title: Anomalous c-axis charge dynamics in copper oxide materials

abstract:
  Within the t-J model, the c-axis charge dynamics of the copper oxide
materials in the underdoped and optimally doped regimes is studied by
considering the incoherent interlayer hopping. It is shown that the c-axis
charge dynamics is mainly governed by the scattering from the in-plane
fluctuation. In the optimally doped regime, the c-axis resistivity is a linear
in temperatures, and shows the metallic-like behavior for all temperatures,
while the c-axis resistivity in the underdoped regime is characterized by a
crossover from the high temperature metallic-like behavior to the low
temperature semiconducting-like behavior, which are consistent with experiments
and numerical simulations.


title: Phase Diagram of Integer Quantum Hall Effect

abstract:
  The phase diagram of integer quantum Hall effect is numerically determined in
the tight-binding model, which can account for overall features of recently
obtained experimental phase diagram. In particular, the quantum Hall plateaus
are terminated by two distinct insulating phases, characterized by the Hall
resistance with classic and quantized values, respectively, which is also in
good agreement with experiments.


title: Finite H2 concentrations in superfluid 4He

abstract:
  We have studied the solubility of molecular hydrogen in bulk liquid $^4$He at
zero temperature using the diffusion Monte Carlo method and realistic
interatomic potentials between the different species of the mixture. Around the
$^4$He equilibrium density, the H_2 molecules clusterize in liquid-like drops
blocking the existence of a uniform dilution. On the contrary, at higher
densities the cluster formation is less feasible and metastable dilute
solutions may exist.


title:
The small polaron crossover transition in colossal magnetoresistance
  (CMR) manganites

abstract:
  Based on the combined model of the double exchange and the polaron, we have
studied the small-to-large polaron crossover transition and explored its
effects on the magnetic and transport properties in colossal magnetoresistance
(CMR) manganites. We have used the variational Lang-Firsov canonical
transformation, and shown that the magnetic and transport properties of both
high and low $T_C$ manganites are well described in terms of a single
formalism. We have reproduced the rapid resistivity drop below $T_C$, a
realistic CMR ratio, and the {\it first-order-like} sharp magnetic phase
transition, which are observed in low $T_C$ manganites.


title: Field-induced breakdown of the quantum Hall effect

abstract:
  A numerical analysis is made of the breakdown of the quantum Hall effect
caused by the Hall electric field in competition with disorder. It turns out
that in the regime of dense impurities, in particular, the number of localized
states decreases exponentially with the Hall field, with its dependence on the
magnetic and electric field summarized in a simple scaling law. The physical
picture underlying the scaling law is clarified. This intra-subband process,
the competition of the Hall field with disorder, leads to critical breakdown
fields of magnitude of a few hundred V/cm, consistent with observations, and
accounts for their magnetic-field dependence \propto B^{3/2} observed
experimentally. Some testable consequences of the scaling law are discussed.


title:
Dimensional Crossover driven by Magnetic Ordering in Optical
  Conductivity of Pr_{1/2}Sr_{1/2}MnO_3

abstract:
  We investigated optical properties of Pr_{0.5}Sr_{0.5}MnO_3, which has the
A-type antiferromagnetic ordering at a low temperature. We found that T-
dependence of spectral weight transfer shows a clear correlation with the
magnetic phase transition. In comparison with the optical conductivity results
of Nd_{0.5}Sr_{0.5}MnO_3, which has the CE-type antiferromagnetic charge
ordering, we showed that optical properties of Pr_{0.5}Sr_{0.5}MnO_3 near the
Neel temperature could be explained by a crossover from 3D to 2D metals.
Details of spectral weight changes are consistent with the polaron picture.


title: Transport on an annealed disordered lattice

abstract:
  We study the diffusion on an annealed disordered lattice with a local
dynamical reorganization of bonds. We show that the typical rearrangement time
depends on the renewal rate like $t_r \sim \tau^{\alpha}$ with $\alpha \neq 1$.
This implies that the crossover time to normal diffusion in a slow
rearrangement regime shows a critical behavior at the percolation threshold.
New scaling relations for the dependence of the diffusion coefficient on the
renewal rate are obtained. The derivation of scaling exponents confirms the
crucial role of singly connected bonds in transport properties. These results
are checked by numerical simulations in two and three dimensions.


title: Aharonov-Bohm effect in circular carbon nanotubes

abstract:
  We study the interference of interacting electrons in toroidal single-wall
carbon nanotubes coupled to metallic electrodes by tunnel junctions. The dc
conductance shows resonant features as a function of the gate voltage and the
magnetic field. The conductance pattern is determined by the interaction
parameter, which in turn can be cross-checked against the exponents governing
the transport at high temperatures. The coordinate dependence of the
conductance reflects electron correlations in one-dimensional space.


title:
Systematic Evolution of the Magnetotransport Properties of
  Bi_{2}Sr_{2-x}La_{x}CuO_{6} with Carrier Concentration

abstract:
  We report that it is possible to obtain a series of high-quality crystals of
La-doped Bi-2201, of which the transport properties have been believed to be
"dirtier" than those of other cuprates. In our crystals, the normal-state
transport properties display behaviors which are in good accord with other
cuprates; for example, in the underdoped region the in-plane resistivity
\rho_{ab} shows the pseudogap feature and in the overdoped region the T
dependence of \rho_{ab} changes to T^n with n > 1. The characteristic
temperatures of the pseudogap deduced from the resistivity and the Hall
coefficient data are presented.


title: Contact phenomena in carbon nanotubes

abstract:
  Poor screening of the long-range Coulomb interaction in one-dimensional
carbon nanotubes results in a peculiar picture of contact phenomena. Being
brought to a contact with a metal, conducting nanotube accumulates electric
charge whose density decays slowly with the distance from the contact. This
should be contrasted to a conventional metal-metal contact where the charge
density decreases exponentially at atomic distances. Implications for
experiments are discussed.


title:
How to evaluate ground-state landscapes of disordered systems
  thermodynamical correctly

abstract:
  Ground states of three-dimensional EA Ising spin glasses are calculated for
sizes up to 14^3 using a combination of a genetic algorithm and cluster-exact
approximation. For each realization several independent ground states are
obtained. Then, by applying ballistic search and T=0 Monte-Carlo simulations,
it is ensured that each ground state appears with the same probability.
Consequently, the results represent the true T=0 thermodynamic behavior. The
distribution P(|q|) of overlaps is evaluated. For increasing size the width of
P(|q|) and the fraction of the distribution below q_0=0.5 converge to zero.
This indicates that for the infinite system P(|q|) is a delta function, in
contrast to previous results. Thus, the ground-state behavior is dominated by
few large clusters of similar ground states.


title: Self-Assembly in Mixtures of Polymers and Small Associating Molecules

abstract:
  The interaction between a flexible polymer in good solvent and smaller
associating solute molecules such as amphiphiles (surfactants) is considered
theoretically. Attractive correlations, induced in the polymer because of the
interaction, compete with intra-chain repulsion and eventually drive a joint
self-assembly of the two species, accompanied by partial collapse of the chain.
Results of the analysis are found to be in good agreement with experiments on
the onset of self-assembly in diverse polymer-surfactant systems. The threshold
concentration for self-assembly in the mixed system (critical aggregation
concentration, cac) is always lower than the one in the polymer-free solution
(critical micelle concentration, cmc). Several self-assembly regimes are
distinguished, depending on the effective interaction between the two species.
For strong interaction, corresponding experimentally to oppositely charged
species, the cac is much lower than the cmc. It increases with ionic strength
and depends only weakly on polymer charge. For weak interaction, the cac is
lower but comparable to the cmc, and the two are roughly proportional over a
wide range of cmc values. Association of small molecules with amphiphilic
polymers exhibiting intra-chain aggregation (polysoaps) is gradual, having no
sharp onset.


title:
Persistence in the Zero-Temperature Dynamics of the Diluted Ising
  Ferromagnet in Two Dimensions

abstract:
  The non-equilibrium dynamics of the strongly diluted random-bond Ising model
in two-dimensions (2d) is investigated numerically.
  The persistence probability, P(t), of spins which do not flip by time t is
found to decay to a non-zero, dilution-dependent, value $P(\infty)$. We find
that $p(t)=P(t)-P(\infty)$ decays exponentially to zero at large times.
Furthermore, the fraction of spins which never flip is a monotonically
increasing function over the range of bond-dilution considered. Our findings,
which are consistent with a recent result of Newman and Stein, suggest that
persistence in disordered and pure systems falls into different classes.
Furthermore, its behaviour would also appear to depend crucially on the
strength of the dilution present.


title: High Magnetic Field ESR in the Haldane Spin Chains NENP and NINO

abstract:
  We present electron spin resonance experiments in the one-dimensional
antiferromagnetic S=1 spin chains NENP and NINO in pulsed magnetic fields up to
50T. The measured field dependence of the quantum energy gap for B||b is
analyzed using the exact diagonalization method and the density matrix
renormalization group method (DMRG). A staggered anisotropy term (-1)^i d(S_i^x
S_i^z + S_i^z S_i^x) was considered for the first time in addition to a
staggered field term (-1)^i S_i^x B_st. We show that the spin dynamics in high
magnetic fields strongly depends on the orthorhombic anisotropy E.


title: The Ashkin-Teller neural network near saturation

abstract:
  The thermodynamic and retrieval properties of the Ashkin-Teller neural
network model storing an infinite number of patterns are examined in the
replica-symmetric mean-field approximation. In particular, for linked patterns
temperature-capacity phase diagrams are derived for different values of the
two-neuron and four-neuron coupling strengths. This model can be considered as
a particular non-trivial generalisation of the Hopfield model and exhibits a
number of interesting new features. Some aspects of replica-symmetry breaking
are discussed.


title: Sand as Maxwell's demon

abstract:
  We consider a dilute gas of granular material inside a box, kept in a
stationary state by shaking. A wall separates the box into two identical
compartments, save for a small hole at some finite height $h$. As the gas is
cooled, a second order phase transition occurs, in which the particles
preferentially occupy one side of the box. We develop a quantitative theory of
this clustering phenomenon and find good agreement with numerical simulations.


