title:
Spatial Correlations of Mobility and Immobility in a Glassforming
  Lennard-Jones Liquid

abstract:
  Using extensive molecular dynamics simulations of an equilibrium,
glass-forming Lennard-Jones mixture, we characterize in detail the local atomic
motions. We show that spatial correlations exist among particles undergoing
extremely large (``mobile'') or extremely small (``immobile'') displacements
over a suitably chosen time interval. The immobile particles form the cores of
relatively compact clusters, while the mobile particles move cooperatively and
form quasi-one-dimensional, string-like clusters. The strength and length scale
of the correlations between mobile particles are found to grow strongly with
decreasing temperature, and the mean cluster size appears to diverge at the
mode-coupling critical temperature. We show that these correlations in the
particle displacements are related to equilibrium fluctuations in the local
potential energy and local composition.


title:
Magnetoresistance of Granular Ferromagnets - Observation of a Magnetic
  Proximity Effect?

abstract:
  We have observed a superparamagnetic to ferromagnetic transition in films of
isolated Ni grains covered by non-magnetic overlayers. The magnetoresistance
(MR) of the films was measured as a function of the overlayer thickness.
Initially, the granular Ni films exhibited negative MR curves peaked at H=0. As
different materials were deposited onto the grains hysteresis developed in the
MR. This behavior is ascribed to an increase of the typical domain size due to
magnetic coupling between grains. The strength of the inter-grain coupling is
found to correlate with the magnetic susceptibility of the overlayer material.
We discuss possible mechanisms for this coupling and suggest that the data may
reflect the existence of a magnetic proximity effect (analogous to the
well-known effect in superconductivity) in which a ferromagnetic moment is
induced in the metallic non-magnetic medium.


title:
Magnetic bound states in the quarter-filled ladder system
  $\alpha'-NaV_{2}O_{5}$}

abstract:
  Raman scattering in the quarter-filled spin ladder system alpha'-NaV_2O_5
shows in the dimerized singlet ground state ($T \leq T_{SP}=35K$) an unexpected
sequence of three magnetic bound states. Our results suggest that the recently
proposed mapping onto an effective spin chain for $T > T_{SP}$ has to be given
up in favor of the full topology and exchange paths of a ladder in the
dimerized phase for $T < T_{SP}$. As the new ground state we propose a dynamic
superposition of energetically nearly degenerate dimer configurations on the
ladder.


title: Universal properties for linelike melting of the vortex lattice

abstract:
  Using numerical results obtained within two models describing vortex matter
(interacting elastic lines (Bose model) and uniformly frustrated XY-model) we
establish universal properties of the melting transition within the linelike
regime. These properties, which are captured correctly by both models, include
the scaling of the melting temperature with anisotropy and magnetic field, the
effective line tension of vortices in the liquid regime, the latent heat, the
entropy jump per entanglement length, and relative jump of Josephson energy at
the transition as compared to the latent heat. The universal properties can
serve as experimental fingerprints of the linelike regime of melting.
Comparison of the models allows us to establish boundaries of the linelike
regime in temperature and magnetic field.


title: Zeroth Sound Modes of Dilute Fermi Gas with Arbitrary Spin

abstract:
  Motivated by the recent success of optical trapping of alkali bosons, we have
studied the zeroth sound modes of dilute Fermi gases with arbitrary spin-f,
which are spin-S excitations ($0\leq S\leq 2f$). The dispersion of the mode (S)
depends on a single Landau parameter $F^{(S)}$, which is related to the
scattering lengths of the system through a simple formula. Measurement of (even
a subset of) these modes in finite magnetic fields will enable one to determine
all the interaction parameters of the system.


title:
New Mechanism of Quantum Oscillations in the Suprcpnducing Mixed State
  (Hc1<<B<<Hc2)

abstract:
  We argue that inhomogeneity inherent to the presence of periodic
supercurrents in the vortex lattice sorts excitations by energies into the ones
that are spatially localized and those that perform motion along large Larmour
orbits. This energy threshold results in a new mechanism for the de Haas-van
Alphen oscillations which enhances oscillations at B<<Hc2, even for an
isotropic superconductor with a constant gap. We suggest that the mechanism is
of a general character and can cause the slow decay of the de Haas-van Alphen
effect when the field, B, decreases below Hc2.


title:
Microwave Resonance and Carrier-Carrier Interaction in Two-Dimensional
  Hole Systems at High Magnetic Field

abstract:
  Microwave frequency conductivity, Re($\sigma_{xx}$), of high quality
two-dimensional hole systems (2DHS) in a large perpendicular magnetic field (B)
is measured with the carrier density ($n_s$) of the 2DHS controlled by a
backgate bias. The high B insulating phase of the 2DHS exhibits a microwave
resonance that remains well-defined, but shifts to higher peak frequency
($f_{pk}$) as $n_s$ is reduced. Over a wide range of $n_s, f_{pk} \propto
n_s^{-1/2}$ is observed for the two samples we measured. The data clearly
indicate that both carrier-carrier interactions and disorder are indispensable
in determining the dynamics of the insulator. The $n_s$ dependence of $f_{pk}$
is consistent with a weakly pinned Wigner crystal in which domain size
increases with $n_s$, due to larger carrier-carrier interaction.


title:
Quantum Open-Systems Approach to Current Noise in Resonant Tunnelling
  Junctions

abstract:
  A quantum Markovian master equation is derived to describe the current noise
in resonant tunnelling devices. This equation includes both incoherent and
coherent quantum tunnelling processes. We show how to obtain the population
master equation by adiabatic elimination of quantum coherences in the presence
of elastic scattering. We calculate the noise spectrum for a double well device
and predict sub-shot noise statistics for strong tunnelling between the wells.
The method is an alternative to Green's functions methods, and population
master equations for very small coherently coupled quantum dots.


title:
Correlation-enhanced Friedel oscillations in amorphous alloys and
  quasicrystals

abstract:
  We show that quantum correlations induced by electron-electron interactions
in the presence of random impurity scattering can play an important role in the
thermal stabilization of amorphous Hume-Rothery systems: When there is strong
backscattering off local, concentrical ion clusters, the static electron
density response $\chi (0,q)$ acquires a powerlaw divergence at $q=2k_F$ even
at elevated temperature. This leads to an enhancement as well as to a
systematical phase shift of the Friedel oscillations, both consistent with
experiments. The possible importance of this effect in icosahedral
quasicrystals is discussed.


title:
Energy Transport in the Integrable System in Contact with Various Types
  of Phonon Reservoirs

abstract:
  We study how energy transport in an integrable system is affected by the
spectral densities of heat reservoirs. The model investigated here is the
quantum harmonic chain whose both ends are in contact with two heat reservoirs
at different temperatures. The master equation for the reduced density matrix
is derived on the assumption that the reservoirs are composed of an infinite
number of independent harmonic oscillators. We evaluate temperature profile and
energy flux in the stationary state for the master equation and discuss how
they depend on the types of spectral densities. When we attach the reservoirs
of the same type of spectral density, we find that the temperature profile is
independent of the types. On the other hand, when the two reservoirs have
different types of spectral densities, the energy profile near the ends of the
chain depends on the types. When the coupling is finite, the temperature
profile near the ends shows wide variation of behavior dependent on spectral
densities and temperatures of reservoirs. This dependence is discussed with the
Fokker-Planck equations obtained in the classical limit.


title: Lorentz's model with dissipative collisions

abstract:
  Propagation of a particle accelerated by an external field through a
scattering medium is studied within the generalized Lorentz model allowing
inelastic collisions. Energy losses at collisions are proportional to
$(1-\alpha^{2})$, where $0\le\alpha\le 1$ is the restitution coefficient. For
$\alpha =1$ (elastic collisions) there is no stationary state. It is proved in
one dimension that when $\alpha <1$ the stationary state exists . The
corresponding velocity distribution changes from a highly asymmetric
half-gaussian ($\alpha =0$) to an asymptotically symmetric distribution $\sim
{\rm exp}[-(1-\alpha)v^{4}/2]$, for $\alpha\to 1$. The identical scaling
behavior in the limit of weak inelasticity is derived in three dimensions by a
self-consistent perturbation analysis, in accordance with the behavior of
rigorously evaluated moments. The dependence on the external field scales out
in any dimension, predicting in particular the stationary current to be
proportional to the square root of the external acceleration.


title: Crashes as Critical Points

abstract:
  We study a rational expectation model of bubbles and crashes. The model has
two components : (1) our key assumption is that a crash may be caused by local
self-reinforcing imitation between noise traders. If the tendency for noise
traders to imitate their nearest neighbors increases up to a certain point
called the ``critical'' point, all noise traders may place the same order
(sell) at the same time, thus causing a crash. The interplay between the
progressive strengthening of imitation and the ubiquity of noise is
characterized by the hazard rate, i.e. the probability per unit time that the
crash will happen in the next instant if it has not happened yet. (2) Since the
crash is not a certain deterministic outcome of the bubble, it remains rational
for traders to remain invested provided they are compensated by a higher rate
of growth of the bubble for taking the risk of a crash. Our model distinguishes
between the end of the bubble and the time of the crash,: the rational
expectation constraint has the specific implication that the date of the crash
must be random. The theoretical death of the bubble is not the time of the
crash because the crash could happen at any time before, even though this is
not very likely. The death of the bubble is the most probable time for the
crash. There also exists a finite probability of attaining the end of the
bubble without crash. Our model has specific predictions about the presence of
certain critical log-periodic patterns in pre-crash prices, associated with the
deterministic components of the bubble mechanism. We provide empirical evidence
showing that these patterns were indeed present before the crashes of 1929,
1962 and 1987 on Wall Street and the 1997 crash on the Hong Kong Stock
Exchange. These results are compared with statistical tests on synthetic data.


title: Thermalization of a particle with dissipative collisions

abstract:
  One considers the motion of a test particle in an homogeneous fluid in
equilibrium at temperature $T$, undergoing dissipative collisions with the
fluid particles. It is shown that the corresponding linear Boltzmann equation
still posseses a stationary Maxwellian velocity distribution, with an effective
temperature smaller than $T$. This effective temperature is explicitly given in
terms of the restitution parameter and the masses.


title:
From spinons to magnons in explicit and spontaneously dimerized
  antiferromagnetic chains

abstract:
  We reconsider the excitation spectra of a dimerized and frustrated
antiferromagnetic Heisenberg chain. This model is taken as the simpler example
of compiting spontaneous and explicit dimerization relevant for Spin-Peierls
compounds. The bosonized theory is a two frequency Sine-Gordon field theory. We
analize the excitation spectrum by semiclassical methods. The elementary
triplet excitation corresponds to an extended magnon whose radius diverge for
vanishing dimerization. The internal oscilations of the magnon give rise to a
series of excited state until another magnon is emited and a two magnon
continuum is reached. We discuss, for weak dimerization, in which way the
magnon forms as a result of a spinon-spinon interaction potential.


