title: Local Impurity Phase Pinning and Pinning Force in Charge Density Waves

abstract:
  Starting from the static Fukuyama-Lee-Rice equation for a three-dimensional
incommensurate charge density wave (CDW) in quasi one-dimensional conductors a
solvable model for local phase pinning by impurities is defined and studied. We
find that average CDW energy and average pinning force show critical behaviour
with respect to the pinning parameter $h$. Specifically the pinning force
exhibits a threshold at $h=1$ with exponent $\beta=2$. Our model examplifies a
general concept of local impurity pinning in which the force exerted by the
impurity on the periodic CDW structure becomes multivalued and metastable
states appear beyond a threshold. It is found that local impurity pinning
becomes less effective at low temperatures and may eventually cease completely.
These results are independent of spatial dimensionality as expected for local
impurity pinning. Comparison with Larkin's model is also made.


title: Adsorption of polydisperse polymer chains

abstract:
  The adsorption of polydisperse ideal polymer chains is shown to be sensitive
to the large N tail of the distribution of chains. If and only if the number of
chains decays more slowly than exponentially then there is an adsorption
transition like that of monodisperse infinite chains. If the number decays
exponentially the adsorption density diverges continuously at a temperature
which is a function of the mean chain length. At low coverages, chains with
repulsive monomer--monomer interactions show the same qualitative behaviour


title: Messages on Networks as a Many-Body Problem

abstract:
  In this paper we initiate an approach that deals with the problem of
calculating average properties of messages traveling on networks, by employing
concepts and methods that are used for the study of the many-body problem in
the field of physics. We set up a framework that simplifies enormously the
problem and, through a concrete example, we show how it can be applied to a
broad class of networks and protocols.


title:
On the magnetoresistance anisotropy of a 2-dimensional electron gas with
  large half-integer filling factors

abstract:
  The unsymmetric confinement of the electrons in a typical 2-dimensional
electron gas (2DEG) breaks the full cubic symmetry of the in-plane transport
properties. A rigorous invariance of the transport under rotation by pi/2 about
the cubic axis perpendicular to the plane of the gas no longer holds, even for
structurally perfect samples, and asymmetries under rotation by pi/2 are
allowed. It is proposed that this symmetry breaking plays a role in the recent
observation, by Little et al., of the pronounced anisotropy of the
magnetoresistance of a 2DEG at very low temperatures, at large half-integer
filling factors of the Landau levels in such a system.


title:
Theory of Chiral Modulations and Fluctuations in Smectic-A Liquid
  Crystals Under an Electric Field

abstract:
  Chiral liquid crystals often exhibit periodic modulations in the molecular
director; in particular, thin films of the smectic-C* phase show a chiral
striped texture. Here, we investigate whether similar chiral modulations can
occur in the induced molecular tilt of the smectic-A phase under an applied
electric field. Using both continuum elastic theory and lattice simulations, we
find that the state of uniform induced tilt can become unstable when the system
approaches the smectic-A--smectic-C* transition, or when a high electric field
is applied. Beyond that instability point, the system develops chiral stripes
in the tilt, which induce corresponding ripples in the smectic layers. The
modulation persists up to an upper critical electric field and then disappears.
Furthermore, even in the uniform state, the system shows chiral fluctuations,
including both incipient chiral stripes and localized chiral vortices. We
compare these predictions with observed chiral modulations and fluctuations in
smectic-A liquid crystals.


title: Path Crossing Exponents and the External Perimeter in 2D Percolation

abstract:
  2D Percolation path exponents $x^{\cal P}_{\ell}$ describe probabilities for
traversals of annuli by $\ell$ non-overlapping paths, each on either occupied
or vacant clusters, with at least one of each type. We relate the probabilities
rigorously to amplitudes of $O(N=1)$ models whose exponents, believed to be
exact, yield $x^{\cal P}_{\ell}=({\ell}^2-1)/12$. This extends to half-integers
the Saleur--Duplantier exponents for $k=\ell/2$ clusters, yields the exact
fractal dimension of the external cluster perimeter, $D_{EP}=2-x^{\cal
P}_3=4/3$, and also explains the absence of narrow gate fjords, as originally
found by Grossman and Aharony.

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


title:
Metal-insulator transition in spatially-correlated random magnetic field
  system

abstract:
  We reexamine the problem of delocalization of two-dimensional electrons in
the presence of random magnetic field. By introducing spatial correlations
among random fluxes, a well-defined metal-insulator transition characterized by
a two-branch scaling of conductance has been demonstrated numerically. Critical
conductance is found non-universal with a value around $e^2/h$. Interesting
connections of this system with the recently observed B=0 two-dimensional
metallic phase (Kravchenko et al., Phys. Rev. B {\bf 50}, 8039 (1994)) are also
discussed.


title: Realistic model of correlated disorder and Anderson localization

abstract:
  A conducting 1D line or 2D plane inside (or on the surface of) an insulator
is considered.Impurities displace the charges inside the insulator. This
results in a long-range fluctuating electric field acting on the conducting
line (plane). This field can be modeled by that of randomly distributed
electric dipoles. This model provides a random correlated potential with
<U(r)U(r+k)> decaying as 1/k . In the 1D case such correlations give essential
corrections to the localization length but do not destroy Anderson
localization.


title: Reentrant charge order transition in the extended Hubbard model

abstract:
  We study the extended Hubbard model with both on-site and nearest neighbor
Coulomb repulsion ($U$ and $V$, respectively) in the Dynamical Mean Field
theory. At quarter filling, the model shows a transition to a charge ordered
phase with different sublattice occupancies $n_A \nen_B$. The effective mass
increases drastically at the critical $V$ and a pseudo-gap opens in the
single-particle spectral function for higher values of $V$. The $V_c(T)$-curve
has a negative slope for small temperatures, i.e. the charge ordering
transition can be driven by increasing the temperature. This is due to the
higher spin-entropy of the charge ordered phase.


title: Spontaneous coalition forming: a model from spin glass

abstract:
  In this paper a simple model is proposed to decribe the spontaneous formation
of coalitions among a group of actors like countries. The basic ingredients are
from the physics of disorder systems. It is the interplay of two different spin
glass models with respectively random bond and random site disorders which is
instrumental in the present approach. The cold war stabilty period is then
given an explanation as well the instabilities produced by the Warsow pact
dissolution. European and Chinese stabilities are also discussed.

categories: cond-mat.dis-nn adap-org cond-mat.stat-mech nlin.AO


title:
A low-temperature dynamic mode scanning force microscope operating in
  high magnetic fields

abstract:
  A scanning force microscope was implemented operating at temperatures below
4.2K and in magnetic fields up to 8T. Piezoelectric quartz tuning forks were
employed for non optical tip-sample distance control in the dynamic operation
mode. Fast response was achieved by using a phase-locked loop for driving the
mechanical oscillator. Possible applications of this setup for various scanning
probe techniques are discussed.


title:
The effect of an external magnetic field on the gas-liquid transition in
  the Ising spin fluid

abstract:
  The theoretical phase diagrams of the magnetic (Ising) lattice fluid in an
external magnetic field is presented. It is shown that, depending on the
strength of the nonmagnetic interaction between particles, various effects of
external field on the Ising fluid take place. In particular, at moderate values
of the nonmagnetic attraction the field effect on the gas-liquid critical
temperature is nonmonotoneous. A justification of such behavior is given. If
short-range correlations are taken into account (within a cluster approach),
the Curie temperature also depend on the nonmagnetic interaction.


title: Imprints of log-periodic self-similarity in the stock market

abstract:
  Detailed analysis of the log-periodic structures as precursors of the
financial crashes is presented. The study is mainly based on the German Stock
Index (DAX) variation over the 1998 period which includes both, a spectacular
boom and a large decline, in magnitude only comparable to the so-called Black
Monday of October 1987. The present example provides further arguments in
favour of a discrete scale-invariance governing the dynamics of the stock
market. A related clear log-periodic structure prior to the crash and
consistent with its onset extends over the period of a few months. Furthermore,
on smaller time-scales the data seems to indicate the appearance of analogous
log-periodic oscillations as precursors of the smaller, intermediate decreases.
Even the frequencies of such oscillations are similar on various levels of
resolution. The related value $\lambda \approx 2$ of preferred scaling ratios
is amazingly consistent with those found for a wide variety of other complex
systems. Similar analysis of the major American indices between September 1998
and February 1999 also provides some evidence supporting this concept but, at
the same time, illustrates a possible splitting of the dynamics that a large
market may experience.


title:
Two-dimensional Dilute Ising Models: Defect Lines and the Universality
  of the Critical Exponent \nu

abstract:
  We consider two-dimensional Ising models with randomly distributed
ferromagnetic bonds and study the local critical behavior at defect lines by
extensive Monte Carlo simulations. Both for ladder and chain type defects,
non-universal critical behavior is observed: the critical exponent of the
defect magnetization is found to be a continuous function of the strength of
the defect coupling. Analyzing corresponding stability conditions, we obtain
new evidence that the critical exponent $\nu$ of the bulk correlation length of
the random Ising model does not depend on dilution, i.e. $\nu=1$.


title: On the self-consistent spin-wave theory of layered Heisenberg magnets

abstract:
  The versions of the self-consistent spin-wave theories (SSWT) of
two-dimensional (2D) Heisenberg ferro- and antiferromagnets with a weak
interlayer coupling and/or magnetic anisotropy, that are based on the
non-linear Dyson-Maleev, Schwinger, and combined boson-pseudofermion
representations, are analyzed. Analytical results for the temperature
dependences of (sublattice) magnetization and short-range order parameter, and
the critical points are obtained. The influence of external magnetic field is
considered. Fluctuation corrections to SSWT are calculated within a
random-phase approximation which takes into account correctly leading and
next-leading logarithmic singularities. These corrections are demonstrated to
improve radically the agreement with experimental data on layered perovskites
and other systems. Thus an account of these fluctuations provides a
quantitative theory of layered magnets.


title: Impurity Dynamics in a Bose Condensate

abstract:
  We estimate the changes in the condensate ground state energy induced by one
or more "sizable" impurities. By "sizable", we mean an impurity whose size is
within a few orders of magnitude of the trap size and not necessarily atomic in
scale. A sizable impurity will "drill" a hole in the condensate wave function
and alter the condensate energy. The question is whether this microscopic
change can be detected macroscopically because of the Bose-Einstein
condensation effect. We discuss the effect of impurity expulsion, condensate
interactions, and the induced interaction between two impurities.