title:
Temperature evolution of the quantum Hall effect in the FISDW state:
  Theory vs Experiment

abstract:
  We discuss the temperature dependence of the Hall conductivity $\sigma_{xy}$
in the magnetic-field-induced spin-density-wave (FISDW) state of the
quasi-one-dimensional Bechgaard salts (TMTSF)_2X. Electronic thermal
excitations across the FISDW energy gap progressively destroy the quantum Hall
effect, so $\sigma_{xy}(T)$ interpolates between the quantized value at zero
temperature and zero value at the transition temperature T_c, where FISDW
disappears. This temperature dependence is similar to that of the superfluid
density in the BCS theory of superconductivity. More precisely, it is the same
as the temperature dependence of the Fr\"ohlich condensate density of a regular
CDW/SDW. This suggests a two-fluid picture of the quantum Hall effect, where
the Hall conductivity of the condensate is quantized, but the condensate
fraction of the total electron density decreases with increasing temperature.
The theory appears to agree with the experimental results obtained by measuring
all three components of the resistivity tensor simultaneously on a
(TMTSF)_2PF_6 sample and then reconstructing the conductivity tensor.


title:
The fraction of condensed counterions around a charged rod: Comparison
  of Poisson-Boltzmann theory and computer simulations

abstract:
  We investigate the phenomenon of counterion condensation in a solution of
highly charged rigid polyelectrolytes within the cell model. A method is
proposed which -- based on the charge distribution function -- identifies both
the fraction of condensed ions and the radial extension of the condensed layer.
Within salt-free Poisson-Boltzmann (PB) theory it reproduces the well known
fraction 1-1/xi of condensed ions for a Manning parameter xi>1. Furthermore, it
predicts a weak salt dependence of this fraction and a breakdown of the concept
of counterion condensation in the high salt limit. We complement our
theoretical investigations with molecular dynamics simulations of a cell-like
model, which constantly yield a stronger condensation than predicted by PB
theory. While the agreement between theory and simulation is excellent in the
monovalent, weakly charged case, it deteriorates with increasing electrostatic
interaction strength and, in particular, increasing valence. For instance, at a
high concentration of divalent salt and large xi our computer simulations
predict charge oscillations, which mean-field theory is unable to reproduce.


title: Metastable states as a key to the dynamics of supercooled liquids

abstract:
  Computer simulations of a model glass-forming system are presented, which are
particularly sensitive to the correlation between the dynamics and the
topography of the potential energy landscape. This analysis clearly reveals
that in the supercooled regime the dynamics is strongly influenced by the
presence of deep valleys in the energy landscape, corresponding to long-lived
metastable amorphous states. We explicitly relate non-exponential relaxation
effects and dynamic heterogeneities to these metastable states and thus to the
specific topography of the energy landscape.


title:
Statistical properties of eigenvectors in non-Hermitian Gaussian random
  matrix ensembles

abstract:
  Statistical properties of eigenvectors in non-Hermitian random matrix
ensembles are discussed, with an emphasis on correlations between left and
right eigenvectors. Two approaches are described. One is an exact calculation
for Ginibre's ensemble, in which each matrix element is an independent,
identically distributed Gaussian complex random variable. The other is a
simpler calculation using $N^{-1}$ as an expansion parameter, where $N$ is the
rank of the random matrix: this is applied to Girko's ensemble. Consequences of
eigenvector correlations which may be of physical importance in applications
are also discussed. It is shown that eigenvalues are much more sensitive to
perturbations than in the corresponding Hermitian random matrix ensembles. It
is also shown that, in problems with time-evolution governed by a non-
Hermitian random matrix, transients are controlled by eigenvector correlations.


title:
The potential energy landscape of a model glass former: thermodynamics,
  anharmonicities, and finite size effects

abstract:
  It is possible to formulate the thermodynamics of a glass forming system in
terms of the properties of inherent structures, which correspond to the minima
of the potential energy and build up the potential energy landscape in the
high-dimensional configuration space. In this work we quantitatively apply this
general approach to a simulated model glass-forming system. We systematically
vary the system size between N=20 and N=160. This analysis enables us to
determine for which temperature range the properties of the glass former are
governed by the regions of the configuration space, close to the inherent
structures. Furthermore, we obtain detailed information about the nature of
anharmonic contributions. Moreover, we can explain the presence of finite size
effects in terms of specific properties of the energy landscape. Finally,
determination of the total number of inherent structures for very small systems
enables us to estimate the Kauzmann temperature.


title: Phonon and Elastic Instabilities in MoC and MoN

abstract:
  We present several results related to the instability of MoC and MoN in the
B1 (sodium chloride) structure. These compounds were proposed as potential
superconductors with moderately high transition temperatures. We show that the
elastic instability in B1-structure MoN, demonstrated several years ago,
persists at elevated pressures, thus offering little hope of stabilizing this
material without chemical doping. For MoC, another material for which
stoichiometric fabrication in the B1-structure has not proven possible, we find
that all of the cubic elastic constants are positive, indicating elastic
stability. Instead, we find X-point phonon instabilities in MoC (and in MoN as
well), further illustrating the rich behavior of carbo-nitride materials. We
also present additional electronic structure results for several transition
metal (Zr, Nb and Mo) carbo-nitride systems and discuss systematic trends in
the properties of these materials. Deviations from strict electron counting
dependencies are apparent.


title: Condensation of `composite bosons' in a rotating BEC

abstract:
  We provide evidence for several novel phases in the dilute limit of rotating
BECs. By exact calculation of wavefunctions and energies for small numbers of
particles, we show that the states near integer angular momentum per particle
are best considered condensates of composite entities, involving vortices and
atoms. We are led to this result by explicit comparison with a description
purely in terms of vortices. Several parallels with the fractional quantum Hall
effect emerge, including the presence of the Pfaffian state.


title: Complex temperatures zeroes of partition function in spin-glass models

abstract:
  An approximate method is proposed for investigating complex-temperature
properties of real-dimensional spin-glass models. The method uses the
complex-temperature data of the ferromagnetic model on the same lattice. The
universality line in the complex-temperature space is obtained.


title: Monolayers of 3He on the Surface of Bulk Superfluid 4He

abstract:
  We have used quantum evaporation to investigate the two-dimensional fermion
system that forms at the free surface of (initially isotopically pure) 4He when
small quantities of 3He are added to it. By measuring the first-arrival times
of the evaporated atoms, we have determined that the 3He-3He potential in this
system is V_3S/k_B=(0.23+/-0.02) K nm^2 (repulsive) and estimated a value of
m_3S=(1.53+/-0.02)m_3 for the zero-coverage effective mass. We have also
observed the predicted second layer-state which becomes occupied once the first
layer-state density exceeds about 0.6 monolayers.


title: Universal Behaviour of Metal-Insulator Transitions in the p-SiGe System

abstract:
  Magnetoresistance measurements are presented for a strained p-SiGe quantum
well sample where the density is varied through the B=0 metal-insulator
transition. The close relationship between this transition, the high field Hall
insulator transition and the filling factor $\nu$=3/2 insulating state is
demonstrated.


title:
Relative Evaporation Probabilities of 3He and 4He from the Surface of
  Superfluid 4He

abstract:
  We report a preliminary experiment which demonstrates that 3He atoms in
Andreev states are evaporated by high-energy (E/k_B ~ 10.2 K) phonons in a
quantum evaporation process similar to that which occurs in pure 4He. Under
conditions of low 3He coverage, high-energy phonons appear to evaporate 3He and
4He atoms with equal probability. However, we have not managed to detect any
3He atoms that have been evaporated by rotons, and conclude that the
probability of a roton evaporating a 3He atom is less than 2% of the
probability that it evaporates a 4He atom.


title:
Formation of the Heavy-Fermion State - an Explanation in a Model
  Traditionally Called Localized

abstract:
  In contrary to widely spread view about the substantial delocalization of f
electrons in heavy- fermion (h-f) compounds it is argued that h-f phenomena can
be understood with localized f electrons. Then the role of crystal-field
interactions is essential and the heavy-fermion behaviour can occur for the
localized Kramers-doublet ground state.


title:
Watching a superfluid untwist itself: Recurrence of Rabi oscillations in
  a Bose-Einstein condensate

abstract:
  The order parameter of a condensate with two internal states can continuously
distort in such a way as to remove twists that have been imposed along its
length. We observe this effect experimentally in the collapse and recurrence of
Rabi oscillations in a magnetically trapped, two-component Bose-Einstein
condensate of ^87Rb.


title: Instabilities in crystal growth by atomic or molecular beams

abstract:
  The planar front of a growing a crystal is often destroyed by instabilities.
In the case of growth from a condensed phase, the most frequent ones are
diffusion instabilities, which will be but briefly discussed in simple terms in
chapter II. The present review is mainly devoted to instabilities which arise
in ballistic growth, especially Molecular Beam Epitaxy (MBE). The reasons of
the instabilities can be geometric (shadowing effect), but they are mostly
kinetic or thermodynamic. The kinetic instabilities which will be studied in
detail in chapters IV and V result from the fact that adatoms diffusing on a
surface do not easily cross steps (Ehrlich-Schwoebel or ES effect). When the
growth front is a high symmetry surface, the ES effect produces mounds which
often coarsen in time according to power laws. When the growth front is a
stepped surface, the ES effect initially produces a meandering of the steps,
which eventually may also give rise to mounds. Kinetic instabilities can
usually be avoided by raising the temperature, but this favours thermodynamic
instabilities. Concerning these ones, the attention will be focussed on the
instabilities resulting from slightly different lattice constants of the
substrate and the adsorbate. They can take the following forms. i) Formation of
misfit dislocations (chapter VIII). ii) Formation of isolated epitaxial
clusters which, at least in their earliest form, are `coherent' with the
substrate, i.e. dislocation-free (chapter X). iii) Wavy deformation of the
surface, which is presumably the incipient stage of (ii) (chapter IX). The
theories and the experiments are critically reviewed and their comparison is
qualitatively satisfactory although some important questions have not yet
received a complete answer.


title: Quasiparticle localization in superconductors with spin-orbit scattering

abstract:
  We develop a theory of quasiparticle localization in superconductors in
situations without spin rotation invariance. We discuss the existence, and
properties of superconducting phases with localized/delocalized quasiparticle
excitations in such systems in various dimensionalities. Implications for a
variety of experimental systems, and to the properties of random Ising models
in two dimensions, are briefly discussed.


title: On the time evolution of the entropic index

abstract:
  We adapt the Kolmogorov-Sinai entropy to the non-extensive perspective
recently advocated by Tsallis. The resulting expression is an average on the
invariant distribution, which should be used to detect the genuine entropic
index Q. We argue that the condition Q >1 is time dependent.


title: Fidelity and leakage of Josephson qubits

abstract:
  The unit of quantum information is the qubit, a vector in a two-dimensional
Hilbert space. On the other hand, quantum hardware often operates in
two-dimensional subspaces of vector spaces of higher dimensionality. The
presence of higher quantum states may affect the accuracy of quantum
information processing. In this Letter we show how to cope with {\em quantum
leakage} in devices based on small Josephson junctions. While the presence of
higher charge states of the junction reduces the fidelity during gate
operations we demonstrate that errors can be minimized by appropriately
designing and operating the gates.