title: Lagrangian method for multiple correlations in passive scalar advection

abstract:
  A Lagrangian method is introduced for calculating simultaneous n-point
correlations of a passive scalar advected by a random velocity field, with
random forcing and finite molecular diffusivity kappa. The method, which is
here presented in detail, is particularly well suited for studying the kappa
tending to 0 limit when the velocity field is not smooth. Efficient Monte Carlo
simulations based on this method are applied to the Kraichnan model of passive
scalar and lead to accurate determinations of the anomalous intermittency
corrections in the fourth-order structure function as a function of the scaling
exponent xi of the velocity field in two and three dimensions. Anomalous
corrections are found to vanish in the limits xi tending to 0 and xi tending to
2, as predicted by perturbation theory.


title: Finite-size scaling for the S=1/2 Heisenberg Antiferromagnetic Chain

abstract:
  Corrections to the asymptotic correlation function in a Heisenberg spin-1/2
antiferromagnetic spin chain are known to vanish slowly (logarithmically) as a
function of the distance r or the chain size L. This leads to significant
differences with numerical results. We calculate the sub-leading logarithmic
corrections to the finite-size correlation function, using renormalization
group improved perturbation theory, and compare the result with numerical data.


title: Density of states, screening and the width of the quantum Hall plateaus

abstract:
  We present a consistent treatment of the quantum Hall effect within the
electrostatic approximation. We derive the form of the density of states (DOS)
which differs from the usual gaussian shape valid for non-interacting
electrons. Below a crossover temperature, this DOS gradually transforms into
the non-interacting DOS near the Fermi energy. We obtain an estimate of this
crossover temperature, which determines the range of validity of this
framework, and allows a reconciliation of the width and temperature dependence
of the quantum Hall plateaus with experiments. The DOS dramatically enhances
the electron-lattice relaxation rate, thus reducing the possibility of electron
overheating.


title:
Critical droplets in Metastable States of Probabilistic Cellular
  Automata

abstract:
  We consider the problem of metastability in a probabilistic cellular
automaton (PCA) with a parallel updating rule which is reversible with respect
to a Gibbs measure. The dynamical rules contain two parameters $\beta$ and $h$
which resemble, but are not identical to, the inverse temperature and external
magnetic field in a ferromagnetic Ising model; in particular, the phase diagram
of the system has two stable phases when $\beta$ is large enough and $h$ is
zero, and a unique phase when $h$ is nonzero. When the system evolves, at small
positive values of $h$, from an initial state with all spins down, the PCA
dynamics give rise to a transition from a metastable to a stable phase when a
droplet of the favored $+$ phase inside the metastable $-$ phase reaches a
critical size. We give heuristic arguments to estimate the critical size in the
limit of zero ``temperature'' ($\beta\to\infty$), as well as estimates of the
time required for the formation of such a droplet in a finite system. Monte
Carlo simulations give results in good agreement with the theoretical
predictions.


title:
Composite Fermion Approach to the Quantum Hall Hierarchy: When it Works
  and Why

abstract:
  The mean field composite Fermion (MFCF) picture has been qualitatively
successful when applied to electrons (or holes) in the lowest Landau level.
Because the energy scales associated with Coulomb interactions and with
Chern-Simons gauge field interactions are different, there is no rigorous
justification of the qualitative success of the MFCF picture. Here we show that
what the MFCF picture does is to select from all the allowed angular momentum
(L) multiplets of N electrons on a sphere, a subset with smaller values of L.
For this subset, the coefficients of fractional parentage for pair states with
small relative angular momentum $R$ (and therefore large repulsion) either
vanish or they are small. This set of states forms the lowest energy sector of
the spectrum.


title:
Hund's Rule for Monopole Harmonics, or Why the Composite Fermion Picture
  Works

abstract:
  The success of the mean field composite Fermion (MFCF) picture in predicting
the lowest energy band of angular momentum multiplets in fractional quantum
Hall systems cannot be found in a cancellation between the Coulomb and
Chern--Simons interactions beyond the mean field, due to their totally
different energy scales. We show that the MFCF approximation can be regarded as
a kind of semi-empirical Hund's rule for monopole harmonics. The plausibility
of the rule is easily established, but rigorous proof relies on comparison with
detailed numerical calculations.


title: Cracking Piles of Brittle Grains

abstract:
  A model which accounts for cracking avalanches in piles of grains subject to
external load is introduced and numerically simulated. The stress is
stochastically transferred from higher layers to lower ones. Cracked areas
exhibit various morphologies, depending on the degree of randomness in the
packing and on the ductility of the grains. The external force necessary to
continue the cracking process is constant in wide range of values of the
fraction of already cracked grains. If the grains are very brittle, the force
fluctuations become periodic in early stages of cracking. Distribution of
cracking avalanches obeys a power law with exponent $\tau = 2.4 \pm 0.1$.


title: Incompressible states of negatively charged magneto-excitons

abstract:
  We study the system of up to four negatively charged magneto-excitons (X-'s)
in the spherical geometry, using the exact-diagonalization techniques. At low
energies, X-'s are bound and behave like charged particles without internal
dynamics. The pseudopotential describing X-:X- scattering is almost identical
to that of electrons and the low-lying few X- states correspond to the
few-electron states. The total angular momentum of the ground state depends on
the effective filling factor nu and vanishes at its special values. The analogs
to the Laughlin nu=1/3 state and the Jain nu=2/5 state of electrons are found.
The X- system is predicted to exhibit the fractional quantum Hall effect.


title:
Excitonic Ions and Pseudopotentials in 2D Systems: Evidence for Quantum
  Hall States of an X- Gas

abstract:
  Systems of up to twelve electrons and six holes on the Haldane sphere are
studied by exact numerical diagonalization. The low lying states of the system
involve bound excitonic complexes such as (X^n)-. The angular momenta of these
complexes and the pseudopotentials describing their interaction are determined.
The similarity to the electron pseudopotential suggests the possibility of
incompressible ground states of a gas of X- ions for nu<=1/3. The nu=1/3 state
of three X-'s is found.


title: Stationary State Skewness in Two Dimensional KPZ Type Growth

abstract:
  We present numerical Monte Carlo results for the stationary state properties
of KPZ type growth in two dimensional surfaces, by evaluating the finite size
scaling (FSS) behaviour of the 2nd and 4th moments, $W_2$ and $W_4$, and the
skewness, $W_3$, in the Kim-Kosterlitz (KK) and BCSOS model. Our results agree
with the stationary state proposed by L\"assig. The roughness exponents
$W_n\sim L^{\alpha_n}$ obey power counting, $\alpha_n= n \alpha$, and the
amplitude ratio's of the moments are universal. They have the same values in
both models: $W_3/W_2^{1.5}= -0.27(1)$ and $W_4/W_2^{2}= +3.15(2)$. Unlike in
one dimension, the stationary state skewness is not tunable, but a universal
property of the stationary state distribution. The FSS corrections to scaling
in the KK model are weak and $\alpha$ converges well to the
Kim-Kosterlitz-L\"assig value $\alpha={2/5} $. The FSS corrections to scaling
in the BCSOS model are strong. Naive extrapolations yield an smaller value,
$\alpha\simeq 0.38(1)$, but are still consistent with $\alpha={2/5}$ if the
leading irrelevant corrections to FSS scaling exponent is of order
$y_{ir}\simeq -0.6(2)$.


title: Landau Ginzburg theory of the d-wave Josephson junction

abstract:
  This letter discusses the Landau Ginzburg theory of a Josephson junction
composed of on one side a pure d-wave superconductor oriented with the $(110)$
axis normal to the junction and on the other side either s-wave or d-wave
oriented with $(100)$ normal to the junction. We use simple symmetry arguments
to show that the Josephson current as a function of the phase must have the
form $j(\phi) = j_1 \sin(\phi) + j_2 \sin(2 \phi)$. In principle $j_1$ vanishes
for a perfect junction of this type, but anisotropy effects, either due to a-b
axis asymmetry or junction imperfections can easily cause $j_1 / j_2$ to be
quite large even in a high quality junction. If $j_1 / j_2$ is sufficiently
small and $j_2$ is negative local time reversal symmetry breaking will appear.
Arbitrary values of the flux would then be pinned to corners between such
junctions and occasionally on junction faces, which is consistent with
experiments by Kirtley et al.


title:
Photonic band gaps and defect states induced by excitations of
  Bose-Einstein condensates in optical lattices

abstract:
  We study the interaction of a Bose-Einstein condensate, which is confined in
an optical lattice, with a largely detuned light field propagating through the
condensate. If the condensate is in its ground state it acts as a periodic
dielectric and gives rise to photonic band gaps at optical frequencies. The
band structure of the combined system of condensed lattice-atoms and photons is
studied by using the concept of polaritons. If elementary excitations of the
condensate are present, they will produce defect states inside the photonic
band gaps. The frequency of localized defect states is calculated using the
Koster-Slater model.


title: On the ground state of fractional quantum Hall effect

abstract: The paper has bene withdrawn by the author.


title: A physical realization of the quantum superalgebra sl_q(2/1)

abstract: The article has been withdrawn by the author.


title: Effect of weak disorder in the Fully Frustrated XY model

abstract:
  The critical behaviour of the Fully Frustrated XY model in presence of weak
positional disorder is studied in a square lattice by Monte Carlo methods. The
critical exponent associated to the divergence of the chiral correlation length
is found to be equal to 1.7 already at very small values of disorder.
Furthermore the helicity modulus jump is found larger than the universal value
expected in the XY model.


title: On the Degeneracy of the Quantum Hall Effect Wavefunction on the torus


title:
Degeneracy of the lowest Landau level and quantum group of the quantum
  Hall effect on the Poincare half plane

abstract: The paper has been withdrawn by the author.


title: Optimal lag in dynamical investments

abstract:
  A portfolio of different stocks and a risk-less security whose composition is
dynamically maintained stable by trading shares at any time step leads to a
growth of the capital with a nonrandom rate. This is the key for the theory of
optimal-growth investment formulated by Kelly. In presence of transaction
costs, the optimal composition changes and, more important, it turns out that
the frequency of transactions must be reduced. This simple observation leads to
the definition of an optimal lag between two rearrangement of the portfolio.
This idea is tested against an investment in a risky asset and a risk-less one.
The price of the first is proportional to NYSE composite index while the price
of the second grows according to the American Discount Rate. An application to
a portfolio of many stochastically equivalent securities is also provided.