title:
Issues and Observations on Applications of the Constrained-Path Monte
  Carlo Method to Many-Fermion Systems

abstract:
  We report several important observations that underscore the distinctions
between the constrained-path Monte Carlo method and the continuum and lattice
versions of the fixed-node method. The main distinctions stem from the
differences in the state space in which the random walk occurs and in the
manner in which the random walkers are constrained. One consequence is that in
the constrained-path method the so-called mixed estimator for the energy is not
an upper bound to the exact energy, as previously claimed. Several ways of
producing an energy upper bound are given, and relevant methodological aspects
are illustrated with simple examples.


title: Dynamics of elastic boundaries

abstract:
  We study, both analytically and numerically, the dynamics of elastic
boundaries such as crack fronts in fracture and surfaces of contact in solid on
solid friction. The elastic waves in the solid give rise to kinks that move
with a characteristic velocity along the boundary. As stopping kinks pass
through they cause moving parts of the boundary to stop. Starting kinks cause
stationary parts of the boundary to move. We study the interaction of these
kinks with disorder that arises from the spatial variations of the friction
constant or fracture energy. In the absence of elastic waves, elastic
boundaries with disorder operate at a critical point leading to a power-law
distribution of slip events and self-affine boundaries. Elastic waves result in
relevant perturbations at this fixed point. Slip events beyond a critical size
run away and the velocity of the boundary jumps to a nonzero value when the
external load is increased above a threshold. We analyze in detail a class of
simple models that capture the essential features of bulk vectorial elasticity
and discuss the implications of our results for friction and fracture dynamics.


title: Molecular Orbital Models of Benzene, Biphenyl and the Oligophenylenes

abstract:
  A two state (2-MO) model for the low-lying long axis-polarised excitations of
poly(p-phenylene) oligomers and polymers is developed. First we derive such a
model from the underlying Pariser-Parr-Pople (P-P-P) model of pi-conjugated
systems. The two states retained per unit cell are the Wannier functions
associated with the valence and conduction bands. By a comparison of the
predictions of this model to a four state model (which includes the non-bonding
states) and a full P-P-P model calculation on benzene and biphenyl, it is shown
quantitatively how the 2-MO model fails to predict the correct excitation
energies. The 2-MO model is then solved for oligophenylenes of up to 15 repeat
units using the density matrix renormalisation group (DMRG) method. It is shown
that the predicted lowest lying, dipole allowed excitation is ca. 1 eV higher
than the experimental result. The failure of the 2-MO model is a consequence of
the fact that the original HOMO and LUMO single particle basis does not provide
an adequate representation for the many body processes of the electronic
system.


title:
Universal Behavior of Quantum Impurity Scattering in Tomonaga-Luttinger
  Liquid

abstract:
  Using bosonization and path integral methods, we study general low
temperature behavior of non-magnetic and magnetic impurity scattering in
Tomonaga-Luttinger liquid, and calculate electron Green function for a general
backward scattering potential. We demonstrate that electron density of state
near the impurity site is suppressed by the backward scattering, but it mainly
remains invariant as far away from the impurity, and at zero temperature the
electrons are completely reflected on the impurity site, the system breaks into
two subsystems but right- and left-moving electron fields have a twisted
boundary condition. We also show that a testing charge can only be partially
screened by conduction electrons, and in strong interaction region the impurity
susceptibility has a 1/T-type low temperature behavior.


title:
A new conjecture extends the GM law for percolation thresholds to
  dynamical situations

abstract:
  The universal law for percolation thresholds proposed by Galam and Mauger
(GM) is found to apply also to dynamical situations. This law depends solely on
two variables, the space dimension d and a coordinance numberq. For regular
lattices, q reduces to the usual coordination number while for anisotropic
lattices it is an effective coordination number. For dynamical percolation we
conjecture that the law is still valid if we use the number q_2 of second
nearest neighbors instead of q. This conjecture is checked for the dynamic
epidemic model which considers the percolation phenomenon in a mobile
disordered system. The agreement is good.


title: Toy model for molecular motors

abstract:
  A hopping model for molecular motors is presented consisting of a state with
asymmetric hopping rates with period 2 and a state with uniform hopping rates.
State changes lead to a stationary unidirectional current of a particle. The
current is explicitly calculated as a function of the rate of state changes,
including also an external bias field. The Einstein relation between the linear
mobility of the particle and its diffusion coefficient is investigated. The
power input into the system is derived, as well as the power output resulting
from the work performed against the bias field. The efficiency of this model is
found to be rather small.


title: Critical Crashes

abstract:
  We argue that the word ``critical'' in the title is not purely literary.
Based on our and other previous work on nonlinear complex dynamical systems, we
summarize present evidence, on the Oct. 1929, Oct. 1987, Oct. 1987 Hong-Kong,
Aug. 1998 global market events and on the 1985 Forex event, for the hypothesis
advanced four years ago that stock market crashes are caused by the slow
buildup of long-range correlations between traders leading to a collapse of the
stock market in one critical instant. We qualify the log-periodic oscillations
using a novel non-parametric method that does not rely on any fit: the
corresponding log-periodogram exhibits a strong statistically significant peak
for all six crashes examined, pointing at approximately the same prefered
scaling ratio around 2.


title: Thickness dependent Curie temperatures of ferromagnetic Heisenberg films

abstract:
  We develop a procedure for calculating the magnetic properties of a
ferromagnetic Heisenberg film with single-ion anisotropy which is valid for
arbitrary spin and film thickness. Applied to sc(100) and fcc(100) films with
spin S=7/2 the theory yields the layer dependent magnetizations and Curie
temperatures of films of various thicknesses making it possible to investigate
magnetic properties of films at the interesting 2D-3D transition.


title: ``Smoke Rings'' in Ferromagnets

abstract:
  It is shown that bulk ferromagnets support propagating non-linear modes that
are analogous to the vortex rings, or ``smoke rings'', of fluid dynamics. These
are circular loops of {\it magnetic} vorticity which travel at constant
velocity parallel to their axis of symmetry. The topological structure of the
continuum theory has important consequences for the properties of these
magnetic vortex rings. One finds that there exists a sequence of magnetic
vortex rings that are distinguished by a topological invariant (the Hopf
invariant). We present analytical and numerical results for the energies,
velocities and structures of propagating magnetic vortex rings in ferromagnetic
materials.


title:
Inversion of Randomly Corrugated Surfaces Structure from Atom Scattering
  Data

abstract:
  The Sudden Approximation is applied to invert structural data on randomly
corrugated surfaces from inert atom scattering intensities. Several expressions
relating experimental observables to surface statistical features are derived.
The results suggest that atom (and in particular He) scattering can be used
profitably to study hitherto unexplored forms of complex surface disorder.


title:
Frequency dependent relaxation rate in the superconducting
  YBa2Cu3O{6+delta}

abstract:
  The submillimeter-wave 3 cm{-1} < nu < 35 cm{-1} complex conductivity of the
reduced YBa2Cu3O{6+delta} film (Tc=56.5 K) was investigated for temperatures 4
K < T < 300 K and compared to the properties of the same film in the optimally
doped state. The frequency dependence of the effective quasiparticle scattering
rate 1/tau*(nu) was extracted from the spectra. 1/tau*(nu) is shown to be
frequency independent at low frequencies and high temperatures. A gradual
change to 1/tau*~nu^{1.5} law is observed as temperature decreases. In order to
explain the observed temperature dependence of the low frequency spectral
weight above Tc, the quasiparticle effective mass is supposed to be temperature
dependent for T>Tc.


title: Mesoscopic Superconducting Disc with Short-Range Columnar Defects

abstract:
  Short-range columnar defects essentially influence the magnetic properties of
a mesoscopic superconducting disc.They help the penetration of vortices into
the sample, thereby decrease the sample magnetization and reduce the upper
critical field. Even the presence of weak defects split a giant vortex state
(usually appearing in a clean disc in the vicinity of the transition to a
normal state) into a number of vortices with smaller topological charges. In a
disc with a sufficient number of strong enough defects vortices are always
placed onto defects. The presence of defects lead to the appearance of
additional magnetization jumps related to the redistribution of vortices which
are already present on the defects and not to the penetration of new vortices.


title:
Monte Carlo Results for Projected Self-Avoiding Polygons: A
  Two-dimensional Model for Knotted Polymers

abstract:
  We introduce a two-dimensional lattice model for the description of knotted
polymer rings. A polymer configuration is modeled by a closed polygon drawn on
the square diagonal lattice, with possible crossings describing pairs of
strands of polymer passing on top of each other. Each polygon configuration can
be viewed as the two- dimensional projection of a particular knot. We study
numerically the statistics of large polygons with a fixed knot type, using a
generalization of the BFACF algorithm for self-avoiding walks. This new
algorithm incorporates both the displacement of crossings and the three types
of Reidemeister transformations preserving the knot topology. Its ergodicity
within a fixed knot type is not proven here rigorously but strong arguments in
favor of this ergodicity are given together with a tentative sketch of proof.
Assuming this ergodicity, we obtain numerically the following results for the
statistics of knotted polygons: In the limit of a low crossing fugacity, we
find a localization along the polygon of all the primary factors forming the
knot. Increasing the crossing fugacity gives rise to a transition from a
self-avoiding walk to a branched polymer behavior.


title:
Finite-size scaling above the upper critical dimension revisited: The
  case of the five-dimensional Ising model

abstract:
  Monte Carlo results for the moments <M^k> of the magnetization distribution
of the nearest-neighbor Ising ferromagnet in a L^d geometry, where L (4 \leq L
\leq 22) is the linear dimension of a hypercubic lattice with periodic boundary
conditions in d=5 dimensions, are analyzed in the critical region and compared
to a recent theory of Chen and Dohm (CD) [X.S. Chen and V. Dohm, Int. J. Mod.
Phys. C (1998)]. We show that this finite-size scaling theory (formulated in
terms of two scaling variables) can account for the longstanding discrepancies
between Monte Carlo results and the so-called ``lowest-mode'' theory, which
uses a single scaling variable tL^{d/2} where t=T/T_c-1 is the temperature
distance from the critical temperature, only to a very limited extent. While
the CD theory gives a somewhat improved description of corrections to the
``lowest-mode'' results (to which the CD theory can easily be reduced in the
limit t \to 0, L \to \infty, tL^{d/2} fixed) for the fourth-order cumulant,
discrepancies are found for the susceptibility (L^d <M^2>). Reasons for these
problems are briefly discussed.


title: Logarithmic Relaxations in a Random Field Lattice Gas Subject to Gravity

abstract:
  A simple lattice gas model with random fields and gravity is introduced to
describe a system of grains moving in a disordered environment. Off equilibrium
relaxations of bulk density and its two time correlation functions are
numerically found to show logarithmic time dependences and "aging" effects.
Similitudes with dry granular media are stressed. The connections with off
equilibrium dynamics in others kinds of "frustrated" lattice models in presence
of a directional driving force (gravity) are discussed to single out the
appearance of universal features in the relaxation process.