title: Dephasing of Local Vibrations in a Planar Lattice of Adsorbed Molecules

abstract:
  We investigate anharmonically coupled high- and low-frequency excitations in
a planar lattice of adsorbed molecules interacting with phonons of a crystal.
Dephasing of high-frequency local vibrations by low-frequency resonance modes
is described in terms of temperature Green's function. The equations obtained
are solved, first, with a small ratio of the anharmonic coupling coefficient
for high- and low-frequency modes to the resonance width, and second, in the
low-temperature limit. High-frequency spectral line positions and widths depend
on dispersion laws and resonance mode lifetimes. It is shown that lateral
interactions of low-frequency modes of adsorbed molecules can lead to a
significant narrowing of high-frequency spectral lines, which is consistent
with experimental data.


title: First-Order Phase Transition in a Quantum Hall Ferromagnet

abstract:
  The single-particle energy spectrum of a two-dimensional electron gas in a
perpendicular magnetic field consists of equally-spaced spin-split Landau
levels, whose degeneracy is proportional to the magnetic field strength. At
integer and particular fractional ratios between the number of electrons and
the degeneracy of a Landau level (filling factors n) quantum Hall effects
occur, characterised by a vanishingly small longitudinal resistance and
quantised Hall voltage. The quantum Hall regime offers unique possibilities for
the study of cooperative phenomena in many-particle systems under
well-controlled conditions. Among the fields that benefit from quantum-Hall
studies is magnetism, which remains poorly understood in conventional material.
Both isotropic and anisotropic ferromagnetic ground states have been predicted
and few of them have been experimentally studied in quantum Hall samples with
different geometries and filling factors. Here we present evidence of
first-order phase transitions in n = 2 and 4 quantum Hall states confined to a
wide gallium arsenide quantum well. The observed hysteretic behaviour and
anomalous temperature dependence in the longitudinal resistivity indicate the
occurrence of a transition between the two distinct ground states of an Ising
quantum-Hall ferromagnet. Detailed many-body calculations allowed the
identification of the microscopic origin of the anisotropy field.


title: Antiferromagnetism in hydrated 123 compounds

abstract:
  Copper nuclear quadrupole resonance (NQR) and zero field nuclear magnetic
resonance (ZFNMR) studies of YBa$_2$Cu$_3$O$_{6.5}$ show that a magnetic phase
appears in underdoped 123 superconductors treated in ambient wet air. The
studies give convincing evidence that the ``empty'' CuO chains play the role of
easy water insertion channels. The reaction occurs first in ordered regions of
the crystallites. The final product of the reaction is a non-superconducting
antiferromagnetic compound characterized by at least two types of magnetically
ordered copper ions with ZFNMR spectra respectively in the frequency ranges of
46-96 and 96-135 MHz respectively. Even for powder samples fixed in an epoxy
resin, the reaction is found to occur partly after a few years.


title:
Positive Magnetoresistance of Composite Fermions in Laterally Modulated
  Structures

abstract:
  Adopting the mean-field composite fermion picture, we describe the
magneto-transport properties of a two-dimensional electron gas with laterally
modulated density around filling factor 1/2. The occurrence of a strong
positive magnetoresistance at low effective magnetic fields as well as Weiss
oscillations, which were observed in recent experiments in such systems, can be
explained within a semi-classical Boltzmann equation approach, provided one
goes beyond a second order approximation in the modulation strength.


title:
Thin superconducting disk with B-dependent Jc: Flux and current
  distributions

abstract:
  The critical state in a superconducting thin circular disk with an arbitrary
magnetic field dependence of the critical sheet current, Jc(B), is analyzed.
With an applied field Ba perpendicular to the disk, a set of coupled integral
equations for the flux and current distributions is derived. The equations are
solved numerically, and flux and current profiles are presented graphically for
several commonly used Jc(B) dependences. It is shown that for small Ba the flux
penetration depth can be described by an effective Bean model with a
renormalized Jc entering the leading term. We argue that these results are
qualitatively correct for thin superconductors of any shape. The results
contrast the parallel geometry behavior, where at small Ba the B-dependence of
the critical current can be ignored.


title: Self-organization of value and demand

abstract:
  We study the dynamics of exchange value in a system composed of many
interacting agents. The simple model we propose exhibits cooperative emergence
and collapse of global value for individual goods. We demonstrate that the
demand that drives the value exhibits non Gaussian "fat tails" and typical
fluctuations which grow with time interval with a Hurst exponent of 0.7.


title:
Spectrum of the Fokker-Planck operator representing diffusion in a
  random velocity field

abstract:
  We study spectral properties of the Fokker-Planck operator that represents
particles moving via a combination of diffusion and advection in a
time-independent random velocity field, presenting in detail work outlined
elsewhere [J. T. Chalker and Z. J. Wang, Phys. Rev. Lett. {\bf 79}, 1797
(1997)]. We calculate analytically the ensemble-averaged one-particle Green
function and the eigenvalue density for this Fokker-Planck operator, using a
diagrammatic expansion developed for resolvents of non-Hermitian random
operators, together with a mean-field approximation (the self-consistent Born
approximation) which is well-controlled in the weak-disorder regime for
dimension d>2. The eigenvalue density in the complex plane is non-zero within a
wedge that encloses the negative real axis. Particle motion is diffusive at
long times, but for short times we find a novel time-dependence of the
mean-square displacement, $<r^2> \sim t^{2/d}$ in dimension d>2, associated
with the imaginary parts of eigenvalues.


title:
Resonant Raman scattering by elementary electronic excitations in
  semiconductor structures

abstract:
  We explain quantitatively why resonant Raman scattering spectroscopy, an
extensively used experimental tool in studying elementary electronic
excitations in doped low dimensional semiconductor nanostructures, always
produces an observable peak at the so-called "single particle" excitation
although the standard theory predicts that there should be no such single
particle peak in the Raman spectra. We have thus resolved an experimental
puzzle which dates back more than twenty-five years.


title:
Spinodal decomposition of an ABv model alloy: Patterns at unstable
  surfaces

abstract:
  We develop mean-field kinetic equations for a lattice gas model of a binary
alloy with vacancies (ABv model) in which diffusion takes place by a vacancy
mechanism. These equations are applied to the study of phase separation of
finite portions of an unstable mixture immersed in a stable vapor. Due to a
larger mobility of surface atoms, the most unstable modes of spinodal
decomposition are localized at the vapor-mixture interface. Simulations show
checkerboard-like structures at the surface or surface-directed spinodal waves.
We determine the growth rates of bulk and surface modes by a linear stability
analysis and deduce the relation between the parameters of the model and the
structure and length scale of the surface patterns. The thickness of the
surface patterns is related to the concentration fluctuations in the initial
state.


title: Phases from Lightly-Doped Two-Legged Spin Ladders

abstract:
  We study two-leg spin ladder systems analytically by using a t-J model which
includes the interchain spin exchange and the interchain hopping term. The spin
part is mapped to a quantum sine-Gordon model via a bosonization method that
fully accounts for the phase fluctuation. The spin gap in the Luther-Emery
phase evolves to gapless Luttinger liquid phases in even-ladders at certain
doping concentration. This transition occurs as doping rate increases and/or
the ratio of the interchain spin exchange and hopping to the intrachain
couplings decreases. We also estimate the transition temperature at which the
conventional electron phase is deconfined to spinon and holon.


title:
Two-dimensional pattern formation in surfactant-mediated epitaxial
  growth

abstract:
  The effects of a surfactant on two-dimensional pattern formation in epitaxial
growth are explored theoretically using a simple model, in which an adatom
becomes immobile only after overcoming a large energy barrier as it exchanges
positions with a surfactant atom, and subsequent growth from such a seed is
further shielded. Within this model, a fractal-to-compact island shape
transition can be induced by either decreasing the growth temperature or
increasing the deposition flux. This and other intriguing findings are in
excellent qualitative agreement with recent experiments.


title: Perturbation study of the conductance through a finite Hubbard chain

abstract:
  Transport through a Hubbard chain of size N (=1,2,3,...) connected to
reservoirs is studied at T = 0 in an electron-hole symmetric case based on the
second-order perturbation theory in U. The result shows a typical even-odd
property corresponding to a Kondo or Mott-Hubbard physics. In this report,
specifically, we study the dependence of the conductance on the coupling
between the chain and reservoirs, which was not examined in detail in our
previous report [Phy. Rev. B 59, 12240 (1999)].


title: Optimal orbitals from energy fluctuations in correlated wave functions

abstract:
  A quantum Monte Carlo method of determining Jastrow-Slater wave functions for
which the energy is stationary with respect to variations in the
single-particle orbitals is presented. A potential is determined by a
least-squares fitting of fluctuations in the energy with a linear combination
of one-body operators. This potential is used in a self-consistent scheme for
the orbitals whose solution ensures that the energy of the correlated wave
function is stationary with respect to variations in the orbitals. The method
is feasible for atoms, molecules, and solids and is demonstrated for the carbon
and neon atoms.


title:
Tunneling-Assisted Acoustic Plasmon-Quasiparticle Excitation Resonances
  in Coupled Q1D Electron Gases

abstract:
  We show that a weak non-resonant tunneling between two quantum wires leads to
splitting of the acoustic plasmon mode at finite wavevector. Two gaps open up
in the dipersion of the acoustic plasmon mode when its frequency is close to
the frequencies of the quasiparticle excitations. In contrast to the Laudau
damping of the collective excitations, these gaps are attributed to tunneling
assisted acoustic plasmon-quasiparticle excitation resonances. We predict that
such a resonance can be observed in inelastic light scattering spectrum.


title:
Inelastic Coulomb scattering rates due to acoustic and optical plasmon
  modes in coupled quantum wires

abstract:
  We report a theoretical study on the inelastic Coulomb scattering rate of an
injected electron in two coupled quantum wires in quasi-one-dimensional doped
semiconductors. Two peaks appear in the scattering spectrum due to the optical
and the acoustic plasmon scattering in the system. We find that the scattering
rate due to the optical plasmon mode is similar to that in a single wire but
the acoustic plasmon scattering depends crucially on its dispersion relation at
small $q$. Furthermore, the effects of tunneling between the two wires are
studied on the inelastic Coulomb scattering rate. We show that a weak tunneling
can strongly affect the acoustic plasmon scattering.


