title:
Violation of the Fluctuation-Dissipation Theorem in a Two-Dimensional
  Ising Model with Dipolar Interactions

abstract:
  The violation of the Fluctuation-Dissipation Theorem (FDT) in a
two-dimensional Ising model with both ferromagnetic exchange and
antiferromagnetic dipolar interactions is established and investigated via
Monte Carlo simulations. Through the computation of the autocorrelation
C(t+t_w,t_w) and the integrated response (susceptibility) functions we obtain
the FDT violation factor X(t+t_w,t_w) for different values of the temperature,
the waiting time t_w and the quotient delta=J_0/J_d, J_0 and J_d being the
strength of exchange and dipolar interactions respectively. For positive values
of delta this system develops a striped phase at low temperatures, in which the
non-equilibrium dynamics presents two different regimes. Our results show that
such different regimes are not reflected in the FDT violation factor, where X
goes always to zero for high values of t_w in the aging regime, a result that
appears in domain growth processes in non-frustrated ordered systems.


title: Berry-phase theory of proper piezoelectric response

abstract:
  Recent theoretical advances have established that the electric polarization
in an insulating crystal can be viewed as a multivalued quantity that is
determined by certain Berry phases associated with the occupied Bloch bands.
The application of this approach to the computation of piezoelectric
coefficients is not entirely straightforward, since a naive determination of
the (``improper'') piezoelectric coefficients from finite differences of the
polarization at nearby strain states leads to a dependence upon the choice of
``branch'' of the polarization. The purpose of the present paper is to clarify
that if one calculates instead the ``proper'' piezoelectric response, the
branch dependence is eliminated. From this analysis, a simplified recipe for
the direct finite-difference computation of the proper piezoelectric
coefficients emerges naturally.


title: Cuprate core level line shapes for different Cu-O networks

abstract:
  We have studied the Cu core level photoemission spectra in the Anderson
impurity model for cuprates with different Cu-O networks, dimensionalities
(zero, one, two and three) and Cu valencies (two and three). We focus on the
shape of the leading peak and obtain very good agreement with the experimental
data. We show how the shape of the spectrum is related to the valence
electronic structure and the Cu-O network but also that other atoms can play a
role.


title:
Series expansion and computer simulation studies of random sequential
  adsorption

abstract:
  We discuss two important techniques, series expansion and Monte Carlo
simulation, for random sequential adsorption study. Random sequential
adsorption is an idealization for surface deposition where the time scale of
particle relaxation is much longer than the time scale of deposition. Particles
are represented as extended objects which are adsorbed to a continuum surface
or lattice sites. Once landed on the surface, the particles stick to the
surface. We review in some details various methods of computing the coverage
theta(t) and present some of the recent and new results in random sequential
adsorption.


title: The interaction of surface acoustic waves with an array of quantum wires

abstract:
  We describe the interaction of surface acoustic waves with electrons in an
array of quantum wires, patterned out of a two dimensional electron gas. Two
specific geometries are considered, in which the surface acoustic wave travels
parallel, or perpendicular to the wires. Although we assume the electron wave
functions in different wires do not overlap, the screening of the phonon
potential by the electrons in the wire array is a collective phenomenon. It is
shown that the surface acoustic wave absorption cannot be described via the ac
conductivity in the usual manner. We derive an integral equation for the
dielectric function of the electrons in the quantum wire system, and solve it
in the narrow wire approximation. Using the dielectric function we find the
absorption and the change in velocity of the surface acoustic waves.


title: Growth of nanostructures by cluster deposition : a review

abstract:
  This paper presents a comprehensive analysis of simple models useful to
analyze the growth of nanostructures obtained by cluster deposition. After
detailing the potential interest of nanostructures, I extensively study the
first stages of growth (the submonolayer regime) by kinetic Monte-Carlo
simulations. These simulations are performed in a wide variety of experimental
situations : complete condensation, growth with reevaporation, nucleation on
defects, total or null cluster-cluster coalescence... The main scope of the
paper is to help experimentalists analyzing their data to deduce which of those
processes are important and to quantify them. A software including all these
simulation programs is available at no cost on request to the author. I
carefully discuss experiments of growth from cluster beams and show how the
mobility of the clusters on the surface can be measured : surprisingly high
values are found. An important issue for future technological applications of
cluster deposition is the relation between the size of the incident clusters
and the size of the islands obtained on the substrate. An approximate formula
which gives the ratio of the two sizes as a function of the melting temperature
of the material deposited is given. Finally, I study the atomic mechanisms
which can explain the diffusion of the clusters on a substrate and the result
of their mutual interaction (simple juxtaposition, partial or total
coalescence...)


title: Critical Crashes?

abstract:
  In this short note we discuss recent attempts to describe pre-crash market
dynamics with analogies from theory of critical phenomena.


title: Skyrmion dynamics in quantum Hall ferromagnets

abstract:
  Exploring a classical solution of the non-linear sigma model for a quantum
Hall ferromagnet, a skyrmion-magnon effective hamiltonian is obtained via the
collective coordinates method. Using the Feynman-Vernon functional integral
formalism for this model we find the temperature dependent transport
coefficients which characterize a single skyrmion dynamics.


title: Markovian approximation in foreign exchange markets

abstract:
  In this paper we test the random walk hypothesis on the high frequency
dataset of the bid--ask Deutschemark/US dollar exchange rate quotes registered
by the inter-bank Reuters network over the period October 1, 1992 to September
30, 1993. Then we propose a stochastic model for price variation which is able
to describe some important features of the exchange market behavior. Besides
the usual correlation analysis we have verified the validity of this model by
means of other approaches inspired by information theory . These techniques are
not only severe tests of the approximation but also evidence some aspects of
the data series which have a clear financial relevance.


title:
Composite Fermions and quantum Hall systems: Role of the Coulomb
  pseudopotential

abstract:
  The mean field composite Fermion (CF) picture successfully predicts angular
momenta of multiplets forming the lowest energy band in fractional quantum Hall
(FQH) systems. This success cannot be attributed to a cancellation between
Coulomb and Chern-Simons interactions beyond the mean field, because these
interactions have totally different energy scales. Rather, it results from the
behavior of the Coulomb pseudopotential V(L) (pair energy as a function of pair
angular momentum) in the lowest Landau level (LL). The class of short range
repulsive pseudopotentials is defined that lead to short range Laughlin like
correlations in many body systems and to which the CF model can be applied.
These Laughlin correlations are described quantitatively using the formalism of
fractional parentage. The discussion is illustrated with an analysis of the
energy spectra obtained in numerical diagonalization of up to eleven electrons
in the lowest and excited LL's. The qualitative difference in the behavior of
V(L) is shown to sometimes invalidate the mean field CF picture when applied to
higher LL's. For example, the nu=7/3 state is not a Laughlin nu=1/3 state in
the first excited LL. The analysis of the involved pseudopotentials also
explains the success or failure of the CF picture when applied to other systems
of charged Fermions with Coulomb repulsion, such as the Laughlin quasiparticles
in the FQH hierarchy or charged excitons in an electron-hole plasma.


title: Non-linear effects and dephasing in disordered electron systems

abstract:
  The calculation of the dephasing time in electron systems is presented. By
means of the Keldysh formalism we discuss in a unifying way both weak
localization and interaction effects in disordered systems. This allows us to
show how dephasing arises both in the particle-particle channel (weak
localization) and in the particle-hole channel (interaction effect). First we
discuss dephasing by an external field. Besides reviewing previous work on how
an external oscillating field suppresses the weak localization correction, we
derive a new expression for the effect of a field on the interaction
correction. We find that the latter may be suppressed by a static electric
field, in contrast to weak localization. We then consider dephasing due to
inelastic scattering. The ambiguities involved in the definition of the
dephasing time are clarified by directly comparing the diagrammatic approach
with the path-integral approach. We show that different dephasing times appear
in the particle-particle and particle-hole channels. Finally we comment on
recent experiments.


