title: Transferable relativistic Dirac-Slater pseudopotentials

abstract:
  We present a method for constructing a scalar-relativistic pseudopotential
which provides exact agreement with relativistic Dirac-Slater all-electron
eigenvalues at the reference configuration. All-electron wave functions are
self-consistently computed in the valence region at the exact all-electron
scalar relativistic eigenvalues. This method improves tr ansferability of the
resulting pseudopotential and presents a better starting point for th e
designed non-local pseudopotential approach {[Phys. Rev. B {\bf 59}, 12471
(1999)]}. We present calculations for the gold atom as an example of the new
approach.


title: Field theory of self-avoiding walks in random media

abstract:
  Based on the analogy with the quantum mechanics of a particle propagating in
a {\em complex} potential, we develop a field-theoretical description of the
statistical properties of a self-avoiding polymer chain in a random
environment. We show that the account of the non-Hermiticity of the quantum
Hamiltonian results in a qualitatively different structure of the effective
action, compared to previous studies. Applying the renormalisation group
analysis, we find a transition between the weak-disorder regime, where the
quenched randomness is irrelevant, and the strong-disorder regime, where the
polymer chain collapses. However, the fact that the renormalised interaction
constants and the chiral symmetry breaking regularisation parameter flow
towards strong coupling raises questions about the applicability of the
perturbative analysis.


title: $\Psi=0$ at a sharp semiconductor/insulator interface. Is this correct?

abstract:
  The generalized boundary conditions for the envelope wave function that take
into account the real structure of an interface were used to investigate the
hole spectrum of the semiconductor quantum dot embedded in an insulator matrix.
An essential influence of the interface levels, which could exist at the top of
the valence band, on the hole spectrum has been demonstrated. It is found that
usually applied boundary conditions, wherein all components of the envelope
wave function vanish at the interface, can be used only in the absence of the
interface levels close to the band edge.


title:
Phase Separation Based on U(1) Slave-boson Functional Integral Approach
  to the t-J Model

abstract:
  We investigate the phase diagram of phase separation for the hole-doped two
dimensional system of antiferromagnetically correlated electrons based on the
U(1) slave-boson functional integral approach to the t-J model. We show that
the phase separation occurs for all values of J/t, that is, whether $0 < J/t <
1$ or $J/t \geq 1$ with J, the Heisenberg coupling constant and t, the hopping
strength. This is consistent with other numerical studies of hole-doped two
dimensional antiferromagnets. The phase separation in the physically
interesting J region, $0 < J/t \lesssim 0.4$ is examined by introducing
hole-hole (holon-holon) repulsive interaction. We find from this study that
with high repulsive interaction between holes the phase separation boundary
tends to remain robust in this low $J$ region, while in the high J region, J/t
> 0.4, the phase separation boundary tends to disappear.


title:
Impurity pinning in transport through 1D Mott-Hubbard and spin gap
  insulators

abstract:
  A low energy crossover (see cond-mat/9711167) induced by Fermi liquid
reservoirs in transport through a 1D Mott-Hubbard insulator of finite length
$L$ is examined in the presence of impurity pinning. Under the assumption that
the Hubbard gap 2M is large enough: $M > T_L \equiv v_c/L$ ($v_c$: charge
velocity in the wire) and the impurity backscattering rate $\Gamma_1 \ll T_L$,
the conductance vs. voltage/temperature displays a zero-energy resonance.
Transport through a spin gapped 1D system is also described availing of duality
between the backscattered current of this system and the direct current of the
Mott-Hubbard insulator.


title: Electronic structure of quasi free 2 dimensional HfS_2 clusters

abstract:
  Cluster research so far only dealt with either 3D free clusters or clusters
on substrates with strong electronic coupling at the interface. Preparation of
free and orientated 2D clusters is usually not possible because one of these
conditions generally excludes the other. We demonstrate a new way to grow 2D
clusters of transition metal dichalcogenides which are only weakly bound to the
substrate, i.e. quasi free, but all show the same orientation thus allowing
angle resolved photoemission experiments. In combination with scanning
tunneling microscopy this provides for the first time the possibility to
investigate the correlation between the size of quasi free 2D clusters and its
associated electronic structure. For 2D HfS_2 clusters grown by van der Waals
epitaxy we demonstrate that k_parallel-dispersion of electronic states already
occurs for cluster diameters above ~100 nm.


title:
Ab-initio calculation of the electronic and optical excitations in
  polythiophene: effects of intra- and interchain screening

abstract:
  We present an calculation of the electronic and optical excitations of an
isolated polythiophene chain as well as of bulk polythiophene. We use the GW
approximation for the electronic self-energy and include excitonic effects by
solving the electron-hole Bethe-Salpeter equation. The inclusion of interchain
screening in the case of bulk polythiophene drastically reduces both the
quasi-particle band gap and the exciton binding energies, but the optical gap
is hardly affected. This finding is relevant for conjugated polymers in
general.


title: Fluctuation - induced forces in critical fluids

abstract:
  The current knowledge about fluctuation - induced long - ranged forces is
summarized. Reference is made in particular to fluids near critical points, for
which some new insight has been obtained recently. Where appropiate, results of
analytic theory are compared with computer simulations and experiments.


title: Perturbation of a lattice spectral band by a nearby resonance

abstract:
  A soluble model of weakly coupled "molecular" and "nuclear" Hamiltonians is
studied in order to exhibit explicitly the mechanism leading to the enhancement
of fusion probability in case of a narrow near-threshold nuclear resonance. We,
further, consider molecular cells of this type being arranged in lattice
structures. It is shown that if the real part of the narrow nuclear resonance
lies within the molecular band generated by the intercellular interaction, an
enhancement, proportional to the inverse width of the nuclear resonance, is to
be expected.

categories: cond-mat.mtrl-sci math-ph math.MP nucl-th


title: Dynamical TAP approach to mean field glassy systems

abstract:
  The Thouless, Anderson, Palmer (TAP) approach to thermodynamics of mean field
spin-glasses is generalised to dynamics. A method to compute the dynamical TAP
equations is developed and applied to the p-spin spherical model. In this
context we show to what extent the dynamics can be represented as an evolution
in the free energy landscape. In particular the relationship between the
long-time dynamics and the local properties of the free energy landscape shows
up explicitly within this approach. Conversely, by an instantaneous normal
modes analysis we show that the local properties of the energy landscape seen
by the system during its dynamical evolution do not change qualitatively at the
dynamical transition.


title: Dewetting on porous media with aspiration

abstract:
  We consider a porous solid covered with a water film (or with a drop) in
situations where the liquid is pumped in, either spontaneously (if the porous
medium is hydrophilic) or mechanically (by an external pump). The dynamics of
dewetting is then strongly modified. We analyse a few major examples: a)
horizontal films, which break at a certain critical thickness, b) the "modified
Landau-Levich problem" where a porous plate moves up from a bath and carries a
film: aspiration towards the plate limits the height H reached by the film, c)
certain situation where the hysteresis of contact angles is important.


title:
Results from bosonisation for resonant tunneling through a quantum dot
  in an Aharanov-Bohm ring

abstract:
  We study coherent charge tunneling through a one-dimensional interacting ring
with a one-dimensional quantum dot embedded in one of its arms through
bosonisation. The symmetries of the effective action explain many of the
features such as phase change between resonances, in-phase successive
resonances and phase-locking, which have been observed in experiments of
coherent transport in mesoscopic rings, with a quantum dot. We also predict
changes in the behaviour of the tunneling conductance in the presence of an
Aharanov-Bohm flux through the ring. We argue that these results hold true in
general for any dot.


title:
O(N)-invariant Hierarchical Renormalization Group Fixed Points by
  Algebraic Numerical Computation and \epsilon-Expansion

abstract:
  Generalizing methods developed by Pinn, Pordt and Wieczerkowski for the
hierarchical model with one component (N=1) and dimensions d between 2 and 4 we
compute O(N)-symmetric fixed points of the hierarchical renormalization group
equation for some N and d with 0 < d < 4 and -2 <= N <= 20. The spectra of the
linearized RG equation at the fixed points are calculated and the critical
exponents \nu are extracted from the spectrum and compared to
Borel-Pade-resummed \epsilon-expansion.


title:
Comment on "Quantum Scattering of Heavy Particles from a 10 K Cu(111)
  Surface"

abstract:
  In the original paper Althoff et al. (see ibid., vol.79, p.4429 (1997))
reported a study of scattering of thermal Ne, Ar, and Kr atoms from a Cu(111)
surface in which they assessed the corresponding Debye-Waller factor (DWF) as a
function of the particle mass m in a wide range of substrate temperature T. The
experiments were interpreted by the semiclassical DWF theory in which the
projectile moves on the classical recoilless trajectory and the surface
vibrations are quantized. Siber and Gumhalter claim that the experiments
described by Althoff et al. were carried out in the quantum scattering regime
in which the semiclassical scalings of Althoff et al. do not hold and the
semiclassical DWE significantly deviates from the exact quantum one both in the
low and high T limits. Hence, it is claimed, the quantum scattering data of
Althoff et al. cannot be reliably interpreted by the semiclassical theory.


title:
Long-range electrostatic interactions between like-charged colloids:
  steric and confinement effects

abstract:
  Within the framework of a Modified Poisson-Boltzmann theory accounting for
steric effects of microions, we prove analytically that the effective pair
interactions between like-charge colloids immersed in a confined electrolyte
are repulsive. Our approach encompasses and extends previously known results to
the case of complete confinement, and further incorporates the finite size of
the microions which is absent in the standard Poisson-Boltzmann theory.


title: Study of energy transfer in helium atom scattering from surfaces

abstract:
  Recently developed quantum mechanical theory of inelastic He atom scattering
(HAS) from solid surfaces is employed to analyze the energy transfer between
projectile particles (thermal energy He-atoms) and vibrational degrees of
freedom (phonons) characteristic of a variety of experimentally studied
surfaces. We have first calculated the angular resolved energy transfer which
can be directly compared with the values deducible from the HAS time-of-flight
spectra and a good agreement with experimental data has been found. This
enabled us to calculate the total or angular integrated energy transfer, which
is of paramount importance in the studies of gas-surface scattering, but is
neither accessible in HAS (which yields only the angular resolved quantities),
nor in the wind tunnel measurements for surfaces whose atomic composition and
cleanliness must be maintained during the experiment. Here we present the
results for prototype collision systems of this kind, viz. He => Cu(001), He =>
Xe/Cu(111) and He => Xe(111) which are representative of the very different
types of surface vibrational dynamics and thereby provide an insight into some
common properties of energy transfer in gas-surface scattering.