title: Booms and Crashes in Self-Similar Markets

abstract:
  Sharp changes in time series representing market dynamics are studied by
means of the self--similar analysis suggested earlier by the authors. These
sharp changes are market booms and crashes. Such crises phenomena in markets
are analogous to critical phenomena in physics. A simple classification of the
market crisis phenomena is given.


title: Density Matrix Renormalization Group of Gapless Systems

abstract:
  We investigate convergence of the density matrix renormalization group (DMRG)
in the thermodynamic limit for gapless systems. Although the DMRG correlations
always decay exponentially in the thermodynamic limit, the correlation length
at the DMRG fixed-point scales as $\xi \sim m^{1.3}$, where $m$ is the number
of kept states, indicating the existence of algebraic order for the exact
system. The single-particle excitation spectrum is calculated, using a
Bloch-wave ansatz, and we prove that the Bloch-wave ansatz leads to the
symmetry $E(k)=E(\pi -k)$ for translationally invariant half-integer
spin-systems with local interactions. Finally, we provide a method to compute
overlaps between ground states obtained from different DMRG calculations.


title:
Shell filling in non-linear magneto-tunneling spectroscopy of vertical
  quantum dots

abstract:
  We report on non-linear magneto-tunneling experiments carried out in single
GaAs vertical quantum dots.
  We show that conduction at low voltage bias can be a spectroscopic tool for
both the ground state and first excited states of few electrons on the dot.
Increasing voltage a large resonant peak is observed and attributed to
tunneling across the quasi-continuum of higher excited states.


title:
Resonant Inelastic X-Ray Scattering from Valence Excitations in
  Insulating Copper-Oxides

abstract:
  We report resonant inelastic x-ray measurements of insulating La$_2$CuO$_4$
and Sr$_2$CuO$_2$Cl$_2$ taken with the incident energy tuned near the Cu K
absorption edge. We show that the spectra are well described in a shakeup
picture in 3rd order perturbation theory which exhibits both incoming and
outgoing resonances, and demonstrate how to extract a spectral function from
the raw data. We conclude by showing {\bf q}-dependent measurements of the
charge transfer gap.


title: Interacting electrons in polygonal quantum dots

abstract:
  The low-lying eigenstates of a system of two electrons confined within a
two-dimensional quantum dot with a hard polygonal boundary are obtained by
means of exact diagonalization. The transition from a weakly correlated charge
distribution for small dots to a strongly correlated "Wigner molecule" for
large dots is studied, and the behaviour at the crossover is determined. In
sufficiently large dots, a recently proposed mapping to an effective
charge-spin model is investigated, and is found to produce the correct ordering
of the energy levels and to give a good first approximation to the size of the
level spacings. We conclude that this approach is a valuable method to obtain
the low energy spectrum of few-electron quantum dots.


title: Comment on ``Roughening Transition of Interfaces in Disordered Media''

abstract:
  Emig and Nattermann (Phys. Rev. Lett. 81, 1469 (1998)) have recently
investigated the competition between lattice pinning and impurity pinning using
a Renormalisation Group (RG) approach. For elastic objects of internal
dimensions $2 < D < 4$, they find, at zero temperature, an interesting second
order phase transition between a flat phase for small disorder and a rough
phase for large disorder. These results contrast with those obtained using the
replica variational approach for the same problem, where a first order
transition between flat and rough phases was predicted. In this comment, we
show that these results can be reconciled by analysing the RG flow for an
arbitrary dimension $N$ for the displacement field.


title: Bose-Einstein condensation in arbitrarily shaped cavities

abstract:
  We discuss the phenomenon of Bose-Einstein condensation of an ideal
non-relativistic Bose gas in an arbitrarily shaped cavity. The influence of the
finite extension of the cavity on all thermodynamical quantities, especially on
the critical temperature of the system, is considered. We use two main methods
which are shown to be equivalent. The first deals with the partition function
as a sum over energy levels and uses a Mellin-Barnes integral representation to
extract an asymptotic formula. The second method converts the sum over the
energy levels to an integral with a suitable density of states factor obtained
from spectral analysis. The application to some simple cavities is discussed.


title:
Self-organization of traffic jams in cities: effects of stochastic
  dynamics and signal periods

abstract:
  We propose a cellular automata model for vehicular traffic in cities by
combining (and appropriately modifying) ideas borrowed from the
Biham-Middleton-Levine (BML) model of city traffic and the Nagel-Schreckenberg
(NS) model of highway traffic. We demonstrate a phase transition from the
"free-flowing" dynamical phase to the completely "jammed" phase at a vehicle
density which depends on the time periods of the synchronized signals and the
separation between them. The intrinsic stochasticity of the dynamics, which
triggers the onset of jamming, is similar to that in the NS model, while the
phenomenon of complete jamming through self-organization as well as the final
jammed configurations are similar to those in the BML model. Using our new
model, we have made an investigation of the time-dependence of the average
speeds of the cars in the "free-flowing" phase as well as the dependence of
flux and jamming on the time period of the signals.

categories: cond-mat.stat-mech adap-org comp-gas nlin.AO nlin.CG nlin.PS patt-sol


title: Interlayer coupling in ferromagnetic semiconductor superlattices

abstract:
  We develop a mean-field theory of carrier-induced ferromagnetism in diluted
magnetic semiconductors. Our approach represents an improvement over standard
RKKY model allowing spatial inhomogeneity of the system, free-carrier spin
polarization, finite temperature, and free-carrier exchange and correlation to
be accounted for self-consistently. As an example, we calculate the electronic
structure of a Mn$_x$Ga$_{1-x}$As/GaAs superlattice with alternating
ferromagnetic and paramagnetic layers and demonstrate the possibility of
semiconductor magnetoresistance systems with designed properties.


title: Thermopower of high-T_c cuprates

abstract:
  We have studied the thermopower of La_2CuO_{4+z} and Nd_2CuO_{4-y} which
undergo an antiferromagnetic transition near room temperature and
Bi_2Sr_{2-x}La_xCuO_{6+z} for which a broad spectrum of doping is possible. The
thermopower of La_2CuO_{4+z} and Nd_2CuO_{4-y} is seen to exhibit an anomaly at
the Neel temperature, whereas the resistivity is not. The extrapolated
zero-temperature intercept of the thermopower, which is known to be positive
for hole-doped cuprate superconductors, is found to become negative for
Bi_2Sr_{2-x}La_xCuO_{6+z} above some critical doping-level. The data strongly
suggest that the thermopower of high-T_c cuprates contains a large amount of
extra contribution in addition to the usual diffusion thermopower. We discuss
origins of the extra contribution in the thermopower.


title: Fractal approach to the beta relaxation in supercooled liquids

abstract:
  In the present work we present a fractal model for the beta relaxation in
supercooled liquids. The macroscopic dynamics is obtained by superposition of
relaxation of independent mesoscopic regions (cages). Scaling relations and
exponents are assumed for the distribution of cage sizes and for the size
dependent response of the independent cages. In this way we obtain some scaling
relations for the average response, in the time and frequency domain. For a
particular choice of the scaling exponents we obtain the scaling relations
predicted by the MCT. The comparison with recent light scattering data reveals
that the scaling exponents of the distribution of cage sizes are universal but
the single cage relaxation depends on the detailed material structure


title:
Bias and temperature dependence of the noise in a single electron
  transistor

abstract:
  A single electron transistor based on Al-AlO_x-Nb tunnel junctions was
fabricated by shadow evaporation and in situ barrier formation. Its output
current noise was measured, using a transimpedance amplifier setup, as a
function of bias voltage, gain, and temperature, in the frequency range 1...300
Hz. The spot noise at 10 Hz is dominated by a gain dependent component,
indicating that the main noise contribution comes from fluctuations at the
input of the transistor. Deviations from ideal input charge noise behaviour are
found in the form of a bias dependence of the differential charge equivalent
noise, i. e. the derivative of current noise with respect to gain. The
temperature dependence of this effect could indicate that heating is activating
the noise sources, and that they are located inside or in the near vicinity of
the junctions.


title: Stretching DNA: Role of electrostatic interaction

abstract:
  The effect of electrostatic interactions on the stretching of DNA is
investigated using a simple worm like chain model. In the limit of small force
there are large conformational fluctuations which are treated using a
self-consistent variational approach. For small values of the external force f,
we find theoreticlly and by a simple blob picture that the extension scales as
fr_D where r_D is the Debye screening length. In the limit of large force the
electrostatic effects can be accounted for within the semiflexible chain model
of DNA by assuming that only small excursions from rod-like conformations are
possible. In this regime the extension approaches the contour length as
f^{-1/2} where f is the magnitude of the external force. The theory is used to
analyze experiments that have measured the extension of double-stranded DNA
subject to tension at various salt concentrations. The theory reproduces nearly
quantitatively the elastic response of DNA at small and large values of f and
for all concentration of the monovalent counterions. The limitations of the
theory are also pointed out.


title: Nonuniversality in short-time critical dynamics

abstract:
  We study behaviour of dynamical critical exponents of the two-dimensional
Ising model with a line of defects. Simulations done at an early time (first
100 Monte Carlo steps) reveal that the critical exponent of Janssen et al (Z.
Phys. B 73 539) depends on the strength of the exchange coupling constant (J')
of the altered line. On the other hand, our simulations permit us to conclude
that the dynamical critical exponent z is not sensitive to changes in J'. In
adition, we investigate the possible invariance of the anomalous dimension of
the magnetization at the beginning of the process.


title: Superfluorescent decay in quasiballistic disordered systems

abstract:
  Superfluorescent decay in a weakly disordered slab is considered. By solving
the Maxwell-Bloch equations we show, that the cooperation number increases with
increasing of the mean free path. Superfluorescent impulse is emitted at a
small angle to the slab, isotropically in a plane direction.


title: Expansion of Liquid 4He Through the Lambda Transition

abstract:
  Zurek suggested [Nature 317 (1985) 505] that the Kibble mechanism, through
which topological defects such as cosmic strings are believed to have been
created in the early Universe, can also result in the formation of topological
defects in liquid 4He, i.e. quantised vortices, during rapid quenches through
the superfluid transition. Preliminary experiments [Hendry et al, Nature 368
(1994) 315] seemed to support this idea in that the quenches produced the
predicted high vortex-densities. The present paper describes a new experiment
incorporating a redesigned expansion cell that minimises vortex creation
arising from conventional hydrodynamic flow. The post-quench line-densities of
vorticity produced by the new cell are no more than 10^10 m^{-2}, a value that
is at least two orders-of-magnitude less than the theoretical prediction. We
conclude that most of the vortices detected in the original experiment must
have been created through conventional flow processes.