title: Depletion forces near curved surfaces

abstract:
  Based on density functional theory the influence of curvature on the
depletion potential of a single big hard sphere immersed in a fluid of small
hard spheres with packing fraction \eta_s either inside or outside of a hard
spherical cavity of radius R_c is calculated. The relevant features of this
potential are analyzed as function of \eta_s and R_c. There is a very slow
convergence towards the flat wall limit R_c \to \infty. Our results allow us to
discuss the strength of depletion forces acting near membranes both in normal
and lateral directions and to make contact with recent experimental results.


title:
Enhancement of Andreev reflection by spin polarization in
  out-of-equilibrium ferromagnet-superconductor junctions

abstract:
  We report on transport measurements on Nb/Al/Gd/Al/Nb junctions.Bulk
Gadolinium is a weakly polarized ferromagnet (5-7%), and is present in the
junction in granular form (superparamagnet). We show that Andreev reflection is
strongly enhanced by a weak polarization, obtained by applying an external
magnetic field. A new model is proposed that accounts for this effect. The
transport is described in terms of ``hot'' carriers that experience Zeeman
splitting due to the spin polarized background. The Landauer formula is
$G=(4e^2)/h A(h_b)$, $h_b$ being the exchange field at the grain boundary, and
A(E) the Andreev current coefficient, that increases with E for resistive
junctions.


title:
Study of a Nonlocal Density scheme for electronic--structure
  calculations

abstract:
  An exchange-correlation energy functional beyond the local density
approximation, based on the exchange-correlation kernel of the homogeneous
electron gas and originally introduced by Kohn and Sham, is considered for
electronic structure calculations of semiconductors and atoms. Calculations are
carried out for diamond, silicon, silicon carbide and gallium arsenide. The
lattice constants and gaps show a small improvement with respect to the LDA
results.
  However, the corresponding corrections to the total energy of the isolated
atoms are not large enough to yield a substantial improvement for the cohesive
energy of solids, which remains hence overestimated as in the LDA.


title: Connection between Dispersive Transport and Statistics of Extreme Events

abstract:
  A length dependence of the effective mobility in the form of a power law, B ~
L^(1-1/alpha) is observed in dispersive transport in amorphous substances, with
0 < \alpha < 1. We deduce this behavior as a simple consequence of the
statistical theory of extreme events. We derive various quantities related to
the largest value in samples of n trials, for the exponential and power-law
probability densities of the individual events.


title:
^{139}La Spectrum and Spin-Lattice Relaxation Measurements of
  La_{2/3}Ca_{1/3}MnO_3 in the Paramagnetic State

abstract:
  ^{139}La NMR measurements of La_{2/3}Ca_{1/3}MnO_3 performed in the
paramagnetic state and high magnetic fields are reported. Analysis of the high
temperature spectrum measured establishes that the spectrum it is a standard
powder pattern broadened by a variation in lattice distortions. At lower
temperatures, the spectrum shifts and broadens. Both the shift and broadening
exhibit Curie-Weiss behavior, indicating that the shift measures the
polarization of the electron spin system, and the broadening reflects a
distribution of magnetic susceptibilities. This distribution may result from
variations of local susceptibility in the bulk of the sample or from
differences in demagnetizing factors among powder grains. The spectrum also
indicates that the lattice distortions do not change as the temperature lowers.
Spectral diffusion measurements suggest that the temperature dependence of the
spectrum shape does not result from freezing out of motion of magnetic
polarons. Variation of the nuclear spin-lattice relaxation across the spectrum
indicate that magnetic fluctuations, not lattice vibrations, dominate nuclear
relaxation. Nuclear spin-lattice relaxation therefore measures electron spin
dynamics in this system. The magnetic field dependence of the spin-lattice
relaxation indicates that the electron spin-spin correlation function adopts
simple single exponential behavior with a slow field-independent correlation
time of 10^{-8} seconds near Tc. The spin-spin correlation function changes
form at higher temperatures, but remains slow. The spin-lattice relaxation
therefore indicates the presence of extremely slow dynamics above Tc.


title: Slow dynamics of water under pressure

abstract:
  We perform lengthy molecular dynamics simulations of the SPC/E model of water
to investigate the dynamics under pressure at many temperatures and compare
with experimental measurements. We calculate the isochrones of the diffusion
constant $D$ and observe power-law behavior of $D$ on lowering temperature with
an apparent singularity at a temperature $T_c(P)$, as observed for water.
Additional calculations show that the dynamics of the SPC/E model are
consistent with slowing down due to the transient caging of molecules, as
described by the mode-coupling theory (MCT). This supports the hypothesis that
the apparent divergences of dynamic quantities along $T_c(P)$ in water may be
associated with ``slowing down'' as described by MCT.


title: Magnetooptical sum rules close to the Mott transition

abstract:
  We derive new sum rules for the real and imaginary parts of the
frequency-dependent Hall constant and Hall conductivity. As an example, we
discuss their relevance to the doped Mott insulator that we describe within the
dynamical mean-field theory of strongly correlated electron systems.


title:
Interplay between spin-relaxation and Andreev reflection in
  ferromagnetic wires with superconducting contacts

abstract:
  We analyze the change in the resistance of a junction between a diffusive
ferromagnetic (F) wire and normal metal electrode, due to the onset of
superconductivity (S) in the latter and a double Andreev scattering process
leading to a complete internal reflection of a large fraction of the
spin-polarized electrons back into the ferromagnet. The superconducting
transition results in an additional contact resistance arising from the
necessity to match spin-polarized current in F-wire to spin-less current in
S-reservoir, which is comparable to the resistance of a piece of a F-wire with
the length equal to the spin-relaxation length.


title: Infinite magnetoresistance of magnetic multilayers

abstract:
  We examine transport properties of a magnetic superlattice with current
perpendicular to the planes. In the limit that the phase-breaking and spin flip
scattering lengths are greater than the system size, a multiple-scattering
approach is used to calculate the 4-probe conductance. We show that by tuning
the strength of tunnel barriers placed between the current and voltage probes
giant magnetoresistance ratios of arbitrary strength and size are achievable.


title:
Topological defects and Goldstone excitations in domain walls between
  ferromagnetic quantum Hall effect liquids

abstract:
  It is shown that the low-energy spectrum of a ferromagnetic quantum Hall
effect liquid in a system with a multi-domain structure generated by an
inhomogeneous bare Zeeman splitting $\epsilon_{Z}$ is formed by excitations
localized at the walls between domains. For a step-like $\epsilon_Z(r)$, the
domain wall spectrum includes a spin-wave with a linear dispersion and a small
gap due to spin-orbit coupling, and a low-energy topological defects. The
latter are charged and may dominate in the transport under conditions that the
percolation through the network of domain walls is provided.


title:
Hole-Density Evolution of the One-Particle Spectral Function in Doped
  Ladders

abstract:
  The spectral function $A({\bf q}, \omega)$ of doped $t-J$ ladders is
presented on clusters with up to $2 \times 20$ sites at zero temperature
applying a recently developed technique that uses up to $\sim 6 \times 10^6$
rung-basis states. Similarities with photoemission results for the 2D cuprates
are observed, such as the existence of a gap at $(\pi,0)$ near half-filling
(caused by hole pair formation) and flat bands in its vicinity. These features
should be observable in ARPES experiments on ladders. The main result of the
paper is the nontrivial evolution of the spectral function from a narrow band
at $x=0$, to a quasi-noninteracting band at $x \geq 0.5$. It was also observed
that the low-energy peaks of a cluster spectra acquire finite line-widths as
their energies move away from the chemical potential.


title:
Study of a generalized Metropolis decision rule in auxiliary field
  quantum Monte Carlo

abstract:
  We consider a generalization of the standard Metropolis algorithm
acceptance/rejection decision rule and numerically explore its properties using
auxiliary field quantum Monte Carlo. The generalization involves a free
parameter which, given a criterion for proposing attempted moves, can be used
to tune the average acceptance rate in a particular way. Such tuning can also
potentially change Monte Carlo autocorrelation times, and the combination of
the changing acceptance rate and autocorrelation times raises the possibility
of more efficient simulations. We explore these issues using primarily
massively parallel quantum Monte Carlo runs of the ``test case''
two-dimensional Hubbard model, and discuss results and applications.


title:
Strongly enhanced inelastic collisions in a Bose-Einstein condensate
  near Feshbach resonances

abstract:
  The properties of Bose-Einstein condensed gases can be strongly altered by
tuning the external magnetic field near a Feshbach resonance. Feshbach
resonances affect elastic collisions and lead to the observed modification of
the scattering length. However, as we report here, this is accompanied by a
strong increase in the rate of inelastic collisions. The observed three-body
loss rate in a sodium Bose-Einstein condensation increased when the scattering
length was tuned to both larger or smaller values than the off-resonant value.
This observation and the maximum measured increase of the loss rate by several
orders of magnitude are not accounted for by theoretical treatments. The strong
losses impose severe limitations for using Feshbach resonances to tune the
properties of Bose-Einstein condensates. A new Feshbach resonance in sodium at
1195 G was observed.


title: The Phase Separation Scenario for Manganese Oxides

abstract:
  Recent computational studies of models for manganese oxides have revealed a
rich phase diagram, not anticipated in early calculations in this context
performed in the 1950's and 60's. In particular, the transition between the
antiferromagnetic insulator state of the hole-undoped limit and the
ferromagnetic metal at finite hole-density was found to occur through a
mixed-phase process. When extended Coulomb interactions are included, a
microscopically charge inhomogeneous state should be stabilized. These phase
separation tendencies, also present at low electronic densities, influence the
properties of the ferromagnetic region by increasing charge fluctuations.
Experimental data reviewed here using several techniques for manganites and
other materials are consistent with this scenario. Similarities with results
previously discussed in the context of cuprates are clear from this analysis,
although the phase segregation tendencies in manganites seem stronger.


title:
Orbital ordering, Jahn-Teller distortion, and anomalous X-Ray scattering
  in Manganates

abstract:
  We demonstrate with LSDA+U calculations that x-ray scattering at the K edge
of Mn is sensitive to orbital ordering in one energy range and Jahn-Teller
distortion in another. Contrary to what is suggested by atomic or cluster
models used to date we show that band structure effects rather than local
Coulomb interactions dominate the polarization dependence of the K edge
scattering and therefore it is sensitive to nearest neighbour bond length
distortions and next nearest neighbour orbital occupation. Based on this we
propose a new mechanism for K edge x-ray scattering in the manganates which we
suggest is also applicable to transition metal compounds in general.