title: Beta-relaxation of non-polymeric liquids close to the glass transition

abstract:
  Dielectric beta-relaxation in a pyridine-toluene solution is studied close to
the glass transition. In the equilibrium liquid state the beta loss peak
frequency is not Arrhenius (as in the glass) but virtually
temperature-independent, while the maximum loss is strongly
temperature-dependent. Both loss peak frequency and maximum loss exhibit
thermal hysteresis. A new annealing-state independent parameter involving loss
and loss peak frequency is identified. This parameter has a simple Arrhenius
temperature-dependence and is unaffected by the glass transition.


title: Pair Connectedness and Shortest Path Scaling in Critical Percolation

abstract:
  We present high statistics data on the distribution of shortest path lengths
between two near-by points on the same cluster at the percolation threshold.
Our data are based on a new and very efficient algorithm. For $d=2$ they
clearly disprove a recent conjecture by M. Porto et al., Phys. Rev. {\bf E 58},
R5205 (1998). Our data also provide upper bounds on the probability that two
near-by points are on different infinite clusters.


title:
Similarity of slow stripe fluctations between Sr-doped cuprates and
  oxygen-doped nickelates

abstract:
  Stripe fluctuations in La2NiO4.17 have been studied by 139La NMR using the
field and temperature dependence of the linewidth and relaxation rates. In the
formation process of the stripes the NMR line intensity is maximal below 230K,
starts to diminish around 140K, disappears around 50K and recovers at 4K. These
results are shown to be consistent with, but completely complementary to
neutron measurements, and to be generic for oxygen doped nickelates and
underdoped cuprates.


title: Does macroscopic disorder imply microscopic chaos?

abstract:
  We argue that Gaspard and coworkers [Nature 394, 865 (1998)] do not give
evidence for microscopic chaos in the sense in which they use the term. The
effectively infinite number of molecules in a fluid can generate the same
macroscopic disorder without any intrinsic instability. But we argue also that
the notion of chaos in infinitely extended systems needs clarification: In a
wider sense, even some systems without local instabilities can be considered
chaotic.


title:
Comment on "Intermittent Synchronization in a Pair of Coupled Chaotic
  Pendula"

abstract:
  In [G. Baker et al., Phys. Rev. Lett. 81, 554 (1998)] a number of supposedly
novel and surprising features were observed in a system composed of two
periodically driven and asymmetrically coupled pendula. In particular it was
claimed that `permanent synchronization ... does not occur except as a
numerical artifact'. It was suggested that this might be related to the
particular type of coupling. In this comment I want to point out that some of
these claims cannot be maintained. The synchronization in this system is
precisely of standard blow-out type. The observed intermittency is exactly the
on-off intermittency well known from the synchronization of multifractal
chaotic attractors.


title:
Exact Eigenstates and Magnetic Response of Spin-1 and Spin-2 Vectorial
  Bose-Einstein Condensates

abstract:
  The exact eigenspectra and eigenstates of spin-1 and spin-2 vectorial
Bose-Einstein condensates (BECs) are found, and their response to a weak
magnetic field is studied and compared with their mean-field counterparts.
Whereas mean-field theory predicts the vanishing population of the zero
magnetic-quantum-number component of a spin-1 antiferromagnetic BEC, the
component is found to become populated as the magnetic field decreases. The
spin-2 BEC exhibits an even richer magnetic response due to quantum correlation
between 3 bosons.


title: The Aharonov-Bohm effect for an exciton

abstract:
  We study theoretically the exciton absorption on a ring shreded by a magnetic
flux. For the case when the attraction between electron and hole is
short-ranged we get an exact solution of the problem. We demonstrate that,
despite the electrical neutrality of the exciton, both the spectral position of
the exciton peak in the absorption, and the corresponding oscillator strength
oscillate with magnetic flux with a period $\Phi_0$---the universal flux
quantum. The origin of the effect is the finite probability for electron and
hole, created by a photon at the same point, to tunnel in the opposite
directions and meet each other on the opposite side of the ring.


title:
Peierls Instabilities in Quasi-One-Dimensional Quantum Double-Well
  Chains

abstract:
  Peierls-type instabilities in quarter-filled ($\bar{n}=1/2$) and half-filled
($\bar{n}=1$) quantum double-well hydrogen-bonded chain are investigated
analytically in the framework of two-stage orientational-tunnelling model with
additional inclusion of the interactions of protons with two different optical
phonon branches. It is shown that when the energy of proton-phonon coupling
becomes large, the system undergoes a transition to a various types of
insulator states. The influence of two different transport amplitudes on ground
states properties is studied. The results are compared with the pressure effect
experimental investigations in superprotonic systems and hydrogen halides at
low temperatures.


title:
Single particle and collective excitations in the one-dimensional charge
  density wave solid K0.3MoO3 probed in real time by femtosecond spectroscopy

abstract:
  Ultrafast transient reflectivity changes caused by collective and single
particle excitations in the quasi one-dimensional charge-density wave (CDW)
semiconductor K0.3MoO3 are investigated with optical pump-probe spectroscopy.
The temperature-dependence of non-equilibrium single particle excitations
across the CDW gap and their recombination dynamics are reported for the first
time. In addition, amplitude mode reflectivity oscillations are observed in
real time. A T-dependent overdamped response is also observed which is
attributed to relaxation of the phason mode.


title:
A Generalization of Metropolis and Heat-Bath Sampling for Monte Carlo
  Simulations

abstract:
  For a wide class of applications of the Monte Carlo method, we describe a
general sampling methodology that is guaranteed to converge to a specified
equilibrium distribution function. The method is distinct from that of
Metropolis in that it is sometimes possible to arrange for unconditional
acceptance of trial moves. It involves sampling states in a local region of
phase space with probability equal to, in the first approximation, the square
root of the desired global probability density function. The validity of this
choice is derived from the Chapman-Kolmogorov equation, and the utility of the
method is illustrated by a prototypical numerical experiment.


title: The Schr\"odinger formulation of the Feynman path centroid density

abstract:
  We present an analysis of the Feynman path centroid density that provides new
insight into the correspondence between the path integral and the Schr\"odinger
formulations of statistical mechanics. The path centroid density is a central
concept for several approximations (centroid molecular dynamics, quantum
transition state theory, and pure quantum self-consistent harmonic
approximation) that are used in path integral studies of thermodynamic and
dynamical properties of quantum particles. The centroid density is related to
the quasi-static response of the equilibrium system to an external force. The
path centroid dispersion is the canonical correlation of the position operator,
that measures the linear change in the mean position of a quantum particle upon
the application of a constant external force. At low temperatures, this
quantity provides an approximation to the excitation energy of the quantum
system. In the zero temperature limit, the particle's probability density
obtained by fixed centroid path integrals corresponds to the probability
density of minimum energy wave packets, whose average energy define the Feynman
effective classical potential.


title:
The Feynman effective classical potential in the Schr\"odinger
  formulation

abstract:
  New physical insight into the correspondence between path integral concepts
and the Schr\"odinger formulation is gained by the analysis of the effective
classical potential, that is defined within the Feynman path integral
formulation of statistical mechanics. This potential is related to the
quasi-static response of the equilibrium system to an external force. These
findings allow for a comprehensive formulation of dynamical approximations
based on this potential.


title: Dephasing in Disordered Conductors due to Fluctuating Electric Fields

abstract:
  We develop a novel eikonal expansion for the Cooperon to study the effect of
space- and time-dependent electric fields on the dephasing rate of disordered
conductors. For randomly fluctuating fields with arbitrary covariance we derive
a general expression for the dephasing rate which is free of infrared
divergencies in reduced dimensions. For time-dependent external fields with
finite wavelength and sufficiently small amplitude we show that the dephasing
rate is proportional to the square root of the electromagnetic power coupled
into the system, in agreement with data by Wang and Lindelof [Phys. Rev. Lett.
{\bf{59}}, 1156 (1987)].


title:
Effects of construction history on the stress distribution under a sand
  pile

abstract:
  We report experiments on cohesionless granular piles to determine the effect
of construction history on the static stress distribution. The stresses beneath
the piles are monitored using a very sensitive capacitive technique. The piles
are formed either by release of granular material from a relatively small
output (localized source), or from a large diameter sieve (homogeneous rain).
The stress profiles resulting from localized source inputs have a clear stress
dip near the center of the pile while the results from an homogeneous rain show
no stress dip. We also show that the stress profiles scale simply with the pile
height. Experiments on wedges-shaped piles show the same effects but to a
lesser degree.


title: Infinite-randomness quantum Ising critical fixed points

abstract:
  We examine the ground state of the random quantum Ising model in a transverse
field using a generalization of the Ma-Dasgupta-Hu renormalization group (RG)
scheme. For spatial dimensionality d=2, we find that at strong randomness the
RG flow for the quantum critical point is towards an infinite-randomness fixed
point, as in one-dimension. This is consistent with the results of a recent
quantum Monte Carlo study by Pich, et al., including estimates of the critical
exponents from our RG that agree well with those from the quantum Monte Carlo.
The same qualitative behavior appears to occur for three-dimensions; we have
not yet been able to determine whether or not it persists to arbitrarily high
d. Some consequences of the infinite-randomness fixed point for the quantum
critical scaling behavior are discussed. Because frustration is irrelevant in
the infinite-randomness limit, the same fixed point should govern both
ferromagnetic and spin-glass quantum critical points. This RG maps the random
quantum Ising model with strong disorder onto a novel type of
percolation/aggregation process.


title: Triviality of the Ground State Structure in Ising Spin Glasses

abstract:
  We investigate the ground state structure of the three-dimensional Ising spin
glass in zero field by determining how the ground state changes in a fixed
finite block far from the boundaries when the boundary conditions are changed.
We find that the probability of a change in the block ground state
configuration tends to zero as the system size tends to infinity. This
indicates a trivial ground state structure, as predicted by the droplet theory.
Similar results are also obtained in two dimensions.


title: Kondo Effect in Systems with Spin Disorder

abstract:
  We consider the role of static disorder in the spin sector of the one- and
two-channel Kondo models. The distribution functions of the disorder-induced
effective energy splitting between the two levels of the Kondo impurity are
derived to the lowest order in the concentration of static scatterers. It is
demonstrated that the distribution functions are strongly asymmetric, with the
typical splitting being parametrically smaller than the average rms value. We
employ the derived distribution function of splittings to study the temperature
dependence of the low-temperature conductance of a sample containing an
ensemble of two-channel Kondo impurities. The results are used to analyze the
consistency of the two-channel Kondo interpretation of the zero-bias anomalies
observed in Cu/(Si:N)/Cu nanoconstrictions.