title:
Semi-Analytical Calculation of the Rouse Dynamics of Randomly Branched
  Polymers

abstract:
  We present a semi-analytical approach to the determination of the dynamic
properties of randomly branched polymers under the Rouse approximation. The
principle procedure is based on examining a spectrum of eigenvalues which
represents the average dynamic behavior of various structures. The calculated
spectra show that the eigenvalue distribution is random even within a single
structure which in turn produces a continuous spectrum of values for the entire
class. The auto-correlation functions for the radius of gyration squared were
calculated based on these spectra, which confirms that the dynamics is
non-exponential as earlier reported. A universal stretched exponent is also
found in this study.


title: Inhibited Recombination of Charged Magnetoexcitons

abstract:
  Time-resolved photoluminescence measurements show that the decay time for
charged excitons in a GaAs two-dimensional electron gas increases by an order
of magnitude at high magnetic fields. Unlike neutral excitons, the charged
exciton center-of-mass is spatially confined in a ``magnetically-adjustable
quantum dot'' by the cyclotron orbit and the quantum well. The inhibited
recombination is explained by a reduced phase coherence volume of the
magnetically-confined charged excitons.


title: Statistical Analysis of Magnetic Field Spectra

abstract:
  We have calculated and statistically analyzed the magnetic-field spectrum
(the ``B-spectrum'') at fixed electron Fermi energy for two quantum dot systems
with classically chaotic shape. This is a new problem which arises naturally in
transport measurements where the incoming electron has a fixed energy while one
tunes the magnetic field to obtain resonance conductance patterns. The
``B-spectrum'', defined as the collection of values ${B_i}$ at which
conductance $g(B_i)$ takes extremal values, is determined by a quadratic
eigenvalue equation, in distinct difference to the usual linear eigenvalue
problem satisfied by the energy levels. We found that the lower part of the
``B-spectrum'' satisfies the distribution belonging to Gaussian Unitary
Ensemble, while the higher part obeys a Poisson-like behavior. We also found
that the ``B-spectrum'' fluctuations of the chaotic system are consistent with
the results we obtained from random matrices.


title:
Ground-State Properties of the Two-Band Model for Halogen-Bridged Metal
  Complexes

abstract:
  Based on a symmetry argument, we systematically reveal Hartree-Fock
broken-symmetry solutions of the one-dimensional two-band extended
Peierls-Hubbard model. Performing numerical investigations as well, the
possibility of novel density-wave states appearing is argued.


title: Magnetic Properties of Quantum Ferrimagnetic Spin Chains

abstract:
  Magnetic susceptibilities of spin-$(S,s)$ ferrimagnetic Heisenberg chains are
numerically investigated. It is argued how the ferromagnetic and
antiferromagnetic features of quantum ferrimagnets are exhibited as functions
of $(S,s)$. Spin-$(S,s)$ ferrimagnetic chains behave like combinations of
spin-$(S-s)$ ferromagnetic and spin-$(2s)$ antiferromagnetic chains provided
$S=2s$.


title: Low-Energy Structure of Heisenberg Ferrimagnetic Spin Chains

abstract:
  Static and dynamic structure factors of Heisenberg ferrimagnetic spin chains
are numerically investigated. There exist two distinct branches of elementary
excitations, which exhibit ferromagnetic and antiferromagnetic aspects. The
ferromagnetic feature is smeared out with the increase of temperature, whereas
the antiferromagnetic one persists up to higher temperatures. The scattering
intensity is remarkably large at lower boundaries of the ferromagnetic and
antiferromagnetic spectra. All these observations are consistent with the
ferromagnetic-to-antiferromagnetic crossover in the thermal behavior which has
recently been reported.


title: Annealing of defects in Fe after MeV Heavy ion irradiation

abstract:
  We report study of recovery dynamics, followed by in-situ resistivity
measurement after 100 MeV oxygen ion irradiation, in cold rolled Fe at 300K.
Scaling behavior with microstructural density and temperature of sample have
been used to establish stress induced defects formed during irradiation as a
new type of sink. The dynamics after irradiation has been shown to be due to
migration of defects to two types of sinks i.e. stress induced defect as
variable sinks and internal surfaces as fixed sinks. Experimental data obtained
under various experimental conditions have been fitted to theoretical curves.
Parameters thus obtained from fitting are employed to establish effect of
electronic energy loss and temperature on recovery dynamics and stress
associated with variable sinks.


title:
Quantum degeneracy and interaction effects in spin-polarized Fermi-Bose
  mixtures

abstract:
  Various features of spin-polarized Fermi gases confined in harmonic traps are
discussed, taking into account possible perspectives of experimental
measurements. The mechanism of the expansion of the gas is explicitly
investigated and compared with the one of an interacting Bose gas. The role of
interactions on the equilibrium and non equilibrium behaviour of the fermionic
component in Fermi-Bose mixtures is discussed. Special emphasis is given to the
case of potassium isotopes mixtures.


title:
Clustering Instabilities, Arching, and Anomalous Interaction
  Probabilities as Examples for Cooperative Phenomena in Dry Granular Media

abstract:
  In a freely cooling granular material fluctuations in density and temperature
cause position dependent energy loss. Due to strong local dissipation, pressure
and energy drop rapidly and material moves from `hot' to `cold' regions,
leading to even stronger dissipation and thus causing the density instability.
The assumption of `molecular chaos' is valid only in the homogeneous cooling
regime. As soon as the density instability occurs, the impact parameter is not
longer uniformly distributed. The pair-correlation and the structure functions
show that the molecular chaos assumption --- together with reasonable excluded
volume modeling --- is important for short distances and irrelevant on large
length scales.
  In this study, the probability distribution of the collision frequency is
examined for pipe flow and for freely cooling granular materials as well.
Uncorrelated events lead to a Poisson distribution for the collision
frequencies. In contrast, the fingerprint of the cooperative phenomena
discussed here is a power-law decay of the probability for many collisions per
unit time.
  Keywords: discrete element method, event driven simulations, clustering
instability, arching, shock waves, power-law distribution, cooperative
phenomena.


title: Sub-Poissonian shot noise in non-degenerate diffusive conductors

abstract:
  A theory is presented for the universal reduction of shot noise by Coulomb
repulsion, which was observed in computer simulations of a disordered
non-degenerate electron gas by Gonzalez et al. (cond-mat/9803372). The
universality of the reduction below the uncorrelated value is explained as a
feature of the high-voltage regime of space-charge limited conduction. The
reduction factor depends on the dimensionality d of the density of states,
being close but not quite equal to 1/d in two and three dimensions.


title: Magnetization plateau in a two-dimensional multiple-spin exchange model

abstract:
  We study a multiple-spin exchange model on a triangular lattice, which is a
possible model for low-density solid 3He films. Due to strong competitions
between ferromagnetic three-spin exchange and antiferromagnetic four-spin one,
the ground states are highly degenerate in the classical limit. At least
2^{L/2}-fold degeneracy exists on the L*L triangular lattice except for the
SO(3) symmetry. In the magnetization process, we found a plateau at
m/m_{sat}=1/2, in which the ground state is "uuud state" (a collinear state
with four sublattices). The 1/2-plateau appears due to the strong four-spin
exchange interaction. This plateau survives against both quantum and thermal
fluctuations. Under a magnetic field which realizes the "uuud" ordered state, a
phase transition occurs at a finite temperature. We predict that low-density
solid 3He thin films may show the 1/2-plateau in the magnetization process.
Experimental observation of the plateau will verify strength of the four-spin
exchange. It is also discussed that this magnetization plateau can be
understood as an insulating-conducting transition in a particle picture.


title: Mean-Field Approximations to the Longest Common Subsequence Problem

abstract:
  The Longest Common Subsequence (LCS) problem is a fundamental problem of
sequence comparison. A natural approximation to this problem is a model in
which every pairs of letters of two ``sequences'' are matched independently of
the other pairs with probability 1/S, $S$ representing the size of the
alphabet. This model is analogous to a mean field version of the LCS problem,
which can be solved with a cavity approach (Eur. Phys. J. B 7-2(1999),pp.
293-308). We refine here this approximation by incorporating in a systematic
way correlations among the matches in the cavity calculation. We obtain a
series of closer and closer approximations to the LCS problem, which we
quantify in the large $S$ limit, both with a perturbative approach and by
Monte-Carlo simulations. We find that, as it happens in the expansion around
mean-field for other disordered systems, the corrections to our approximations
depend upon long-ranged correlation effects which render the large $S$
expansion non-perturbative.


title:
Direct Observation of Lateral Coupling Between Self-Assembled Qauntum
  Dots

abstract:
  Scattering of carriers between spatially separated zero dimensional states
has been observed in a system of self-assembled In_{0.55}Al_{0.45}As quantum
dots. We believe the interdot tunneling is mediated by localized states below
the barrier band edge. The experiment was performed by taking photoluminescence
excitation spectra at 4.2K using a near-field scanning optical microscope.
Surprisingly, the excitation spectrum from individual dots does not quench to
zero at any energy, even when the energy of the exciting light is tuned below
the barrier band edge. On top of this continuum, narrow resonances are observed
in the emission lines of individual dots. These resonances tend to occur
simultaneously in several emission lines, originating from different quantum
dots, evincing interdot scattering.


title: Infrared absorption from Charge Density Waves in magnetic manganites

abstract:
  The infrared absorption of charge density waves coupled to a magnetic
background is first observed in two manganites La{1-x}Ca{x}MnO{3} with x = 0.5
and x = 0.67. In both cases a BCS-like gap 2 Delta (T), which for x=0.5 follows
the hysteretic ferro-antiferromagnetic transition, fully opens at a finite T{0}
< T{Neel}, with 2 Delta(T{0})/kT{c} close to 5. These results may also explain
the unusual coexistence of charge ordering and ferromagnetism in
La{0.5}Ca{0.5}MnO{3}.


title:
Magnetic anisotropy and low-energy spin waves in the
  Dzyaloshinskii-Moriya spiral magnet Ba_2 Cu Ge_2 O_7

abstract:
  New neutron diffraction and inelastic scattering experiments are used to
investigate in detail the field dependence of the magnetic structure and
low-energy spin wave spectrum of the Dzyaloshinskii-Moriya helimagnet Ba_2 Cu
Ge_2 O_7. The results suggest that the previously proposed model for the
magnetism of this compound (an ideal sinusoidal spin spiral, stabilized by
isotropic exchange and Dzyaloshinskii-Moriya interactions) needs to be refined.
Both new and previously published data can be quantitatively explained by
taking into account the Kaplan-Shekhtman-Entin-Wohlman-Aharony (KSEA) term, a
special magnetic anisotropy term that was predicted to always accompany
Dzyaloshinskii-Moriya interactions in insulators.