title:
Recovery temperature for nonclassical energy transfer in atom-surface
  scattering

abstract:
  Nonperturbative expressions are derived for the angular resolved energy
transfer spectra in the quantum regime of multiphonon scattering of inert gas
atoms from surfaces. Application to He atom scattering from a prototype
heatbath Xe/Cu(111) shows good agreement with experiments. This enables a full
quantum calculation of the total energy transfer $\mu$ and therefrom the much
debated recovery or equilibrium temperature $T_{r}$ characteristic of zero
energy transfer in gas-surface collisions in the free molecular flow regime.
Classical universal character of $\mu$ and $T_{r}$ is refuted.


title: Magnetization of noncircular quantum dots

abstract:
  We calculate the magnetization of quantum dots deviating from circular
symmetry for noninteracting electrons or electrons interacting according to the
Hartree approximation. For few electrons the magnetization is found to depend
on their number, and the shape of the dot. The magnetization is an ideal probe
into the many-electron state of a quantum dot.


title: Self-organization, resources and strategies in a minority game

abstract:
  We find that the existence of self-organization of the members of a recently
proposed minority game, depends on the type of update rules used. The resulting
resource distribution is studied in some detail, and a related strategy scheme
is considered, as a tool to improve the understanding of the model


title: Magnetic Field Dependent Tunneling in Glasses

abstract:
  We report on experiments giving evidence for quantum effects of
electromagnetic flux in barium alumosilicate glass. In contrast to expectation,
below 100 mK the dielectric response becomes sensitive to magnetic fields. The
experimental findings include both, the complete lifting of the dielectric
saturation by weak magnetic fields and oscillations of the dielectric response
in the low temperature resonant regime. As origin of these effects we suggest
that the magnetic induction field violates the time reversal invariance leading
to a flux periodicity in the energy levels of tunneling systems. At low
temperatures, this effect is strongly enhanced by the interaction between
tunneling systems and thus becomes measurable.


title: Fermion zero modes on vortices in chiral superconductors

abstract:
  The energy levels of the fermions bound to the vortex core are considered for
the general case of chiral superconductors. There are two classes of chiral
superconductivity: in the superconducting state of class I the axisymmetric
singly quantized vortex has the same energy spectrum of bound states as in
s-wave superconductor: E=(n+1/2)\omega_0 with integral n. In the class II the
corresponding spectrum is E=n\omega_0 and thus contains the state with exactly
zero energy. The effect of a single impurity on the spectrum of bound state is
also considered. For the class I the spectrum acquires the double period \Delta
E=2\omega_0 and consists of two equidistant sets of levels in accordance with
A.I. Larkin and Yu.N. Ovchinnikov, Phys. Rev. B57 (1998) 5457. The spectrum is
not influenced by a single impurity if the same approximation is applied for
vortices in the class II superconducting states.


title: Stochastic Theory of Foreign Exchange Market Dynamics

abstract:
  A new stochastic theory of a foreign exchange markets dynamics is developed.
As a result we have the new probability distribution which well describes
statistical and scaling dependencies ''experimentally'' observed in foreign
exchange markets in recent years. The developed dynamic theory is compared with
well-known phenomenological Levy distribution approach which is widely applied
to this problem. It is shown that the developed stochastic dynamics and
phenomenological approach based on the Levy distribution give the same
statistical and scaling dependencies.


title:
Dynamics of Learning with Restricted Training Sets II: Tests and
  Applications

abstract:
  We apply a general theory describing the dynamics of supervised learning in
layered neural networks in the regime where the size p of the training set is
proportional to the number of inputs N, as developed in a previous paper, to
several choices of learning rules. In the case of (on-line and batch) Hebbian
learning, where a direct exact solution is possible, we show that our theory
provides exact results at any time in many different verifiable cases. For
non-Hebbian learning rules, such as Perceptron and AdaTron, we find very good
agreement between the predictions of our theory and numerical simulations.
Finally, we derive three approximation schemes aimed at eliminating the need to
solve a functional saddle-point equation at each time step, and assess their
performance. The simplest of these schemes leads to a fully explicit and
relatively simple non-linear diffusion equation for the joint field
distribution, which already describes the learning dynamics surprisingly well
over a wide range of parameters.


title:
The influence of the Hall force on the vortex dynamics in type II
  superconductors

abstract:
  The effect of the Hall force on the pinning of vortices in type II
superconductors is considered. A field theoretic formulation of the pinning
problem allows a non-perturbative treatment of the influence of quenched
disorder. A self-consistent theory is constructed using the diagrammatic
functional method for the effective action, and an expression for the pinning
force for independent vortices as well as vortex lattices is obtained. We find
that the pinning force for a single vortex is suppressed by the Hall force at
low temperatures while it is increased at high temperatures. The effect of the
Hall force is more pronounced on a single vortex than on a vortex lattice. The
results of the self-consistent theory are shown to be in good agreement with
numerical simulations.


title:
The Symmetries and Origins of the Order Parameters for Pairing and Phase
  Coherence in Cuprates

abstract:
  The symmetries and origins of the order parameters (OPs) for pairing and
long-range phase coherence in hole-doped cuprates are discussed. By analyzing
tunneling, inelastic neutron scattering and torque measurements we conclude
that the Cooper pairs in cuprates consist of spinons, and the pairing OP has an
anisotropic s-wave symmetry, whereas the OP for long-range phase coherence has
the magnetic origin due to spin fluctuations and, has the d-wave symmetry. All
conclusions are based exclusively on experimental facts.


title:
Staggered-vorticity correlations in a lightly doped t-J model: a
  variational approach

abstract:
  We report staggered vorticity correlations of current in the d-wave
variational wave function for the lightly-doped t-J model. Such correlations
are explained from the SU(2) symmetry relating d-wave and staggered-flux
mean-field phases. The correlation functions computed by the variational Monte
Carlo method suggest that pairs are formed of holes circulating in opposite
directions.


title:
Fermionisation of the Spin-S Uimin-Lai-Sutherland Model: Generalisation
  of Supersymmetric t-J Model to Spin-S

abstract:
  The spin-1 Uimin-Lai-Sutherland (ULS) isotropic chain model is expressed in
terms of fermions and the equivalence of the fermionic representation to the
supersymmetric t-J model is established directly at the level of
Hamiltonians.The spin-S ULS model is fermionized and the Hamiltonian of the
corresponding generalisation of the t-J model is written down.


title: Photoemission Quasi-Particle Spectra of Sr$_2$RuO$_4$

abstract:
  Multi-band quasi-particle calculations based on perturbation theory and
dynamical mean field methods show that the creation of a photoemission hole
state in Sr$_2$RuO$_4$ is associated with a highly anisotropic self-energy.
Since the narrow Ru-derived $d_{xz,yz}$ bands are more strongly distorted by
Coulomb correlations than the wide $d_{xy}$ band, charge is partially
transferred from $d_{xz,yz}$ to $d_{xy}$, thereby shifting the $d_{xy}$ van
Hove singularity close to the Fermi level.


title:
Path Integral Monte Carlo Simulations for Fermion Systems: Pairing in
  the Electron-Hole Plasma

abstract:
  We review the path integral method wherein quantum systems are mapped with
Feynman's path integrals onto a classical system of "ring-polymers" and then
simulated with the Monte Carlo technique. Bose or Fermi statistics correspond
to possible "cross-linking" of polymers. As proposed by Feynman, superfluidity
and Bose condensation result from macroscopic exchange of bosons. To map
fermions onto a positive probability distribution, one must restrict the paths
to lie in regions where the fermion density matrix is positive. We discuss a
recent application to the two-component electron-hole plasma. At low
temperature excitons and bi-excitons form. We have used nodal surfaces
incorporating paired fermions and see evidence of a Bose condensation in the
energy, specific heat and superfluid density. In the restricted path integral
picture, pairing appears as intertwined electron-hole paths. Bose condensation
occurs when these intertwined paths wind around the periodic boundaries.


title:
Spin picture of the one-dimensional Hubbard model: Two-fluid structure
  and phase dynamics

abstract:
  We propose a scheme for investigating the quantum dynamics of interacting
electron models by means of time-dependent variational principle and spin
coherent states of space lattice operators. We apply such a scheme to the
one-dimensional hubbard model, and solve the resulting equations in different
regimes. In particular, we find that at low densities the dynamics is mapped
into two coupled nonlinear Schroedinger equations, whereas near half-filling
the model is described by two coupled Josephson junction arrays. Focusing then
to the case in which only the phases of the spin variables are dynamically
active, we examine a number of different solutions corresponding to the
excitations of few macroscopic modes. Based on fixed point equation of the
simpler among them, we show that the standard one-band ground state phase space
is found.


title: Sliding blocks with random friction and absorbing random walks

abstract:
  With the purpose of explaining recent experimental findings, we study the
distribution $A(\lambda)$ of distances $\lambda$ traversed by a block that
slides on an inclined plane and stops due to friction. A simple model in which
the friction coefficient $\mu$ is a random function of position is considered.
The problem of finding $A(\lambda)$ is equivalent to a First-Passage-Time
problem for a one-dimensional random walk with nonzero drift, whose exact
solution is well-known. From the exact solution of this problem we conclude
that: a) for inclination angles $\theta$ less than $\theta_c=\tan(\av{\mu})$
the average traversed distance $\av{\lambda}$ is finite, and diverges when
$\theta \to \theta_c^{-}$ as $\av{\lambda} \sim (\theta_c-\theta)^{-1}$; b) at
the critical angle a power-law distribution of slidings is obtained:
$A(\lambda) \sim \lambda^{-3/2}$. Our analytical results are confirmed by
numerical simulation, and are in partial agreement with the reported
experimental results. We discuss the possible reasons for the remaining
discrepancies.


title:
Effects of dilute nonmagnetic impurities on the Q=(pi,pi)
  spin-fluctuation spectrum in YBa2Cu3O7

abstract:
  The effects of nonmagnetic impurities on the Q=(pi,pi) spin-fluctuation
spectral weight Im chi(Q,omega) are studied within the framework of the
two-dimensional Hubbard model using the random phase approximation. In the
first part of the paper, the effects of the nonmagnetic impurities on the
magnetic susceptibility of the noninteracting (U=0) system, chi_0(Q,omega), are
calculated with the self-energy and the vertex corrections using various forms
of the effective electron-impurity interaction. Here, the range and the
strength of the impurity potential are varied as well as the concentration of
the impurities. It is shown that the main effect of dilute impurities on
chi_0(Q,omega) is to cause a weak smearing. In the second part, Im chi(Q,omega)
is calculated for the interacting system. Here, the calculations are
concentrated on the processes which involve the impurity scattering of the spin
fluctuations with finite momentum transfers. In order to make comparisons with
the experimental data on the frequency dependence of Im chi(Q,omega) in Zn
substituted YBa2Cu3O7, results are given for various values of the model
parameters.