title:
Slope of the superconducting gap function in
  $Bi_2Sr_2CaCu_2O_{8+\delta}$ measured by vacuum tunneling spectroscopy

abstract:
  Reproducible scanning tunneling microscope (STM) spectra of
$Bi_2Sr_2CaCu_2O_{8+\delta}$ consistently exhibit asymmetric tunneling
characteristics, with the higher peak conductance corresponding to a negatively
biased sample. We consider various possible sources of this asymmetry that are
not intrinsic to the superconducting state, including energy dependence of the
normal state densities of states of sample and/or tip, existence of bandwidth
cutoffs, unequal work functions of tip and sample, and energy-dependent
transmission probability. None of these effects can explain the sign and
temperature dependence of the observed asymmetry. This indicates that the
observed asymmetry reflects an intrinsic property of the superconducting state:
an energy-dependent superconducting gap function with non-zero slope at the
Fermi energy. Such a sloped gap function will also give rise to a
thermoelectric effect in STM experiments, resulting in a $positive$
thermopower. We discuss the feasibility of observing this thermoelectric effect
with an STM and conclude that it is easily observable. An analysis of
thermoelectric currents and voltages together with the tunneling spectra as
function of temperature and tip- sample distance would allow for accurate
determination of the slope of the gap function. It is suggested that it would
be very worthwhile to perform these experiments, because the slope of the gap
function reflects a fundamental property of the superconducting state. The
theory of hole superconductivity has predicted the existence of such a slope,
of universal sign, in all superconductors. It is furthermore argued that recent
experimental results on vortex lattice imaging provide further strong evidence
for the existence of the gap slope discussed here.


title: Scaling for domain growth in the Ising model with competing dynamics

abstract:
  We study the domain growth of the one-dimensional kinetic Ising model under
the competing influence of Glauber dynamics at temperature T and Kawasaki
dynamics with a configuration-independent rate. The scaling of the structure
factor is shown to have the form for nonconserved dynamics with the corrections
arising from the spin-exchange process, i.e., $S(k,t)=Lg_0(kL,t/\tau
)+g_1(kL,t/\tau)+... $, and the corresponding scaling functions are calculated
analytically. A correction to the Porod law at zero temperature is also given.


title: Solidity of viscous liquids II: Anisotropic flow events

abstract:
  Recent findings on the displacements in the surroundings of isotropic flow
events in viscous liquids [Phys. Rev. E, to appear Feb. 1999] are generalized
to the anisotropic case. Also, it is shown that a flow event is characterized
by a dimensionless number reflecting the degree of anisotropy.


title: Comment on "A landscape theory of aggregation"

abstract:
  The problem of aggregation processes in alignments is the subject of a paper
published in a Statistical Physics Journal (Physica A 230 (1996) 174-188). Two
models are presented and discussed in that paper. First the energy landscape
model proposed by Axelrod and Bennett (B. J. Pol. S. 23 (1993) 211-233) is
analysed. The model is shown not to include most of its claimed results. Then a
second model is presented to reformulate correctly the problem within
statistical physics and to extend it beyond the initial Axelrod-Bennett
analogy.


title: Quantum Ferrimagnets

abstract:
  We study quantum ferrimagnets in one, two, and three dimensions by using a
variety of methods and approximations. These include: (i) a treatment based on
the spin coherent state path-integral formulation of quantum ferrimagnets by
taking into account the leading order quantum and thermal fluctuations (ii) a
field-theoretical (non-linear $\sigma$-model type) formulation of the special
case of one-dimensional quantum ferrimagnets at zero temperature (iii) an
effective description in terms of dimers and quantum rotors, and (iv) a quantum
renormalization group study of ferrimagnetic Heisenberg chains. Some of the
formalism discussed here can be used for a unified treatment of both
ferromagnets and antiferromagnets in the semiclassical limit. We show that the
low (high) energy effective Hamiltonian of a (S_1, S_2) Heisenberg ferrimagnet
is a ferromagnetic (antiferromagnetic) Heisenberg model. We also study the
phase diagram of quantum ferrimagnets in the presence of an external magnetic
field h ($h_{c1} < h < h_{c2}$) and show that the low- and the high-field
phases correspond respectively to the classical N\'eel and the fully polarized
ferromagnetic states. We also calculate the transition temperature for the
Berezinskii-Kosterlitz-Thouless phase transition in the special case of
two-dimensional quantum ferrimagnets.


title: A Model for Ordinary Levy Motion

abstract:
  We propose a simple model based on the Gnedenko limit theorem for simulation
and studies of the ordinary Levy motion, that is, a random process, whose
increments are independent and distributed with a stable probability law. We
use the generalized structure function for characterizing anomalous diffusion
rate and propose to explore the modified Hurst method for empirical rescaled
range analysis. We also find that the structure function being estimated from
the ordinary Levy motion sample paths as well as the (ordinary) Hurst method
lead to spurious ''pseudo-Gaussian'' relations.


title: Anisotropy and magnetism of high temperature oxides superconductors

abstract:
  Phonon or electron mediated weak BCS attraction is enough to have high
critical temperature if a van Hove anomaly is at work. This could apply to
electron doped compounds and also to compounds with CuO$_2$ planes overdoped in
holes, where $T_c$ decreases with increasing doping. If phonons dominate, it
should lead to an anisotropic but mainly $s$ superconductive gap, as observed
recently in overdoped LaSrCuO, and probably also in electron doped compounds.
If electrons dominate, a $d$ gap should develop as observed in a number of
cases. In the underdoped range, the observed decrease of $T_c$ with hole doping
can be related in all cases to the development of antiferromagnetic
fluctuations which produces a magnetic pseudogap, thus lowering the density of
states at the Fermi level. The observed mainly $d$ superconductive gap then can
be due to a prevalent superconductive coupling through antiferromagnetic
fluctuations; it could also possibly be attributed to the same phonon coupling
as in the overdoped range, now acting on Bloch functions scattered in the
magnetic pseudogap. More systematic studies of superconductive gap anisotropy
and of magnetic fluctuations would be in order.


title:
Confined Harmonically Interacting Spin-Polarized Fermions in a Magnetic
  Field: Thermodynamics

abstract:
  We investigate the combined influence of a magnetic field and a harmonic
interparticle interaction on the thermodynamic properties of a finite number of
spin polarized fermions in a confiment potential. This study is an extension
using our path integral approach of symmetrized density matrices for identical
particles. The thermodynamical properties are calculated for a three
dimensional model of N harmonically interacting spin polarized fermions in a
parabolic potential well in the presence of a magnetic field. The free energy
and the internal energy are obtained for a limited number of particles.
Deviations from the thermodynamical limit become negligible for about 100 or
more particles, but even for a smaller number of fermions present in the well,
scaling relations similar to those of the continuum approximation to the
density of states are already satisfied.


title:
Static and Dynamic Properties of a Viscous Silica Melt Molecular
  Dynamics Computer Simulations

abstract:
  We present the results of a large scale molecular dynamics computer
simulation in which we investigated the static and dynamic properties of a
silica melt in the temperature range in which the viscosity of the system
changes from O(10^-2) Poise to O(10^2) Poise. We show that even at temperatures
as high as 4000 K the structure of this system is very similar to the random
tetrahedral network found in silica at lower temperatures. The temperature
dependence of the concentration of the defects in this network shows an
Arrhenius law. From the partial structure factors we calculate the neutron
scattering function and find that it agrees very well with experimental neutron
scattering data. At low temperatures the temperature dependence of the
diffusion constants $D$ shows an Arrhenius law with activation energies which
are in very good agreement with the experimental values. With increasing
temperature we find that this dependence shows a cross-over to one which can be
described well by a power-law, D\propto (T-T_c)^gamma. The critical temperature
T_c is 3330 K and the exponent gamma is close to 2.1. Since we find a similar
cross-over in the viscosity we have evidence that the relaxation dynamics of
the system changes from a flow-like motion of the particles, as described by
the ideal version of mode-coupling theory, to a hopping like motion. We show
that such a change of the transport mechanism is also observed in the product
of the diffusion constant and the life time of a Si-O bond, or the space and
time dependence of the van Hove correlation functions.


title: Long range Neel order in the triangular Heisenberg model

abstract:
  We have studied the Heisenberg model on the triangular lattice using several
Quantum Monte Carlo (QMC) techniques (up to 144 sites), and exact
diagonalization (ED) (up to 36 sites). By studying the spin gap as a function
of the system size we have obtained a robust evidence for a gapless spectrum,
confirming the existence of long range Neel order. Our best estimate is that in
the thermodynamic limit the order parameter m= 0.41 +/- 0.02 is reduced by
about 59% from its classical value and the ground state energy per site is
e0=-0.5458 +/- 0.0001 in unit of the exchange coupling. We have identified the
important ground state correlations at short distance.


title: Theory of sound attenuation in glasses: The role of thermal vibrations

abstract:
  Sound attenuation and internal friction coefficients are calculated for a
realistic model of amorphous silicon. It is found that, contrary to previous
views, thermal vibrations can induce sound attenuation at ultrasonic and
hypersonic frequencies that is of the same order or even larger than in
crystals. The reason is the internal-strain induced anomalously large
Gr\"uneisen parameters of the low-frequency resonant modes.


title: Semiclassical theory of transport in a random magnetic field

abstract:
  We study the semiclassical kinetics of 2D fermions in a smoothly varying
magnetic field $B({\bf r})$. The nature of the transport depends crucially on
both the strength $B_0$ of the random component of $B({\bf r})$ and its mean
value $\bar{B}$. For $\bar{B}=0$, the governing parameter is $\alpha=d/R_0$,
where $d$ is the correlation length of disorder and $R_0$ is the Larmor radius
in the field $B_0$. While for $\alpha\ll 1$ the Drude theory applies, at
$\alpha\gg 1$ most particles drift adiabatically along closed contours and are
localized in the adiabatic approximation. The conductivity is then determined
by a special class of trajectories, the "snake states", which percolate by
scattering at the saddle points of $B({\bf r})$ where the adiabaticity of their
motion breaks down. The external field also suppresses the diffusion by
creating a percolation network of drifting cyclotron orbits. This kind of
percolation is due only to a weak violation of the adiabaticity of the
cyclotron rotation, yielding an exponential drop of the conductivity at large
$\bar{B}$. In the regime $\alpha\gg 1$ the crossover between the snake-state
percolation and the percolation of the drift orbits with increasing $\bar{B}$
has the character of a phase transition (localization of snake states) smeared
exponentially weakly by non-adiabatic effects. The ac conductivity also
reflects the dynamical properties of particles moving on the fractal
percolation network. In particular, it has a sharp kink at zero frequency and
falls off exponentially at higher frequencies. We also discuss the nature of
the quantum magnetooscillations. Detailed numerical studies confirm the
analytical findings. The shape of the magnetoresistivity at $\alpha\sim 1$ is
in good agreement with experimental data in the FQHE regime near $\nu=1/2$.