title: Thermal Contraction and Disordering of the Al(110) Surface

abstract:
  Al(110) has been studied for temperatures up to 900 K via ensemble
density-functional molecular dynamics. The strong anharmonicity displayed by
this surface results in a negative coefficient of thermal expansion, where the
first interlayer distance decreases with increasing temperature. Very shallow
channels of oscillation for the second-layer atoms in the direction
perpendicular to the surface support this anomalous contraction, and provide a
novel mechanism for the formation of adatom-vacancy pairs, preliminary to the
disordering and premelting transition. Such characteristic behavior originates
in the free-electron-gas bonding at a loosely packed surface.


title: Monte Carlo study of a two-dimensional quantum ferromagnet

abstract:
  We present quantum Monte Carlo results for the field and temperature
dependence of the magnetization and the spin-lattice relaxation rate $1/T_1$ of
a two-dimensional $S=1/2$ quantum Heisenberg ferromagnet. The Monte Carlo
method, which yields results free of systematic errors, is described in detail.
The high accuracy of the calculated magnetization allows for stringent tests of
recent approximate analytical calculations. We also compare our results with
recent experimental data for a $\nu=1$ quantum Hall ferromagnet, which is
expected to be well described by the Heisenberg model. The dynamic response
function needed to extract $1/T_1$ is obtained using maximum-entropy analytic
continuation of the corresponding imaginary-time dependent correlation
function. We discuss the reliability of this approach.


title:
Metal-insulator transition in the one-dimensional Holstein model at half
  filling

abstract:
  We study the one-dimensional Holstein model with spin-1/2 electrons at
half-filling. Ground state properties are calculated for long chains with great
accuracy using the density matrix renormalization group method and extrapolated
to the thermodynamic limit. We show that for small electron-phonon coupling or
large phonon frequency, the insulating Peierls ground state predicted by
mean-field theory is destroyed by quantum lattice fluctuations and that the
system remains in a metallic phase with a non-degenerate ground state and
power-law electronic and phononic correlations. When the electron-phonon
coupling becomes large or the phonon frequency small, the system undergoes a
transition to an insulating Peierls phase with a two-fold degenerate ground
state, long-range charge-density-wave order, a dimerized lattice structure, and
a gap in the electronic excitation spectrum.


title:
An SU(4) Approach of High-Temperature Superconductivity and
  Antiferromagnetism

abstract:
  We present an SU(4) model of high-$T_c$ superconductivity. One dynamical
symmetry of this model corresponds to the previously proposed SO(5) model for
unification of superconductivity and antiferromagnetism, but there are two
additional dynamical symmetries: SO(4), associated with antiferromagnetic order
and SU(2), associated with a D-wave pairing condensate. These provide a 3-phase
microscopic model of high-$T_c$ superconductivity and permit a clear
understanding of the role played by the SO(5) symmetry.


title:
Photoemission Evidence for a Remnant Fermi Surface and d-Wave-Like
  Dispersion in Insulating Ca2CuO2Cl2

abstract:
  An angle resolved photoemission study on Ca2CuO2Cl2, a parent compound of
high Tc superconductors is reported. Analysis of the electron occupation
probability, n(k) from the spectra shows a steep drop in spectral intensity
across a contour that is close to the Fermi surface predicted by the band
calculation. This analysis reveals a Fermi surface remnant even though
Ca2CuO2Cl2 is a Mott insulator. The lowest energy peak exhibits a dispersion
with approximately the |cos(kxa)-cos(kya)| form along this remnant Fermi
surface. Together with the data from Dy doped Bi2Sr2CaCu2O(8 + delta) these
results suggest that this d-wave like dispersion of the insulator is the
underlying reason for the pseudo gap in the underdoped regime.


title:
Quasiparticles in the vortex lattice of unconventional superconductors:
  Bloch waves or Landau levels?

abstract:
  A novel singular gauge transformation is developed for quasiparticles in the
mixed state of a strongly type-II superconductor which permits a full solution
of the problem at low and intermediate fields. For a periodic vortex lattice
the natural low-energy quasiparticle states are Bloch waves rather than Landau
levels discussed in the recent literature. The new representation elucidates
the physics considerably and provides fresh insights into the spectral
properties of such systems.


title:
Reply to Simon's Comment on "Evidence for an Anisotropic State of
  Two-Dimensional Electrons in High Landau Levels"

abstract:
  We recently reported [PRL 82, 394 (1999)] large transport anisotropies in a
two-dimensional electron gas in high Landau levels. These observations were
made utilizing both square and Hall bar sample geometries. Simon recently
commented [cond-mat/9903086] that a classical calculation of the current flow
in the sample shows a magnification of an underlying anisotropy when using a
square sample. In this reply we present more recent data obtained with a very
high mobility sample, and reiterate that, with or without magnification, an
anisotropic state develops in high Landau levels at very low temperatures.


title:
Low-energy singlet and triplet excitations in the spin-liquid phase of
  the two-dimensional J1-J2 model

abstract:
  We analyze the stability of the spontaneously dimerized spin-liquid phase of
the frustrated Heisenberg antiferromagnet - the J1-J2 model. The lowest triplet
excitation, corresponding to breaking of a singlet bond, is found to be stable
in the region 0.38 < J2/J1 < 0.62. In addition we find a stable low-energy
collective singlet mode, which is closely related to the spontaneous violation
of the discrete symmetry. Both modes are gapped in the quantum disordered phase
and become gapless at the transition point to the Neel ordered phase
(J2/J1=0.38). The spontaneous dimerization vanishes at the transition and we
argue that the disappearance of dimer order is related to the vanishing of the
singlet gap. We also present exact diagonalization data on a small (4x4)
cluster which indeed show a structure of the spectrum, consistent with that of
a system with a four-fold degenerate (spontaneously dimerized) ground state.


title:
Analytic computation of the Instantaneous Normal Modes spectrum in low
  density liquids

abstract:
  We analytically compute the spectrum of the Hessian of the Hamiltonian for a
system of N particles interacting via a purely repulsive potential in one
dimension. Our approach is valid in the low density regime, where we compute
the exact spectrum also in the localized sector. We finally perform a numerical
analysis of the localization properties of the eigenfunctions.


title:
Molecular dynamics simulation of binary hard-sphere crystal/melt
  interfaces

abstract:
  We examine, using molecular dynamics simulation, the structure and
thermodynamics of the (100) and (111) disordered face-centered cubic (FCC)
crystal/melt interfaces for a binary hard-sphere system. This study is an
extension of our previous work, [Phys. Rev. E 54, R5905 (1996)], in which
preliminary data for the (100) interface were reported. Density and diffusion
profiles on both fine- and course-grained scales are calculated and analyzed
leading to the conclusion that equilibrium interfacial segregation is minimal
in this system.


title: Avalanche Merging and Continuous Flow in a Sandpile Model

abstract:
  A dynamical transition separating intermittent and continuous flow is
observed in a sandpile model, with scaling functions relating the transport
behaviors between both regimes. The width of the active zone diverges with
system size in the avalanche regime but becomes very narrow for continuous
flow. The change of the mean slope, Delta z, on increasing the driving rate, r,
obeys Delta z ~ r^{1/theta}. It has nontrivial scaling behavior in the
continuous flow phase with an exponent theta given, paradoxically, only in
terms of exponents characterizing the avalanches theta = (1+z-D)/(3-D).