title: Evidence for Anisotropic Kondo Behavior in Ce0.8La0.2Al3

abstract:
  We have performed an inelastic neutron scattering study of the low energy
spin dynamics of the heavy fermion compound Ce0.8La0.2Al3 as a function of
temperature and external pressure up to 5 kbar. At temperatures below 3 K, the
magnetic response transforms from a quasi-elastic form, common to many heavy
fermion systems, to a single well-defined inelastic peak, which is extremely
sensitive to external pressure. The scaling of the spin dynamics and the
thermodynamic properties are in agreement with the predictions of the
anisotropic Kondo model.


title:
Market Fluctuations: multiplicative and percolation models, size effects
  and predictions

abstract:
  We present a set of models of the main stylized facts of market price
fluctuations. These models comprise dynamical evolution with threshold dynamics
and Langevin price equation with multiplicative noise, percolation models to
describe the interaction between traders and hierarchical cascade models to
unravel the possible correlation accross time scales, including the
log-periodic signatures associated to financial crashes. The main empirical
knowledge is summarized and some key empirical tests are presented.


title: Phase Diagram of the 1/2-1/2-1-1 Spin Chain by the Nonlinear Sigma Model

abstract:
  We examine a periodic mixed spin chain with spin magnitudes 1/2 and 1 which
are arrayed as 1/2-1/2-1-1. The three independent parameters are ratios of the
four exchange couplings. We determine phase boundaries in the parameter space
by using the gapless condition which was previously derived by mapping a
general inhomogeneous spin chain to the nonlinear sigma model. We find two
gapless boundaries separating three disordered phases. The features of the
phases are explained in terms of singlet clusters.


title: The London field in bulk layered superconductors

abstract:
  In a composite superconductor in uniform rotation, the London field strength
at equilibrium is given by the usual expression B_L = 2m Omega/e; here m
corresponds to the bare electron mass, although the effective mass m* can be
different in different layers. In the presence of strong angular accelerations,
however, transient phases and differences in m* can become relevant. In
particular, if the layers are orthogonal to the rotation axis and one of them
is much thinner than the others, the superconductor may enter metastable states
characterized by intense "slide" supercurrents in this layer. We investigate
this phenomenon theoretically and with reference to current experiments.


title:
Enhancement of Josephson quasiparticle current in coupled
  superconducting single-electron transistors

abstract:
  The Josephson quasiparticle (JQP) cycle in a voltage-biased superconducting
single-electron transistor (SSET) combines coherent Cooper pair tunneling with
incoherent quasiparticle decay. We have measured the influence of current flow
through an independently-biased SSET on the JQP cycle when the two SSET's have
a strong mutual capacitive coupling. We find, among other effects, that the JQP
current in one SSET is enhanced by the presence of a quasiparticle current in
the other SSET. A simplified model of the coupled-SSET system is presented
which reproduces the enhancement effect.


title: Thresholds in layered neural networks with variable activity

abstract:
  The inclusion of a threshold in the dynamics of layered neural networks with
variable activity is studied at arbitrary temperature. In particular, the
effects on the retrieval quality of a self-controlled threshold obtained by
forcing the neural activity to stay equal to the activity of the stored paterns
during the whole retrieval process, are compared with those of a threshold
chosen externally for every loading and every temperature through optimisation
of the mutual information content of the network. Numerical results, mostly
concerning low activity networks are discussed.


title: Interband Light Absorption at a Rough Interface

abstract:
  Light absorption at the boundary of indirect-band-gap and direct-forbidden
gap semiconductors is analyzed. It is found that the possibility of the
electron momentum nonconservation at the interface leads to essential
enhancement of absorption in porous and microcrystalline semiconductors. The
effect is more pronounced at a rough boundary due to enlargement of the share
of the interface atoms.


title:
Transport in semiconductor superlattices: from quantum kinetics to
  terahertz-photon detectors

abstract:
  Semiconductor superlattices are interesting for two distinct reasons: the
possibility to design their structure (band-width(s),doping, etc.) gives access
to a large parameter space where different physical phenomena can be explored.
Secondly, many important device applications have been proposed, and then
subsequently successfully fabricated. A number of theoretical approaches has
been used to describe their current-voltage characteristics, such as miniband
conduction, Wannier-Stark hopping, and sequential tunneling. The choice of a
transport model has often been dictated by pragmatic considerations without
paying much attention to the strict domains of validity of the chosen model. In
the first part of this paper we review recent efforts to map out these
boundaries, using a first-principles quantum transport theory, which
encompasses the standard models as special cases. In the second part, focusing
in the mini-band regime, we analyze a superlattice device as an element in an
electric circuit, and show that its performance as a THz-photon detector allows
significant optimization, with respect to geometric and parasitic effects, and
detection frequency. The key physical mechanism enhancing the responsivity is
the excitation of hybrid Bloch-plasma oscillations.


title:
Resonant inelastic x-ray scattering as a probe of optical scale
  excitations in strongly electron-correlated systems: quasi-localized view

abstract:
  An application of resonant inelastic x-ray scattering technique for studying
of optical scale excitations in electron-correlated materials is discussed.
Examples are given including data obtained for 3d transition metal, lanthanide,
and actinide systems. In some cases, the data are compared with the results of
crystal-field multiplet and Anderson impurity model calculations. Advantages of
this technigue are pointed out, such as an ability to probe an extended
multiplet structure of the ground state configuration, which is not fully
accessible by other spectroscopies, an extreme sensitivity of spectral profiles
to the chemical state of the element in question and to the crystal-field
strength, and a great potential in probing the ground state character (for
example, ground state J-mixing in rare-earths) due to the technique's elemental
selectivity and strict selection rules. Issues are addressed, such as a
possible deviations from the linear dispersion of inelastic scattering
structures, corresponding to charge-transfer excitations, with varying
excitation energies and an estimation of values for model parameters, involved
in the description of charge-transfer processes


title: Liquid induced transitions in granular media

abstract:
  We investigate the effect of interstitial liquid on the physical properties
of granular media by measuring the angle of repose as a function of the liquid
content. The resultant adhesive forces lead to three distinct regimes in the
observed behavior as the liquid content is increased: a granular regime in
which the grains move individually, a correlated regime in which the grains
move in correlated clusters, and a plastic regime in which the grains flow
coherently. We discuss these regimes in terms of two proposed theories
describing the effects of liquid on the physical properties of granular media.


title:
Analytic structure factors and pair-correlation functions for the
  unpolarized homogeneous electron gas

abstract:
  We propose a simple and accurate model for the electron static structure
factors (and corresponding pair-correlation functions) of the 3D unpolarized
homogeneous electron gas. Our spin-resolved pair-correlation function is built
up with a combination of analytic constraints and fitting procedures to quantum
Monte Carlo data, and, in comparison to previous attempts (i) fulfills more
known integral and differential properties of the exact pair-correlation
function, (ii) is analytic both in real and in reciprocal space, and (iii)
accurately interpolates the newest, extensive diffusion-Monte Carlo data of
Ortiz, Harris and Ballone [Phys. Rev. Lett. 82, 5317 (1999)]. This can be of
interest for the study of electron correlations of real materials and for the
construction of new exchange and correlation energy density functionals.


title:
Microscopic derivation of transport coefficients and boundary conditions
  in discrete drift-diffusion models of weakly coupled superlattices

abstract:
  A discrete drift-diffusion model is derived from a microscopic sequential
tunneling model of charge transport in weakly coupled superlattices provided
temperatures are low or high enough. Realistic transport coefficients and novel
contact current--field characteristic curves are calculated from microscopic
expressions, knowing the design parameters of the superlattice. Boundary
conditions clarify when possible self-sustained oscillations of the current are
due to monopole or dipole recycling.


title: A Droplet State in an Interacting Two-Dimensional Electron System

abstract:
  It is well known that the dielectric constant of two-dimensional (2D)
electron system goes negative at low electron densities. A consequence of the
negative dielectric constant could be the formation of the droplet state. The
droplet state is a two-phase coexistence region of high density liquid and low
density "gas". In this paper, we carry out energetic calculations to study the
stability of the droplet ground state. The possible relevance of the droplet
state to recently observed 2D metal-insulator transition is also discussed.


title: Meron-Cluster Simulation of Quantum Spin Ladders in a Magnetic Field

abstract:
  Numerical simulations of numerous quantum systems suffer from the notorious
sign problem. Meron-cluster algorithms lead to an efficient solution of sign
problems for both fermionic and bosonic models. Here we apply the meron concept
to quantum spin systems in an arbitrary external magnetic field, in which case
standard cluster algorithms fail. As an example, we simulate antiferromagnetic
quantum spin ladders in a uniform external magnetic field that competes with
the spin-spin interaction. The numerical results are in agreement with analytic
predictions for the magnetization as a function of the external field.


title:
Parallel magnetic field induced giant magnetoresistance in low density
  {\it quasi}-two dimensional layers

abstract:
  We provide a possible theoretical explanation for the recently observed giant
positive magnetoresistance in high mobility low density {\it quasi}-two
dimensional electron and hole systems. Our explanation is based on the strong
coupling of the parallel field to the {\it orbital} motion arising from the
{\it finite} layer thickness and the large Fermi wavelength of the {\it
quasi}-two dimensional system at low carrier densities.


title: Self-similar chain conformations in polymer gels

abstract:
  We use molecular dynamics simulations to study the swelling of randomly
end-cross-linked polymer networks in good solvent conditions. We find that the
equilibrium degree of swelling saturates at Q_eq = N_e**(3/5) for mean strand
lengths N_s exceeding the melt entanglement length N_e. The internal structure
of the network strands in the swollen state is characterized by a new exponent
nu=0.72. Our findings are in contradiction to de Gennes' c*-theorem, which
predicts Q_eq proportional N_s**(4/5) and nu=0.588. We present a simple Flory
argument for a self-similar structure of mutually interpenetrating network
strands, which yields nu=7/10 and otherwise recovers the classical Flory-Rehner
theory. In particular, Q_eq = N_e**(3/5), if N_e is used as effective strand
length.


title: The Effect of Splayed Pins on Vortex Creep and Critical Currents

abstract:
  We study the effects of splayed columnar pins on the vortex motion using
realistic London Langevin simulations. At low currents vortex creep is strongly
suppressed, whereas the critical current j_c is enhanced only moderately.
Splaying the pins generates an increasing energy barrier against vortex
hopping, and leads to the forced entanglement of vortices, both of which
suppress creep efficiently. On the other hand splaying enhances kink nucleation
and introduces intersecting pins, which cut off the energy barriers. Thus the
j_c enhancement is strongly parameter sensitive. We also characterize the angle
dependence of j_c, and the effect of different splaying geometries.