title:
A Jordan scenario is behind the Van Hove scenario. Towards a quantum
  chemical theory of high temperature superconductivity

abstract:
  It is suggested that the appearance of Jordan blocks in the normal form of
the recursion (transfer) matrix is responsible for high temperature
superconductivity and related phenomena. Several published observations are
discussed to support this idea. The draft of a quantum chemical theory is given
for a more complete verification. Keywords: Jordan scenario, Van Hove scenario,
high temperature superconductivity. Short title: Jordan scenario of high
temperature superconductivity.


title: The Stress Transmission Universality Classes of Periodic Granular Arrays

abstract:
  The transmission of stress is analysed for static periodic arrays of rigid
grains, with perfect and zero friction. For minimal coordination number (which
is sensitive to friction, sphericity and dimensionality), the stress
distribution is soluble without reference to the corresponding displacement
fields. In non-degenerate cases, the constitutive equations are found to be
simple linear in the stress components. The corresponding coefficients depend
crucially upon geometrical disorder of the grain contacts.


title: Vortex Density of States and Absorption in Clean Layered Superconductors

abstract:
  We study the spectrum of the states localized in the vortex cores in the
mixed state of clean layered superconductors. S-wave coupling is assumed. It is
found that in a large region of parameters adjacent to the superclean case a
new universal (i.e. independent of the density of impurities) class of level
statistics arises. It is the circular unitary random matrix ensemble. The
density of states for such conditions is calculated. The absorption resulting
from the Landau-Zener transitions between these levels is different from the
classical result for an isotropic three-dimensional system.


title: Zero Temperature Phases of the Electron Gas

abstract:
  The stability of different phases of the three-dimensional non-relativistic
electron gas is analyzed using stochastic methods. With decreasing density, we
observe a {\it continuous} transition from the paramagnetic to the
ferromagnetic fluid, with an intermediate stability range ($25\pm 5 \leq
r_s\leq 35 \pm 5$) for the {\it partially} spin-polarized liquid. The freezing
transition into a ferromagnetic Wigner crystal occurs at $r_s=65 \pm 10$. We
discuss the relative stability of different magnetic structures in the solid
phase, as well as the possibility of disordered phases.


title: The possibility of two-exciton pairing in singlet and in triplet states

abstract:
  We propose a two-dimensional exciton model showing the possibility of
two-exciton pairing in cuprates, both in singlet and in triplet states. The
former is assumed to be responsible for the superconductivity and the latter
for the unusual normal state properties. The ground state is found to be a
projection of the Bose condensation of N/2 mobile bosons with a \eta projection
operator which is a generalization of the Gutzwiller projection operator. The
phase diagram of the model shows insulator-superconductivity and
superconductivity-metal transitions with doping.


title: Small denominators and anomalous behaviour in the Holstein-Hubbard model

abstract:
  We consider a system of interacting fermions on a chain in a periodic
potential incommensurate with the chain spacing. We derive a convergent
perturbative expansion, afflicted by a small denominator problem and based on
renormalization group, for the two point Schwinger function. We obtain the
large distance behavior of the Schwinger function, which is anomalous and
described by critical indices, related to the gap and the wave function
renormalization.


title: Renormalization group for the XYZ model

abstract:
  We study in a rigorous way the XYZ spin model by Renormalization Group
methods.


title: Theory of spin excitations in undoped and underdoped cuprates

abstract:
  We consider the magnetic properties of high Tc cuprates from a gauge theory
point of view, with emphasis on the underdoped regime. Underdoped cuprates
possess certain antiferromagnetic correlations, as evidenced, for example, by
different temperature dependence of the Cu and O site NMR relaxation rates,
that are not captured well by slave boson mean field theories of the t-J model.
We show that the inclusion of gauge fluctuations will remedy the deficiencies
of the mean field theories. As a concrete illustration of the gauge-fluctuation
restoration of the antiferromangetic correlation and the feasibility of the 1/N
perturbation theory, the Heisenberg spin chain is analyzed in terms of a 1+1D
U(1) gauge theory with massless Dirac fermions. The 1/N-perturbative treatment
of the same gauge theory in 2+1D (which can be motivated from the mean field
pi-flux phase of the Heisenberg model) leads to a dynamical mass generation
corresponding to an antiferromagnetic ordering. On the other hand, it is argued
that in a similar gauge theory with an additional coupling to a Bose (holon)
field, symmetry breaking does not occur, but antiferromagnetic correlations are
enhanced, which is the situation in the underdoped cuprates.

categories: cond-mat.str-el cond-mat.supr-con hep-ph hep-th


title: Shift Operators for XXX-Heisenberg Chain

abstract:
  The shift operators for XXX-Heisenberg chain are found. They are formed by
local Yangian operators and the amplitutes of the eigenfunctions obeying Bethe
ansatz for the Hamiltonian. The physical implication of the shift operators are
also explained.


title:
Bending and Base-Stacking Interactions in Double-Stranded Semiflexible
  Polymer

abstract:
  Simple expressions for the bending and the base-stacking energy of
double-stranded semiflexible biopolymers (such as DNA and actin) are derived.
The distribution of the folding angle between the two strands is obtained by
solving a Schr\"{o}dinger equation variationally. Theoretical results based on
this model on the extension versus force and extension versus degree of
supercoiling relations of DNA chain are in good agreement with the experimental
observations of Cluzel {\it et al.} [Science {\bf 271}, 792 (1996)], Smith {\it
et al.} [{\it ibid.} {\bf 271}, 795 (1996)], and Strick {\it et al.} [{\it
ibid.} {\bf 271}, 1835 (1996)].

categories: cond-mat.soft cond-mat.stat-mech physics.bio-ph q-bio


title:
Effects of Dissipation on Quantum Phase Slippage in Charge Density Wave
  Systems

abstract:
  We study the effect of the dissipation on the quantum phase slippage via the
creation of ``vortex ring'' in charge density wave (CDW) systems. The
dissipation is assumed to come from the interaction with the normal electron
near and inside of the vortex core. We describe the CDW by extracted
macroscopic degrees of freedom, that is, the CDW phase and the radius of the
``vortex ring'', assume the ohmic dissipation, and investigate the effect in
the context of semiclassical approximation.
  The obtained results are discussed in comparison with experiments. It turns
out that the effect of such a dissipation can be neglected in experiments.


title: Quantum fluctuations in the cohesive force of metallic nanowires

abstract:
  Based on the recent free electron model for cohesion in narrow metallic
constrictions by Stafford et al., we calculate the quantum fluctuations in the
cohesive force versus elongation. The fluctuations are dominated by states near
the Fermi energy, thus explaining their apparently universal magnitude of order
epsilon_F/lambda_F. We present numerical results for the force fluctuations in
a simple geometry and show that they are well described by the contributions of
a few classical periodic orbits in the Balian-Bloch trace formula for the
density of states of transverse motion.


title:
Novel Position-Space Renormalization Group for Bond Directed Percolation
  in Two Dimensions

abstract:
  A new position-space renormalization group approach is investigated for bond
directed percolation in two dimensions. The threshold value for the bond
occupation probabilities is found to be $p_c=0.6443$. Correlation length
exponents on time (parallel) and space (transverse) directions are found to be
$\nu_\parallel=1.719$ and $\nu_\perp=1.076$, respectively, which are in very
good agreement with the best known series expansion results.


title: Analytical results for random walk persistence

abstract:
  In this paper, we present the detailed calculation of the persistence
exponent $\theta$ for a nearly-Markovian Gaussian process $X(t)$, a problem
initially introduced in [Phys. Rev. Lett. 77, 1420 (1996)], describing the
probability that the walker never crosses the origin. New resummed perturbative
and non-perturbative expressions for $\theta$ are obtained, which suggest a
connection with the result of the alternative independent interval
approximation (IIA). The perturbation theory is extended to the calculation of
$\theta$ for non-Gaussian processes, by making a strong connection between the
problem of persistence and the calculation of the energy eigenfunctions of a
quantum mechanical problem. Finally, we give perturbative and non-perturbative
expressions for the persistence exponent $\theta(X_0)$, describing the
probability that the process remains bigger than $X_0\sqrt{<X^2(t)>}$.


title:
Optical conductivity of the Kondo insulator YbB_12: Gap formation and
  low-energy excitations

abstract:
  Optical reflectivity experiments have been conducted on single crystals of
the Kondo insulator YbB_12 in order to obtain its optical conductivity,
\sigma(\omega). Upon cooling below 70 K, a strong supression of \sigma(\omega)
is seen in the far-infrared region, indicating the opening of an energy gap of
~ 25 meV. This gap development is coincident with a rapid decrease in the
magnetic susceptibility, which shows that the gap opening has significant
influence on magnetic properties. A narrow, asymmetric peak is observed at ~40
meV in \sigma(\omega), which is attributed to optical transitions between the
Yb 4f-derived states across the gap. In addition, a broad peak is observed at
~0.25 eV. This peak is attributed to transitions between Yb 4f-derived states
and p-d band, and is reminiscent of similar peaks previously observed for
rare-earth hexaborides.


title: Fully Frustrated Ising System on a 3D Simple Cubic Lattice: Revisited

abstract:
  Using extensive Monte Carlo simulations, we clarify the critical behaviour of
the 3 dimensional simple cubic Ising Fully Frustrated system. We find two
transition temperatures and two long range ordered phases. Within the present
numerical accuracy, the transition at higher temperature is found to be second
order and we have extracted the standard critical exponent using finite size
scaling method. On the other hand, the transition at lower temperature is found
to be first order. It is argued that entropy plays a major role on determining
the low temperature state.


title: Discontinuous Crack Fronts of Three-Dimensional Fractures

abstract:
  The relation between fracture surface morphology and the three-dimensional
structure of crack fronts is investigated through direct observation of brittle
cracks in gels. A key notion in this investigation is the discontinuity of the
crack front, whose advancement creates the fracture surface. We discuss the
significance of our findings in the studies of general three-dimensional
brittle fractures.


title: Extended Fractons and Localized Phonons on Percolation Clusters

abstract:
  We investigate the localization behavior of vibrational modes of infinite
percolation clusters above the critical concentration in two and three
dimensions using the method of level statistics. We find that all eigenstates
are localized in d=2, including the low frequency phonon states. In d=3 we
obtain evidence for a localization-delocalization transition. But contrary to
the common view this transition occurs for frequencies above the phonon fracton
crossover giving rise to a new regime of extended fracton states.


