methods, provides atomic resolution
without requiring sophisticated instrumentation. The process is simple,
controllable, reversible, and robust, allowing rapid fabrication of electrode
pairs with high yield. We expect the method to prove useful in interfacing
molecular-scale structures to macroscopic probes and electronic devices .


title:
Electronic structure and properties of pure and doped $\epsilon$-FeSi
  from ab-initio local density theory

abstract:
  Local density calculations of the electronic structure of FeSi,
FeSi_{1-x}Al_x and Fe_{1-x}Ir_xSi systems in the B20 structure are presented.
Pure FeSi has a semi-conducting gap of 6 mRy at 0 K.
  Effects of temperature (T) in terms of electronic and vibrational excitations
are included. Various measurable properties, such as magnetic susceptibility
chi(T), electronic specific heat C(T), thermoelectric power S(T), relative
variations in resistivity rho(T), and peak positions in the density-of-states
(DOS) are calculated. The feedback from vibrational disorder onto the
electronic structure is found to be essential for a good description of most
properties, although the results for S(T) in undoped FeSi can be described up
to about 150 K without considerations of disorder. Above this T, only the
filling of the gap due to disorder accompanied by exchange enhancement, can
explain the large susceptiblity. The overall good agreement with experimental
data for most properties in doped and pure FeSi suggests that this system is
well described by LDA even at large T. Doped FeSi can be described quite well
from rigid-band shifts of the Fermi energy on the DOS of pure FeSi.
Spin-polarization in Ir doped FeSi leads to a semi-metallic magnetic state at
low T. (Submitted to Phys. Rev. B)


title:
Neutral evolution of model proteins: diffusion in sequence space and
  overdispersion

abstract:
  We simulate the evolution of model protein sequences subject to mutations. A
mutation is considered neutral if it conserves 1) the structure of the ground
state, 2) its thermodynamic stability and 3) its kinetic accessibility. All
other mutations are considered lethal and are rejected. We adopt a lattice
model, amenable to a reliable solution of the protein folding problem. We prove
the existence of extended neutral networks in sequence space -- sequences can
evolve until their similarity with the starting point is almost the same as for
random sequences. Furthermore, we find that the rate of neutral mutations has a
broad distribution in sequence space. Due to this fact, the substitution
process is overdispersed (the ratio between variance and mean is larger than
one). This result is in contrast with the simplest model of neutral evolution,
which assumes a Poisson process for substitutions, and in qualitative agreement
with biological data.


title: Comment on ``Superconducting PrBa_2Cu_3O_x''

abstract:
  Recently, Zou et al. (Phys. Rev. Lett. 80, 1074, 1998) reported the
observation of bulk superconductivity (SC) for a PrBa_2Cu_3O_x (Pr123) single
crystal grown by the traveling-solvent floating zone (TSFZ) method. The aim of
this Comment is to show the inconsistency of the value of effective magnetic
moment \mu_{eff} reported by Zou et al. (2.92\mu_B) with their magnetic
susceptibility data. The estimation made directly from their data points gives
a considerably smaller value of \mu_{eff}=2.09\mu_B. At the same time the
values of mu_{eff}=2.9\mu_B and 3.1\mu_B were obtained for our Pr123 single
crystals grown by flux method for H||ab-plane and H||c-axis, respectively. This
suggests that Pr occupies only about a half of the RE sites in TSFZ crystal.
The other half of the RE sites is occupied most probably by the nonmagnetic Ba.
Noteworthy, SC with T_c=43 K was observed earlier for
Pr_{0.5}Ca_{0.5}Ba_2Cu_3O_{7-y} thin films. Ba^{2+} has a larger ionic radius
than Pr^{3+} and so the substitution of Ba for Pr could give a natural
explanation not only for the SC in TSFZ Pr123 but also for the elongation of
the distance between the CuO_2 planes observed by Zou et al.


title:
Effect of combining a DC bias current with an AC transport current on AC
  losses in a High Temperature Superconductor

abstract:
  Creating complex flux configurations by superposing a dc current or magnetic
field onto the ac current in a type II superconducting tape should lead to a
variety of peculiar behaviors. An example is the appearance of the Clem valley,
a minimum in the ac losses as a function of the dc bias amplitude, which has
been theoretically studied by LeBlanc et al., in the continuation of Clem's
calculations. These situations have been investigated by applying a dc current
to a silver-gold sheathed Bi-2223 tape at 77 K (critical current 29 A), in
addition to the usual ac transport current. The ac losses were measured by the
null calorimetric method to ensure that the total losses were being accounted
for. These were recorded for different values of the ac and dc currents,
leading to the observation of two different behaviors depending on the ac
current. Our revelation of the Clem valley is, to our knowledge, the first
experimental validation of this phenomenon in high temperature superconductors,
and may provide a simple way of reducing the ac loss in industrial applications
of these materials.