title:
Anderson transition and thermal effects on electron states in amorphous
  silicon

abstract:
  I discuss the properties of electron states in amorphous Si based on large
scale calculations with realistic several thousand atom models. A relatively
simple model for the localized to extended (Anderson) transition is reviewed.
Then, the effect of thermal disorder on localized electron states is
considered. It is found that under readily accessible conditions, localized
(midgap or band tail) states and their conjugate energies may fluctuate
dramatically. The possible importance of non-adiabatic atomic dynamics to doped
or photo-excited systems is briefly discussed.


title:
Stochastic versus dynamic approach to Levy statistics in the presence of
  an external perturbation

abstract:
  We study the influence of a dissipation process on diffusion dynamics
triggered by slow fluctuations. We study both strong- and weak-friction regime.
When the latter regime applies, the system is attracted by the basin of either
Gauss or Levy statistics according to whether the fluctuation correlation
function is integrable or not. We analyze with a numerical calculation the
border between the two basins of attraction.


title:
Computer simulation of low-energy excitations in amorphous silicon with
  voids

abstract:
  We use empirical molecular dynamics technique to study the low-energy
vibrations in a large 4096 atom model for pure amorphous silicon and a set of
models with voids of different size based on it. Numerical vibrational
eigenvalues and eigenvectors for our models are obtained by exact
diagonalization of their dynamical matrices. Our calculations show that
localized low-energy vibrational excitations of rather complex structure are
present in amorphous silicon models with voids. According to their spatial
localization patterns we make an attempt to classify these excitations as modes
associated with the void and "mixed" modes associated with the interaction of
the void with strained regions of silicon network.


title:
Metallic low-temperature resistivity in 2D over an extended temperature
  range

abstract:
  We report measurements of the zero-field resistivity in dilute 2D electron
system in silicon at temperatures down to 35 mK. This extends the previously
explored range of temperatures by almost an order of magnitude. On the metallic
side, the resistivity near the metal-insulator transition continues to decrease
with decreasing temperature and shows no low-temperature up-turn. At the
critical electron density, the resistivity is found to be
temperature-independent in the entire temperature range 35 mK to 1 K.


title:
Electrostatically-Driven Granular Media: Phase Transitions and
  Coarsening

abstract:
  We report the experimental and theoretical study of electrostatically driven
granular material. We show that the charged granular medium undergoes a
hysteretic first order phase transition from the immobile condensed state
(granular solid) to a fluidized dilated state (granular gas) with a changing
applied electric field. In addition we observe a spontaneous precipitation of
dense clusters from the gas phase and subsequent coarsening - coagulation of
these clusters. Molecular dynamics simulations shows qualitative agreement with
experimental results.


title:
Multicritical Phenomena of Superconductivity and Antiferromagnetism in
  Organic Conductor $\kappa$-(BEDT-TTF)$_2$X

abstract:
  We study theoretically the multicritical phenomena of the superconductivity
(SC) and antiferromagnetism (AF) in organic conductors $\kappa$-BEDT salts. The
phase diagram and the experimental data on the NMR relaxation rate $1/T_1$ is
analysed in terms of the renormalization group method. The bicritical
phenomenon observed experimentally indicates the rotational symmetry, i.e.,
SO(5) symmetry, of the SC and the AF. The critical exponent $x$ for the
divergence of $1/T_1$ is well explained by $ x = \nu (z - 1 - \eta)$ with the
dynamical exponent $z = 3/2$ for the AF region while $z = \phi/\nu \sim 1.84 $
at the bicritical point. These results strongly suggest that the origin of the
SC is in common with that of the AF and that its symmetry is d-wave.


title: Bandgap renormalization of modulation doped quantum wires

abstract:
  We measure the photoluminescence spectra for an array of modulation doped,
T-shaped quantum wires as a function of the 1d density n_e which is modulated
with a surface gate. We present self-consistent electronic structure
calculations for this device which show a bandgap renormalization which, when
corrected for excitonic energy and its screening, are largely insensitive to
n_e and which are in quantitatively excellent agreement with the data. The
calculation (cf. cond-mat/9908349) shows the importance of including
orthogonality between the screening electrons and the electron(s) bound to the
hole. The calculations show that electron and hole remain bound up to 3 x 10^6
cm^-1 and that therefore the stability of the exciton far exceeds the
conservative Mott criterion.


title:
Carrier relaxation dynamics in intra-gap states: the case of the
  superconductor Y Ba_2 Cu_3O_7-delta and the charge-density-wave semiconductor
  K_0.3MoO_3

abstract:
  The unusual slow carrier relaxation dynamics - observed in femtosecond
pump-probe experiments on high-temperature superconductors and recently also in
a charge-density-wave system - is analyzed in terms of a model for relaxation
of carriers in intra-gap states. The data on YBa_2Cu_3O_7-delta near optimum
doping and K_0.3$MoO_3 are found to be described very well with the model using
a BCS-like gap which closes at T_c. From the analysis of the data we conclude
that a significant intra-gap density of localized states exists in these
materials, which can be clearly distinguished from quasiparticle states by the
time-resolved optical experiments because of the different time- and
temperature-dependences of the photoinduced transmission or reflection.
Localized charges are suggested to be the most likely origin of the intra-gap
states, while the similarity of the response in the two materials appear to
exclude spin and vortex excitations.


title: Une methode Monte-Carlo pour les meandres sur ordinateur parallele

abstract:
  Pour modeliser le repliement compact d'un polymere, on etudie la statistique
des meandres, definis comme configurations d'un circuit automobile traversant n
fois une riviere. Grace a une methode Monte-Carlo adaptee a un ordinateur
massivement parallele, les meandres sont simules jusqu'a la taille n=400. Le
comportement asymptotique du denombrement et de l'enroulement moyen des
meandres sont presentes.


title: A novel iterative strategy for protein design

abstract:
  We propose and discuss a novel strategy for protein design. The method is
based on recent theoretical advancements which showed the importance to treat
carefully the conformational free energy of designed sequences. In this work we
show how computational cost can be kept to a minimum by encompassing negative
design features, i.e. isolating a small number of structures that compete
significantly with the target one for being occupied at low temperature. The
method is succesfully tested on minimalist protein models and using a variety
of amino acid interaction potentials.


title: Theory of Coherent and Incoherent Nuclear Spin-Dephasing in the Heart

abstract:
  We present an analytical theory of susceptibility induced nuclear spin
dephasing in the capillary network of myocardium. Using a strong collision
approach, equations are obtained for the relaxation rate of the free induction
and the spin echo decay. Simulation and experimental data are well predicted by
the theory. Since paramagnetic deoxyhemoglobin as the origin of nuclear spin
dephasing has a higher tissue concentration in myocardium supplied by a
stenotic, i. e. significantly narrowed, coronary artery, spin dephasing might
serve as a diagnostic tool. Our approach can be modified for capillary networks
in other tissues than myocardium and may be applied in material science.

categories: cond-mat.soft cond-mat.mtrl-sci physics.bio-ph physics.med-ph


title:
Magnetic Properties of the transition to localized superconductivity
  around columnar defects

abstract:
  We consider flux lines in presence of a dense square array of columnar
defects, with insulating core and radius of the order of the coherence length.
The properties of the phase transition from the flux line regime to the
localized superconducting state are determined here. We show that the localized
superconducting state is a new vortex state because of the counterclockwise and
of the clockwise supercurrent flows found around each defect. We propose here a
general operator to count the vorticity of any cavity inside a superconductor
and give a new method to obtain the reversible magnetization of superconductors
with multiply connected geometries.


title:
Impurity scattering in metallic carbon nanotubes with superconducting
  pair potentials

abstract:
  Effects of the superconducting pair potential on the impurity scattering
processes in metallic carbon nanotubes are studied theoretically. The backward
scattering of electrons vanishes in the normal state. In the presence of the
superconducting pair correlations, the backward scatterings of electron- and
hole-like quasiparticles vanish, too. The impurity gives rise to backward
scatterings of holes for incident electrons, and it also induces backward
scatterings of electrons for incident holes. Negative and positive currents
induced by such the scatterings between electrons and holes cancel each other.
Therefore, the nonmagnetic impurity does not hinder the supercurrent in the
regions where the superconducting proximity effects occur. Relations with
experiments are discussed.


title: Categorization in fully connected multi-state neural network models

abstract:
  The categorization ability of fully connected neural network models, with
either discrete or continuous Q-state units, is studied in this work in replica
symmetric mean-field theory. Hierarchically correlated multi-state patterns in
a two level structure of ancestors and descendents (examples) are embedded in
the network and the categorization task consists in recognizing the ancestors
when the network is trained exclusively with their descendents. Explicit
results for the dependence of the equilibrium properties of a Q=3-state model
and a $Q=\infty$-state model are obtained in the form of phase diagrams and
categorization curves. A strong improvement of the categorization ability is
found when the network is trained with examples of low activity. The
categorization ability is found to be robust to finite threshold and synaptic
noise. The Almeida-Thouless lines that limit the validity of the
replica-symmetric results, are also obtained.


title:
Properties of excitations in systems with a spinor Bose-Einstein
  condensate

abstract:
  General theory in case of homogenous Bose-Einstein condensed systems with
spinor condensate is presented for the correlation functions of density and
spin fluctuations and for the one-particle propagators as well. The random
phase approximation is investigated and the damping of the modes is given in
the intermediate temperature region. It is shown that the collective and the
one-particle excitation spectra do not coincide fully.


title: Arrested States of Solids

abstract:
  Solids produced as a result of a fast quench across a freezing or a
structural transition get stuck in long-lived metastable configurations of
distinct morphology, sensitively dependent on the processing history. {\it
Martensites} are particularly well studied examples of nonequilibrium
solid-solid transformations. Since there are some excellent reviews on the
subject, we shall, in this brief article, mainly present a summary of our work
on the nonequilibrium statistical mechanics of Martensites.


title: Pairing Correlations in a Generalized Hubbard Model for the Cuprates

abstract:
  Using numerical diagonalization of a 4x4 cluster, we calculate on-site s,
extended s and d pairing correlation functions (PCF) in an effective
generalized Hubbard model for the cuprates, with nearest-neighbor correlated
hopping and next nearest-neighbor hopping t'. The vertex contributions (VC) to
the PCF are significantly enhanced, relative to the t-t'-U model. The behavior
of the PCF and their VC, and signatures of anomalous flux quantization,
indicate superconductivity in the d-wave channel for moderate doping and in the
s-wave channel for high doping and small U.