title: Evolutionary Dynamics of the World Wide Web

abstract:
  We present a theory for the growth dynamics of the World Wide Web that takes
into account the wide range of stochastic growth rates in the number of pages
per site, as well as the fact that new sites are created at different times.
This leads to the prediction of a universal power law in the distribution of
the number of pages per site which we confirm experimentally by analyzing data
from large crawls made by the search engines Alexa and Infoseek. The existence
of this power law not only implies the lack of any length scale for the Web,
but also allows one to determine the expected number of sites of any given size
without having to exhaustively crawl the Web.


title: Spin domains in ground state spinor Bose-Einstein condensates

abstract:
  Bose-Einstein condensates of dilute atomic gases, characterized by a
macroscopic population of the quantum mechanical ground state, are a new,
weakly interacting quantum fluid. In most experiments condensates in a single
weak field seeking state are magnetically trapped. These condensates can be
described by a scalar order parameter similar to the spinless superfluid 4He.
Even though alkali atoms have angular momentum, the spin orientation is not a
degree of freedom because spin flips lead to untrapped states and are therefore
a loss process. In contrast, the recently realized optical trap for sodium
condensates confines atoms independently of their spin orientation. This opens
the possibility to study spinor condensates which represent a system with a
vector order parameter instead of a scalar. Here we report a study of the
equilibrium state of spinor condensates in an optical trap. The freedom of spin
orientation leads to the formation of spin domains in an external magnetic
field. The structure of these domains are illustrated in spin domain diagrams.
Combinations of both miscible and immiscible spin components were realized.


title:
A Unified Algebraic Approach to Few and Many-Body Hamiltonians having
  Linear Spectra

abstract:
  We develop an algebraic approach for finding the eigenfunctions of a large
class of few and many-body Hamiltonians, in one and higher dimensions, having
linear spectra. The method presented enables one to exactly map these
interacting Hamiltonians to decoupled oscillators, thereby, giving a precise
correspondence between the oscillator eigenspace and the wavefunctions of these
quantum systems. The symmetries behind the degeneracy structure and the
commuting constants of motion responsible for the quantum integrability of some
of these models are made transparent. Apart from analysing a number of
well-known dynamical systems like planar oscillators with commensurate
frequencies, both with or without singular inverse-square terms and generalized
Calogero-Sutherland type models, we also point out a host of other examples
where the present approach can be profitably employed. We further study
Hamiltonians having Laughlin wavefunction as the ground-state and establish
their equivalence to free oscillators. This reveals the underlying linear
$W_{1+\infty}$ symmetry algebra unambiguously and establishes the Laughlin
wavefunction as the highest weight vector of this algebra.

categories: cond-mat hep-th nlin.SI quant-ph solv-int


title: Integrable impurities in Hubbard chain with the open boundary condition

abstract:
  The Kondo problem of two impurities in 1D strongly correlated electron system
within the framework of the open boundary Hubbard chain is solved and the
impurities, coupled to the ends of the electron system, are introduced by their
scattering matrices with electrons so that the boundary matrices satisfy the
reflecting integrability condition. The finite size correction of the ground
state energy is obtained due to the impurities. Exact expressions for the low
temperature specific heat contributed by the charge and spin parts of the
magnetic impurities are derived. The Pauli susceptibility and the Kondo
temperature are given explicitly. The Kondo temperature is inversely
proportional to the density of electrons.


title: New Approach on the General Shape Equation of Axisymmetric Vesicles

abstract:
  The general Helfrich shape equation determined by minimizing the curvature
free energy describes the equilibrium shapes of the axisymmetric lipid bilayer
vesicles in different conditions. It is a non-linear differential equation with
variable coefficients. In this letter, by analyzing the unique property of the
solution, we change this shape equation into a system of the two differential
equations. One of them is a linear differential equation. This equation system
contains all of the known rigorous solutions of the general shape equation. And
the more general constraint conditions are found for the solution of the
general shape equation.


title: Theory of spin wave excitation in manganites

abstract:
  The role of the orbital degrees of freedom is studied theoretically for the
spin dynamics of $R_{1-x}A_x$MnO$_3$. Based on the meanfield solution, an RPA
calculation has been done and it is found that the $d_{x^2-y^2}$-type orbital
is essential for the double-exchange (DE) interactions, i.e., the DE is
basically two-dimensional interaction. Based on this results compared with
experiments, we propose that the orbital wavefunction is $d_{x^2-y^2}$-type
locally even in the metallic ferromagnetic state, which fluctuate quantum
mechanically. Well agreement of the estimation with experiments suggest that
the Jahn-Teller phonon has less importance on the spin dynamics.


title:
Rotational relaxation in Brownian rotors with frustrated interactions on
  a square lattice

abstract:
  We present simulation results on the equilibrium relaxation of Brownian
planar rotors based on a uniformly frustrated XY model on a square lattice. The
rotational relaxation exhibits typical dynamic features of fragile supercooled
liquids including the two-step relaxation. We observe a dynamic cross-over from
high temperature regime with Arrhenius behavior to low temperaure regime with
temperature-dependent activation energy. A consistent picture for the observed
slow dynamics can be given in terms of caging effect and thermal activation
across potential barriers in the energy landscapes.


title: Temperature dependence of the resistivity in the double-exchange model

abstract:
  The resistivity around the ferromagnetic transition temperature in the double
exchange model is studied by the Schwinger boson approach. The spatial spin
correlation responsible for scattering of conduction electrons are taken into
account by adopting the memory function formalism. Although the correlation
shows a peak lower than the transition temperature, the resistivity in the
ferromagnetic state monotonically increases with increasing temperature due to
a variation of the electronic state of the conduction electron. In the
paramagnetic state, the resistivity is dominated by the short range correlation
of scattering and is almost independent of the temperature. It is attributed to
a cancellation between the nearest-neighbor spin correlation, the fermion
bandwidth, and the fermion kinetic energy. This result implies the importance
of the temperature dependence of the electronic states of the conduction
electron as well as the localized spin states in both ferromagnetic and
paramagnetic phases.


title: Liesegang patterns : Studies on the width law

abstract:
  The so-called "width law" for Liesegang patterns, which states that the
positions x_n and widths w_n of bands verify the relation x_n \sim w_n^{\alpha}
for some \alpha>0, is investigated both experimentally and theoretically. We
provide experimental data exhibiting good evidence for values of \alpha close
to 1. The value \alpha=1 is supported by theoretical arguments based on a
generic model of reaction-diffusion.


title:
Density matrix renormalization group for disordered bosons in one
  dimension

abstract:
  We calculate the zero-temperature phase diagram of the disordered
Bose-Hubbard model in one dimension using the density matrix renormalization
group. For integer filling the Mott insulator is always separated from the
superfluid by a Bose glass phase. There is a reentrance of the Bose glass both
as a function of the repulsive interaction and of disorder. At half-filling
where no Mott insulator exists, the superfluid density has a maximum where the
kinetic and repulsive energies are about the same. Superfluidity is suppressed
both for small and very strong repulsion but is always monotonic in disorder.


title:
Negative magnetoresistance in La(0.6)Y(0.1)Ca(0.3)MnO(3): Evidence for
  charge localization governed by the Curie-Weiss law

abstract:
  Colossal negative magnetoresistance \Delta \rho (T,B) observed in
La(0.6)Y(0.1)Ca(0.3)MnO(3) at B=1T shows a nearly perfect symmetry around
T_0=160K suggesting a universal field-induced transport mechanism in this
material. Attributing this symmetry to strong magnetic fluctuations (triggered
by the Y substitution and further enhanced by magnetic field, both above and
below the field-dependent Curie temperature T_C(B)=T_0), the data are
interpreted in terms of the nonthermal spin hopping and magnetization M
dependent charge carrier localization scenario leading to \Delta \rho (T,B)=
-\rho_s(1-exp(-\gamma M^2)) with M(T,B)=CB/|T-T_C|^n. The separate fits through
all the data points above and below T_C yield C^{+}\simeq C^{-} and n^{+}\simeq
n^{-}\simeq 1. The obtained results corroborate the importance of fluctuation
effects in this material recently found (cond-mat/9812219) to dominate its
magneto-thermopower behavior far beyond T_C.


title: Nonlinear dynamics in superlattices driven by high frequency ac-fields

abstract:
  We investigate the dynamical processes taking place in nanodevices driven by
high-frequency electromagnetic fields. We want to elucidate the role of
different mechanisms that could lead to loss of quantum coherence. Our results
show how the dephasing effects of disorder that destroy after some periods
coherent oscillations, such as Rabi oscillations, can be overestimated if we do
not consider the electron-electron interactions that can reduce dramatically
the decoherence effects of the structural imperfections. Experimental
conditions for the observation of the predicted effects are discussed.


title: Tunneling problems by quantum Monte Carlo

abstract:
  We develop a new numerical scheme which allows precise solution of coherent
tunneling problems, i.e., problems with exponentially small transition
amplitudes between quasidegenerate states. We explain how this method works for
the single-particle (tunneling in the double-well potential) and many-body
systems (e.g., vacuum-to-vacuum transitions), and gives directly the instanton
shape and tunneling amplitude. Most importantly, transition amplitudes may be
calculated to arbitrary accuracy (being limited solely by statistical errors)
no matter how small are their absolute values.


title:
Coexistence of ferromagnetism and superconductivity in the hybrid
  ruthenate-cuprate compound RuSr_2GdCu_2O_8 studied by muon spin rotation (\mu
  SR) and DC-magnetization

abstract:
  We have investigated the magnetic and the superconducting properties of the
hybrid ruthenate-cuprate compound RuSr_{2}GdCu_{2}O_{8} by means of zero-field
muon spin rotation- (ZF-$\mu $SR) and DC magnetization measurements. The
DC-magnetisation data establish that this material exhibits ferromagnetic order
of the Ru-moments ($\mu (Ru) \approx 1 \mu_{B}$) below T_{Curie} = 133 K and
becomes superconducting at a much lower temperature T_c = 16 K. The ZF-$\mu $SR
experiments indicate that the ferromagnetic phase is homogeneous on a
microscopic scale and accounts for most of the sample volume. They also suggest
that the magnetic order is not significantly modified at the onset of
superconductivity.