title: Rheology of a Supercooled Polymer Melt

abstract:
  Molecular dynamics simulations are performed for a polymer melt composed of
short chains in quiescent and sheared conditions. The stress relaxation
function $G(t)$ exhibits a stretched exponential form in a relatively early
stage and ultimately follows the Rouse function in quiescent supercooled state.
Transient stress evolution after application of shear obeys the linear growth
$\int_0^t dt'G(t')$ for strain less than 0.1 and then saturates into a
non-Newtonian viscosity. In steady states, strong shear-thinning and elongation
of chains into ellipsoidal shapes are found at extremely small shear. A glassy
component of the stress is much enhanced in these examples.


title: Phase Transition in the Two-Dimensional Gauge Glass

abstract:
  The two-dimensional XY gauge glass, which describes disordered
superconducting grains in strong magnetic fields, is investigated, with regard
to the possibility of a glass transition. We compute the glass susceptibility
and the correlation function of the system via extensive numerical simulations
and perform the finite-size scaling analysis. This gives strong evidence for a
finite-temperature transition, which is expected to be of a novel type.


title: Josephson junction between anisotropic superconductors

abstract:
  The sin-Gordon equation for Josephson junctions with arbitrary misaligned
anisotropic banks is derived. As an application, the problem of Josephson
vortices at twin planes of a YBCO-like material is considered. It is shown that
for an arbitrary orientation of these vortices relative to the crystal axes of
the banks, the junctions should experience a mechanical torque which is
evaluated. This torque and its angular dependence may, in principle, be
measured in small fields, since the flux penetration into twinned crystals
begins with nucleation of Josephson vortices at twin planes.


title: Superconductivity, magnetic ordering, and its interplay in HoNi2B2C

abstract:
  We investigated the low-temperature properties of polycrystalline
HoNi_2^11B_2C by means of ac-susceptibility and neutron powder diffraction in
the temperature range of 1.5 to 10 K and zero magnetic field. A sample with
pronounced re-entrant behaviour was chosen since it exhibits a superconducting
state which is strongly affected by the intrinsic magnetism, the formation of
long range magnetic order, via pair-breaking. The evolution and volume
fractions of the three different magnetic structures were observed and
correlated to the superconducting behaviour.


title: A Monte-Carlo study of meanders

abstract:
  We study the statistics of meanders, i.e. configurations of a road crossing a
river through "n" bridges, and possibly winding around the source, as a toy
model for compact folding of polymers. We introduce a Monte-Carlo method which
allows us to simulate large meanders up to n = 400. By performing large "n"
extrapolations, we give asymptotic estimates of the connectivity per bridge R =
3.5018(3), the configuration exponent gamma = 2.056(10), the winding exponent
nu = 0.518(2) and other quantities describing the shape of meanders.
  Keywords : folding, meanders, Monte-Carlo, tree


title: Proximity effect in planar Superconductor/Semiconductor junction

abstract:
  We have measured the very low temperature (down to 30 mK) subgap resistance
of Titanium Nitride (Superconductor, Tc = 4.6 K)/highly doped Silicon
(Semiconductor) SIN junction (the insulating layer stands for the Schottky
barrier). As the temperature is lowered, the resistance increases as expected
in SIN junction. Around 300 mK, the resistance shows a maximum and decreases at
lower temperature. This observed behavior is due to coherent backscattering
towards the interface by disorder in Silicon ("Reflectionless tunneling"). This
effect is also observed in the voltage dependence of the resistance (Zero Bias
Anomaly) at low temperature (T<300 mK). The overall resistance behavior (in
both its temperature and voltage dependence) is compared to existing theories
and values for the depairing rate, the barrier resistance and the effective
carrier temperature are extracted.


title:
Correlation effects in the ground state charge density of
  Mott-insulating NiO: a comparison of ab-initio calculations and high-energy
  electron diffraction measurements

abstract:
  Accurate high-energy electron diffraction measurements of structure factors
of NiO have been carried out to investigate how strong correlations in the Ni
3d shell affect electron charge density in the interior area of nickel ions and
whether the new ab-initio approaches to the electronic structure of strongly
correlated metal oxides are in accord with experimental observations. The
generalized gradient approximation (GGA) and the local spin density
approximation corrected by the Hubbard U term (LSDA+U) are found to provide the
closest match to experimental measurements. The comparison of calculated and
observed electron charge densities shows that correlations in the Ni 3d shell
suppress covalent bonding between the oxygen and nickel sublattices.


title:
Relativistic Effects of Light in Moving Media with Extremely Low Group
  Velocity

abstract:
  A moving dielectric medium acts as an effective gravitational field on light.
One can use media with extremely low group velocities [Lene Vestergaard Hau et
al., Nature 397, 594 (1999)] to create dielectric analogs of astronomical
effects on Earth. In particular, a vortex flow imprints a long-ranging
topological effect on incident light and can behave like an optical black hole.


title: c-axis Optical Conductivity in Cuprates

abstract:
  We investigate the c-axis optical conductivity and d.c. resistivity of
cuprate superconductors in the normal state. Assuming that the interlayer
hopping is incoherent we express the conductivity with planar spectral
functions obtained (i) from angle-resolved photoemission experiments, (ii)
using marginal Fermi liquid ansatz, and (iii) with the finite-temperature
Lanczos method for finite two-dimensional systems described by the t-J model.
Here in the low doping regime a pseudo-gap opening in the density of states
appears to be responsible for a semimetallic-like behavior of the D.C.
resistivity. In the optimally doped regime we find an anomalous relaxation
rate. Analytically this result is reproduced with the use of the marginal Fermi
liquid ansatz for the self energy with parameters obtained from the exact
diagonalization results.


title:
Effects of temperature upon the collapse of a Bose-Einstein condensate
  in a gas with attractive interactions

abstract:
  We present a study of the effects of temperature upon the excitation
frequencies of a Bose-Einstein condensate formed within a dilute gas with a
weak attractive effective interaction between the atoms. We use the
self-consistent Hartree-Fock Bogoliubov treatment within the Popov
approximation and compare our results to previous zero temperature and
Hartree-Fock calculations The metastability of the condensate is monitored by
means of the $l=0$ excitation frequency. As the number of atoms in the
condensate is increased, with $T$ held constant, this frequency goes to zero,
signalling a phase transition to a dense collapsed state. The critical number
for collapse is found to decrease as a function of temperature, the rate of
decrease being greater than that obtained in previous Hartree-Fock
calculations.


title:
Electronic Spectra and Their Relation to the (pi,pi) Collective Mode in
  High-Tc Superconductors

abstract:
  Photoemission spectra of Bi2Sr2CaCu2O8 reveal that the high energy feature
near (pi,0), the "hump", scales with the superconducting gap and persists above
Tc in the pseudogap phase. As the doping decreases, the dispersion of the hump
increasingly reflects the wavevector (pi,pi) characteristic of the undoped
insulator, despite the presence of a large Fermi surface. This can be
understood from the interaction of the electrons with a collective mode,
supported by our observation that the doping dependence of the resonance
observed by neutron scattering is the same as that inferred from our data.


title: Ideal glass transitions for hard ellipsoids

abstract:
  For hard ellipsoids of revolution we calculate the phase diagram for the
idealized glass transition. Our equations cover the glass physics in the full
phase space, for all packing fractions and all aspect ratios X$_0$. With
increasing aspect ratio we find the idealized glass transition to become
primarily be driven by orientational degrees of freedom. For needle or plate
like systems the transition is strongly influenced by a precursor of a nematic
instability. We obtain three types of glass transition lines. The first one
($\phi_c^{(B)}$) corresponds to the conventional glass transition for spherical
particles which is driven by the cage effect. At the second one
($\phi_c^{(B')}$) which occurs for rather non-spherical particles a glass phase
is formed which consists of domains. Within each domain there is a nematic
order where the center of mass motion is quasi--ergodic, whereas the
inter--domain orientations build an orientational glass. The third glass
transition line ($\phi_c^{(A)}$) occurs for nearly spherical ellipsoids where
the orientational degrees of freedom with odd parity, e.g. 180$^o$ flips,
freeze independently from the positions.


title: Levy-Nearest-Neighbors Bak-Sneppen Model

abstract:
  We study a random neighbor version of the Bak-Sneppen model, where "nearest
neighbors" are chosen according to a probability distribution decaying as a
power-law of the distance from the active site, P(x) \sim |x-x_{ac
}|^{-\omega}. All the exponents characterizing the self-organized critical
state of this model depend on the exponent \omega. As \omega tends to 1 we
recover the usual random nearest neighbor version of the model. The pattern of
results obtained for a range of values of \omega is also compatible with the
results of simulations of the original BS model in high dimensions. Moreover,
our results suggest a critical dimension d_c=6 for the Bak-Sneppen model, in
contrast with previous claims.


title:
Enhancement of Coulomb interactions in semiconductor nanostructures by
  dielectric confinement

abstract:
  We present a theoretical analysis of the effect of dielectric confinement on
the Coulomb interaction in dielectrically modulated quantum structures. We
discuss the implications of the strong enhancement of the electron-hole and
electron-electron coupling for two specific examples: (i) GaAs-based quantum
wires with remote oxide barriers, where combined quantum and dielectric
confinements are predicted to lead to room temperature exciton binding, and
(ii) semiconductor quantum dots in colloidal environments, where the many-body
ground states and the addition spectra are predicted to be drastically altered
by the dielectric environment.


title: Poisson Bracket Formulation of Nematic Polymer Dynamics

abstract:
  We formulate the dynamical theory of nematic polymers, starting from a
microscopic Poisson bracket approach. We find that the Poisson bracket between
the nematic director and momentum depends on the (Maier-Saupe) order parameter
of the nematic phase. We use this to derive reactive couplings of the nematic
director to the strain rates. Additionally, we find that local dynamics breaks
down as the polymers begin to overlap. We offer a physical picture for both
results.


title:
Evidence of exactness of the mean field theory in the nonextensive
  regime of long-range spin models

abstract:
  The q-state Potts model with long-range interactions that decay as 1/r^alpha
subjected to an uniform magnetic field on d-dimensional lattices is analized
for different values of q in the nonextensive regime (alpha between 0 and d).
We also consider the two dimensional antiferromagnetic Ising model with the
same type of interactions. The mean field solution and Monte Carlo calculations
for the equations of state for these models are compared. We show that, using a
derived scaling which properly describes the nonextensive thermodynamic
behaviour, both types of calculations show an excellent agreement in all the
cases here considered, except for alpha=d. These results allow us to extend to
nonextensive magnetic models a previous conjecture which states that the mean
field theory is exact for the Ising one.