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


title: Weighted-density approximation for general nonuniform fluid mixtures

abstract:
  In order to construct a general density-functional theory for nonuniform
fluid mixtures, we propose an extension to multicomponent systems of the
weighted-density approximation (WDA) of Curtin and Ashcroft [Phys. Rev. A 32,
2909 (1985)]. This extension corrects a deficiency in a similar extension
proposed earlier by Denton and Ashcroft [Phys. Rev. A 42, 7312 (1990)], in that
that functional cannot be applied to the multi-component nonuniform fluid
systems with spatially varying composition, such as solid-fluid interfaces. As
a test of the accuracy of our new functional, we apply it to the calculation of
the freezing phase diagram of a binary hard-sphere fluid, and compare the
results to simulation and the Denton-Ashcroft extension.


title:
Functional Schr\"odinger Representations of Holstein-Primakoff Boson and
  Slave-Boson Theories for Heisenberg Antiferromagnets

abstract:
  We present functional Schr\"odinger representations of Holstein-Primakoff
boson and slave-boson theories for the Heisenberg Hamiltonian and the $t-J$
Hamiltonian respectively. Based on these representations we obtain the
dispersion relations of magnons for two dimensional antiferromagnets. By
applying the functional Schr\"odinger representation of the Holstein-Primakoff
boson theory to the Heisenberg Hamiltonian, the exchange energy is correctly
predicted and the self-energy of quasi-hole is obtained. From the use of the
functional Schr\"odinger representation of the $t-J$ Hamiltonian it is shown
that at half-filling the dispersion relation obtained from the slave-boson
theory leads to that obtained from the Holstein-Primakoff boson approach.


title:
Strongly Anisotropic Electronic Transport at Landau Level Filling Factor
  $\nu = 9/2$ and $\nu = 5/2$ Under Tilted Magnetic Field

abstract:
  We have investigated the influence of an increasing in-plane magnetic field
on the states at half-filling of Landau levels ($\nu$ = 11/2, 9/2, 7/2, and
5/2) of a two-dimensional electron system. In the electrically anisotropic
phase at $\nu$ = 9/2 and 11/2 an in-plane magnetic field of $\sim$ 1-2 T
overcomes its initial pinning to the crystal lattice and {\it reorient} this
phase. In the initially isotropic phases at $\nu$ = 5/2 and 7/2 an in-plane
magnetic field {\it induces} a strong electrical anisotropy. In all cases, for
high in-plane fields, the high resistance axis is parallel to the direction of
the in-plane field.


title: Real space renormalisation for reaction-diffusion systems

abstract:
  The stationary state of stochastic processes such as reaction-diffusion
systems can be related to the ground state of a suitably defined quantum
Hamiltonian. Using this analogy, we investigate the applicability of a real
space renormalisation group approach, originally developped for quantum spin
systems, to interacting particle systems. We apply the technique to an exactly
solvable reaction-diffusion system and to the contact process (both in $d=1$).
In the former case, several exact results are recovered. For the contact
process, surprisingly good estimates of critical parameters are obtained from a
small-cell renormalisation.


title: Large Thermopower in a Layered Oxide NaCo_2O_4

abstract:
  A transition-metal oxide NaCo_2O_4 is a layered oxide in which CoO_2 and Na
alternately stack along the c axis. Recently we have found that this compound
shows unusually large thermopower with low resistivity, which is comparable to
those of Bi_2Te_3. The negative transverse magnetoresistance and the strongly
temperature-dependent Hall coefficient suggest that electron correlation
dominates the conduction mechanism in NaCo_2O_4.


title:
Ground states of lattice gases with ``almost'' convex repulsive
  interactions

abstract:
  To our best knowledge there is only one example of a lattice system with
long-range two-body interactions whose ground states have been determined
exactly: the one-dimensional lattice gas with purely repulsive and strictly
convex interactions. Its ground-state particle configurations do not depend on
the rate of decay of the interactions and are known as the generalized Wigner
lattices or the most homogenenous particle configurations. The question of
stability of this beautiful and universal result against certain perturbations
of the repulsive and convex interactions seems to be interesting by itself.
Additional motivations for studying such perturbations come from surface
physics (adsorbtion on crystal surfaces) and theories of correlated fermion
systems (recent results on ground-state particle configurations of the
one-dimensional spinless Falicov-Kimball model).
  As a first step we have studied a one-dimensional lattice gas whose two-body
interactions are repulsive and strictly convex only from distance 2 on while
its value at distance 1 is fixed near its value at infinity. We show that such
a modification makes the ground-state particle configurations sensitive to the
decay rate of the interactions: if it is fast enough, then particles form
2-particle lattice-connected aggregates that are distributed in the most
homogeneous way. Consequently, despite breaking of the convexity property, the
ground state exibits the feature known as the complete devil's staircase.


title: Enhanced winnings in a mixed-ability population playing a minority game

abstract:
  We study a mixed population of adaptive agents with small and large memories,
competing in a minority game. If the agents are sufficiently adaptive, we find
that the average winnings per agent can exceed that obtainable in the
corresponding pure populations. In contrast to the pure population, the average
success rate of the large-memory agents can be greater than 50 percent. The
present results are not reproduced if the agents are fed a random history,
thereby demonstrating the importance of memory in this system.


title:
Raman scattering in a two-dimensional electron gas: Boltzmann equation
  approach

abstract:
  The inelastic light scattering in a 2-d electron gas is studied theoretically
using the Boltzmann equation techniques. Electron-hole excitations produce the
Raman spectrum essentially different from the one predicted for the 3-d case.
In the clean limit it has the form of a strong non-symmetric resonance due to
the square root singularity at the electron-hole frequency $\omega = vk$ while
in the opposite dirty limit the usual Lorentzian shape of the cross section is
reestablished. The effects of electromagnetic field are considered
self-consistently and the contribution from collective plasmon modes is found.
It is shown that unlike 3-d metals where plasmon excitations are unobservable
(because of very large required transfered frequencies), the two-dimensional
electron system gives rise to a low-frequency ($\omega \propto k^{1/2}$)
plasmon peak. A measurement of the width of this peak can provide data on the
magnitude of the electron scattering rate.


title: Enhancement of pair correlation in a one-dimensional hybridization model

abstract:
  We propose an integrable model of one-dimensional (1D) interacting electrons
coupled with the local orbitals arrayed periodically in the chain. Since the
local orbitals are introduced in a way that double occupation is forbidden, the
model keeps the main feature of the periodic Anderson model with an interacting
host. For the attractive interaction, it is found that the local orbitals
enhance the effective mass of the Cooper-pair-like singlets and also the pair
correlation in the ground state. However, the persistent current is depressed
in this case. For the repulsive interaction case, the Hamiltonian is
non-Hermitian but allows Cooper pair solutions with small momenta, which are
induced by the hybridization between the extended state and the local orbitals.


title: Second magnetization peak in flux lattices: the decoupling scenario

abstract:
  The second peak phenomena of flux lattices in layered superconductors is
described in terms of a disorder induced layer decoupling transition. For weak
disorder the tilt mudulus undergoes an apparent discontinuity which leads to an
enhanced critical current and reduced domain size in the decoupled phase. The
Josephson plasma frequency is reduced by decoupling and by Josephson glass
pinning; in the liquid phase it varies as 1/[BT(T+T_0)] where T is temperature,
B is field and T_0 is the disorder dependent temperature of the multicritical
point.


title: Geometric Phase Transitions

abstract:
  A model in statistical mechanics, characterised by the corresponding Gibbs
measure, is a subset of the totality of probability distributions on the phase
space. The shape of this subset, i.e., the geometry, then plays an important
role in statistical analysis of the model. It is known that this subset has the
structure of a manifold equipped with a Riemannian metric, given by the Fisher
information matrix. Invariant quantities such as thermodynamic curvature have
been studied extensively in the literature, although a satisfactory physical
interpretation of the geometry has not hitherto been established. In this
article, we investigate the thermodynamic curvature for one and two dimensional
Ising models and report the existence of a geometric phase transition
associated with a change in the signature of the curvature. This transition is
of a continuous type, and exists for finite systems. The effect may be tested
in principle by mesoscopic scale experiments.