title:
Superconductivity and spin triplet collective mode in the t-J model
  close to antiferromagnetic instability

abstract:
  To investigate relations between long-range antiferromagnetic (AF) order,
superconductivity and two particle triplet collective excitations we consider a
modified two dimensional t-J model at doping close to half filling. The model
includes additional hopping t'' and nearest sites Coulomb repulsion V. The
additional parameters allow us to control closeness of the system to the AF
instability. We demonstrate the possibility of co-existence of long-range AF
order and d-g-wave superconductivity. In the phase with long-range AF order we
find, analytically, superconducting gaps and spin wave renormalization. We
demonstrate that at approaching the point of the AF instability the spin
triplet collective excitation arises with energy below the superconducting gap.


title: Self-Organized Segregation within an Evolving Population

abstract:
  An evolving population, in which individual members (`agents') adapt their
behaviour according to past experience, is of central importance to many
disciplines. Because of their limited knowledge and capabilities, agents are
forced to make decisions based on inductive, rather than deductive, thinking.
We show that a population of competing agents with similar capabilities and
knowledge will tend to self-segregate into opposing groups characterized by
extreme behavior. Cautious agents perform poorly and tend to become rare.


title:
Thermodynamic properties of spin-1/2 transverse XY chain with
  Dzyaloshinskii-Moriya interaction: Exact solution for correlated Lorentzian
  disorder

abstract:
  We extend the consideration of the spin-1/2 transverse XY chain with
correlated Lorentzian disorder (Phys. Rev. B {\bf 55,} 14298 (1997)) for the
case of additional Dzyaloshinskii-Moriya interspin interaction. It is shown how
the averaged density of states can be calculated exactly. Results are presented
for the density of states and the transverse magnetization.


title: Relaxation in graph coloring and satisfiability problems

abstract:
  Using T=0 Monte Carlo simulation, we study the relaxation of graph coloring
(K-COL) and satisfiability (K-SAT), two hard problems that have recently been
shown to possess a phase transition in solvability as a parameter is varied. A
change from exponentially fast to power law relaxation, and a transition to
freezing behavior are found. These changes take place for smaller values of the
parameter than the solvability transition. Results for the coloring problem for
colorable and clustered graphs and for the fraction of persistent spins for
satisfiability are also presented.


title: Semiclassical description of Heisenberg models via spin-coherent states

abstract:
  We use spin-coherent states as a time-dependent variational ansatz for a
semiclassical description of a large family of Heisenberg models. In addition
to common approaches we also evaluate the square variance of the Hamiltonian in
terms of coherent states. This quantity turns out to have a natural
interpretation with respect to time-dependent solutions of the equations of
motion and allows for an estimate of quantum fluctuations in a semiclassical
regime. The general results are applied to solitons, instantons and vortices in
several one- and two-dimensional models.


title:
Fixed-N Superconductivity: The Crossover from the Bulk to the
  Few-Electron Limit

abstract:
  We present a truly canonical theory of superconductivity in ultrasmall
metallic grains by variationally optimizing fixed-N projected BCS
wave-functions, which yields the first full description of the entire crossover
from the bulk BCS regime (mean level spacing $d \ll$ bulk gap $\tilde\Delta$)
to the ``fluctuation-dominated'' few-electron regime ($d\gg\tilde\Delta$). A
wave-function analysis shows in detail how the BCS limit is recovered for $d\ll
\tilde \Delta$, and how for $d \gg \tilde \Delta$ pairing correlations become
delocalized in energy space. An earlier grand-canonical prediction for an
observable parity effect in the spectral gaps is found to survive the fixed-N
projection.


title: Spin-3/2 models on the Cayley tree -- optimum ground state approach

abstract:
  We present a class of optimum ground states for spin-3/2 models on the Cayley
tree with coordination number 3. The interaction is restricted to nearest
neighbours and contains 5 continuous parameters. For all values of these
parameters the Hamiltonian has parity invariance, spin-flip invariance, and
rotational symmetry in the xy-plane of spin space. The global ground states are
constructed in terms of a 1-parametric vertex state model, which is a direct
generalization of the well-known matrix product ground state approach. By using
recursion relations and the transfer matrix technique we derive exact
analytical expressions for local fluctuations and longitudinal and transversal
two-point correlation functions.


title:
S-matrix network models for coherent waves in random media: construction
  and renormalization

abstract:
  Networks of random quantum scatterers (S-matrices) form paradigmatic models
for the propagation of coherent waves in random S-matrix network models cover
universal localization-delocalization properties and have some advantages over
more traditional Hamiltonian models. In particular, a straightforward
implementation of real space renormalization techniques is possible. Starting
from a finite elementary cell of the S-matrix network, hierarchical network
models can be constructed by recursion. The localization-delocalization
properties are contained in the flow of the forward scattering strength
('conductance') under increasing system size. With the aid of 'small scale'
numerics qualitative aspects of the localization-delocalization properties of
S-matrix network models can be worked out.


title:
Glassy Relaxation and Breakdown of the Stokes-Einstein Relation in the
  Two Dimensional Lattice Coulomb Gas of Fractional Charges

abstract:
  We present Monte Carlo simulation results on the equilibrium relaxation of
the two dimensional lattice Coulomb gas with fractional charges, which exhibits
a close analogy to the primary relaxation of fragile supercooled liquids.
Single particle and collective relaxation dynamics show that the
Stokes-Einstein relation is violated at low temperatures, which can be
characterized by a fractional power law relation between the self-diffusion
coefficient and the characteristic relaxation time. The microscopic spatially
heterogeneous structure responsible for the violation is identified.


title: Quantum Kinetic Theory for a Condensed Bosonic Gas

abstract:
  We present a kinetic theory for Bose-Einstein condensation of a weakly
interacting atomic gas in a trap. Starting from first principles, we establish
a Markovian kinetic description for the evolution towards equilibrium. In
particular, we obtain a set of self-consistent master equations for mean
fields, normal densities, and anomalous fluctuations. These kinetic equations
generalize the Gross-Pitaevskii mean-field equations, and merge them
consistently with a quantum-Boltzmann equation approach.


title: Liquid Crystal Phases of Quantum Hall Systems

abstract:
  Mean-field calculations for the two dimensional electron gas (2DEG) in a
large magnetic field with a partially filled Landau level with index $N\geq 2$
consistently yield ``stripe-ordered'' charge-density wave ground-states, for
much the same reason that frustrated phase separation leads to stripe ordered
states in doped Mott insulators. We have studied the effects of quantum and
thermal fluctuations about such a state and show that they can lead to a set of
electronic liquid crystalline states, particularly a stripe-nematic phase which
is stable at $T>0$. Recent measurements of the longitudinal resistivity of a
set of quantum Hall devices have revealed that these systems spontaneously
develop, at low temepratures, a very large anisotropy. We interpret these
experiments as evidence for a stripe nematic phase, and propose a general phase
diagram for this system.


title: Anisotropic in-plane thermal conductivity of single-crystal YBa_2Cu_4O_8

abstract:
  We report measurements of the in-plane thermal conductivity ($\kappa$) of
YBa_2Cu_4O_8 (Y-124) single crystals in the temperature range $4K\leq T\leq
300K$ and compare them with previous results on YBa_2Cu_3O_{6+x} (Y-123). For
transport perpendicular to the CuO chains, $\kappa_a(300K)\simeq 10$W/mK, and
along the chains, $\kappa_b(300K)\simeq 40$W/mK. The temperature dependence of
$\kappa$ for both transport directions is much stronger than in
YBa_2Cu_3O_{6+x} (Y-123), indicative of substantially superior lattice heat
conduction in Y-124, and resulting in maximum values for $\kappa$ at $T\sim
20$K exceeding 200W/mK. The data imply a surprisingly large anisotropy in the
lattice conduction. $\kappa_a$ and $\kappa_b$ are enhanced in the
superconducting state as in other cuprates. The magnitude and anisotropy of the
enhancement are discussed and compared to those of Y-123.


title: Microcanonical Transfer Matrix Study of the Q-state Potts Model

abstract:
  The microcanonical transfer matrix is used to study the zeros of the
partition function of the Q-state Potts model. Results are presented for the
Yang-Lee zeros of the 3-state model, the Fisher zeros of the 3-state model in
an external field $H_q<0$, and the spontaneous magnetization of the 2-state
model. In addition, we are able to calculate the ground-state entropy of the
3-state model and find $s_0=0.43153(3)$ in excellent agreement with the exact
value, 0.43152...


title: Application of Polynomial Algorithms to a Random Elastic Medium

abstract:
  A randomly pinned elastic medium in two dimensions is modeled by a disordered
fully-packed loop model. The energetics of disorder-induced dislocations is
studied using exact and polynomial algorithms from combinatorial optimization.
Dislocations are found to become unbound at large scale, and the elastic phase
is thus unstable giving evidence for the absence of a Bragg glass in two
dimensions.


title:
Classification and Stability of Phases of the Multicomponent
  One-Dimensional Electron Gas

abstract:
  The classification of the ground-state phases of complex one-dimensional
electronic systems is considered in the context of a fixed-point strategy.
Examples are multichain Hubbard models, the Kondo-Heisenberg model, and the
one-dimensional electron gas in an active environment. It is shown that, in
order to characterize the low-energy physics, it is necessary to analyze the
perturbative stability of the possible fixed points, to identify all discrete
broken symmetries, and to specify the quantum numbers and elementary wave
vectors of the gapless excitations. Many previously-proposed exotic phases of
multichain Hubbard models are shown to be unstable because of the ``spin-gap
proximity effect.'' A useful tool in this analysis is a new generalization of
Luttinger's theorem, which shows that there is a gapless even-charge mode in
any incommensurate N-component system.


title: Spin relaxation in Mn12-acetate

abstract:
  We present a comprehensive derivation of the magnetization relaxation in a
Mn12-acetate crystal based on thermally assisted spin tunneling induced by
quartic anisotropy and weak transverse magnetic fields. The overall relaxation
rate as function of the magnetic field is calculated and shown to agree well
with data including all resonance peaks. The Lorentzian shape of the resonances
is also in good agreement with recent data. A generalized master equation
including resonances is derived and solved exactly. It is shown that many
transition paths with comparable weight exist that contribute to the relaxation
process. Previously unknown spin-phonon coupling constants are calculated
explicitly.


title: On the Thomas-Fermi approximation for Bose condensates in traps

abstract:
  Thomas-Fermi theory for Bose condesates in inhomogeneous traps is revisited.
The phase-space distribution function in the Thomas-Fermi limit is
$f_0(\bold{R},\bold{p})$ $\alpha$ $\delta(\mu - H_{cl})$ where $H_{cl}$ is the
classical counterpart of the self-consistent Gross-Pitaevskii Hamiltonian. No
assumption on the large N-limit is introduced and, e.g the kinetic energy is
found to be in good agreement with the quantal results even for low and
intermediate particle numbers N. The attractive case yields conclusive results
as well.