title:
The density dependence of the transition temperature in a homogenous
  Bose flui

abstract:
  Transition temperature data obtained as a function of particle density in the
$^4$He-Vycor system are compared with recent theoretical calculations for 3D
Bose condensed systems. In the low density dilute Bose gas regime we find, in
agreement with theory, a positive shift in the transition temperature of the
form $\Delta T/T_0 = \gamma(na^{3})^{1/3}$. At higher densities a maximum is
found in the ratio of $T_c /T_0$ for a value of the interaction parameter,
na$^3$, that is in agreement with path-integral Monte Carlo calculations.


title:
Imaging of Microwave Permittivity, Tunability, and Damage Recovery in
  (Ba,Sr)TiO3 Thin Films

abstract:
  We describe the use of a near-field scanning microwave microscope to
quantitatively image the dielectric permittivity and tunability of thin-film
dielectric samples on a length scale of 1 micron. We demonstrate this technique
with permittivity images and local hysteresis loops of a 370 nm thick barium
strontium titanate thin film at 7.2 GHz. We also observe the role of annealing
in the recovery of dielectric tunability in a damaged region of the thin film.
We can measure changes in relative permittivity as small as 2 at 500, and
changes in dielectric tunability as small as 0.03 V$^{-1}$.


title: Stable and unstable vortices in multicomponent Bose-Einstein condensates

abstract:
  We study the stability and dynamics of vortices in two-species condensates as
prepared in the recent JILA experiment (M. R. Andrews {\em et al.}, Phys. Rev.
Lett. 83 (1999) 2498). We find that of the two available configurations, in
which one specie has vorticity $m=1$ and the other one has $m=0$, only one is
linearly stable, which agrees with the experimental results. However, it is
found that in the unstable case the vortex is not destroyed by the instability,
but may be transfered from one specie to the other or display complex
spatiotemporal dynamics.


title: Relationship Between Conductivity and Phase Coherence Length in Cuprates

abstract:
  The large ($10^2 - 10^5$) and strongly temperature dependent resistive
anisotropy $\eta = (\sigma_{ab}/\sigma_c)^{1/2}$ of cuprates perhaps holds the
key to understanding their normal state in-plane $\sigma_{ab}$ and out-of-plane
$\sigma_{c}$ conductivities. It can be shown that $\eta$ is determined by the
ratio of the phase coherence lengths $\ell_i$ in the respective directions:
$\sigma_{ab}/\sigma_c = \ell_{ab}^2/\ell_{c}^2$. In layered crystals in which
the out-of-plane transport is incoherent, $\ell_{c}$ is fixed, equal to the
interlayer spacing. As a result, the T-dependence of $\eta$ is determined by
that of $\ell_{ab}$, and vice versa, the in-plane phase coherence length can be
obtained directly by measuring the resistive anisotropy. We present data for
hole-doped $YBa_2Cu_3O_y$ ($6.3 < y < 6.9$) and $Y_{1-x}Pr_xBa_2Cu_3O_{7-\delta
}$ ($0 < x \leq 0.55$) and show that $\sigma_{ab}$ of crystals with different
doping levels can be well described by a two parameter universal function of
the in-plane phase coherence length. In the electron-doped
$Nd_{2-x}Ce_{x}CuO_{4-y}$, the dependence $\sigma_{ab}(\eta)$ indicates a
crossover from incoherent to coherent transport in the c-direction.


title: Transverse depinning in strongly driven vortex lattices with disorder

abstract:
  Using numerical simulations we investigate the transverse depinning of moving
vortex lattices interacting with random disorder. We observe a finite
transverse depinning barrier for vortex lattices that are driven with high
longitudinal drives, when the vortex lattice is defect free and moving in
correlated 1D channels. The transverse barrier is reduced as the longitudinal
drive is decreased and defects appear in the vortex lattice, and the barrier
disappears in the plastic flow regime. At the transverse depinning transition,
the vortex lattice moves in a staircase pattern with a clear transverse
narrow-band voltage noise signature.


title:
Magnetoresistance and conductivity exponents of quench-condensed
  ultra-thin films of Bi

abstract:
  We have studied the magnetoresistance (MR) and evolution of conductivity with
thickness of quench-condensed Bismuth films on substrates of various dielectric
constants. Our results indicate a negative intial MR proportional to the square
of the magnetic field. The conductance shows a power-law kind of dependence on
thickness, with an exponent close to 1.33, characterisitic of a 2-D percolating
system, only when the films are grown on a thin ($\sim 10${\rm \AA} Germanium
underlayer but not otherwise.


title: Temperature dependence of spin polarizations at higher Landau Levels

abstract:
  We report our results on temperature dependence of spin polarizations at
$\nu=1$ in the lowest as well as in the next higher Landau level that compare
well with recent experimental results. At $\nu=3$, except having a much smaller
magnitude the behavior of spin polarization is not much influenced by higher
Landau levels. In sharp contrast, for filling factor $\nu=\frac83$ we predict
that unlike the case of $\nu=\frac23$ the system remains fully spin polarized
even at vanishingly small Zeeman energies.


title: On the Abrikosov transition in disordered superconducting films

abstract:
  We consider a disordered superconducting film in the magnetic field close to
the upper critical field. Assuming that the phase transition to the glassy
superconducting state exists and it is of the second order we show that it is
exactly described by a zero-dimensional replica model having a form of one
describing the phase transition in the SK (Sherrington-Kirkpatrick) spin-glass
model. The dependence of the magnetic moment on the external magnetic field is
shown to be more smooth than the Abrikosov mean field result for the clean
case. While the total magnetic moment of the film close to the phase transition
is diamagnetic, the part of it emerging due to the phase transition is
paramagnetic, this shows tendency to the paramagnetic Meissner effect.


title:
Hysteretic Response of an Anti-Ferromagnetic Random-Field Ising Model in
  One Dimension at Zero Temperature

abstract:
  We consider the hysteretic response of a one-dimensional anti-ferromagnetic
random-field Ising model at zero temperature for a uniform bounded distribution
of quenched random fields, and present analytic results in a limited range of
the applied field.


title:
Nonequilibrium Fluctuations, Travelling Waves, and Instabilities in
  Active Membranes

abstract:
  The stability of a flexible fluid membrane containing a distribution of
mobile, active proteins (e.g. proton pumps) is shown to depend on the structure
and functional asymmetry of the proteins. A stable active membrane is in a
nonequilibrium steady state with height fluctuations whose statistical
properties are governed by the protein activity. Disturbances are predicted to
travel as waves at sufficiently long wavelength, with speed set by the normal
velocity of the pumps. The unstable case involves a spontaneous, pump-driven
undulation of the membrane, with clumping of the proteins in regions of high
activity.


title:
Phase Transitions Between Topologically Distinct Gapped Phases in
  Isotropic Spin Ladders

abstract:
  We consider various two-leg ladder models exhibiting gapped phases. All of
these phases have short-ranged valence bond ground states, and they all exhibit
string order. However, we show that short-ranged valence bond ground states
divide into two topologically distinct classes, and as a consequence, there
exist two topologically distinct types of string order. Therefore, not all
gapped phases belong to the same universality class. We show that phase
transitions occur when we interpolate between models belonging to different
topological classes, and we study the nature of these transitions.


title:
First principles calculation of structural and magnetic properties for
  Fe monolayers and bilayers on W(110)

abstract:
  Structure optimizations were performed for 1 and 2 monolayers (ML) of Fe on a
5 ML W(110) substrate employing the all-electron full-potential linearized
augmented plane-wave (FP-LAPW) method. The magnetic moments were also obtained
for the converged and optimized structures. We find significant contractions
($\sim$ 10 %) for both the Fe-W and the neighboring Fe-Fe interlayer spacings
compared to the corresponding bulk W-W and Fe-Fe interlayer spacings. Compared
to the Fe bcc bulk moment of 2.2 $\mu_B$, the magnetic moment for the surface
layer of Fe is enhanced (i) by 15% to 2.54 $\mu_B$ for 1 ML Fe/5 ML W(110), and
(ii) by 29% to 2.84 $\mu_B$ for 2 ML Fe/5 ML W(110). The inner Fe layer for 2
ML Fe/5 ML W(110) has a bulk-like moment of 2.3 $\mu_B$. These results agree
well with previous experimental data.


title: Comprehensive study of Frohlich polaron

abstract:
  A detailed study of the Frohlich polaron model is performed on the basis of
diagrammatic quantum Monte Carlo method. The method is further developed both
quantitatively (performance) and qualitatively (new estimators), and is
enhanced by spectral analysis of the polaron Green function, within a novel
approach. We present the best up to date results for the binding energy, and
for the first time make available precise data for the effective mass,
including the region of intermediate and strong couplings. We look at the
structure of the polaron cloud and answer such questions as the average number
of phonons in the cloud and their number/momentum distribution. The spectral
analysis reveals non-trivial structure of the spectral density at intermediate
and large coupling: the spectral continuum features pronounced peaks that we
attribute to unstable excited states of the polaron.


title:
Role of two-dimensional electronic state in superconductivity in
  La$_{2-x}$Sr$_{x}$CuO$_{4}$

abstract:
  We have measured out-of-plane resistivity $\rho_c$ for
La$_{2-x}$Sr$_{x}$CuO$_{4}$ under anisotropic pressure. c-axis compression,
which decreases $\rho_c$, reduces $T_{\rm c}$ drastically, whereas c-axis
extention, which increases $\rho_c$, enhances $T_{\rm c}$ from 38K at ambient
pressure to 51.6K at 8GPa. We find that the variation of $T_{\rm c}$ scales as
a function of $\rho_c$, and that the c-axis pressure coefficient is much
stronger than the ab-axis one. These imply that $T_{\rm c}$ depends primarily
on the interlayer, rather than the in-plane, lattice parameter.