title:
Vortex motion in superconducting YBCO inferred from the damping of the
  oscillations of a levitating magnetic microsphere

abstract:
  The damping of the oscillations of a small permanent magnet (spherical shape,
radius 0.1 mm) levitating between two parallel YBCO surfaces is measured as a
function of oscillation amplitude and temperature. The losses in the samples
(epitaxial thin films, bulk granular and bulk melt-textured) are analyzed in
terms of oscillating shielding currents flowing through trapped flux lines
whose motion gives rise to electric fields. We find dissipation to originate
from different mechanisms of flux dynamics. At small amplitudes there is a
linear regime described by a surface resistance varying from 10^-9 Ohm for bulk
samples down to 10^-13 Ohm for the thin films at low temperatures. With
increasing amplitude various nonlinear regimes are observed, firstly collective
pinning with diverging energy barriers, secondly in bulk samples above 85 K
hysteretic damping, and finally in thin films exponentially large losses which
can be described by pinning energies vanishing linearly at large currents.


title: Aging and Levy distributions in sandpiles

abstract:
  Aging in complex systems is studied via the sandpile model. Relaxation of
avalanches in sandpiles is observed to depend on the time elapsed since the
begining of the relaxation. Levy behavior is observed in the distribution of
characteristic times. In this way, aging and self-organized criticality appear
to be closely related.


title: Vortex Rings and Mutual Drag in Trapped Bose-Einstein Condensates

abstract:
  We study the drag on an object moving through a trapped Bose-Einstein
condensate, and show that finite compressibility leads to a mutual drag, which
is subsequently suppressed by the formation of a vortex ring.


title:
Controlled Drift of Indirect Excitons in Coupled Quantum Wells: Toward
  Bose Condensation in a Two-Dimensional Trap

abstract:
  We have succeeded in trapping indirect excitons in coupled quantum wells in a
harmonic potential minimum via inhomogeneous applied stress and electric field.
These excitons exhibit a strong Stark shift (over 60 meV), long lifetime (100
ns), and high diffusivity (1000 cm$^2$/s). This approach is very promising for
obtaining the high exciton density needed for Bose condensation of excitons in
two dimensions.


title:
Frequency Dependent Specific Heat of Amorphous Silica: A Molecular
  Dynamics Computer Simulation

abstract:
  We use molecular dynamics computer simulations to calculate the frequency
dependence of the specific heat of a SiO_2 melt. The ions interact with the BKS
potential and the simulations are done in the NVE ensemble. We find that the
frequency dependence of the specific heat shows qualitatively the same behavior
as the one of structural quantities, in that at high frequencies a microscopic
peak is observed and at low frequencies an alpha-peak, the location of which
quickly moves to lower frequencies when the temperature is decreased.


title:
SQUID based resistance bridge for shot noise measurement on low
  impedance samples

abstract:
  We present a resistance bridge which uses a SQUID to measure the shot noise
in low impedance samples. The experimental requirements are high DC bias
currents (typically 10mA) together with high AC sensitivity (pA/VHz). This
system is used to investigate the shot noise in
Superconductor/Normal/Superconductor junctions where Andreev reflection
enhanced shot noise is expected. Because our setup has an intrinsic noise much
smaller than the thermal noise of the resistance bridge at 4.2K, reliable
results can be obtained on impedances out of the range of classical measurement
schemes.


title: Algebraic properties of an integrable t-J model with impurities

abstract:
  We investigate the algebraic structure of a recently proposed integrable
$t-J$ model with impurities. Three forms of the Bethe ansatz equations are
presented corresponding to the three choices for the grading. We prove that the
Bethe ansatz states are highest weight vectors of the underlying $gl(2|1)$
supersymmetry algebra. By acting with the $gl(2|1)$ generators we construct a
complete set of states for the model.


title: Effect of disorder on the Kondo behavior of thin Cu(Mn) films

abstract:
  We have studied the influence of disorder on the Kondo effect in thin films
of Cu(Mn), i.e., Cu doped with a small amount of Mn. We find that the Kondo
contribution to the resistivity is suppressed when the elastic mean-free-path,
$\lambda$, is reduced. While this is qualitatively similar to results found
previously by our group in a different material, our new experiments reveal in
detail how this suppression depends on both film thickness and $\lambda$. These
results are compared with the theory of Martin, Wan, and Phillips. While there
is general qualitative agreement with this theory, there appear to be some
quantitative discrepancies.


title: Delocalization Transition of a Rough Adsorption-Reaction Interface

abstract:
  We introduce a new kinetic interface model suitable for simulating
adsorption-reaction processes which take place preferentially at surface
defects such as steps and vacancies. As the average interface velocity is taken
to zero, the self- affine interface with Kardar-Parisi-Zhang like scaling
behaviour undergoes a delocalization transition with critical exponents that
fall into a novel universality class. As the critical point is approached, the
interface becomes a multi-valued, multiply connected self-similar fractal set.
The scaling behaviour and critical exponents of the relevant correlation
functions are determined from Monte Carlo simulations and scaling arguments.


title: Magnetic-Field-Induced Exciton Tunneling in Shallow Quantum Wells

abstract:
  We study the effect of the magnetic field orientation on the
electroabsorption spectra of excitons confined in extremely shallow quantum
wells. When the applied electric field is parallel to the quantum well plane,
we demonstrate that, for in-plane magnetic field orientation, the discrete
confined exciton peak undergoes a transition into a continuum resonance. In
contrast, for perpendicular magnetic fields, the exciton peak exhibits the
usual Stark red-shift. We show that such a dramatic dependence on the magnetic
field orientation originates from a resonant coupling between the confined and
the bulk-like excitons. Such coupling is caused by the interplay between the
quantum-well potential and a velocity-dependent two-body interaction between
the exciton center-of-mass and relative motion degrees of freedom induced by
the in-plane magnetic field. As a result, the exciton tunnels out of the
quantum well as a whole without being ionized. We discuss possible experimental
applications of this effect.


title: Orientational order on curved surfaces - the high temperature region

abstract:
  We study orientational order, subject to thermal fluctuations, on a fixed
curved surface. We derive, in particular, the average density of zeros of
Gaussian distributed vector fields on a closed Riemannian manifold. Results are
compared with the density of disclination charges obtained from a Coulomb gas
model. Our model describes the disordered state of two dimensional objects with
orientational degrees of freedom, such as vector ordering in Langmuir
monolayers and lipid bilayers above the hexatic to fluid transition.


title:
Phenomenology of the Term Structure of Interest Rates with Pade
  Approximants

abstract:
  The classical approach in finance attempts to model the term structure of
interest rates using specified stochastic processes and the no arbitrage
argument. Up to now, no universally accepted theory has been obtained for the
description of experimental data. We have chosen a more phenomenological
approach. It is based on results obtained some twenty years ago by physicists,
results which show that Pad\'e Approximants are very suitable for approximating
large classes of functions in a very precise and coherent way. In this paper,
we have chosen to compare Pad\'e Approximants with very low indices with the
experimental densities of interest rates variations. We have shown that the
data published by the Federal Reserve System in the United States are very well
reproduced with two parameters only. These parameters are rather simple
functions of the lag and of the maturity and are directly related to the
moments of the distributions.


title:
Berry Phase and the Symmetry of the Vibronic Ground State in Dynamical
  Jahn-Teller Systems

abstract:
  Due to the frequent presence of a Berry phase, in most cases of dynamical
Jahn-Teller systems the symmetry of the ground state is the same as that of the
electronic state. However, the H x h icosahedral case, relevant for the physics
of fullerene ions, provides a first example of linear coupling leading, at
strong coupling, to a change in symmetry of the ground state to a totally
symmetric nondegenerate state. We generalize this observation and show through
detailed examples that the absence of a Berry phase can, but does not
necessarily, lead to a nondegenerate ground state.


title: Coulomb Drag in Double Layers with Correlated Disorder

abstract:
  We study the effect of correlations between impurity potentials in different
layers on the Coulomb drag in a double-layer electron system. It is found that
for strongly correlated potentials the drag in the diffusive regime is
considerably enhanced as compared to conventional predictions. The appropriate
experimental conditions are discussed, and the new experiments are suggested.


title:
Critical and off-critical studies of the Baxter-Wu model with general
  toroidal boundary conditions

abstract:
  The operator content of the Baxter-Wu model with general toroidal boundary
conditions is calculated analytically and numerically. These calculations were
done by relating the partition function of the model with the generating
function of a site-colouring problem in a hexagonal lattice. Extending the
original Bethe-ansatz solution of the related colouring problem we are able to
calculate the eigenspectra of both models by solving the associated
Bethe-ansatz equations. We have also calculated, by exploring the conformal
invariance at the critical point, the mass ratios of the underlying massive
theory governing the Baxter-Wu model in the vicinity of its critical point.

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


title: A unified description of static and dynamic properties of Fermi liquids

abstract:
  In Landau's phenomenological Fermi-liquid theory (FLT), most physical
quantities are derived from the knowledge of the energy variation $\delta
E[\delta n]$ corresponding to a change $\delta n$ of the quasi-particle (QP)
distribution function $n \equiv {n_{k \sigma}}$. We show that the internal
energy $E[n]$ (or, more precisely, the thermodynamic potential $\Phi[n]$),
expressed as a function of the QP distribution $n$, can be interpreted as an
effective potential (in the sense of field theory), which is obtained from the
free energy by a Legendre transformation. This allows to obtain explicitly
$\delta\Phi$ (or $\delta E$) starting from a microscopic Hamiltonian and to
relate the Landau $f$ function to the forward-scattering two-particle vertex
without considering the collective modes as in the standard diagrammatic
derivation of FLT. Out-of-equilibrium properties are obtained by extending the
definition of the effective potential to space- and time-dependent
configurations. $\Phi[n]$ is then a functional of the Wigner distribution
function $n \equiv {n_{k \sigma}(r,t)}$. It contains information about both the
static and dynamic properties of the Fermi liquid. In particular, it yields the
quantum Boltzmann equation satisfied by $n_{k \sigma}(r,t)$. Finally, we show
how $\delta\Phi[\delta n]$ can be derived (in the static case) using a
finite-temperature renormalization-group approach. In agreement with previous
results based on this technique, we find that the Landau $f$ function is
defined by the fixed-point value of the $\Omega$-limit of the
forward-scattering two-particle vertex.