title: Multiple quantum phases in artificial double-dot molecules

abstract:
  We study coupled semiconductor quantum dots theoretically through a
generalized Hubbard approach, where intra- and inter-dot Coulomb Correlation,
as well as tunneling effects are described on the basis of realistic electron
wavefunctions. We find that the ground-state configuration of
vertically-coupled double dots undergoes non-trivial quantum transitions as a
function of the inter-dot distance d; at intermediate values of d we predict a
new phase that should be observable in the addition spectra and in the
magnetization changes.


title:
Femtosecond Coherent Dynamics of the Fermi Edge Singularity and Exciton
  Hybrid

abstract:
  We study theoretically the coherent nonlinear optical response of doped
semiconductors with partially occupied subbands. When the Fermi energy of an
occupied subband approaches the exciton level of an upper subband, the
absorption spectrum acquires a characteristic double-peak shape originating
from the interference between the Fermi-edge singularity and the exciton
resonance. We demonstrate that, for the off-resonant pump excitation, the
pump/probe spectrum undergoes a dramatic transformation in the coherent regime,
with a strong time-dependent exchange of oscillator strength between Fermi edge
singularity and exciton peaks. We show that this effect originates from the
many-body electron-hole correlations which determine the dynamical response of
the Fermi see. To calculate the nonlinear absorption spectrum, we use the
coupled cluster method combined with time-dependent Schrieffer-Wolff
transformation. Possible experimental applications are discussed.


title: Indeterminacy in foreign exchange market

abstract:
  We discuss price variations distributions in foreign exchange markets,
characterizing them both in calendar and business time frameworks. The price
dynamics is found to be the result of two distinct processes, a multi-variance
diffusion and an error process. The presence of the latter, which dominates at
short time scales, leads to indeterminacy principle in finance. Furthermore,
dynamics does not allow for a scheme based on independent probability
distributions, since volatility exhibits a strong correlation even at the
shortest time scales.


title: Anomalous condensate fluctuations in strongly interacting superfluids

abstract:
  We show that the condensate occupation of a superfluid Bose liquid quite
generally exhibits anomalously large fluctuations at finite temperatures. In
three dimensions, the variance scales like T^2 V^{4/3} at low T, generalizing
the result obtained by Giorgini, Pitaevskii, and Stringari for a weakly
interacting Bose gas. In two dimensions there is only a quasicondensate, and
the fluctuations are of the same order as the mean value.


title: The underscreened Kondo effect in ladder systems

abstract:
  We introduce a new Kondo ladder model by coupling the well-known
Takhtajan-Babujan S=1 chain to a half-filled one-dimensional electron gas, and
we solve it using quantum field theory techniques. Through the so-called
underscreened Kondo effect, this gives novel insights into low-dimensional spin
liquid systems. For instance, the optical conductivity reveals only a pseudogap
at $k=\pi$ despite of the insulating state, a coherent magnon peak in the
dynamical spin susceptibility coexists with an incoherent spinon background,
the magnetization yields a fractional-like quantization. More generally, this
problem belongs to the class of Luttinger liquids in active environments, a
topic of importance not only to the Kondo physics, but also to striped phases
or carbon nanotubes. Experimental consequences on heavy-fermion systems are
also discussed.


title:
Surface versus crystal-termination effects in the optical properties of
  surfaces

abstract:
  We prove, by realistic microscopic calculations within the sp^3s^* Tight
Binding method for GaAs (110) and (100), that the surface optical properties
are not influenced by long-range crystal termination effects, and hence that
they can be consistently studied considering slabs of limited thickness (20 -
30 \AA). The origin of derivative-like and bulk-like lineshapes in Reflection
Anisotropy Spectra is also discussed, analyzing the effects arising from
possible surface-induced reduction, broadening, and shifting of the bulk
spectrum near the surface.


title: Apparent multifractality in financial time series

abstract:
  We present a exactly soluble model for financial time series that mimics the
long range volatility correlations known to be present in financial data.
Although our model is `monofractal' by construction, it shows apparent
multiscaling as a result of a slow crossover phenomenon on finite time scales.
Our results suggest that it might be hard to distinguish apparent and true
multifractal behavior in financial data. Our model also leads to a new family
of stable laws for sums of correlated random variables.


title:
Micro-Raman and resistance measurements of epitaxial La0.7Sr0.3MnO3
  films

abstract:
  The Channel-Spark method was used for deposition of highly oriented
ferromagnetic La0.7Sr0.3MnO3 films on NdGaO3 substrates. It was found that
additional oxygen decreases the film quality suppressing the Curie temperature
and metal-insulator transition below the room temperature. To achieve the best
quality of the films the samples were either annealed in high vacuum at
deposition temperature or even deposited in argon atmosphere with no oxygen
annealing. For such films the resistive measurements showed a metallic
behaviour in the interval 10-300 K in accordance with the high Curie point (Tc
350 K). Micro-Raman analysis indicate that the La0.7Sr0.3MnO3 films are well
ordered, while some outgrowths show stoichiometrical deviations.


title:
Violation of Luttinger's theorem in strongly correlated electronic
  systems within a 1/N expansion

abstract:
  We study the 1/N expansion of a generic, strongly correlated electron model
(SU(N) symmetric Hubbard model with $U=\infty$ and N degrees of freedom per
lattice site) in terms of X operators. The leading order of the expansion
describes a usual Fermi liquid with renormalized, stable particles. The
next-to-leading order violates Luttinger's theorem if a finite convergence
radius for the 1/N expansion for a fixed and non-vanishing doping away from
half-filling is assumed. We find that the volume enclosed by the Fermi surface,
is at large, but finite N's and small dopings larger than at $N=\infty$. As a
by-product an explicit expression for the electronic self-energy in O(1/N) is
given which cannot be obtained by factorization or mode-coupling assumptions
but contains rather sophisticated vertex corrections.


title: Scaling functions for Tsallis non--extensive statistics

abstract:
  We study the one-dimensional Ising model with long-range interactions in the
context of Tsallis non-extensive statistics by computing numerically the number
of states with a given energy. We find that the internal energy, magnetization,
entropy and free energy follow non-trivial scaling laws with the number of
constituents $N$ and temperature $T$. Each of the scaling functions for the
internal energy, the magnetization and the free energy, adopts three different
forms corresponding to $q>1$, $q=1$ and $q<1$, being $q$ the non-extensivity
parameter of Tsallis statistics.


title:
The Solution of the Modified Helmholtz Equation in a Wedge and an
  Application to Diffusion-Limited Coalescence

abstract:
  The general solution of the modified Helmholtz equation,
q_{xx}(x,y)+q_{yy}(x,y)-4b^2q(x,y)=0, in the wedge 0 < x < y < infinity, is
presented. This solution is used to find the explicit steady-state of
diffusion-limited coalescence, A+A<-->A, on the half-line, with a trap-source
at the origin.


title:
Capture on High Curvature Region: Aggregation of Colloidal Particle
  Bound to Giant Phospholipid Vesicles

abstract:
  A very recent observation on the membrane mediated attraction and ordered
aggregation of colloidal particles bound to giant phospholipid vesicles (I.
Koltover, J. O. R\"{a}dler, C. R. Safinya, Phys. Rev. Lett. {\bf 82},
1991(1999)) is investigated theoretically within the frame of Helfrich
curvature elasticity theory of lipid bilayer fluid membrane. Since the concave
or waist regions of the vesicle possess the highest local bending energy
density, the aggregation of colloidal beads on these places can reduce the
elastic energy in maximum. Our calculation shows that a bead in the concave
region lowers its energy $\sim 20 k_B T$. For an axisymmetrical dumbbell
vesicle, the local curvature energy density along the waist is equally of
maximum, the beads can thus be distributed freely with varying separation
distance.


title:
Spin-orbit coupling effect on quantum Hall ferromagnets with vanishing
  Zeeman energy

abstract:
  We present the phase diagram of a ferromagnetic quantum Hall effect liquid in
a narrow quantum well with vanishing single-particle Zeeman splitting,
$\epsilon_{{\rm Z}}$ and pronounced spin-orbit coupling. Upon decreasing
$\epsilon_{{\rm Z}}$, the spin-polarization field of a liquid takes, first, the
easy-axis configuration, followed by the formation of a helical state, which
affects the transport and NMR properties of a liquid and the form of
topological defects in it. The analysis is extended over high odd integer
filling factors.


title:
Bloch Oscillation under a Bichromatic Laser: Quasi-Miniband Formation,
  Collapse, and Dynamical Delocalization and Localization

abstract:
  A novel DC and AC driving configuration is proposed for semiconductor
superlattices, in which the THz AC driving is provided by an intense
bichromatic cw laser. The two components of the laser, usually in the visible
light range, are near but not exactly resonant with interband Wannier-Stark
transitions, and their frequency difference equals the Wannier-Stark ladder
spacing. Multi-photon processes with the intermediate states in the conduction
(valence) band cause dynamical delocalization and localization of valence
(conduction) electrons, and the corresponding formation and collapse of the
quasi-minibands.


title: Bayes-optimal performance in a discrete space

abstract:
  We study a simple model of unsupervised learning where the single symmetry
breaking vector has binary components $\pm 1$. We calculate exactly the
Bayes-optimal performance of an estimator which is required to lie in the same
discrete space. We also show that, except for very special cases, such an
estimator cannot be obtained by minimization of a class of variationally
optimal potentials.


title:
Coherent dynamics of Bose-Einstein condensates in high-finesse optical
  cavities

abstract:
  We study the mutual interaction of a Bose-Einstein condensed gas with a
single mode of a high-finesse optical cavity. We show how the cavity
transmission reflects condensate properties and calculate the self-consistent
intra-cavity light field and condensate evolution. Solving the coupled
condensate-cavity equations we find that while falling through the cavity, the
condensate is adiabatically transfered into the ground state of the periodic
optical potential. This allows time dependent non-destructive measurements on
Bose-Einstein condensates with intriguing prospects for subsequent controlled
manipulation.


title:
Two energy gaps observed in tunneling measurements on hole-doped
  cuprates: pairing gap and coherent gap

abstract:
  There is a clear discrepancy among the energy-gap values for different 90 K
cuprates, inferred from tunneling measurements. By using the phase diagram for
hole-doped cuprates we show that tunneling measurements performed on 90 K
cuprates, simply, detect two different energy gaps: the pairing gap and the
coherent gap, which are identical in conventional superconductors. We find that
there are two reasons why tunneling measurements show in one cuprate
exclusively the coherent gap while, in another cuprate, they show the pairing
gap: (i) the number of CuO2 planes per unit cell in the cuprate, and (ii) the
directionality of the tunneling current (along c-axis or ab-plane).