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

abstract:
  The magnetoresistance of a granular superconductor in a strong magnetic field
is considered. It is assumed that this field destroys the superconducting gap
in each grain, such that all interesting effects considered in the paper are
due to superconducting fluctuations. The conductance of the system is assumed
to be large, which allows us to neglect all localization effects as well as the
Coulomb interaction. It is shown that at low temperatures the superconducting
fluctuations reduce the one-particle density of states but do not contribute to
transport. As a result, the resistivity of the normal state exceeds the
classical resistivity approaching the latter only in the limit of extremely
strong magnetic fields, and this leads to a negative magnetoresistance. We
present detailed calculations of physical quatities relevant for describing the
effect and make a comparison with existing experiments.


title: Formation of Liesegang Patterns

abstract:
  It has been recently shown that precipitation bands characteristic of
Liesegang patterns emerge from spinodal decomposition of reaction products in
the wake of moving reaction fronts. This mechanism explains the geometric
sequence of band positions x_n ~ Q(1+p)^n and, furthermore, it yields a spacing
coefficient, p, that is in agreement with the experimentally observed
Matalon-Packter law. Here I examine the assumptions underlying this theory and
discuss the choice of input parameters that leads to experimentally observable
patterns. I also show that the so called width law relating the position and
the width of the bands w_n ~ x_n follows naturally from this theory.


title: Dimensional reduction in a model with infinitely many absorbing states

abstract:
  Using Monte Carlo method we study a two-dimensional model with infinitely
many absorbing states. Our estimation of the critical exponent beta=0.273(5)
suggests that the model belongs to the (1+1) rather than (2+1)
directed-percolation universality class. We also show that for a large class of
absorbing states the dynamic Monte Carlo method leads to spurious dynamical
transitions.


title: Vortex states in superconducting rings

abstract:
  The superconducting state of a thin superconducting disk with a hole is
studied within the non-linear Ginzburg-Landau theory in which the
demagnetization effect is accurately taken into account. We find that the flux
through the hole is not quantized, the superconducting state is stabilized with
increasing size of the hole for fixed radius of the disk, and a transition to a
multi-vortex state is found if the disk is sufficiently large. Breaking the
circular summetry through a non central location of the hole in the disk
enhances the multi-vortex state.


title: Longitudinal magnetic excitations in classical spin systems

abstract:
  Using spin dynamics simulations we predict the splitting of the longitudinal
spin wave peak in all antiferromagnets with single site anisotropy into two
peaks separated by twice the energy gap at the Brillouin zone center. This
phenomenon has yet to be observed experimentally but can be easily investigated
through neutron scattering experiments on MnF$_2$ and FeF$_2$. We have also
determined that for all classical Heisenberg models the longitudinal
propagative excitations are entirely multiple spin-wave in nature.


title:
Numerical studies of the vibrational isocoordinate rule in chalcogenide
  glasses

abstract:
  Many properties of alloyed chalcogenide glasses can be closely correlated
with the average coordination of these compounds. This is the case, for
example, of the ultrasonic constants, dilatometric softening temperature and
the vibrational densities of states. What is striking, however, is that these
properties are nevertheless almost independent of the composition at given
average coordination. Here, we report on some numerical verification of this
experimental rule as applied to vibrational density of states.


title:
Normal State Resistivity of Underdoped YBa2Cu3Ox Thin Films and
  La2-xSrxCuO4 Ultra-Thin Films under Epitaxial Strain

abstract:
  The normal state resistivity of high temperature superconductors can be
probed in the region below Tc by suppressing the superconducting state in high
magnetic fields. Here we present the normal state properties of YBa2Cu3Ox thin
films in the underdoped regime and the normal state resistance of La2-xSrxCuO4
thin films under epitaxial strain, measured below Tc by applying pulsed fields
up to 60 T. A universal rho(T) behaviour is reported. We interpret these data
in terms of the recently proposed 1D quantum transport model with the 1D paths
corresponding to the charge stripes.


title:
Replica symmetry breaking solution for the fermionic Ising spin glass
  and the Ghatak-Sherrington model

abstract:
  We solve the fermionic version of the Ising spin glass for arbitrary filling
\mu and temperature T taking into account replica symmetry breaking. Using a
simple exact mapping from \mu to the anisotropy parameter D, we also obtain the
solution of the S=1 Sherrington-Kirkpatrick model. An analytic expression for
T=0 gives an improved critical value for the first-order phase transition. We
revisit the question of stability against replica-diagonal fluctuations and
find that the appearance of complex eigenvalues of the Almeida-Thouless matrix
is not an artifact of the replica-symmetric approximation.


title: Measurements of flux dependent screening in Aharonov-Bohm rings

abstract:
  In order to investigate the effect of electronic phase coherence on screening
we have measured the flux dependent polarizability of isolated mesoscopic rings
at 350 MHz. At low temperature (below 100 mK) both non-dissipative and
dissipative parts of the polarizability exhibit flux oscillations with a period
of half a flux quantum in a ring. The sign and amplitude of the effect are in
good agreement with recent theoretical predictions. The observed positive
magneto-polarizability corresponds to an enhancement of screening when time
reversal symmetry is broken. The effect of electronic density and temperature
are also measured.


title:
The Cu NQR Study of the Stripe Phase Local Structure in the Lanthanum
  Cuprates

abstract:
  Using Cu NQR in Eu-doped La_{2- x}Sr_xCuO_4 we show the evidence of the
pinned stripe phase for moderate doping at 1.3K. The pinned fraction increases
by one order of magnitude near hole doping x=1/8. The NQR lineshape reveals
three inequivalent Cu positions: i) sites in the charged stripe; ii)
nonmagnetic sites outside the stripes; iii) sites with a magnetic moment of
0.29 Bohr magnethon in the AF correlated regions. A dramatic change of the NQR
signal for x > 0.18 correlating with the onset of bulk superconductivity
corresponds to the depinning of the stripe phase. .


title: Local Magnetic Order vs. Superconductivity in a Layered Cuprate

abstract:
  We report on the phase diagram for charge-stripe order in
La(1.6-x)Nd(0.4)Sr(x)CuO(4), determined by neutron and x-ray scattering studies
and resistivity measurements. From an analysis of the in-plane resistivity
motivated by recent nuclear-quadrupole-resonance studies, we conclude that the
transition temperature for local charge ordering decreases monotonically with
x, and hence that local antiferromagnetic order is uniquely correlated with the
anomalous depression of superconductivity at x = 1/8. This result is consistent
with theories in which superconductivity depends on the existence of
charge-stripe correlations.


title: One-Particle Spectral Properties of 1D Mott-Hubbard Insulators

abstract:
  We use an exact {\it holon} and {\it spinon} Landau-liquid functional which
describes the holon - spinon interactions at all scattering orders, to study
correlation functions of integrable multicomponent many-particle problems
showing both linear and non-linear energy bands. Motivated by recent
photoemission experiments, we consider specific cases when the dominant
non-linear band terms are quadratic and apply our results to the evaluation of
the half-filling 1D Hubbard model one-particle spectral functions beyond
conformal-field theory.


title: The diffusion equation and the principle of minimum Fisher information

abstract:
  It is shown that the diffusion equation and its adjoint (time reversed)
equation can be derived with only a few assumptions, using an
information-theoretic approach based on the principle of minimum Fisher
information


title:
Annealed disorder, rare regions, and local moments: A novel mechanism
  for metal-insulator transitions

abstract:
  Local magnetic moments in disordered sytems can be described in terms of
annealed magnetic disorder, in addition to the underlying quenched disorder. It
is shown that for noninteracting electron systems at zero temperature, the
annealed disorder leads to a new mechanism, and a new universality class, for a
metal-insulator transition. The transition is driven by a vanishing of the
thermodynamic density susceptibility rather than by localization effects. The
critical behavior near two-dimensions is determined, and the underlying physics
is discussed.


title: Spin Susceptibility in Underdoped $\bf YBa_2Cu_3O_{6+x}$

abstract:
  We report a comprehensive polarized and unpolarized neutron scattering study
of the evolution of the dynamical spin susceptibility with temperature and
doping in three underdoped single crystals of the \YBCO{6+x} high temperature
superconductor: \YBCO{6.5} (Tc = 52 K), \YBCO{6.7} (Tc = 67 K), and \YBCO{6.85}
(T_c = 87 K). Theoretical implications of these data are discussed, and a
critique of recent attempts to relate the spin excitations to the
thermodynamics of high temperature superconductors is given.


title:
Absolute Negative Conductivity and Spontaneous Current Generation in
  Semiconductor Superlattices with Hot Electrons

abstract:
  We study electron transport through a semiconductor superlattice subject to
an electric field parallel to and a magnetic field perpendicular to the growth
axis. Using a single miniband, semiclassical balance equation model with both
elastic and inelastic scattering, we find that (1) the current-voltage
characteristic becomes multistable in a large magnetic field; and (2) ``hot''
electrons display novel features in their current-voltage characteristics,
including absolute negative conductivity (ANC) and, for sufficiently strong
magnetic fields, a spontaneous dc current at zero bias. We discuss possible
experimental situations providing the necessary hot electrons to observe the
predicted ANC and spontaneous dc current generation.


title:
Analytical Results of the One-Dimensional Hubbard Model in the High
  Temperature Limit

abstract:
  We investigate the grand potential of the one-dimensional Hubbard model in
the high temperature limit, calculating the coefficients of the high
temperature expansion ($\beta$-expansion) of this function up to order
$\beta^4$ by an alternative method. The results derived are analytical and do
not involve any perturbation expansion in the hopping constant, being valid for
arbitrary density of electrons in the one-dimensional model.
  In the half-filled case, we compare our analytical results for the specific
heat and the magnetic susceptibility, in the high-temperature limit, with the
ones obtained by Beni {\it et al.} and Takahashi's integral equations, showing
that the latter result does not take into account the complete energy spectrum
of the one-dimensional Hubbard model. The exact integral solution by J\"uttner
{\it et al}. is applied to the determination of the range of validity of our
expansion in $\beta$ in the half-filled case, for several different values of
$U$.


title: Luminescence from Laser-Created Bubbles in Cryogenic Liquids

abstract:
  A luminescence pulse has been observed from a laser-created bubble in liquid
nitrogen and liquid argon at the first collapse point of the bubble. An unusual
feature is that the width of the pulse is of order 100-1000 ns, much longer
than the 2-8 ns pulses observed when the same experiment is carried out with a
water sample.


title: Quenched Kosterlitz-Thouless Superfluid Transitions

abstract:
  The properties of rapidly quenched superfluid phase transitions are computed
for two-dimensional Kosterlitz-Thouless (KT) systems. The decay in the
vortex-pair density and the recovery of the superfluid density after a quench
are found by solving the Fokker-Planck equation describing the vortex dynamics,
in conjunction with the KT recursion relations. The vortex density is found to
decay approximately as the inverse of the time from the quench, in agreement
with computer simulations and with scaling theories.


title: Vortex Core Size in $^3$He-$^4$He films with Monolayer Superfluid $^4$He

abstract:
  The superfluid transition of $^3$He-$^4$He mixture films adsorbed on alumina
powder is studied, with a $^4$He superfluid coverage near one layer. With up to
1.3 layers of $^3$He added, the transition becomes strongly broadened,
indicating a linear increase in the vortex core size for $^3$He coverages below
one layer. Annealing of the sample mixture at 4.2 K is found to be critically
important in ensuring a homogeneous film across the porous substrate.