title: Classical and Quantum Dynamics in a Random Magnetic Field

abstract:
  Using the supersymmetry approach, we study spectral statistical properties of
a two-dimensional quantum particle subject to a non-uniform magnetic field. We
focus mainly on the problem of regularisation of the field theory. Our analysis
begins with an investigation of the spectral properties of the purely classical
evolution operator. We show that, although the kinetic equation is formally
time-reversible, density relaxation is controlled by {\em irreversible}
classical dynamics. In the case of a weak magnetic field, the effective kinetic
operator corresponds to diffusion in the angle space, the diffusion constant
being determined by the spectral resolution of the inhomogeneous magnetic
field. Applying these results to the quantum problem, we demonstrate that the
low-lying modes of the field theory are related to the eigenmodes of the
irreversible classical dynamics, and the higher modes are separated from the
zero mode by a gap associated with the lowest density relaxation rate. As a
consequence, we find that the long-time properties of the system are
characterised by universal Wigner-Dyson statistics. For a weak magnetic field,
we obtain a description in terms of the quasi one-dimensional non-linear
$\sigma$-model.


title:
A scalable, tunable qubit, based on a clean DND or grain boundary D-D
  junction

abstract:
  Unique properties of a ballistic DND or grain boundary D-D junction,
including doubly degenerate ground state with tunable potential barrier between
the "up" and "down" states and non-quantized spontaneous magnetic flux, make it
a good candidate for a solid state qubit. The role of quantum "spin" variable
is played by the sign of equilibrium superconducting phase difference on the
junction, which is revealed in the direction of spontaneous supercurrent flow
in equilibrium.
  Possibilities of design-specific simultaneous operations with several
integrated qubits are discussed.


title:
Phase-field model for Hele-Shaw flows with arbitrary viscosity contrast.
  I. Theoretical approach

abstract:
  We present a phase-field model for the dynamics of the interface between two
inmiscible fluids with arbitrary viscosity contrast in a rectangular Hele-Shaw
cell. With asymptotic matching techniques we check the model to yield the right
Hele-Shaw equations in the sharp-interface limit and compute the corrections to
these equations to first order in the interface thickness. We also compute the
effect of such corrections on the linear dispersion relation of the planar
interface. We discuss in detail the conditions on the interface thickness to
control the accuracy and convergence of the phase-field model to the limiting
Hele-Shaw dynamics. In particular, the convergence appears to be slower for
high viscosity contrasts.

categories: cond-mat.soft cond-mat.mtrl-sci cond-mat.stat-mech physics.flu-dyn


title:
Effects of resonant tunneling in electromagnetic wave propagation
  through a polariton gap

abstract:
  We consider tunneling of electromagnetic waves through a polariton band gap
of a 1-D chain of atoms. We analytically show that a defect embedded in the
structure gives rise to the resonance transmission at the frequency of a local
polariton state associated with the defect. Numerical Monte-Carlo simulations
are used to examine properties of the electromagnetic band arising inside the
polariton gap due to finite concentration of defects.


title:
Phase-field model for Hele-Shaw flows with arbitrary viscosity contrast.
  II. Numerical study

abstract:
  We implement a phase-field simulation of the dynamics of two fluids with
arbitrary viscosity contrast in a rectangular Hele-Shaw cell. We demonstrate
the use of this technique in different situations including the linear regime,
the stationary Saffman-Taylor fingers and the multifinger competition dynamics,
for different viscosity contrasts. The method is quantitatively tested against
analytical predictions and other numerical results. A detailed analysis of
convergence to the sharp interface limit is performed for the linear dispersion
results. We show that the method may be a useful alternative to more
traditional methods.


title: Crossover from thermal hopping to quantum tunneling in Mn_{12}Ac

abstract:
  The crossover from thermal hopping to quantum tunneling is studied. We show
that the decay rate $\Gamma$ with dissipation can accurately be determined near
the crossover temperature. Besides considering the Wentzel-Kramers-Brillouin
(WKB) exponent, we also calculate contribution of the fluctuation modes around
the saddle point and give an extended account of a previous study of crossover
region. We deal with two dangerous fluctuation modes whose contribution can't
be calculated by the steepest descent method and show that higher order
couplings between the two dangerous modes need to be taken into considerations.
At last the crossover from thermal hopping to quantum tunneling in the
molecular magnet Mn_{12}Ac is studied.


title: Macroscopic quantum coherence in mesoscopic ferromagnetic systems

abstract:
  In this paper we study the Macroscopic Quantum Oscillation (MQO) effect in
ferromagnetic single domain magnets with a magnetic field applied along the
hard anistropy axis. The level splitting for the ground state, derived with the
conventional instanton method, oscillates with the external field and is
quenched at some field values. A formula for quantum tunneling at excited
levels is also obtained. The existence of topological phase accounts for this
kind of oscillation and the corresponding thermodynamical quantities exhibit
similar interference effects which resembles to some extent the electron
quantum phase interference induced by gauge potential in the Aharonov-Bohm
effect and the $\Theta $-vacuum in Yang-Mills field theory..


title: Dielectric anomalies of solid CO and N_2 in the audio frequency range

abstract:
  We report the first audio frequency dielectric constant measurements of CO
and N_2 in their solid phases. We have observed several new features, including
(1) strong hysteresis effects above an onset temperature $T_{h} \simeq 42 K$ in
the \beta-phase of pure N_2, (2) absence of the expected short range
antiferroelectric dipolar ordering in CO down to 4.2 K, and (3) anomalous
temperature dependence of the dielectric constant in the \alpha-phases of both
CO and N_2. Quantum mechanical treatment of the molecular rotation explains
some of the observed anomalies in the \alpha-phase, but the strong hysteresis
indicates that pure geometrical frustration plays a significant role in the
\beta-phase.


title:
Electronic Structure and Thermoelectric Prospects of Phosphide
  Skutterudites

abstract:
  The prospects for high thermoelectric performance in phosphide skutterudites
are investigated based on first principles calculations. We find that
stoichiometric CoP_3 differs from the corresponding arsenide and antimonide in
that it is metallic. As such the band structure must be modified if high
thermopowers are to be achieved. In analogy to the antimonides it is expected
that this may be done by filling with La. Calculations for LaFe_4P_12 show that
a gap can in fact be opened by La filling, but that the valence band is too
light to yield reasonable p-type thermopowers at appropriate carrier densities;
n-type La filled material may be more favorable.


title:
The effect of substrate induced strain on the charge-ordering transition
  in Nd_{0.5}Sr_{0.5}MnO_{3} thin films

abstract:
  We report the synthesis and characterization of Nd_{0.5}Sr_{0.5}MnO_{3} thin
films grown by the Pulsed Laser Deposition technique on 100 -oriented LaAlO_{3}
substrates. X-ray diffraction (XRD) studies show that the films are 101
-oriented, with a strained and quasi-relaxed component, the latter increasing
with film thickness. We observe that transport properties are strongly
dependent on the thickness of the films. Variable temperature XRD down to 100 K
suggests that this is caused by substrate induced strain on the films.


title:
Response to Parallel Magnetic Field of a Dilute 2D Electron System
  across the Metal-Insulator Transition

abstract:
  The response to a parallel magnetic field of the very dilute insulating
two-dimensional system of electrons in silicon MOSFET's is dramatic and similar
to that found on the conducting side of the metal-insulator transition: there
is a large initial increase in resistivity with increasing field, followed by
saturation to a value that is approximately constant above a characteristic
magnetic field of about one Tesla. This is unexpected behavior in an insulator
that exhibits Efros-Shklovskii variable-range hopping in zero field, and
appears to be a general feature of very dilute electron systems.


title: Effect of coulomb long range interactions on the Mott transition

abstract:
  We reconsider the Mott transition problem in the presence of long range
Coulomb interactions. Using an extended DMFT, that sums an important class of
diagrams absent in ordinary DMFT, we show that in the presence of Coulomb the
Mott transition in two and three dimensions is discontinuous as envisioned by
Mott.