title: Critical Exponents without beta-Function

abstract:
  We point out that the recently developed strong-coupling theory enables us to
calculate the three main critical exponents nu, eta, omega, from the knowledge
of only the two renormalization constants Z_phi of wave function and Z_m of
mass. The renormalization constant of the coupling strength is superfluous, and
with it also the beta-function, the crucial quantity of the renormalization
group approach to critical phenomena.


title: Third-order phase transition and superconductivity in thin films

abstract:
  We have found a new mean field solution in the BCS theory of
superconductivity. This unconventional solution indicates the existence of
superconducting phase transitions of third order in thin films, or in bulk
matter with a layered structure. The critical temperature increases with
decreasing thickness of the layer, and does not exhibit the isotope effect. The
electronic specific heat is a continuous function of temperature with a
discontinuity in its derivative.


title: A diagrammatic approach for open chaotic systems

abstract:
  A semiclassical diagrammatic approach is constructed for calculating
correlation functions of observables in open chaotic systems with time reversal
symmetry. The results are expressed in terms of classical correlation functions
involving Wigner representations of the observables. The formalism is used to
explain a recent microwave experiment on the 4-disk problem, and to
characterize the two-point function of the photodissociation cross section of
complex molecules.


title: Do columnar defects produce bulk pinning?

abstract:
  From magneto-optical imaging performed on heavy-ion irradiated YBaCuO single
crystals, it is found that at fields and temperatures where strong single
vortex pinning by individual irradiation-induced amorphous columnar defects is
to be expected, vortex motion is limited by the nucleation of vortex kinks at
the specimen surface rather than by half-loop nucleation in the bulk. In the
material bulk, vortex motion occurs through (easy) kink sliding. Depinning in
the bulk determines the screening current only at fields comparable to or
larger than the matching field, at which the majority of moving vortices is not
trapped by an ion track.


title:
Does the Quenched Kardar-Parisi-Zhang Equation Describe the Directed
  Percolation Depinning Models?

abstract:
  The roughening of interfaces moving in inhomogeneous media is investigated by
numerical integration of the phenomenological stochastic differential equation
proposed by Kardar, Parisi, and Zhang [Phys. Rev. Lett. 56, 889, (1986)] with
quenched noise (QKPZ). We express the evolution equations for the mean height
and the roughness into two contributions: the local and the lateral one. We
compare this two contributions with the ones obtained for two directed
percolation deppining models (DPD): the Tang and Leschhorn model [Phys. Rev A
45, R8309 (1992)] and the Buldyrev et al. model [Phys. Rev. A 45, R8313 (1992)]
by Braunstein al. [J. Phys. A 32, 1801 (1999); Phys. Rev. E 59, 4243 (1999)].
Even these models have being classified in the same universality class that the
QKPZ the contributions to the growing mechanisms are quite different. The
lateral contribution in the DPD models, leads to an increasing of the roughness
near the criticality while in the QKPZ equation this contribution always
flattens the roughness. These results suggest that the QKPZ equation does not
describe properly the DPD models even when the exponents derived from this
equation are similar to the one obtained from simulations of these models.


title: Phase Diagram of the Lattice Restricted Primitive Model

abstract:
  We present a comprehensive study of the lattice restricted primitive model,
i.e., a lattice gas consisting of an equal number of positively and negatively
charged particles interacting via on-site exclusion and a 1/r potential. On the
cubic lattice, Monte Carlo simulations show a line of Neel points separating a
disordered, high-temperature phase from a phase with global antiferromagnetic
order. At low temperatures the (high-density) ordered phase coexists with the
(low-density) disordered phase. The Neel line meets the coexistence curve at a
tricritical point, T_t \simeq 0.14, rho_t \simeq 0.4. A simple mean-field
analysis is in qualitative agreement with simulations.


title: Topological Excitations in Doped Spin Ladders

abstract:
  We study low-energy magnetic excitations of doped spin-ladders, based on an
effective Hamiltonian describing interactions of mobile spin and background
spins. The helicity modulus against fluctuations in the ladder plane as well as
out-of-the-plane are calculated in terms of the doping concentration, the leg
number and interaction strengths in the ladder.
  The doping range for spiral phase is computed and its relation to spin gap
structure is discussed.
  The spiral order can enhance existing antiferromagnetic order at certain
doping or reduce it at very low density of holes.
  For odd-legged ladders, formation of spin gap at certain narrow doping range
becomes possible via coupling of spirals and background spins.
  Our results of the doping range for gapped mode in odd-ladders agree well
with those existing numerical studies.


title: Anomalous behaviour of S/N mesoscopic structures near T_c

abstract:
  We observe a maximum in the conductance of Aluminum / n-GaAs junctions at
temperatures 20 mK lower than the superconducting transition temperature. This
is the first observation of a non- monotonic behavior of the conductance near
the superconducting transition in superconducting normal junctions. To
accommodate this effect we calculate the full temperature dependence of the
conductance of these structures, invoking quasiclassical Green's functions in
the diffusive limit. In addition to the well-known low temperature peak at
temperatures on the order of the Thouless energy, we find a maximum near the
critical temperature. This peak has the same origin as the subgap conductance
observed in superconducting-normal junctions at low temperatures. Its
calculated magnitude and position are in good agreement with our experimental
observation.


title: Fractional-differential equations of motion

abstract:
  An elementary system leading to the notions of fractional integrals and
derivatives is considered. Various physical situations whose description is
associated with fractional differential equations of motion are discussed.


title: Ultrafast Coherent Spectroscopy of the Fermi Edge Singularity

abstract:
  In this work we present a theoretical description of the transient response
of the Fermi Edge Singularity (FES). We study the linear and the nonlinear
response of an n-doped QW to laser pulses in the Coherent Control (CC) and Four
Wave Mixing (FWM) Configurations. By means of a bosonization formalism we
calculate the FWM signal emitted by the sample when it is excited by pulses
spectrally peaked around the FES and we show that the long time behavior of the
nonlinear signal is very similar to the linear case.


title:
Various ordered states in a 2D interacting electron system close to an
  electronic topological transition

abstract:
  We consider a 2D electron system on a square lattice with hopping beyond
nearest neighbors. The existence of the quantum critical point associated with
an electronic topological transition in the noninteracting system results in
density wave (DW) and high temperature d-wave superconducting (dSC)
instabilities in the presence of an exchange interaction J. We analyse
different
  DW ordering such as isotropic Spin DW (SDW), d-wave SDW, isotropic Charge DW
(CDW) and d-wave CDW. The coexistence of dSC and SDW orders leads necessary to
the existence of a third order which is a pi triplet superconducting (PTS)
order. A new phase diagram with a mixed phase of SDW, dSC and PTS order is
found. The theory is applied to high-Tc cuprates.


title:
Multiscale Random-Walk Algorithm for Simulating Interfacial Pattern
  Formation

abstract:
  We present a novel computational method to simulate accurately a wide range
of interfacial patterns whose growth is limited by a large scale diffusion
field. To illustrate the computational power of this method, we demonstrate
that it can be used to simulate three-dimensional dendritic growth in a
previously unreachable range of low undercoolings that is of direct
experimental relevance.


title: Suppression of $\bbox{T_c}$ in superconducting amorphous wires

abstract:
  The suppression of the mean field temperature of the superconducting
transition, $T_c$, in homogeneous amorphous wires is studied. We develop a
theory that gives $T_c$ in situations when the dynamically enhanced Coulomb
repulsion competes with the contact attraction. The theory accurately describes
recent experiments on $T_c$--suppression in superconducting wires, after a
procedure that minimizes the role of nonuniversal mechanisms influencing $T_c$
is applied.


title: Van Hove Singularity and Superconductivity in Disordered Sr2RuO4

abstract:
  On the basis of a simple model we analyse the influence of disorder on
critical temperature T_C in p-wave superconductors. The disorder is treated by
means of the Coherent Potential Approximation (CPA) and we focus our attention
on the effect of a van Hove singularity near Fermi energy E_F. For the
appropriate values of its parameters our model reproduces the experimentally
found behaviour of Sr2RuO4.


title: Facet ridge end points in crystal shapes

abstract:
  Equilibrium crystal shapes (ECS) near facet ridge end points (FRE) are
generically complex. We study the body-centered solid-on-solid model on a
square lattice with an enhanced uniaxial interaction range to test the
stability of the so-called stochastic FRE point where the model maps exactly
onto one dimensional Kardar-Parisi-Zhang type growth and the local ECS is
simple. The latter is unstable. The generic ECS contains first-order ridges
extending into the rounded part of the ECS, where two rough orientations
coexist and first-order faceted to rough boundaries terminating in
Pokrovsky-Talapov type end points.


title:
Finite Temperature Behavior of the $\nu=1$ Quantum Hall Effect in
  Bilayer Electron Systems

abstract:
  An effective field theoretic description of $\nu=1$ bilayer electron systems
stabilized by Coulomb repulsion in a single wide quantum well is examined using
renormalization group techniques. The system is found to undergo a crossover
from a low temperature strongly correlated quantum Hall state to a high
temperature compressible state. This picture is used to account for the recent
experimental observation of an anomalous transition in bilayer electron systems
(T. S. Lay, {\em et al.} Phys. Rev. B {\bf 50}, 17725 (1994)). An estimate for
the crossover temperature is provided, and it is shown that its dependence on
electron density is in reasonable agreement with i the experiment.


title: Mechanisms of decoherence in weakly anisotropic molecular magnets

abstract:
  Decoherence mechanisms in crystals of weakly anisotropic magnetic molecules,
such as V15, are studied. We show that an important decohering factor is the
rapid thermal fluctuation of dipolar interactions between magnetic molecules. A
model is proposed to describe the influence of this source of decoherence.
Based on the exact solution of this model, we show that at relatively high
temperatures, about 0.5 K, the quantum coherence in a V15 molecule is not
suppressed, and, in principle, can be detected experimentally. Therefore, these
molecules may be suitable prototype systems for study of physical processes
taking place in quantum computers.


title:
Contrasts between Equilibrium and Non-equilibrium Steady States:
  Computer Aided Discoveries in Simple Lattice Gases

abstract:
  A century ago, the foundations of equilibrium statistical mechanics were
laid. For a system in equilibrium with a thermal bath, much is understood
through the Boltzmann factor, exp{-H[C]/kT}, for the probability of finding the
system in any microscopic configuration C. In contrast, apart from some special
cases, little is known about the corresponding probabilities, if the same
system is in contact with more than one reservoir of energy, so that, even in
stationary states, there is a constant energy flux through our system. These
non-equilibrium steady states display many surprising properties. In
particular, even the simplest generalization of the Ising model offers a wealth
of unexpected phenomena. Mostly discovered through Monte Carlo simulations,
some of the novel properties are understood while many remain unexplained. A
brief review and some recent results will be presented, highlighting the sharp
contrasts between the equilibrium Ising system and this non-equilibrium
counterpart.