title: Fundamental Framework for Technical Analysis

abstract:
  Starting from the characterization of the past time evolution of market
prices in terms of two fundamental indicators, price velocity and price
acceleration, we construct a general classification of the possible patterns
characterizing the deviation or defects from the random walk market state and
its time-translational invariant properties. The classification relies on two
dimensionless parameters, the Froude number characterizing the relative
strength of the acceleration with respect to the velocity and the time horizon
forecast dimensionalized to the training period. Trend-following and contrarian
patterns are found to coexist and depend on the dimensionless time horizon. The
classification is based on the symmetry requirements of invariance with respect
to change of price units and of functional scale-invariance in the space of
scenarii. This ``renormalized scenario'' approach is fundamentally
probabilistic in nature and exemplifies the view that multiple competing
scenarii have to be taken into account for the same past history. Empirical
tests are performed on on about nine to thirty years of daily returns of twelve
data sets comprising some major indices (Dow Jones, SP500, Nasdaq, DAX, FTSE,
Nikkei), some major bonds (JGB, TYX) and some major currencies against the US
dollar (GBP, CHF, DEM, JPY). Our ``renormalized scenario'' exhibits
statistically significant predictive power in essentially all market phases. In
constrast, a trend following strategy and trend + acceleration following
strategy perform well only on different and specific market phases. The value
of the ``renormalized scenario'' approach lies in the fact that it always finds
the best of the two, based on a calculation of the stability of their predicted
market trajectories.


title: A theory of \pi/2 superconducting Josephson junctions

abstract:
  We consider theoretically a Josephson junction with a superconducting
critical current density which has a random sign along the junction's surface.
We show that the ground state of the junction corresponds to the phase
difference equal to \pi/2. Such a situation can take place in superconductor-
ferromagnet junction.


title:
Field theoretical representation of the Hohenberg-Kohn free energy for
  fluids

abstract:
  To go beyond Gaussian approximation to the Hohenberg-Kohn free energy playing
the key role in the density functional theory (DFT), the density functional
\textit{integral} representation would be relevant, because field theoretical
approach to perturbative calculations becomes available. Then the present
letter first derives the associated Hamiltonian of density functional,
explicitly including logarithmic entropy term, from the grand partition
function expressed by configurational integrals. Moreover, two things are done
so that the efficiency of the obtained form may be revealed: to demonstrate
that this representation facilitates the field theoretical treatment of the
perturbative calculation, and further to compare our perturbative formulation
with that of the DFT.


title: Calorimetric Investigation of CeRu2Ge2 up to 8 GPa

abstract:
  We have developed a calorimeter able to give a qualitative picture of the
specific heat of a sample under high pressure up to approximately 10 GPa. The
principle of ac-calorimetry was adapted to the conditions in a high pressure
clamp. The performance of this technique was successfully tested with the
measurement of the specific heat of CeRu2Ge2 in the temperature range 1.5
K<T<12 K. The phase diagram of its magnetic phases is consistent with previous
transport measurements.


title:
Sign Reversal of the Quantum Hall Effect and Helicoidal
  Magnetic-Field-Induced Spin-Density Waves in Organic Conductors

abstract:
  Within the framework of the quantized nesting model, we study the effect of
umklapp scattering on the magnetic-field-induced spin-density-wave (SDW) phases
which are experimentally observed in the quasi-one-dimensional organic
conductors (TMTSF)$_2$X. We discuss the conditions under which umklapp
processes may explain the sign reversals (Ribault anomaly) of the quantum Hall
effect (QHE) observed in these conductors. We find that the `Ribault phase' is
characterized by the coexistence of two SDWs with comparable amplitudes. This
gives rise to additional long wavelength collective modes besides the Goldstone
modes due to spontaneous translation and rotation symmetry breaking. These
modes strongly affect the optical conductivity. We also show that the Ribault
phase may become helicoidal (i.e with circularly polarized SDWs) if the
strength of umklapp processes is sufficiently strong. The QHE vanishes in the
helicoidal phases, but a magnetoelectric effect appears.


title:
Collective modes in a system with two spin-density waves: the `Ribault'
  phase of quasi-one-dimensional organic conductors

abstract:
  We study the long-wavelength collective modes in the magnetic-field-induced
spin-density-wave (FISDW) phases experimentally observed in organic conductors
of the Bechgaard salts family, focusing on phases that exhibit a sign reversal
of the quantum Hall effect (Ribault anomaly). We have recently proposed that
two SDW's coexist in the Ribault phase, as a result of Umklapp processes. When
the latter are strong enough, the two SDW's become circularly polarized
(helicoidal SDW's). In this paper, we study the collective modes which result
from the presence of two SDW's. We find two Goldstone modes, an out-of-phase
sliding mode and an in-phase spin-wave mode, and two gapped modes. The sliding
Goldstone mode carries only a fraction of the total optical spectral weight,
which is determined by the ratio of the amplitude of the two SDW's. In the
helicoidal phase, all the spectral weight is pushed up above the SDW gap. We
also point out similarities with phase modes in two-band or bilayer
superconductors. We expect our conclusions to hold for generic two-SDW systems.


title:
Local Non-Fermi Liquid Theory of Magnetic Impurity Effects in Carbon
  Nanotubes

abstract:
  Magnetic impurity effects in carbon nanotubes are studied theoretically. The
multi channel Kondo effect is investigated with the band structure of the
metallic nanotubes. The local non-Fermi liquid behavior is realized at
temperatures lower than the Kondo temperature T_K. The density of states of
localized electron has a singularity |omega|^1/2 which gives rise to a pseudo
gap at the Kondo resonance in low temperatures. The temperature dependence of
the electronic resistivity is predicted as T^1/2, and the imaginary part of
dynamical susceptibilities has the |omega|^1/2 dependence.


title: The evolution of vibrational excitations in glassy systems

abstract:
  The equations of the mode-coupling theory (MCT) for ideal liquid-glass
transitions are used for a discussion of the evolution of the
density-fluctuation spectra of glass-forming systems for frequencies within the
dynamical window between the band of high-frequency motion and the band of
low-frequency-structural-relaxation processes. It is shown that the strong
interaction between density fluctuations with microscopic wave length and the
arrested glass structure causes an anomalous-oscillation peak, which exhibits
the properties of the so-called boson peak. It produces an elastic modulus
which governs the hybridization of density fluctuations of mesoscopic wave
length with the boson-peak oscillations. This leads to the existence of
high-frequency sound with properties as found by X-ray-scattering spectroscopy
of glasses and glassy liquids. The results of the theory are demonstrated for a
model of the hard-sphere system. It is also derived that certain schematic MCT
models, whose spectra for the stiff-glass states can be expressed by elementary
formulas, provide reasonable approximations for the solutions of the general
MCT equations.


title: Ion-Conducting Polymers - Quenched to Dynamic Disorder

abstract:
  Ion conducting polymers have a biphasic character with crystalline as well as
amorphous phases. There is moreover, a dynamic disorder due to motion of
polymer chain segments. The PEO-NH$_4$ClO$_4$ system undergoes a crossover from
a DLA-type morphology for low salt fraction (X) to a structure with polygonal
spherulites. In the present communication we show that the low X regime
exhibits a variation of diffusivity with crystallinity typical of a quenched
system, whereas the high X regime has dynamic disorder with rapid
rearrangement.


title: Absence of a Bulk Meissner State in RuSr2GdCu2O8

abstract:
  We have systematically investigated the magnetic, electrical, and structural
properties of RuSr2GdCu2O8, in which a long-range ferromagnetic order and
superconductivity have been previously reported to coexist. Based on the
reversible magnetization results, we conclude that the bulk Meissner state does
not exist in this compound and that the condensation energy associated with
superconductivity is negligible. The absence of a bulk Meissner state and the
superconductivity detected are thus attributed to the possible appearance of a
sponge-like crypto-superconducting fine structure in RuSr2GdCu2O8 samples that
are found to be chemically homogeneous to 1-2 mu m and electrically uniform to
\~ 10 mu m across the sample.


title:
Magnetization plateaus as insulator-superfluid transitions in quantum
  spin systems

abstract:
  We study the magnetization process in two-dimensional S=1/2 spin systems, to
discuss the appearance of a plateau structure. The following three cases are
considered: (1) the Heisenberg antiferromagnet and multiple-spin exchange model
on the triangular lattice, (2) Shastry-Sutherland type lattice, [which is a
possible model for SrCu2(BO3)2,] (3) 1/5-depleted lattice (for CaV4O9). We find
in these systems that magnetization plateaus can appear owing to a transition
from superfluid to a Mott insulator of magnetic excitations. The plateau states
have CDW order of the excitations. The magnetizations of the plateaus depend on
components of the magnetic excitations, range of the repulsive interaction, and
the geometry of the lattice.


title: Universal Susceptibility Variations in 1+1 Dimensional Vortex Glass

abstract:
  We model a planar array of fluxlines as a discrete solid-on-solid model with
quenched disorder. Simulations at finite temperatures are made possible by a
new algorithm which circumvents the slow glassy dynamics encountered by
traditional Metropolis Monte Carlo algorithms. Numerical results on magnetic
susceptibility variations support analytic predictions.


title:
Pseudogaps and Charge Band in the Parisi Solution of Insulating and
  Superconducting Electronic Spin Glasses at Arbitrary Fillings

abstract:
  We report progress in understanding the fermionic Ising spin glass with
arbitrary filling. A crossover from a magnetically disordered single band phase
via two intermediate bands just below the freezing temperature to a 3-band
structure at still lower temperatures - beyond an almost random field
instability - is shown to emerge in the magnetic phase. An attempt is made to
explain the exact solution in terms of a quantum Parisi phase. A central
nonmagnetic band is found and seen to become sharply separated at T=0 by gaps
from upper and lower magnetic bands. The gap sizes tend towards zero as the
number of replica symmetry breaking steps increases towards infinity. In an
extended model, the competition between local pairing superconductivity and
spin glass order is discussed.


title: Macroscopic Quantum Dynamics of a Free Domain Wall in a Ferromagnet

abstract:
  We study macroscopic quantum dynamics of a free domain wall in a
quasi-one-dimensional ferromagnet by use of the spin-coherent-state path
integral in {\it discrete-time} formalism. Transition amplitudes between
typical states are quantitatively discussed by use of {\it stationary-action
approximation} with respect to collective degrees of freedom representing the
center position and the chirality of the domain wall. It is shown that the
chirality may be loosely said to be canonically conjugate to the center
position; the latter moves with a speed depending on the former. It is
clarified under what condition the center position can be regarded as an
effective free-particle position, which exhibits the phenomenon of wave-packet
spreading. We demonstrate, however, that in some case the non-linear character
of the spin leads to such a dramatic phenomenon of a non-spreading wave packet
as to completely invalidate the free-particle analogy. In the course of the
discussion, we also point out various difficulties associated with the
continuous-time formalism.