title: Spin-charge separation and Kondo effect in an open quantum dot

abstract:
  We study a quantum dot connected to the bulk by single-mode junctions at
almost perfect conductance. Although the average charge $e\langle N \rangle$ of
the dot is not discrete, its spin remains quantized: $s=1/2$ or $s=0$,
depending (periodically) on the gate voltage. This drastic difference from the
conventional mixed-valence regime stems from the existence of a broad-band,
dense spectrum of discrete levels in the dot. In the doublet state, the Kondo
effect develops at low temperatures. We find the Kondo temperature $T_K$ and
the conductance at $T\lesssim T_K$.


title: Frenkel and charge transfer excitons in C60

abstract:
  We have studied the low energy electronic excitations of C60 using momentum
dependent electron energy-loss spectroscopy in transmission. The momentum
dependent intensity of the gap excitation allows the first direct experimental
determination of the energy of the 1Hg excitation and thus also of the total
width of the multiplet resulting from the gap transition. In addition, we could
elucidate the nature of the following excitations - as either Frenkel or charge
transfer excitons.


title:
Ordering and Fluctuation of Orbital and Lattice Distortion in Perovskite
  Manganese Oxides

abstract:
  Roles of orbital and lattice degrees of freedom in strongly correlated
systems are investigated to understand electronic properties of perovskite Mn
oxides such as La_{1-x}Sr_{x}MnO_{3}. An extended double-exchange model
containing Coulomb interaction, doubly degenerate orbitals and Jahn-Teller
coupling is derived under full polarization of spins with two-dimensional
anisotropy. Quantum fluctuation effects of Coulomb interaction and orbital
degrees of freedom are investigated by using the quantum Monte Carlo method. In
undoped states, it is crucial to consider both the Coulomb interaction and the
Jahn-Teller coupling in reproducing characteristic hierarchy of energy scales
among charge, orbital-lattice and spin degrees of freedom in experiments. Our
numerical results quantitatively reproduce the charge gap amplitude as well as
the stabilization energy and the amplitude of the cooperative Jahn-Teller
distortion in undoped compounds. Upon doping of carriers, in the absence of the
Jahn-Teller distortion, critical enhancement of both charge compressibility and
orbital correlation length is found with decreasing doping concentration. These
are discussed as origins of strong incoherence in charge dynamics. With the
Jahn-Teller coupling in the doped region, collapse of the Jahn-Teller
distortion and instability to phase separation are obtained and favorably
compared with experiments. These provide a possible way to understand the
complicated properties of lightly doped manganites.


title: Directed percolation depinning models: Evolution equations

abstract:
  We present the microscopic equation for the growing interface with quenched
noise for the model first presented by Buldyrev et al. [Phys. Rev. A 45, R8313
(1992)]. The evolution equation for the height, the mean height, and the
roughness are reached in a simple way. The microscopic equation allows us to
express these equations in two contributions: the contact and the local one. We
compare this two contributions with the ones obtained for the Tang and
Leschhorn model [Phys. Rev A 45, R8309 (1992)] by Braunstein et al. [Physica A
266, 308 (1999)]. Even when the microscopic mechanisms are quiet different in
both model, the two contribution are qualitatively similar. An interesting
result is that the diffusion contribution, in the Tang and Leschhorn model, and
the contact one, in the Buldyrev model, leads to an increase of the roughness
near the criticality.


title:
Far-infrared induced current in a ballistic channel -- potential barrier
  structure

abstract:
  We consider electron transport in a ballistic multi-mode channel structure in
the presence of a transversely polarized far-infrared (FIR) field. The channel
structure consists of a long resonance region connected to an adiabatic
widening with a potential barrier at the end. At frequencies that match the
mode energy separation in the resonance region we find distinct peaks in the
photocurrent, caused by Rabi oscillations in the mode population. For an
experimental situation in which the width of the channel is tunable via gates,
we propose a method for reconstructing the spectrum of propagating modes,
without having to use a tunable FIR source. With this method the change in the
spectrum as the gate voltage is varied can be monitored.


title: Island diffusion on metal fcc(100) surfaces

abstract:
  We present Monte Carlo simulations for the size and temperature dependence of
the diffusion coefficient of adatom islands on the Cu(100) surface. We show
that the scaling exponent for the size dependence is not a constant but a
decreasing function of the island size and approaches unity for very large
islands. This is due to a crossover from periphery dominated mass transport to
a regime where vacancies diffuse inside the island. The effective scaling
exponents are in good agreement with theory and experiments.


title:
Hamiltonian Description of Composite Fermions: Magnetoexciton
  Dispersions

abstract:
  A microscopic Hamiltonian theory of the FQHE, developed by Shankar and myself
based on the fermionic Chern-Simons approach, has recently been quite
successful in calculating gaps in Fractional Quantum Hall states, and in
predicting approximate scaling relations between the gaps of different
fractions. I now apply this formalism towards computing magnetoexciton
dispersions (including spin-flip dispersions) in the $\nu=1/3$, 2/5, and 3/7
gapped fractions, and find approximate agreement with numerical results. I also
analyse the evolution of these dispersions with increasing sample thickness,
modelled by a potential soft at high momenta. New results are obtained for
instabilities as a function of thickness for 2/5 and 3/7, and it is shown that
the spin-polarized 2/5 state, in contrast to the spin-polarized 1/3 state,
cannot be described as a simple quantum ferromagnet.


title:
Landau Theory of the Mott Transition in the Fully Frustrated Hubbard
  Model in Infinite Dimensions

abstract:
  We discuss the solution of the Mott transition problem in a fully frustrated
lattice with a semicircular density of states in the limit of infinite
dimensions from the point of view of a Landau free energy functional. This
approach provides a simple relation between the free energy of the lattice
model and that of its local description in terms of an impurity model. The
character of the Mott transition in infinite dimensions, (as reviewed by
Georges Kotliar Krauth and Rozenberg, RMP 68, 1996, 13) follows simply from the
form of the free energy functional and the physics of quantum impurity models.
At zero temperature, below a critical value of the interaction U, a Mott
insulator with a finite gap in the one particle spectrum, becomes unstable to
the formation of a narrow band near the Fermi energy. Using the insights
provided by the Landau approach we answer questions raised about the dynamical
mean field solution of the Mott transition problem, and comment on its
applicability to three dimensional transition metal oxides.


title:
Induced Luttinger Liquid Behavior in an Exactly Solvable Model of
  Stripes

abstract:
  We study an exactly solvable model describing a stripe consisting of a Toda
array of $N$ anharmonic elastic chains sandwiched between two conducting
chains. It is shown that the presence of a charge on one chain generates a
gapless excitation branch (Luttinger liquid) on the other and leads to increase
in the phonon frequencies.


title:
Breakdown of the resistor model of CPP-GMR in magnetic multilayered
  nanostructures

abstract:
  We study the effect on CPP GMR of changing the order of the layers in a
multilayer. Using a tight-binding simple cubic two band model (s-d),
magneto-transport properties are calculated in the zero-temperature, zero-bias
limit, within the Landauer-Buttiker formalism. We demonstrate that for layers
of different thicknesses formed from a single magnetic metal and multilayers
formed from two magnetic metals, the GMR ratio and its dependence on disorder
is sensitive to the order of the layers. This effect disappears in the limit of
large disorder, where the results of the widely-used Boltzmann approach to
transport are restored.


title: Coarsening in a Driven Ising Chain with Conserved Dynamics

abstract:
  We study the low-temperature coarsening of an Ising chain subject to
spin-exchange dynamics and a small driving force. This dynamical system reduces
to a domain diffusion process, in which entire domains undergo nearest-neighbor
hopping, except for the shortest domains -- dimers -- which undergo long-range
hopping. This system is characterized by two independent length scales: the
average domain length L(t)~t^{1/2} and the average dimer hopping distance l(t)~
t^{1/4}. As a consequence of these two scales, the density C_k(t) of domains of
length k does not obey scaling. This breakdown of scaling also leads to the
density of short domains decaying as t^{-5/4}, instead of the t^{-3/2} decay
that would arise from pure domain diffusion.