title: Thermodynamically admissible form for discrete hydrodynamics

abstract:
  We construct a discrete model of fluid particles according to the GENERIC
formalism. The model has the form of Smoothed Particle Hydrodynamics including
correct thermal fluctuations. A slight variation of the model reproduces the
Dissipative Particle Dynamics model with any desired thermodynamic behavior.
The resulting algorithm has the following properties: mass, momentum and energy
are conserved, entropy is a non-decreasing function of time and the thermal
fluctuations produce the correct Einstein distribution function at equilibrium.


title: The center-of-mass response of confined systems

abstract:
  For confined systems of identical particles, either bosons or fermions, we
argue that the parabolic nature of the confinement potential is a prerequisite
for the non-dissipative character of the center of mass response to a uniform
probe. For an excitation in a parabolic confining potential, the half width of
the density response function depends nevertheless quantitatively on properties
of the internal degrees of freedom, as is illustrated here for an ideal
confined gas of identical particles with harmonic interparticle interactions.


title:
Transformation of Thernal Energy in Electric Energy in an Inhomogeneous
  Superconducting Ring

abstract:
  It is shown that a heat energy can be transformed in an electric energy of
direct current by the use of an inhomogeneous superconducting ring. This
trasformation is possible both at an iterative change of temperature and at an
unaltered temperature. In the first case the ring is a classical heat engine
with a maximum efficiency in the Carno cycle. The second case contradicts to
the second thermodynamic law formulated in the middle of last century.
Nevertheless this result is correct because the thermal fluctuation is an
opposite (relatively of the second thermodynamic law) consequence of the law of
entropy increase. In order to introduce some order into the chaotic energy of
the thermal fluctuations a preferential direction should exist. Such
preferential direction of superconducting electron motion exists in a ring if a
magnetic flux is not divisible by the flux quantum.


title: Hydrodynamic lift on bound vesicles

abstract:
  Bound vesicles subject to lateral forces such as arising from shear flow are
investigated theoretically by combining a lubrication analysis of the bound
part with a scaling approach to the global motion. A minor inclination of the
bound part leads to significant lift due to the additive effects of lateral and
tank-treading motions. With increasing shear rate, the vesicle unbinds from the
substrate at a critical value. Estimates are in agreement with recent
experimental data.


title:
Fermi Statistics of Weakly Exicted Granular Systems in a Vibrating Bed
  I: Molecular Dynamics Simulations

abstract:
  Molecular dynamics simulations were carried out to test the thermodynamic
theory of weakly excited, two-dimensional granular systems [Hayakawa and Hong,
Phys. Rev. Lett. 78, 2764 (1997)], where granular materials are viewed as a
collection of spinless Fermions. We first determine the global temperature T by
fitting the steady state density profile to the Fermi distribution function,
and then measure the center of mass, <z(T)>, and its fluctuations, <(\Delta
z(T))^2> as a function of T. We find a fairly good agreement between theory and
simulations, in particular, in the estimation of the temperature and the
scaling behavior of <z(T)> and <(\Delta z(T))^2>.


title: Elementary mechanisms governing the dynamics of silica

abstract:
  A full understanding of glasses requires an accurate atomistic picture of the
complex activated processes that constitute the low-temperature dynamics of
these materials. To this end, we generate over five thousand activated events
in silica glass, using the activation-relaxation technique; these atomistic
mechanisms are analysed and classified according to their activation energies,
their topological properties and their spatial extend. We find that these are
collective processes, involving ten to hundreds of atoms with a continuous
range of activation energies; that diffusion and relaxation occurs through the
creation, annihilation and motion of single dangling bonds; and that silicon
and oxygen have essentially the same diffusivity.


title: Statistical properties of fractures in damaged materials

abstract:
  We introduce a model for the dynamics of mud cracking in the limit of of
extremely thin layers. In this model the growth of fracture proceeds by
selecting the part of the material with the smallest (quenched) breaking
threshold. In addition, weakening affects the area of the sample neighbour to
the crack. Due to the simplicity of the model, it is possible to derive some
analytical results. In particular, we find that the total time to break down
the sample grows with the dimension L of the lattice as L^2 even though the
percolating cluster has a non trivial fractal dimension. Furthermore, we obtain
a formula for the mean weakening with time of the whole sample.


title: Low energy collective excitations in a superfluid trapped Fermi gas

abstract:
  We study low energy collective excitations in a trapped superfluid Fermi gas,
that describe slow variations of the phase of the superfluid order parameter.
Well below the critical temperature the corresponding eigenfrequencies turn out
to be of the order of the trap frequency, and these modes manifest themselves
as the eigenmodes of the density fluctuations of the gas sample. The latter
could provide an experimental evidence of the presence of the superfluid phase.


title: Bragg spectroscopy of a Bose-Einstein condensate

abstract:
  Properties of a Bose-Einstein condensate were studied by stimulated,
two-photon Bragg scattering. The high momentum and energy resolution of this
method allowed a spectroscopic measurement of the mean-field energy and of the
intrinsic momentum uncertainty of the condensate. The coherence length of the
condensate was shown to be equal to its size. Bragg spectroscopy can be used to
determine the dynamic structure factor over a wide range of energy and momentum
transfers.


title: Wigner-Dyson Statistics from the Replica Method

abstract:
  We compute the correlation functions of the eigenvalues in the Gaussian
unitary ensemble using the fermionic replica method. We show that non--trivial
saddle points, which break replica symmetry, must be included in the
calculation in order to reproduce correctly the exact results for the
correlation functions at large distance.


title:
Interfacial tension behavior of binary and ternary mixtures of partially
  miscible Lennard-Jones fluids: a molecular dynamics simulation

abstract:
  By means of extensive equilibrium molecular dynamics simulations we have
investigated, the behavior of the interfacial tension $\gamma$ of two
immiscible symmetrical Lennard-Jones fluids. This quantity is studied as
function of reduced temperature $T^{*}={{k_{_B} T}\over \epsilon}$ in the range
$0.6 \leq T^{*} \leq 3.0$. We find that, unlike the monotonic decay obtained
for the liquid-vapor interfacial tension, for the liquid-liquid interface,
$\gamma (T)$ has a maximum at a specific temperature. We also investigate the
effect that surfactant-like particles has on the thermodynamic as well as the
structural properties of the liquid-liquid interface. It is found that $\gamma$
decays monotonically as the concentration of the surfactant-like particles
increases.


title: Paramagnetic instability at normal-metal - superconductor interfaces

abstract:
  We study the proximity coherence in a mesoscopic normal-metal film (N) in
contact with a superconductor (S). Accounting for a repulsive interaction
between the electrons in the normal metal, we find an enhanced local density of
states close to the NS interface. The sharp peak in the density is pinned to
the Fermi energy and leads to spontaneous paramagnetic interface currents. The
induced orbital magnetic moments exhibit the characteristic features of
paramagnetic reentrance observed in normal-metal coated superconducting
cylinders [Phys. Rev. Lett. 65, 1514 (1990)].


title:
Fermi Statistics of Weakly Excited Granular Materials in a Vibrating Bed
  II: One Dimensional Experiment

abstract:
  A one dimensional experiment in granular dynamics is carried out to test the
thermodynamic theory of weakly excited granular systems [Hayakawa and Hong,
Phys. Rev. Lett. {\bf 78}, 2764(1997)] where granular particles are treated as
spinless Fermions. The density profile is measured and then fit to the Fermi
distribution function, from which the global temperature of the system, T, is
determined. Then the center of mass, <z(T)>, and its fluctuations, <(\Delta
z(T))^2>, are measured and plotted as functions of T. The Fermi function fits
the density profile fairly well, with the value of T being fairly close to the
predicted value. The scaling behavior of <z(T)> and <(\Delta z(T))^2> is in
excellent agreement with the theory.


title: Holes in the t-J_z model: a thorough study

abstract:
  The t-J_z model is the strongly anisotropic limit of the t-J model which
captures some general properties of the doped antiferromagnets (AF). The
absence of spin fluctuations simplifies the analytical treatment of hole motion
in an AF background and allows us to calculate the single- and two-hole spectra
with high accuracy using regular diagram technique combined with real-space
approach. At the same time, numerical studies of this model via exact
diagonalization (ED) on small clusters show negligible finite size effects for
a number of quantities, thus allowing a direct comparison between analytical
and numerical results. Both approaches demonstrate that the holes have tendency
to pair in the p- and d-wave channels at realistic values of t/J. The
interactions leading to pairing and effects selecting p and d waves are
thoroughly investigated. The role of transverse spin fluctuations is considered
using perturbation theory. Based on the results of the present study, we
discuss the pairing problem in the realistic t-J-like model. Possible
implications for preformed pairs formation and phase separation are drawn.


title:
A theoretical investigation of the low lying electronic structure of
  poly(p-phenylene vinylene)

abstract:
  The two-state molecular orbital model of the one-dimensional phenyl-based
semiconductors is applied to poly(p-phenylene vinylene). The energies of the
low-lying excited states are calculated using the density matrix
renormalization group method. Calculations of both the exciton size and the
charge gap show that there are both Bu and Ag excitonic levels below the band
threshold. The energy of the 1Bu exciton extrapolates to 2.60 eV in the limit
of infinite polymers, while the energy of the 2Ag exciton extrapolates to 2.94
eV. The calculated binding energy of the 1Bu exciton is 0.9 eV for a 13
phenylene unit chain and 0.6 eV for an infinite polymer. This is expected to
decrease due to solvation effects. The lowest triplet state is calculated to be
at ca. 1.6 eV, with the triplet-triplet gap being ca. 1.6 eV. A comparison
between theory, and two-photon absorption and electroabsorption is made,
leading to a consistent picture of the essential states responsible for most of
the third-order nonlinear optical properties. An interpretation of the
experimental nonlinear optical spectroscopies suggests an energy difference of
ca. 0.4 eV between the vertical energy and ca. 0.8 eV between the relaxed
energy, of the 1Bu exciton and the band gap, respectively.


title:
Possibility of direct Mott insulator-to-superfluid transitions in weakly
  disordered boson systems

abstract:
  We study the zero-temperature phase transitions of a two-dimensional
disordered boson Hubbard model at incommensurate boson densities. Via matrix
diagonalization and quantum Monte Carlo simulations, we construct the phase
diagram and evaluate the correlation length exponent nu. In the presence of
weak disorder, we obtain nu=0.5 +- 0.1, the same value as that in the pure
model, near the tip of a Mott insulator lobe, using the dynamical critical
exponent z=2. As the strength of disorder is increased beyond a certain value,
however, the value of nu is found to change to 0.9 +- 0.1. This result strongly
suggests that there exist direct Mott insulator-to-superfluid transitions
around the tip of a Mott insulator lobe in the weak disorder regime.