title:
Canonical Transformation of the Three-Band Hubbard Model and Hole
  Pairing

abstract:
  We propose a canonical tranformation approach to the effective interaction
$W_{eff}$ between two holes, based on the three-band Hubbard model but ready to
include extra interactions as well. An effective two-body Hamiltonian can in
principle be obtained including any kind of virtual intermediate states. We
derive the closed-form analytic expression of the effective interaction
including 4-body virtual states, describing the exchange of an electron-hole
pair to all orders. The resulting integral equation, valid for the full plane,
leads to a Cooper-like instability of the Fermi liquid. The two-hole bound
states generalize those reported earlier in cluster calculations by exact
diagonalisation methods.


title:
Stability of the maximum density droplet in quantum dots at high
  magnetic fields

abstract:
  We have measured electron transport through a vertical quantum dot containing
a tunable number of electrons between 0 and 40. Over some region in magnetic
field the electrons are spin polarized and occupy successive angular momentum
states, i.e. the maximum density droplet (MDD) state. The stability region
where the MDD state is the ground state, decreases for increasing electron
number. The instability of the MDD is accompanied by a redistribution of charge
which increases the area of the electron droplet.


title: Relaxation of the field-cooled magnetization of an Ising spin glass

abstract:
  The time and temperature dependence of the field-cooled magnetization of a
three dimensional Ising spin glass, Fe_{0.5}Mn_{0.5}TiO_{3}, has been
investigated. The temperature and cooling rate dependence is found to exhibit
memory phenomena that can be related to the memory behavior of the low
frequency ac-susceptibility. The results add some further understanding on how
to model the three dimensional Ising spin glass in real space.


title:
The Origin of the Pseudogap from Tunneling Spectroscopy Measurements on
  Bi-2212 Single Crystals

abstract:
  We present electron-tunneling spectroscopy of oxygen over- and under-doped
Bi-2212 single crystals in a temperature range between 14 K and 290 K using a
break-junction technique. In over- and under-doped samples we observe a
pseudogap with T* = 280 - 290 K and T* > 290 K, respectively. In order to
explain the data we consider possible models for the pseudogap, namely, (i) a
precursor pairing; (ii) a charge-density-wave gap; (iii) a spin-gap due to
antiferromagnetic (AF) correlations. Analyzing all data we show unambiguously
that the only model which fits the data is a gap due to AF correlations.


title: A generalized spin model of financial markets

abstract:
  We reformulate the Cont-Bouchaud model of financial markets in terms of
classical "super-spins" where the spin value is a measure of the number of
individual traders represented by a portfolio manager of an investment agency.
We then extend this simplified model by switching on interactions among the
super-spins to model the tendency of agencies getting influenced by the opinion
of other managers. We also introduce a fictitious temperature (to model other
random influences), and time-dependent local fields to model slowly changing
optimistic or pessimistic bias of traders. We point out close similarities
between the price variations in our model with $N$ super-spins and total
displacements in an $N$-step Levy flight. We demonstrate the phenomena of
natural and artificially created bubbles and subsequent crashes as well as the
occurrence of "fat tails" in the distributions of stock price variations.


title: Biased diffusion in a one-dimensional adsorbed monolayer

abstract:
  We study dynamics of a probe particle, which performs biased diffusive motion
in a one-dimensional adsorbed monolayer of mobile hard-core particles
undergoing continuous exchanges with a vapor phase. In terms of a
mean-field-type approach, based on the decoupling of the third-order
correlation functions into a product of the pairwise correlations, we determine
analytically the density profiles of the monolayer particles, as seen from the
stationary moving probe, and calculate the terminal velocity, mobility and the
self-diffusion coefficient of the probe. Our analytical results are confirmed
by Monte Carlo simulations.


title: Bose-Einstein condensation in harmonic double wells

abstract:
  We discuss Bose-Einstein condensation in harmonic traps where the confinement
has undergone a splitting along one direction. We mostly consider the 3D
potentials consisting of two cylindrical wells separated a distance 2a along
the z-axis. For ideal gases, the thermodynamics of the confined bosons has been
investigated performing exact numerical summations to describe the major
details of the transition and comparing the results with the semiclassical
density-of-states approximation. We find that for large particle number and
increasing well separation, the condensation temperature evolves from the
thermodynamic limit value T_c^{(0)}(N) to T_c^{(0)}(N/2). The effects of adding
a repulsive interaction between atoms has been examined resorting to the
Gross-Pitaevskii-Popov procedure and it is found that the shift of the
condensation temperature exhibits different signs according to the separation
between wells. In particular, for sufficiently large splitting, the trend
opposes the well known results for harmonic traps, since the critical
temperature appears to increase with growing repulsion strength.


title: Collective Dynamics of Random Polyampholytes

abstract:
  We consider the Langevin dynamics of a semi-dilute system of chains which are
random polyampholytes of average monomer charge $q$ and with a fluctuations in
this charge of the size $Q^{-1}$ and with freely floating counter-ions in the
surrounding. We cast the dynamics into the functional integral formalism and
average over the quenched charge distribution in order to compute the dynamic
structure factor and the effective collective potential matrix. The results are
given for small charge fluctuations. In the limit of finite $q$ we then find
that the scattering approaches the limit of polyelectrolyte solutions.


title:
Effect of Ordering on Spinodal Decomposition of Liquid-Crystal/Polymer
  Mixtures

abstract:
  Partially phase-separated liquid-crystal/polymer dispersions display highly
fibrillar domain morphologies that are dramatically different from the typical
structures found in isotropic mixtures. To explain this, we numerically explore
the coupling between phase ordering and phase separation kinetics in model
two-dimensional fluid mixtures phase separating into a nematic phase, rich in
liquid crystal, coexisting with an isotropic phase, rich in polymer. We find
that phase ordering can lead to fibrillar networks of the minority polymer-rich
phase.


title:
Thermal Transport as a Probe of Localized Charge and Lattice Distortions
  in Manganites and Cuprates

abstract:
  The thermal resistivity of manganites and cuprates is shown to correlate with
local lattice distortions and the presence of localized holes as determined
from neutron diffraction and nuclear quadrupole resonance. Oxygen doping
dependent studies of heat conduction in Y-123 and Hg cuprates suggest that a
fraction of the holes in the CuO$_2$ planes are localized near the planar hole
concentration $p=1/8$. The results are consistent with the formation of
short-ranged, static stripe domains that are pinned near oxygen vacancy
clusters.


title:
Universal structure of the edge states of the fractional quantum Hall
  states

abstract:
  We present an effective theory for the bulk fractional quantum Hall states on
the Jain sequences on closed surfaces and show that it has a universal form
whose structure does not change from fraction to fraction. The structure of
this effective theory follows from the condition of global consistency of the
flux attachment transformation on closed surfaces. We derive the theory of the
edge states on a disk that follows naturally from this globally consistent
theory on a torus. We find that, for a fully polarized two-dimensional electron
gas, the edge states for all the Jain filling fractions $\nu=p/(2np+1)$ have
only one propagating edge field that carries both energy and charge, and two
non-propagating edge fields of topological origin that are responsible for the
statistics of the excitations. Explicit results are derived for the electron
and quasiparticle operators and for their propagators at the edge. We show that
these operators create states with the correct charge and statistics. It is
found that the tunneling density of states for all the Jain states scales with
frequency as $|\omega|^{(1-\nu)/\nu}$.


title:
Correlation Lengths in the Vortex Line Liquid of a High Tc
  Superconductor

abstract:
  We use the three dimensional uniformly frustrated XY model, as a model for a
high temperature superconductor in an applied magnetic field, to explicitly
measure the longitudinal correlation length xi_z in the vortex line liquid
phase. We determine the scaling of xi_z with magnetic field and system
anisotropy close to the vortex lattice melting transition. We apply our results
to determine the extent of longitudinal correlations in YBCO just above
melting.


title: Berry phases in superconducting transitions

abstract:
  I generalize the concept of Berry's geometrical phase for quasicyclic
Hamiltonians to the case in which the ground state evolves adiabatically to an
excited state after one cycle, but returns to the ground state after an integer
number of cycles. This allows to extend the charge Berry phase gamma_c related
to the macroscopic polarization, to many-body systems with fractional number of
particles per site. Under certain conditions, gamma_c and the spin Berry phase
gamma_s jump in pi at the boundary of superconducting phases. In the extended
Hubbard chain with on-site attraction U and nearest-neighbor interaction V at
quarter filling, the transitions detected agree very well with exact results in
two limits solved by the Bethe ansatz, and with previous numerical studies. In
chains with spin SU(2) symmetry, gamma_s jumps when a spin gap opens.


title:
Glass Transition Temperature and Dynamics of $\alpha$-Process in Thin
  Polymer Films

abstract:
  The glass transition temperature $T_g$ and the temperature $T_{\alpha}$
corresponding to the peak in the dielectric loss due to the $\alpha$-process
have been simultaneously determined as functions of film thickness $d$ through
dielectric measurements for thin films of polystyrene. A decrease of $T_g$ was
observed with decreasing film thickness, while $T_{\alpha}$ was found to remain
almost constant for $d > d_c$ and decrease drastically for $d < d_c$. Here,
$d_c$ is a critical thickness dependent on molecular weight. The thickness
dependence of $T_g$ is related to the distribution of relaxation times of the
$\alpha$-process, not to the relaxation time itself.


title: The Quantized Hall Insulator: A New Insulator in Two-Dimensions

abstract:
  Quite generally, an insulator is theoretically defined by a vanishing
conductivity tensor at the absolute zero of temperature. In classical
insulators, such as band insulators, vanishing conductivities lead to diverging
resistivities. In other insulators, in particular when a high magnetic field
(B) is added, it is possible that while the magneto-resistance diverges, the
Hall resistance remains finite, which is known as a Hall insulator. In this
letter we demonstrate experimentally the existence of another, more exotic,
insulator. This insulator, which terminates the quantum Hall effect series in a
two-dimensional electron system, is characterized by a Hall resistance which is
approximately quantized in the quantum unit of resistance h/e^2. This insulator
is termed a quantized Hall insulator. In addition we show that for the same
sample, the insulating state preceding the QHE series, at low-B, is of the HI
kind.


title:
Phase separation in the 2D Hubbard model: a challenging application of
  fixed-node QMC

abstract:
  We discuss the main aspects of the fixed-node quantum Monte Carlo method for
lattice fermions and its recent application to the problem of phase separation
in the 2D Hubbard model, along with virtues, limitations and perspectives of
this kind of approach.