title:
D-wave Bose-Einstein condensation and the London penetration depth in
  superconducting cuprates

abstract:
  We show that bipolaron formation leads to a d-wave Bose-Einstein condensate
in cuprates. It is the bipolaron energy dispersion rather than a particular
pairing interaction which is responsible for the d-wave symmetry. The unusual
low-temperature dependence of the magnetic field penetration depth in cuprates
is explained by the localisation of bosons in the random potential. The
temperature dependence of the penetration depth is linear with positive or
negative slope depending on the random field profile.


title:
Anomalous magnetic ordering in PrBa_2Cu_3O_{7-y} single crystals:
  Evidence for magnetic coupling between the Cu and Pr sublattices

abstract:
  In Al-free PrBa_2Cu_3O_{7-y} single crystals the kink in the temperature
dependence of magnetic susceptibility chi_{ab}(T), connected with Pr
antiferromagnetic ordering, disappears after field cooling (FC) in a field H ||
ab-plane. The kink in chi_c(T) remains unchanged after FC in H || c-axis. As a
possible explanation, freezing of the Cu magnetic moments, lying in the
ab-plane, caused by FC in H || ab, hinders their reorientation and, due to
coupling between the Pr and Cu(2) sublattices, ordering of the Pr^{3+} moments.
A field induced phase transition and a field dependence of the Pr^{3+} ordering
temperature have been found for both H || c and H || ab.


title:
Persistent currents in mesoscopic cavities and the effect of level
  crossings as random variables

abstract:
  In the present article we perform analytical and numerical calculations
related to persistent currents in 2D isolated mesoscopic annular cavities
threaded by a magnetic flux. The system considered has a high number of open
channels and therefore the single particle spectrum exhibits many level
crossings as the flux varies. We determine the effect of the distribution of
level crossings in the typical persistent current.


title: Hall-effect in LuNi_2B_2C in normal and superconducting mixed states

abstract:
  The Hall resistivity rho_{xy} of LuNi_2B_2C is negative in the normal as well
as in the mixed state and has no sign reversal typical for high-T_c
superconductors. A distinct nonlinearity in the rho_{xy} dependence on field H
was found in the normal state for T < 40K, accompanied by a large
magnetoresistance reaching +90% for mu_0H=16T at T=20K. The scaling relation
rho_{xy} ~ \rho_{xx}^\beta (\rho_{xx} is the longitudinal resistivity) was
found in the mixed state, the value of \beta being dependent on the degree of
disorder.


title: Barrier resonances in Bose-Einstein condensation

abstract:
  We study the dynamics of the mean field model of a Bose-Einstein condensed
atom cloud in a parametrically forced trap by using analytical and numerical
techniques. The dynamics is related to a classical Mathieu oscillator in a
singular potential. It is found that there are wide resonances which can
strongly affect the dynamics even when dissipation is present. Different
geometries of the forcing are discussed, as well as the implications of our
results.


title:
Absence of Fragmentation and Broken Spatial Symmetry in the Ground State
  of Dilute Bose Gases

abstract: Paper temporarily withdrawn for remodeling


title: Critical behavior of the planar magnet model in three dimensions

abstract:
  We use a hybrid Monte Carlo algorithm in which a single-cluster update is
combined with the over-relaxation and Metropolis spin re-orientation algorithm.
Periodic boundary conditions were applied in all directions. We have calculated
the fourth-order cumulant in finite size lattices using the single-histogram
re-weighting method. Using finite-size scaling theory, we obtained the critical
temperature which is very different from that of the usual XY model. At the
critical temperature, we calculated the susceptibility and the magnetization on
lattices of size up to $42^3$. Using finite-size scaling theory we accurately
determine the critical exponents of the model and find that $\nu$=0.670(7),
$\gamma/\nu$=1.9696(37), and $\beta/\nu$=0.515(2). Thus, we conclude that the
model belongs to the same universality class with the XY model, as expected.


title:
A Stochastic Liouvillian Algorithm to Simulate Dissipative Quantum
  Dynamics With Arbitrary Precision

abstract:
  An exact and efficient new method to simulate dynamics in dissipative quantum
systems is presented. A stochastic Liouville equation, deduced from Feynman and
Vernon's path-integral expression of the reduced density matrix, is used to
describe the exact dynamics at any dissipative strength and for arbitrarily low
temperatures. The utility of the method is demonstrated by applications to a
damped harmonic oscillator and a double-well system immersed in an Ohmic bath
at low temperatures.