title: Imaging of Low Compressibility Strips in the Quantum Hall Liquid

abstract:
  Using Subsurface Charge Accumulation scanning microscopy we image strips of
low compressibility corresponding to several integer Quantum Hall filling
factors. We study in detail the strips at Landau level filling factors $\nu =$
2 and 4. The observed strips appear significantly wider than predicted by
theory. We present a model accounting for the discrepancy by considering a
disorder-induced nonzero density of states in the cyclotron gap.


title: Decoherence of the Superconducting Persistent Current Qubit

abstract:
  Decoherence of a solid state based qubit can be caused by coupling to
microscopic degrees of freedom in the solid. We lay out a simple theory and use
it to estimate decoherence for a recently proposed superconducting persistent
current design. All considered sources of decoherence are found to be quite
weak, leading to a high quality factor for this qubit.


title: Vortices in magnetically coupled superconducting layered systems

abstract:
  Pancake vortices in stacks of thin superconducting films or layers are
considered. It is stressed that in the absence of Josephson coupling
topological restrictions upon possible configurations of vortices are removed
and various examples of structures forbidden in bulk superconductors are given.
In particular, it is shown that vortices may skip surface layers in samples of
less than a certain size R_c which might be macroscopic. The Josephson coupling
suppresses R_c estimates.


title:
Observation of two time scales in the ferromagnetic manganite
  La(1-x)Ca(x)MnO(3), x = 0.3

abstract:
  We report new zero-field muon spin relaxation and neutron spin echo
measurements in ferromagnetic (FM) (La,Ca)MnO3 which taken together suggest two
spatially separated regions in close proximity possessing very different Mn-ion
spin dynamics. One region corresponds to an extended cluster which displays
'critical slowing down' near Tc and an increasing volume fraction below Tc. The
second region possesses more slowly fluctuating spins and a decreasing volume
fraction below Tc. These data are discussed in terms of the growth of small
polarons into overlapping regions of correlated spins below Tc, resulting in a
microscopically inhomogeneous FM transition.


title:
Meissner state in finite superconducting cylinders with uniform applied
  magnetic field

abstract:
  We study the magnetic response of superconductors in the presence of low
values of a uniform applied magnetic field. We report measurements of DC
magnetization and AC magnetic susceptibility performed on niobium cylinders of
different length-to-radius ratios, which show a dramatic enhance of the initial
magnetization for thin samples, due to the demagnetizing effects. The
experimental results are analyzed by applying a model that calculates the
magnetic response of the superconductor, taking into account the effects of the
demagnetizing fields. We use the results of magnetization and current and field
distributions of perfectly diamagnetic cylinders to discuss the physics of the
demagnetizing effects in the Meissner state of type-II superconductors.


title:
A tetragonal-to-monoclinic phase transition in a ferroelectric
  perovskite: the structure of PbZr(0.52)Ti(0.48)O3

abstract:
  The perovskite-like ferroelectric system PbZr(1-x)Ti(x)O3 (PZT) has a nearly
vertical morphotropic phase boundary (MPB) around x=0.45-0.50. Recent
synchrotron x-ray powder diffraction measurements by Noheda et al. [Appl. Phys.
Lett. 74, 2059 (1999)] have revealed a new monoclinic phase between the
previously-established tetragonal and rhombohedral regions. In the present work
we describe a Rietveld analysis of the detailed structure of the tetragonal and
monoclinic PZT phases on a sample with x= 0.48 for which the lattice parameters
are respectively: at= 4.044 A, ct= 4.138 A, at 325 K, and am= 5.721 A, bm=
5.708 A, cm= 4.138 A, beta= 90.496 deg., at 20K. In the tetragonal phase the
shifts of the atoms along the polar [001] direction are similar to those in
PbTiO3 but the refinement indicates that there are, in addition, local
disordered shifts of the Pb atoms of ~0.2 A perpendicular to the polar axis..
The monoclinic structure can be viewed as a condensation along one of the <110>
directions of the local displacements present in the tetragonal phase. It
equally well corresponds to a freezing-out of the local displacements along one
of the <100> directions recently reported by Corker et al.[J. Phys. Condens.
Matter 10, 6251 (1998)] for rhombohedral PZT. The monoclinic structure
therefore provides a microscopic picture of the MPB region in which one of the
"locally" monoclinic phases in the "average" rhombohedral or tetragonal
structures freezes out, and thus represents a bridge between these two phases.


title: Aharonov-Bohm effect in one-channel weakly disordered rings

abstract:
  A new diagrammatic method, which is a reformulation of Berezinskii's
technique, is constructed to study the density of electronic states
$\rho(\epsilon,\phi)$ of a one-channel weakly disordered ring, threaded by an
external magnetic flux. The exact result obtained for the density of states
shows an oscillation of $\rho(\epsilon,\phi)$ with a period of the flux quantum
$\phi_0= hc/e $. As the sample length (or the impurity concentration) is
reduced, a transition takes place from the weak localization regime (L >> l) to
the ballistic one (L <= l). The analytical expression for the density of states
shows the exact dependence of $\rho(\epsilon,\phi)$ on the ring's circumference
and on disorder strength for both regimes.


title:
On Chemical Potential of a Generalized Hubbard Model with Correlated
  Hopping at Half-Filling

abstract:
  In the present paper we study chemical potential of a generalized Hubbard
model with correlated hopping at half-filling using a generalized mean-field
approximation. For the special case of the model the approach reproduces the
exact results: metal-insulator transition, ground state energy. Chemical
potential of generelized Hubbard model with correlated hoppig as function of
hopping integrals at zero temperature is found. It is shown that chemical
potential of the model is temperature-dependent. The dependences of chemical
potential of a generalized Hubbard model with correlated hopping on the model
parameters are different in metallic and insulating phases leading to a kink at
the point of metal-insulator transition in the ground state. With the increase
of temperature the kink in the chemical potential curve disappears. The
obtained results differ from those of the Hubbard model indicating the
important role of correlated hopping.


title:
Effects of Strong Correlation and Randomness in the Vicinity of the Mott
  Transition in the Quasi-One-Dimensional Hubbard Model

abstract:
  We study the strong correlation effects in the vicinity of the Mott
metal-insulator transition using coupled clean or disordered Hubbard chains
with a infinitely large coordinate number $D_{\perp}\to\infty$ in the direction
perpendicular to the chains and with a long-range transverse hopping. Strong
electron correlation effects are treated partially non-perturbatively with the
use of the exact results for the 1D Hubbard model. In the case of clean
systems, the thermodynamic and transport quantities which characterize the Mott
transition from the Fermi liquid state, such as the specific heat coefficient,
the Drude weight, and the compressibility, are obtained as functions of
hole-doping $\delta=1-n$ ($n$, electron density) by the systematic expansion in
terms of the inverse of the transverse hopping range $l_{\perp}$. We find that
the $\delta$-dependence of these quantities shows non-universal behaviors with
exponents depending on the strength of electron-electron interaction. In the
presence of disorder, it is shown that the frequency-dependence of the
dynamical conductivity obeys the Mott's law, implying the possibility of {\it
Mott glass state} for $\delta \to 0$, provided that there exists a finite range
interaction.


title:
Kinetic equation for liquids with a multistep potential of interaction.
  II: Calculation of transport coefficients

abstract:
  We consider a new kinetic equation for systems with a multistep potential of
interaction proposed by us recently in Physica A 234 (1996) 89. This potential
consists of the hard sphere part and a system of attractive and repulsive
walls. Such a model is a generalization of many previous semi-phenomenological
kinetic theories of dense gases and liquids. In this article a normal solution
to the new kinetic equation has been obtained, integral conservation laws in
the first order on gradients of hydrodynamic parameters have been derived as
well. The expressions for transport coefficients are calculated for the case of
stationary process. We also consider limiting cases for this kinetic equation.
For specific parameters of model interaction potential in shape of the
multistep function, the obtained results rearrange to those of previous kinetic
theories by means of the standard Chapman-Enskog method. In view of this, new
theory can be considered as a generalized one which in some specific cases
arrives at the results of previous ones and in such a way displays the
connection of these theories between themselves. At the end of this article we
present results of numerical computation of transport coefficients for Argon
along curve of saturation and their comparison with experimental data available
and MD simulations.


title: Large-q asymptotics of the random bond Potts model

abstract:
  We numerically examine the large-q asymptotics of the q-state random bond
Potts model. Special attention is paid to the parametrisation of the critical
line, which is determined by combining the loop representation of the transfer
matrix with Zamolodchikov's c-theorem. Asymptotically the central charge seems
to behave like c(q) = 1/2 log_2(q) + O(1). Very accurate values of the bulk
magnetic exponent x_1 are then extracted by performing Monte Carlo simulations
directly at the critical point. As q -> infinity, these seem to tend to a
non-trivial limit, x_1 -> 0.192 +- 0.002.


title: Trader dynamics in a model market

abstract:
  We explore various extensions of Challet and Zhang's Minority Game in an
attempt to gain insight into the dynamics underlying financial markets. First
we consider a heterogeneous population where individual traders employ
differing `time horizons' when making predictions based on historical data. The
resulting average winnings per trader is a highly non-linear function of the
population's composition. Second, we introduce a threshold confidence level
among traders below which they will not trade. This can give rise to large
fluctuations in the `volume' of market participants and the resulting market
`price'.


title: Umklapp scattering at reconstructed quantum-Hall edges

abstract:
  We study the low-lying excitations of a quantum-Hall sample that has
undergone edge reconstruction such that there exist three branches of chiral
edge excitations. Among the interaction processes that involve electrons close
to the three Fermi points is a new type of Umklapp-scattering process which has
not been discussed before. Using bosonization and a refermionization technique,
we obtain exact results for electronic correlation functions and discuss the
effect Umklapp scattering has on the Luttinger-liquid properties of
quantum-Hall edges.


title: Quantum Spin Systems: From Spin Gaps to Pseudo Gaps

abstract:
  Many low dimensional spin systems with a dimerized or ladder-like
antiferromagnetic exchange coupling have a gapped excitation spectrum with
magnetic bound states within the spin gap. For spin ladders with an even number
of legs the existence of spin gaps and within the t-J model a tendency toward
superconductivity with d-wave symmetry is predicted. In the following we will
characterize the spin excitation spectra of different low dimensional spin
systems taking into account strong spin phonon interaction ($\rm CuGeO_3$),
charge ordering ($\rm NaV_2O_5$) and doping on chains and ladders (\ladder).
The spectroscopic characterization of the model systems mentioned above has
been performed using magnetic inelastic light scattering originating from a
spin conserving exchange scattering mechanism. This is also bound to yield more
insight into the interrelation between these spin gap excitations and the
origin of the pseudo gap in high temperature superconductors.