title: Density-matrix spectra for integrable models

abstract:
  The spectra which occur in numerical density-matrix renormalization group
(DMRG) calculations for quantum chains can be obtained analytically for
integrable models via corner transfer matrices. This is shown in detail for the
transverse Ising chain and the uniaxial XXZ Heisenberg model and explains in
particular their exponential character in these cases.


title: Strongly correlated electrons: Dynamical vertex renormalization

abstract:
  Single-band Hubbard model at criticality of the metal-insulator transition is
studied using approximations derived from parquet theory. It is argued that
only the electron-hole and interaction two-particle channels in the parquet
algebra are relevant. A scheme is proposed how to reduce the parquet equations
to a manageable form the complexity of which is comparable with single-channel
approximations such as the renormalized RPA. The newly derived approximation,
however, contains dynamical vertex corrections, remains self-consistent at the
two-particle level and allows only for integrable singularities. A
qualitatively new approach for studying two-particle singularities at zero
temperature is obtained.


title:
Levy Flights in External Force Fields: Langevin and Fractional
  Fokker-Planck Equations, and their Solutions

abstract:
  We consider Levy flights subject to external force fields. This anomalous
transport process is described by two approaches, a Langevin equation with Levy
noise and the corresponding generalized Fokker-Planck equation containing a
fractional derivative in space. The cases of free flights, constant force and
linear Hookean force are analyzed in detail, and we corroborate our findings
with results from numerical simulations. We discuss the non-Gibbsian character
of the stationary solution for the case of the Hookean force, i.e. the
deviation from Boltzmann equilibrium for long times. The possible connection to
Tsallis's q-statistics is studied.


title:
Origin of the Hall conductivity below the vortex-lattice melting in
  twinned single crystal YBCO

abstract:
  In his the Comment, P. Ao affirms that his vortex many-body theory explains
the observations reported in G. D'Anna et al., Phys. Rev. Lett. 81, 2530
(1998). Indeed, we provide here some arguments to show the lack of a
satisfactory explanation within the known theoretical frameworks.


title: Contact forces in regular 3D granular pile

abstract:
  We present exact results for the contact forces in a three dimensional static
piling of identical, stiff and frictionless spheres. The pile studied is a
pyramid of equilateral triangular base (``stack of cannonballs'') with a FCC
(face centered cubic) structure. We show in particular that, as for the two
dimensional case, the pressure on the base of such a pile is uniform.


title: Shock propagation in a granular chain

abstract:
  We numerically solve the propagation of a shock wave in a chain of elastic
beads with no restoring forces under traction (no-tension elasticity). We find
a sequence of peaks of decreasing amplitude and velocity. Analyzing the main
peak at different times we confirm a recently proposed scaling law for its
decay.


title: Topological aspects of geometrical signatures of phase transitions

abstract:
  Certain geometric properties of submanifolds of configuration space are
numerically investigated for classical lattice phi^4 models in one and two
dimensions. Peculiar behaviors of the computed geometric quantities are found
only in the two-dimensional case, when a phase transition is present. The
observed phenomenology strongly supports, though in an indirect way, a recently
proposed topological conjecture about a topology change of the configuration
space submanifolds as counterpart of a phase transition.

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


title:
Dynamical electron correlations in metals: TB-LMTO and multiband Hubbard
  Hamiltonian

abstract:
  We study electronic properties of solids with correlated d electrons which
could be described by a multiband Hubbard Hamiltonian in the weak-interaction
case, $U/w<1$. The one-electron part of the many-body Hamiltonian is described
by a tight-binding LMTO method. The many-body part is treated by
non-selfconsistent FLEX-type approximations with adjusted chemical potential
not to change the LMTO band filling. The calculated DOS gets narrower but is
only little influenced by electron correlations at the Fermi energy. A
precursor of a satellite band in the paramagnetic bcc Ni is found at about 6 eV
below the Fermi level in agreement with experiment.


title: Ordering in magnetic films with surface anisotropy

abstract:
  Effects of the surface exchange anisotropy on ordering of ferromagnetic films
are studied for the exactly solvable classical spin-vector model with D \to
\infty components. For small surface anisotropy \eta'_s << 1 (defined relative
to the exchange interaction), the shift of T_c in a film consisting of N >> 1
layers behaves as T_c^{\rm bulk} - T_c(N) ~ (1/N)\ln(1/\eta'_s) in three
dimensions. The finite-size-scaling limit T_c^{\rm bulk} - T_c(N) \propto
1/(\eta'^{1/2}N^2), which is realized for the model with a bulk anisotropy
\eta' << 1 in the range N\eta'^{1/2} >~ 1, never appears for the model with the
pure surface anisotropy. Here for N\exp(-1/\eta'_s) >~ 1 in three dimensions,
film orders at a temperature above T_c^{\rm bulk} (the surface phase
transition). In the semi-infinite geometry, the surface phase transition occurs
for whatever small values of \eta'_s (i.e., the special phase transition
corresponds to T_c^{\rm bulk}) in dimensions three and lower.


title:
Anisotropic renormalized fluctuations in the microwave resistivity in
  YBCO

abstract:
  We discuss the excess conductivity above Tc due to renormalized
order-parameter fluctuations in YBCO at microwave frequencies. We calculate the
effects of the uniaxial anisotropy on the renormalized fluctuations in the
Hartree approximation, extending the isotropic theory developed by Dorsey
[Phys. Rev. B 43, 7575 (1991)]. Measurements of the real part of the microwave
resistivity at 24 and 48 GHz and of the dc resistivity are performed on
different YBCO films. The onset of the superconducting transition and the
deviation from the linear temperature behavior above Tc can be fully accounted
for by the extended theory. According to the theoretical calculation here
presented, a departure from gaussian toward renormalized fluctuations is
observed. Very consistent values of the fundamental parameters (critical
temperature, coherence lenghts, penetration depth) of the superconducting state
are obtained.


title: Disorder effects in cellular automata for two-lane traffic

abstract:
  For single-lane traffic models it is well known that particle disorder leads
to platoon formation at low densities. Here we discuss the effect of slow cars
in two-lane systems. Surprisingly, even a small number of slow cars can
initiate the formation of platoons at low densities. The robustness of this
phenomenon is investigated for different variants of the lane-changing rules as
well as for different variants on the single-lane dynamics. It is shown that
anticipation of drivers reduces the influence of slow cars drastically.


title:
Jahn-Teller distortion and electronic correlation effects in undoped
  manganese perovskites

abstract:
  The formation of a long-range Jahn-Teller distortion in undoped manganites is
studied within the dynamical mean-field theory taking both electron-phonon and
electron-electron interactions into account. We find that the observed
insulating behavior at temperatures above the Jahn-Teller transition and the
bond-length changes below it can most naturally by explained by assuming a
strong electronic repulsion and a rather weak electron-phonon coupling.


title:
Analogy between two-dimensional dislocation systems and layered
  superconductors

abstract:
  We propose a consistent treatment of the applied external stress on a
two-dimensional dislocation systems by using the analogy between our model and
the layered superconductors. Using the results of a mathematically rigorous,
real-space renormalization-group (RG) calculations in the latter model we
extend the original model developed by Khantha {\it et al.} From our recursion
relations a nonlinear equation is derived for the transition temperature as a
function of the applied stress. Our results are compared with the predictions
of Khantha's model for three particular materials and the reasons for the
deviations are discussed. Possible extensions of our model are outlined.


title: Quasi-particle spectrum around a single vortex in s-wave superconductors

abstract:
  Making use of the Bogoliubov-de Gennes equation, we study the quasi-particle
spectrum and the vortex core structure of a single vortex in quasi 2D s-wave
superconductors for small $p_F\xi_0$, where $p_F$ is the Fermi momentum and
$\xi_0=v_F/\Delta_0$ is the coherence length($\hbar=1$). In particular we find
that the number of bound states decreases rapidly for decreasing $p_F\xi_0$.
Also for $p_F\xi_0\sim 1$, the Kramer-Pesch effect stops around $T/T_c\simeq
0.3$.


title: Quantum Theory of Chiral Interactions in Cholesteric Liquid Crystals

abstract:
  We study the effective chiral interaction between molecules arising from
quantum dispersion interactions within a model in which a) the dominant excited
states of a molecule form a band whose width is small compared to the average
excitation energy and b) biaxial orientational correlation between adjacent
molecules can be neglected. Previous treatments of quantum chiral interactions
were based on a multipole expansion of the intermolecular interaction. However,
because real liquid crystals are composed of elongated molecules, we utilize an
expansion in terms of only coordinates transverse to the long molecular axes.
We identify two distinct physical limits depending on whether one or both of
the interacting molecules are excited in the virtual state. When both molecules
are excited, our results are similar to those found previously by van der Meer
et al. Previously unidentified terms in which only one molecule is excited
involve the interactions of local dipole moments, which exist even when the
global dipole moment of the molecule vanishes. We present analytic and
numerical results for helical molecules. Our results do not indicate whether
the dominant chiral interaction in cholesterics results from quantum or from
steric interactions.


title: Universal Dynamics of Phase-Field Models for Dendritic Growth

abstract:
  We compare time-dependent solutions of different phase-field models for
dendritic solidification in two dimensions, including a thermodynamically
consistent model and several ad hoc models. The results are identical when the
phase-field equations are operating in their appropriate sharp interface limit.
The long time steady state results are all in agreement with solvability
theory. No computational advantage accrues from using a thermodynamically
consistent phase-field model.


title: Testing self-organized criticality by induced seismicity

abstract:
  We examine the hypothesis proposed in recent years by several authors that
the crust is in a self-organized critical (SOC) state by exploring how the SOC
concept can help in understanding the observed earthquake clustering on
relatively narrow fault domains and the phenomenon of induced seismicity. We
review the major reported cases of induced seismicity in various parts of the
world and find that both pore pressure changes (+/-p) and mass transfers (+/-m)
leading to incremental deviatoric stresses of <<1 MPa are sufficient to trigger
seismic instabilities in the uppermost crust with magnitude ranging up to 7.0
in otherwise historically aseismic areas. Once triggered, stress variations of
at least 1 order of magnitude less but still larger than the ~0.01 MPa tidal
stress are enough to sustain seismic activity. We argue that these observations
are in accord with the SOC hypothesis as they show that a significant fraction
of the crust is not far from instability and can thus be made unstable by
minute perturbations. Not all perturbations, however, trigger seismic activity;
this is also compatible with the SOC hypothesis which embodies naturally the
existence of large heterogeneities in the stress field. In conclusion, the
properties of induced seismicity and their rationalization in terms of the SOC
concept provide further evidence that potential seismic hazards extend over a
much larger area than that where earthquakes are frequent.