title:
Thermally activated escape rates of uniaxial spin systems with
  transverse field

abstract:
  Classical escape rates of uniaxial spin systems are characterized by a
prefactor differing from and much smaller than that of the particle problem,
since the maximum of the spin energy is attained everywhere on the line of
constant latitude: theta=const, 0 =< phi =< 2*pi. If a transverse field is
applied, a saddle point of the energy is formed, and high, moderate, and low
damping regimes (similar to those for particles) appear. Here we present the
first analytical and numerical study of crossovers between the uniaxial and
other regimes for spin systems. It is shown that there is one HD-Uniaxial
crossover, whereas at low damping the uniaxial and LD regimes are separated by
two crossovers.


title: Localization by interference: Square billiard with a magnetic flux

abstract:
  Eigenstates and energy levels of a square quantum billiard in a magnetic
field, or with an Aharonov-Bohm flux line, are found in quasiclassical
approximation, that is, for high enough energy. Explicit formulas for the
energy levels and wavefunctions are found. There are localized states, never
before noticed in this well studied problem, whose localization is due to phase
interference, even though there is no or negligible classical effect of the
magnetic field. These and related states account almost entirely for the
magnetic response in certain temperature ranges, and thus have a bearing on the
experiments of Levy et al.


title: Influence of rare regions on magnetic quantum phase transitions

abstract:
  The effects of quenched disorder on the critical properties of itinerant
quantum magnets are considered. Particular attention is paid to locally ordered
rare regions that are formed in the presence of quenched disorder even when the
bulk system is still in the nonmagnetic phase. It is shown that these local
moments or instantons destroy the previously found critical fixed point in the
case of antiferromagnets. In the case of itinerant ferromagnets, the critical
behavior is unaffected by the rare regions due to an effective long-range
interaction between the order parameter fluctuations.


title:
Resonant neutron scattering on the high Tc cuprates and pi and eta
  excitation of the t-J and Hubbard models

abstract:
  We review the explanation for resonant neutron scattering experiments in
$YBa_2Cu_3O_{6+\delta}$ and $Bi_2 Sr_2 Ca_2 O_{8+\delta}$ materials from the
point of view of triplet excitation in the particle-particle channel, the pi
excitation. Relation of these resonances to the superconducting condensation
energy and their role in stabilizing the superconducting state is discussed.
Due to the superconducting fluctuations, the pi resonance may appear as a broad
peak above Tc. Analogue problem to the pi excitation for the case of s-wave
pairing, the eta excitation, is considered in the strong coupling limit with an
emphasis on the resonance precursors in a state with no long range order.


title:
Anisotropic States of Two-Dimensional Electron Systems in High Landau
  Levels: Effect of an In-Plane Magnetic Field

abstract:
  We report the observation of an acute sensitivity of the anisotropic
longitudinal resistivity of two-dimensional electron systems in half-filled
high Landau levels to the magnitude and orientation of an in-plane magnetic
field. In the third and higher Landau levels, at filling fractions nu=9/2,
11/2, etc., the in-plane field can lead to a striking interchange of the "hard"
and "easy" transport directions. In the second Landau level the normally
isotropic resistivity and the weak nu=5/2 quantized Hall state are destroyed by
a large in-plane field and the transport becomes highly anisotropic.


title: Bound-State Instability of the Chiral Luttinger Liquid in One-Dimension

abstract:
  We have developed a new boot-strap method for solving a class of interacting
one-dimensional chiral fermions. The conventional model for interacting
right-moving electrons with spin has an SO(4) symmetry, and can be written as
four interacting Majorana fermions, each with the same velocity. We have found
a method for solving some cases when the velocities of these Majorana fermions
are no longer equal. We demonstrate in some detail the remarkable result that
corrections to the non skeleton self-energy identically vanish for these
models, and this enables us to solve them exactly. For the cases where the
model can be solved by bosonization, our method can be explicitly checked.
However, we are also able to solve some new cases where the excitation spectrum
differs qualitatively from a Luttinger liquid.
  Of particular interest, is the so-called SO(3) model, where a triplet of
Majorana fermions moving at one velocity, interact with a single Majorana
fermion moving at another velocity. We show using our method, that a sharp
bound (or anti-bound) state splits off from the original Luttinger liquid
continuum, cutting off the X-ray singularity to form a broad incoherent
excitation with a lifetime that grows linearly with frequency.


title:
Observation of spin freezing and relaxation at microwave frequencies in
  the spin ladder compound $Sr_{14-x}Ca_{x}Cu_{24}O_{41}$

abstract:
  We report the observation of a frequency ($\omega $) and temperature (T)
-dependent loss peak in $\chi''$ and accompanying dispersion in $\chi'$ from
microwave (2-18GHz) measurements of the complex susceptibility
$\tilde{\chi}(\omega, T)=\chi'+i\chi''$ of $Sr_{14-x}Ca_{x}Cu_{24}O_{41}$. We
associate this phenomenon with a rapid decrease of spin disorder and
corresponding spin relaxation rate, representing a ``spin freezing transition''
accompanying charge ordering which occurs at temperatures $\sim 250K$. Our
results enable direct quantitative measurements of the spin relaxation rate,
and yield information on spin dynamics in these materials and the related
compounds $SrCuO_{2}$ and $Sr_{2}CuO_{3}$.


title:
Bond-orientational ordering and shear rigidity in modulated colloidal
  liquids

abstract:
  From Landau-Alexander-McTague theory and Monte-Carlo simulation results we
show that the modulated liquid obtained by subjecting a colloidal system to a
periodic laser modulation has long range bond-orientational order and non-zero
shear rigidity. From infinite field simulation results we show that in the
modulated liquid phase, the translational order parameter correlation function
decays to zero exponentially while the correlation function for the
bond-orientational order saturates to a finite value at large distances.


title: Quasienergy Spectroscopy of Excitons

abstract:
  We theoretically study nonlinear optics of excitons under intense THz
irradiation. In particular, the linear near infrared absorption and resonantly
enhanced nonlinear sideband generation are described. We predict a rich
structure in the spectra which can be interpreted in terms of the quasienergy
spectrum of the exciton, via a remarkably transparent expression for the
susceptibility, and show that the effects of strongly avoided quasienergy
crossings manifest themselves directly, both in the absorption and transmitted
sidebands.


title:
Determination of the electrical conductivity and the elctroosmotic
  transfer in the concentrated dispersions on the basis of the cell theory of
  the electroosmosis

abstract:
  Theoretical calculations of the electrical conductivity and electroosmotic
transfer as functions of the disperse phase volume fraction and non-dissolving
boundary layer thickness were provided on the basis of the cell theory of
electroosmosis for the limiting case of large degree of electric double layers
overlapping in interparticle space. The obtained results are in qualitative
agreement with the experimental data and describe the main features of the
latter


title: Open t-J chain with boundary impurities

abstract:
  We study integrable boundary conditions for the supersymmetric t-J model of
correlated electrons which arise when combining static scattering potentials
with dynamical impurities carrying an internal degree of freedom. The latter
differ from the bulk sites by allowing for double occupation of the local
orbitals. The spectrum of the resulting Hamiltonians is obtained by means of
the algebraic Bethe Ansatz.


title:
"Nonlinear" covariance matrix and portfolio theory for non-Gaussian
  multivariate distributions

abstract:
  This paper offers a precise analytical characterization of the distribution
of returns for a portfolio constituted of assets whose returns are described by
an arbitrary joint multivariate distribution. In this goal, we introduce a
non-linear transformation that maps the returns onto gaussian variables whose
covariance matrix provides a new measure of dependence between the non-normal
returns, generalizing the covariance matrix into a non-linear fractional
covariance matrix. This nonlinear covariance matrix is chiseled to the specific
fat tail structure of the underlying marginal distributions, thus ensuring
stability and good-conditionning. The portfolio distribution is obtained as the
solution of a mapping to a so-called phi-q field theory in particle physics, of
which we offer an extensive treatment using Feynman diagrammatic techniques and
large deviation theory, that we illustrate in details for multivariate Weibull
distributions. The main result of our theory is that minimizing the portfolio
variance (i.e. the relatively ``small'' risks) may often increase the large
risks, as measured by higher normalized cumulants. Extensive empirical tests
are presented on the foreign exchange market that validate satisfactorily the
theory. For ``fat tail'' distributions, we show that an adequete prediction of
the risks of a portfolio relies much more on the correct description of the
tail structure rather than on their correlations.