title:
Reply to the Comment on ``Charged impurity scattering limited low
  temperature resistivity of low density silicon inversion layers''

abstract:
  This is a Reply to the Comment (cond-mat/9812331) by Kravchenko et al. on our
earlier work (cond-mat/9812216).


title:
Superconducting Electrometer Based on the Resistively Shunted Bloch
  Transistor

abstract:
  We have fabricated the Bloch transistor shunted on-chip by a small-sized Cr
resistor with Rs about 1 kOhm. The Bloch transistor normally consists of two
small Josephson junctions connected in series, which in our case have been
replaced by two superconducting interferometer loops, each with two junctions
in parallel. A capacitively coupled gate is supplied to control the induced
charge of the small intermediate electrode (island) of the transistor. The
measured I-V curves show no hysteresis and correspond to the operation of a
effective Josephson junction at the high-damping and strong-noise limits. The
critical current of the system was found to be close to its nominal value, that
is in accordance with the electromagnetic environment theory. The I-V curves
were modulated by the gate with a period of e and a maximum swing of about 2
/mu_V. Such rather moderate modulation results from the Josephson-to- charging
energies ratio, Ej/Ec about 9, in our sample being far from its optimum value
of 0.3 up to 1.


title: Macroscopic Simulation of Widely Scattered Synchronized Traffic States

abstract:
  Recently, a phase transition to synchronized congested traffic has been
observed in empirical highway data [B. S. Kerner and H. Rehborn, Phys. Rev.
Lett. 79, 4030 (1997)]. This hysteretic transition has been described by a
non-local, gas-kinetic-based traffic model [D. Helbing and M. Treiber, Phys.
Rev. Lett. 81, 3042 (1998)] that, however, did not display the wide scattering
of synchronized states. Here, it is shown that the latter can be reproduced by
a mixture of different vehicle types like cars and trucks. The simulation
results are in good agreement with Dutch highway data.


title: Influence of uniaxial stress on the lamellar spacing of eutectics

abstract:
  Directional solidification of lamellar eutectic structures submitted to
uniaxial stress is investigated. In the spirit of an approximation first used
by Jackson and Hunt, we calculate the stress tensor for a two-dimensional
crystal with triangular surface, using a Fourier expansion of the Airy
function. crystal with triangular surface in contact with its melt, given that
a uniaxial external stress is applied. The effect of the resulting change in
chemical potential is introduced into the standard model for directional
solidification of a lamellar eutectic. This calculation is motivated by an
observation, made recently [I. Cantat, K. Kassner, C. Misbah, and H.
M\"uller-Krumbhaar, Phys. Rev. E, in press] that the thermal gradient produces
similar effects as a strong gravitational field in the case of dilute-alloy
solidification. Therefore, the coupling between the Grinfeld and the
Mullins-Sekerka instabilities becomes strong, as the critical wavelength of the
former instability gets reduced to a value close to that of the latter.
Analogously, in the case of eutectics, the characteristic length scale of the
Grinfeld instability should be reduced to a size not extremely far from typical
lamellar spacings. In a Jackson-Hunt like approach we average the undercooling,
including the stress term, over a pair of lamellae. Following Jackson and Hunt,
we assume the selected wavelength to be determined by the minimum undercooling
criterion and compute its shift due to the external stress. we realize the
shifting of the wavelength by the application of external stress. In addition,
we find that in general the volume fraction of the two solid phases is changed
by uniaxial stress. Implications for experiments on eutectics are discussed.


title: Bosons in a Toroidal Trap: Ground State and Vortices

abstract:
  We study the Bose-Einstein condensate (BEC) in a 3-D toroidal Mexican hat
trap. By changing the parameters of the potential, or the number of bosons, it
is possible to modify strongly the density profile of the BEC. We consider the
ground state properties for positive and negative scattering length and
calculate the spectrum elementary excitations. We also discuss the macroscopic
phase coherence and superfluidity of the BEC by analyzing vortex states and
their stability.


title:
Crossing lattices, vortex chains, and angular dependence of melting line
  in layered superconductors

abstract:
  We investigate vortex structure and melting transition in very anisotropic
layered superconductors at fields tilted with respect to the c-axis. We show
that even a small in-plane field does not homogeneously tilt the vortex lattice
but, instead, penetrates inside the superconductor in the form of Josephson
vortices (JVs) similar to the Meissner state. At high c-axis magnetic field the
phase field of the JV is built up from the phase perturbations created by
displacements of pancake vortices. The crossing-lattices ground state leads to
linear dependencies of the melting field and melting temperature on the
in-plane field, in agreement with recent experimental observations. At small
fields stacks of JVs accumulate additional pancake strings creating vortex rows
with enhanced density. This mechanism explains the mixed chains-lattice state
observed by Bitter decorations.


title: A Brief History of Our Understanding of BEC: From Bose to Beliaev

abstract:
  We review how our current ideas about BEC developed in the early period
1925-1965, which had the specific goal of understanding superfluid $^4$He. This
history is presented by commenting on the key contributions made by Einstein,
Fritz London, Tisza, Landau, Bogoliubov, Oliver Penrose and Feynman. We
emphasize the emergence of the concept of a macroscopic wavefunction describing
the condensate. Starting with the fundamental work of Beliaev in 1957, the
period 1957-1965 was a golden era for theoretical studies of interacting
Bose-condensed gases. This work provided a sound conceptual basis for
understanding the properties of trapped atomic gases which were discovered
thirty years later.


title:
Ripplonic Polaron in the Quasi-One-Dimensional Electron System over
  Liquid Helium

abstract:
  We study the energetics and transport properties of polarons in the quasi-one
dimensional electron system over the liquid helium surface. The localization
lengths and the energy of the ground and excited states are calculated within
the hydrodynamic model of the polaron. The polaron bound energy is found and
the temperature for the polaron formation is estimated below 0.1 K. We examine
the possibility of observation of the polarons by measuring both the
spectroscopic transitions and the mobility.


title: The Theory of Superfluidity of $^4$Helium

abstract:
  I present here a microscopic theory for the superfluidity of $^4$He (He II)
derived from experiments, and answer its essential questions. With a "momenton"
model, the superfluid is shown to feature as a "harmonic superfluid". In which
a new bonding type, the "superfluid bond", is formed. Its activation causes the
anomalous thermal excitation, the large excess of specific heat, etc. The
superfluidity mechanism is recognized being connected to a quantum confinement
effect. The theory predicts the basic properties of He II in overall agreements
with experiments. The series novel concepts evolved here reverse the current
perceptions of $^4$He, give significant impact to the understandings of other
superfluids.


title:
Electron-lattice relaxation, and soliton structures and their
  interactions in polyenes

abstract:
  Density matrix renormalisation group calculations of a suitably parametrised
model of long polyenes (polyacetylene oligomers), which incorporates both long
range Coulomb interactions and adiabatic lattice relaxation, are presented. The
triplet and 2Ag states are found to have a 2-soliton and 4-soliton form,
respectively, both with large relaxation energies. The 1Bu state forms an
exciton-polaron and has a very small relaxation energy. The relaxed energy of
the 2Ag state lies below that of the 1Bu state. The soliton/anti-soliton pairs
are bound.


title:
Anisotropy and temperature dependence of the optical conductivity in
  LaMnO_3

abstract:
  A tight binding parameterization of the band structure, along with a mean
field treatment of Hund, electron-electron, and electron-lattice couplings, is
used to obtain the full optical conductivity tensor of LaMnO_3 as a function of
temperature. We predict striking changes with temperature in the functional
form and magnitude of the optical absorption. Comparison of our results to data
will determine the Hund, electron-lattice, and electron-electron interactions.


title:
Colored noise in the fractional Hall effect: duality relations and exact
  results

abstract:
  We study noise in the problem of tunneling between fractional quantum Hall
edge states within a four probe geometry. We explore the implications of the
strong-weak coupling duality symmetry existent in this problem for relating the
various density-density auto-correlations and cross-correlations between the
four terminals. We identify correlations that transform as either ``odd'' or
``anti-symmetric'', or ``even'' or ``symmetric'' quantities under duality. We
show that the low frequency noise is colored, and that the deviations from
white noise are exactly related to the differential conductance. We show
explicitly that the relationship between the slope of the low frequency noise
spectrum and the differential conductance follows from an identity that holds
to {\it all} orders in perturbation theory, supporting the results implied by
the duality symmetry. This generalizes the results of quantum supression of the
finite frequency noise spectrum to Luttinger liquids and fractional statistics
quasiparticles.


title:
Multiple Andreev Reflection and Giant Excess Noise in Diffusive
  Superconductor/Normal-Metal/Superconductor Junctions

abstract:
  We have studied superconductor/normal metal/superconductor (SNS) junctions
consisting of short Au or Cu wires between Nb or Al banks. The Nb based
junctions display inherent electron heating effects induced by the high thermal
resistance of the NS boundaries. The Al based junctions show in addition
subharmonic gap structures in the differential conductance dI/dV and a
pronounced peak in the excess noise at very low voltages V. We suggest that the
noise peak is caused by fluctuations of the supercurrent at the onset of
Josephson coupling between the superconducting banks. At intermediate
temperatures where the supercurrent is suppressed a noise contribution ~1/V
remains, which may be interpreted as shot noise originating from large multiple
charges.


title: Dynamic scaling in the spatial distribution of persistent sites

abstract:
  The spatial distribution of persistent (unvisited) sites in one dimensional
$A+A\to\emptyset$ model is studied. The `empty interval distribution' $n(k,t)$,
which is the probability that two consecutive persistent sites are separated by
distance $k$ at time $t$ is investigated in detail. It is found that at late
times this distribution has the dynamical scaling form $n(k,t)\sim
t^{-\theta}k^{-\tau}f(k/t^{z})$. The new exponents $\tau$ and $z$ change with
the initial particle density $n_{0}$, and are related to the persistence
exponent $\theta$ through the scaling relation $z(2-\tau)=\theta$. We show by
rigorous analytic arguments that for all $n_{0}$, $1< \tau< 2$, which is
confirmed by numerical results.


title:
Classical diffusion of N interacting particles in one dimension: General
  results and asymptotic laws

abstract:
  I consider the coupled one-dimensional diffusion of a cluster of N classical
particles with contact repulsion. General expressions are given for the
probability distributions, allowing to obtain the transport coefficients. In
the limit of large N, and within a gaussian approximation, the diffusion
constant is found to behave as N^{-1} for the central particle and as (\ln
N)^{-1} for the edge ones. Absolute correlations between the edge particles
increase as (\ln N)^{2}. The asymptotic one-body distribution is obtained and
discussed in relation of the statistics of extreme events.