title: Composability and Generalized Entropy

abstract:
  We address in this paper how tightly the composability nature of systems:
$S_{A+B} =\Omega (S_A, S_B)$ constrains definition of generalized entropies and
investigate explicitly the composability in some ansatz of the entropy form.


title:
Critical exponents of random XX and XY chains: Exact results via random
  walks

abstract:
  We study random XY and (dimerized) XX spin-1/2 quantum spin chains at their
quantum phase transition driven by the anisotropy and dimerization,
respectively. Using exact expressions for magnetization, correlation functions
and energy gap, obtained by the free fermion technique, the critical and
off-critical (Griffiths-McCoy) singularities are related to persistence
properties of random walks. In this way we determine exactly the decay
exponents for surface and bulk transverse and longitudinal correlations,
correlation length exponent and dynamical exponent.


title:
Interplay of Correlation, Randomness and Dimensionality Effects in
  Weakly-Coupled Half-Filled Random Hubbard Chains

abstract:
  We study interplay of electronic correlation, randomness and dimensionality
effects in half-filled random Hubbard chains weakly coupled via an interchain
one-particle hopping. Based on the two-loop renormalization-group approach,
phase diagrams are given in terms of temperature vs. strengths of the
intrachain electron-electron umklapp scattering, the random scattering and the
interchain one-particle hopping.


title: Theoretical study of the (3x2) reconstruction of beta-SiC(001)

abstract:
  By means of ab initio molecular dynamics and band structure calculations, as
well as using calculated STM images, we have singled out one structural model
for the (3x2) reconstruction of the Si-terminated (001) surface of cubic SiC,
amongst several proposed in the literature. This is an alternate dimer-row
model, with an excess Si coverage of 1/3, yielding STM images in good accord
with recent measurements [F.Semond et al. Phys. Rev. Lett. 77, 2013 (1996)].


title: Scale-invariant Truncated L\'evy Process

abstract:
  We develop a scale-invariant truncated L\'evy (STL) process to describe
physical systems characterized by correlated stochastic variables. The STL
process exhibits L\'evy stability for the probability density, and hence shows
scaling properties (as observed in empirical data); it has the advantage that
all moments are finite (and so accounts for the empirical scaling of the
moments). To test the potential utility of the STL process, we analyze
financial data.


title:
Structure and relevant dimension of the Heisenberg model and
  applications to spin rings

abstract:
  For the diagonalization of the Hamilton matrix in the Heisenberg model
relevant dimensions are determined depending on the applicable symmetries.
Results are presented, both, by general formulae in closed form and by the
respective numbers for a variety of special systems. In the case of cyclic
symmetry, diagonalizations for Heisenberg spin rings are performed with the use
of so-called magnon states. Analytically solvable cases of small spin rings are
singled out and evaluated.


title:
Spin-Density-Wave Phase Transitions in Quasi-One-Dimensional Dimerized
  Quarter-Filled Organic Conductors

abstract:
  We have studied spin density wave (SDW) phase transitions in dimerized
quarter-filled Hubbard chains weakly coupled via interchain one-particle
hopping, $t_{b0}$. It is shown that there exists a critical value of $t_{b0}$,
$t_{b}^\ast$, between the incoherent metal regime ($t_{b0}<t_{b}^\ast$) and the
Fermi liquid regime ($t_{b0}>t_{b}^\ast$) in the metallic phase above the SDW
transition temperature. By using the 2-loop perturbative renormalization-group
approach together with the random-phase-approximation, we propose a SDW phase
diagram covering both of the regimes. The SDW phase transition from the
incoherent metal phase for $t_{b0}<t_{b}^\ast$ is caused by growth of the
intrachain electron-electron umklapp scattering toward low temperatures, which
is regarded as preformation of the Mott gap. We discuss relevance of the
present result to the SDW phase transitions in the quasi-one-dimensional
dimerized quarter-filled organic conductors, (TMTTF)$_2$X and (TMTSF)$_2$X.


title:
Kosterlitz-Thouless vs Ginzburg-Landau description of 2D superconducting
  fluctuations

abstract:
  We evaluate the charge and spin susceptibilities of the 2D attractive Hubbard
model and we compare our results with Montecarlo simulations on the same model.
We discuss the possibility to include topological Kosterlitz-Thouless
superconducting fluctuations in a standard perturbative approach substituting
in the fluctuation propagator the Ginzburg-Landau correlation length with the
Kosterlitz-Thouless correlation length.


title: Direct exchange in the edge-sharing spin-1/2 compound MgVO_3

abstract:
  Bandstructure calculations with different spin arrangement for the spin-chain
compound MgVO_3 have been performed, and paramagnetic as well as magnetic
solutions with ferro- and antiferromagnetically ordered chains are found, the
magnetic solutions being by 0.22 eV per formula unit lower than the
paramagnetic one. The orbital analysis of the narrow band crossing the Fermi
level in the paramagnetic solution reveals that the band has almost pure
vanadium 3d character, the lobes of the relevant d-orbitals at the neighboring
in-chain sites being directed towards each other, which suggests direct
exchange. The tight-binding analysis of the band confirms the strong exchange
transfer between neighboring in-chain V-ions. Besides, some additional
superexchange transfer terms are found, which give rise both to in-plane
coupling between the chains and to frustration, the dominant frustration
occuring due to the interchain interaction.


title:
Charge Relaxation in the Presence of Shot Noise in Coulomb Coupled
  Mesoscopic Systems

abstract:
  In the presence of shot noise the charge on a mesoscopic conductor
fluctuates. We are interested in the charge fluctuations which arise if the
conductor is in the proximity of a gate to which it is coupled by long range
Coulomb forces only. Specifically we consider a gate coupled to the edge of a
Hall bar subject to a quantizing magnetic field which contains a quantum point
contact. The gate is located away from the quantum point contact. We evaluate
the charge relaxation resistance for this geometry. The charge relaxation
resistance determines the current fluctuations and potential fluctuations
induced into the gate. If there is only one edge channel the charge relaxation
resistance is determined by transmission and reflection probabilities alone,
but in the presence of many channels the density of states of all edge states
determines this resistance.


title:
Classical orbit bifurcation and quantum interference in mesoscopic
  magnetoconductance

abstract:
  We study the magnetoconductance of electrons through a mesoscopic channel
with antidots. Through quantum interference effects, the conductance maxima as
functions of the magnetic field strength and the antidot radius (regulated by
the applied gate voltage) exhibit characteristic dislocations that have been
observed experimentally. Using the semiclassical periodic orbit theory, we
relate these dislocations directly to bifurcations of the leading classes of
periodic orbits.


title: Topological Defects in Nematic Droplets of Hard Spherocylinders

abstract:
  Using computer simulations we investigate the microscopic structure of the
singular director field within a nematic droplet. As a theoretical model for
nematic liquid crystals we take hard spherocylinders. To induce an overall
topological charge, the particles are either confined to a two-dimensional
circular cavity with homeotropic boundary or to the surface of a
three-dimensional sphere. Both systems exhibit half-integer topological point
defects. The isotropic defect core has a radius of the order of one particle
length and is surrounded by free-standing density oscillations. The effective
interaction between two defects is investigated. All results should be
experimentally observable in thin sheets of colloidal liquid crystals.


title:
Anomalous temperature dependence of surface tension and capillary waves
  at liquid gallium

abstract:
  The temperature dependence of surface tension \gamma(T) at liquid gallium is
studied theoretically and experimentally using light scattering from capillary
waves. The theoretical model based on the Gibbs thermodynamics relates the
temperature derivative of \gamma to the surface excess entropy -\Delta S.
Although capillary waves contribute to the surface entropy with a positive sign
the effect of dipole layer on \Delta S is negative. Experimental data collected
at a free Ga surface in the temperature range from 30 to 160 C show that the
temperature derivative of the tension changes sign near 100 C.


title: Resonant electron transfer between quantum dots

abstract:
  An interaction of electromagnetic field with a nanostructure composed of two
quantum dots is studied theoretically. An effect of a resonant electron
transfer between the localized low-lying states of quantum dots is predicted. A
necessary condition for such an effect is the existence of an excited bound
state whose energy lies close to the top of the barrier separating the quantum
dots. This effect may be used to realize the reversible quantum logic gate NOT
if the superposition of electron states in different quantum dots is viewed as
the superposition of bits 0 and 1.


title:
The Effect of Shear on Phase-Ordering Dynamics with
  Order-Parameter-Dependent Mobility: The Large-n Limit

abstract:
  The effect of shear on the ordering-kinetics of a conserved order-parameter
system with O(n) symmetry and order-parameter-dependent mobility
\Gamma({\vec\phi}) \propto (1- {\vec\phi} ^2/n)^\alpha is studied analytically
within the large-n limit. In the late stage, the structure factor becomes
anisotropic and exhibits multiscaling behavior with characteristic length
scales (t^{2\alpha+5}/\ln t)^{1/2(\alpha+2)} in the flow direction and (t/\ln
t)^{1/2(\alpha+2)} in directions perpendicular to the flow. As in the \alpha=0
case, the structure factor in the shear-flow plane has two parallel ridges.


title: Detection of the BCS transition of a trapped Fermi Gas

abstract:
  We investigate theoretically the properties of a trapped gas of fermionic
atoms in both the normal and the superfluid phases. Our analysis, which
accounts for the shell structure of the normal phase spectrum, identifies two
observables which are sensitive to the presence of the superfluid: the response
of the gas to a modulation of the trapping frequency, and the heat capacity.
Our results are discussed in the context of experiments on trapped Fermi gases.


title: Relaxation of Inter-Landau-level excitations in the Quatized Hall Regime

abstract:
  Relaxation of collective plasmon inter-Landau-level excitation is determined
by emission of LO-phonons or by Auger-like processes when this emission is
suppressed off the magneto-phonon resonance conditions. The decay of
"one-cyclotron" magneto-plasmons with wave-vectors near the roton minimum is
studied under the condition of filling $\nu=1$. Some features of this
relaxation should be helpful for the experimental detection of the
magneto-rotons in a strongly correlated 2D electron gas.


title: Conventional mechanisms for "exotic" superconductivity

abstract:
  We consider the pairing state due to the usual BCS mechanism in substances of
cubic and hexagonal symmetry where the Fermi surface forms pockets around
several points of high symmetry. We find that the symmetry imposed on the
multiple pocket positions could give rise to a multidimensional nontrivial
superconducting order parameter. The time reversal symmetry in the pairing
state is broken. We suggest several candidate substances where such ordering
may appear, and discuss means by which such a phase may be identified.