title:
Symmetry Constraints and Variational Principles in Diffusion Quantum
  Monte Carlo Calculations of Excited-State Energies

abstract:
  Fixed-node diffusion Monte Carlo (DMC) is a stochastic algorithm for finding
the lowest energy many-fermion wave function with the same nodal surface as a
chosen trial function. It has proved itself among the most accurate methods
available for calculating many-electron ground states, and is one of the few
approaches that can be applied to systems large enough to act as realistic
models of solids. In attempts to use fixed-node DMC for excited-state
calculations, it has often been assumed that the DMC energy must be greater
than or equal to the energy of the lowest exact eigenfunction with the same
symmetry as the trial function. We show that this assumption is not justified
unless the trial function transforms according to a one-dimensional irreducible
representation of the symmetry group of the Hamiltonian. If the trial function
transforms according to a multi-dimensional irreducible representation,
corresponding to a degenerate energy level, the DMC energy may lie below the
energy of the lowest eigenstate of that symmetry. Weaker variational bounds may
then be obtained by choosing trial functions transforming according to
one-dimensional irreducible representations of subgroups of the full symmetry
group.


title:
Conductance enhancement in quantum point
  contact-semiconductor-superconductor devices

abstract:
  We present numerical calculations of the conductance of an interface between
a phase-coherent two-dimensional electron gas and a superconductor with a
quantum point contact in the normal region. Using a scattering matrix approach
we reconsider the geometry of De Raedt, Michielsen, and Klapwijk [Phys. Rev. B,
50, 631 (1994)] which was studied within the time-dependent Bogoliubov-de
Gennes formalism. We find that the factor-of-two enhancement of the conductance
G_NS compared to the normal state conductance G_N for ideal interfaces may be
suppressed for interfaces with a quantum point contact with only a few
propagating modes. The suppression is found to depend strongly on the position
of the Fermi level. We also study the suppression due to a barrier at the
interface and find an anomalous behavior caused by quasiparticle interference.
Finally, we consider the limit of sequential tunneling and find a suppression
of the factor-of-two enhancement which may explain the absence of conductance
enhancement in experiments on metal-superconductor structures.


title:
Probing the field-induced variation of the chemical potential in
  Bi(2)Sr(2)CaCu(2)O(y) via the magneto-thermopower measurements

abstract:
  Approximating the shape of the measured in $Bi_2Sr_2CaCu_2O_y$
magneto-thermopower (TEP) $\Delta S(T,H)$ by asymmetric linear triangle of the
form $\Delta S(T,H)\simeq S_p(H)\pm B^{\pm}(H)(T_c-T)$ with positive $B
^{-}(H)$ and $B ^{+}(H)$ defined below and above $T_c$, we observe that $B
^{+}(H)\simeq 2B ^{-}(H)$. In order to account for this asymmetry, we
explicitly introduce the field-dependent chemical potential of holes $\mu (H)$
into the Ginzburg-Landau theory and calculate both an average $\Delta
S_{av}(T,H)$ and fluctuation $\Delta S_{fl}(T,H)$ contributions to the total
magneto-TEP $\Delta S(T,H)$. As a result, we find a rather simple relationship
between the field-induced variation of the chemical potential in this material
and the above-mentioned magneto-TEP data around $T_c$, viz. $\Delta \mu
(H)\propto S_p(H)$.


title:
On the equivalence between the Barkhausen effect and directed Abelian
  sandpile models

abstract:
  The existence of self-organized criticality in the Barkhausen effect and its
analogy with sandpile models is investigated. It is demonstrated that a model
recently introduced to describe the dynamics of a domain wall [Cizeau et al,
Phys. Rev. Lett. 79, 4669 (1997)] belongs to the universality class of
undirected Abelian sandpile models. In this way it is shown that the Barhausen
effect can be taken as an experimental observation of self-organized critical
phenomena.


title:
Pseudogap Formation in an Electronic System with d-wave Attraction at
  Low-density

abstract:
  On the basis of an electronic model with separable attractive interaction,
the precursors at high temperature and strong coupling of the d-wave
superconducting state are investigated in the one-particle spectral function
$A({\bf k},\omega)$ and the total density of states $\rho(\omega)$, with the
use of the self-consistent $t$-matrix approximation. In the low-density region,
it is found that a gap-like structure at the Fermi level appears in $A({\bf
k},\omega)$ and $\rho(\omega)$ above the superconducting transition
temperature. It is shown that the pseudogap energy scale is determined by the
binding energy of the Cooper-pair.


title: Spin accumulation in FS junctions

abstract:
  We consider Andreev reflection in ferromagnet-superconductor junctions and
extend the treatment by de Jong and Beenakker to incorporate the effect of a
repulsive potential at the interface. We show the existence of a spin
accumulation at a finite voltage in this model.


title:
On the Coexistence in RuSr2GdCu2O8 of Superconductivity and
  Ferromagnetism

abstract:
  We review the reasons that make superconductivity unlikely to arise in a
ferromagnet. Then, in light of the report by Tallon and collaborators that
RuSr2GdCu2O8 becomes superconducting at approximately 35 K which is well below
the Curie temperature of 132 K, we consider whether the objections really apply
to this compound. Our considerations are supported by local spin density
calculations for this compound, which indeed indicate a ferromagnetic RuO2
layer. The Ru moment resides in t_2g orbitals but is characteristic of
itinerant magnetism (and is sensitive to choice of exchange-correlation
potential and to the atomic positions). Based on the small exchange splitting
that is induced in the Cu-O layers, the system seems capable of supporting
singlet superconductivity an FFLO-type order parameter and possibly a pi-phase
alternation between layers. If instead the pairing is triplet in the RuO2
layers, it can be distinguished by a spin-polarized supercurrent. Either type
of superconductivity seems to imply a spontaneous vortex phase if the
magnetization is rotated out of the plane.


title:
A Closed Class of Hydrodynamical Solutions for the Collective
  Excitations of a Bose-Einstein Condensate

abstract:
  A trajectory approach is taken to the hydrodynamical treatment of collective
excitations of a Bose-Einstein condensate in a harmonic trap. The excitations
induced by linear deformations of the trap are shown to constitute a broad
class of solutions that can be fully described by a simple nonlinear matrix
equation. An exact closed-form expression is obtained for the solution
describing the mode {n=0, m=2} in a cylindrically symmetric trap, and the
calculated amplitude-dependent frequency shift shows good agreement with the
experimental results of the JILA group.


title: Inelastic Coulomb scattering rate of a multisubband Q1D electron gas

abstract:
  In this work, the Coulomb scattering lifetimes of electrons in two coupled
quantum wires have been studied by calculating the quasiparticle self-energy
within a multisubband model of quasi-one-dimensional (Q1D) electron system. We
consider two strongly coupled quantum wires with two occupied subbands. The
intrasubband and intersubband inelastic scattering rates are caculated for
electrons in different subbands. Contributions of the intrasubband,
intersubband plasmon excitations, as well as the quasiparticle excitations are
investigated. Our results shows that the plasmon exictations of the first
subband are the most important scattering mechanism for electrons in both
subbands.


title:
Phase Diagrams from Topological Transitions: The Hubbard Chain with
  Correlated Hopping

abstract:
  The quantum phase diagram of the Hubbard chain with correlated hopping is
accurately determined through jumps in $\pi$ in the charge and spin Berry
phases. The nature of each thermodynamic phase, and the existence of charge and
spin gaps, is confirmed by calculating correlation functions and other
fundamental quantities using numerical methods, and symmetry arguments.
Remarkably we find striking similarities between the stable phases for moderate
on-site Coulomb repulsion: spin Peierls, spin-density-wave and triplet
superconductor, and those measured in (TMTSF)$_2$X.


title:
Electronic Characteristics of Quasi-2D Metallochloronitrides: Na(x)HfNCL
  (T_c=25 K)

abstract:
  Local density functional results are presented for the electron-doped
metallochloronitrides A(x)ZrNCl and A(x)HfNCl, A = Li or Na, which superconduct
up to 25K. The alkali non-stoichiometry is treated in a virtual crystal
approximation. The electronic structure is strongly two dimensional, especially
in the conduction band region occupied by the carriers, because the states are
formed from the in-plane orbitals d_xy, d_{x^2-y^2} of the metal ion and the
p_x, p_y orbitals of the N ion. We predict a change of behavior at a doping
level of x=0.3.


title: Spectroscopic Properties and STM Images of Carbon Nanotubes

abstract:
  We present a theoretical study of the role of the local environment in the
electronic properties of carbon nanotubes: isolated single- and multi-wall
nanotubes, nanotube-ropes, tubes supported on gold and cutted to finite length.
Interaction with the substrate or with other tubes does not alter the
scanning-tunneling-microscopy (STM) patterns observed for isolated tubes.
STM-topographic images of topological defects (pentagon/heptagon pair) and
tube-caps have also been studied. In both cases the obtained image depends on
the sign of the applied voltage and it can be described in terms of the
previous catalog of STM-images (interference between electronic waves scattered
by the defect). We also have computed the electronic density of states for
isolated tubes with different chiralities and radii confirming a correlation
between the peak-structure in the DOS and the nanotube diameter, however the
metallic plateau in the DOS also depends on the nanotube chirality.
Furthermore, the conduction and valence band structures are not fully
symmetrical to one another. In contrast to STM images, the interaction with the
substrate does modify the energy levels of the nanotube. We observe opening of
small pseudogaps around the Fermi level and broadening of the sharp van Hove
singularities of the isolated single-walled-nanotubes that can be used to
extract useful information about the tube structure and bonding. The
combination of STM and spectroscopic studies opens a new technique to address
the electronic and structural properties of carbon and composite nanotubes.


title: Macroscopic Polarization from Electronic Wavefunctions

abstract:
  The dipole moment of any finite and neutral system, having a
square-integrable wavefunction, is a well defined quantity. The same quantity
is ill-defined for an extended system, whose wavefunction invariably obeys
periodic (Born-von Karman) boundary conditions. Despite this fact, macroscopic
polarization is a theoretically accessible quantity, for either uncorrelated or
correlated many-electron systems: in both cases, polarization is a rather
"exotic" observable. For an uncorrelated-either Hartree-Fock or
Kohn-Sham-crystalline solid, polarization has been expressed and computed as a
Berry phase of the Bloch orbitals (since 1993). The case of a correlated and/or
disordered system received a definitive solution only very recently (1998):
this latest development allows us present here the whole theory from a novel,
and very general, viewpoint. The modern theory of polarization is even relevant
to the foundations of density functional theory in extended systems.


title: Colossal magnetooptical conductivity in doped manganites

abstract:
  We show that the current carrier density collapse in doped manganites, which
results from bipolaron formation in the paramagnetic phase, leads to a colossal
change of the optical conductivity in an external magnetic field at
temperatures close to the ferromagnetic transition. As with the colossal
magnetoresistance (CMR) itself, the corresponding magnetooptical effect is
explained by the dissociation of localized bipolarons into mobile polarons
owing to the exchange interaction with the localized Mn spins in the
ferromagnetic phase. The effect is positive at low frequencies and negative in
the high-frequency region. The present results agree with available
experimental observations.


title: Thermodynamics of CeNiSn at low temperatures and in weak magnetic fields

abstract:
  Detailed experimental and theoretical studies of the low-temperature specific
heat, magnetic susceptibility, thermal expansion and magnetostriction of the
orthorhombic compound CeNiSn are presented. All anomalies observed in the
thermodynamic and magnetic properties of CeNiSn are explained in a framework of
a model of metallic Kondo lattice with well developed spin-liquid-type
excitations. The pseudogap behavior of these excitations appears due to
interplay between spin-fermions and soft crystal-field (CF) states. The
thermodynamic relations for the spin liquid are derived. Together with the
explanation of inelastic neutron scattering spectra given earlier within a same
approach these studies of the low-temperature thermodynamics and magnetic
response give a consistent description of the nature of anomalies in the
low-temperature thermodynamics of perfect and imperfect CeNiSn crystals.


title:
Quantum critical phenomena of long-range interacting bosons in a
  time-dependent random potential

abstract:
  We study the superfluid-insulator transition of a particle-hole symmetric
system of long-range interacting bosons in a time-dependent random potential in
two dimensions, using the momentum-shell renormalization-group method. We find
a new stable fixed point with non-zero values of the parameters representing
the short- and long-range interactions and disorder when the interaction is
asymptotically logarithmic. This is contrasted to the non-random case with a
logarithmic interaction, where the transition is argued to be first-order, and
to the $1/r$ Coulomb interaction case, where either a first-order transition or
an XY-like transition is possible depending on the parameters. We propose that
our model may be relevant in studying the vortex liquid-vortex glass transition
of interacting vortex lines in point-disordered type-II superconductors.


title: Correlations in the Bond-Future Market

abstract:
  We analyze the time series of overnight returns for the bund and btp futures
exchanged at LIFFE (London). The overnight returns of both assets are mapped
onto a one-dimensional symbolic-dynamics random walk: The `bond walk'. During
the considered period (October 1991 - January 1994) the bund-future market
opened earlier than the btp-future one. The crosscorrelations between the two
bond walks, as well as estimates of the conditional probability, show that they
are not independent; however each walk can be modeled by means of a trinomial
probability distribution. Monte Carlo simulations confirm that it is necessary
to take into account the bivariate dependence in order to properly reproduce
the statistical properties of the real-world data. Various investment
strategies have been devised to exploit the `prior' information obtained by the
aforementioned analysis.


title: Volatility in the Italian Stock Market: an Empirical Study

abstract:
  We study the volatility of the MIB30-stock-index high-frequency data from
November 28, 1994 through September 15, 1995. Our aim is to empirically
characterize the volatility random walk in the framework of continuous-time
finance. To this end, we compute the index volatility by means of the
log-return standard deviation. We choose an hourly time window in order to
investigate intraday properties of volatility. A periodic component is found
for the hourly time window, in agreement with previous observations.
Fluctuations are studied by means of detrended fluctuation analysis, and we
detect long-range correlations. Volatility values are log-stable distributed.
We discuss the implications of these results for stochastic volatility
modelling.


title:
Quantum critical point and scaling in a layered array of ultrasmall
  Josephson junctions

abstract:
  We have studied a quantum Hamiltonian that models an array of ultrasmall
Josephson junctions with short range Josephson couplings, $E_J$, and charging
energies, $E_C$, due to the small capacitance of the junctions. We derive a new
effective quantum spherical model for the array Hamiltonian. As an application
we start by approximating the capacitance matrix by its self-capacitive limit
and in the presence of an external uniform background of charges, $q_x$. In
this limit we obtain the zero-temperature superconductor-insulator phase
diagram, $E_J^{\rm crit}(E_C,q_x)$, that improves upon previous theoretical
results that used a mean field theory approximation. Next we obtain a
closed-form expression for the conductivity of a square array, and derive a
universal scaling relation valid about the zero--temperature quantum critical
point. In the latter regime the energy scale is determined by temperature and
we establish universal scaling forms for the frequency dependence of the
conductivity.


title: Sum Rule of the Hall Conductance in Random Quantum Phase Transition

abstract:
  The Hall conductance $\sigma_{xy}$ of two-dimensional {\it lattice} electrons
with random potential is investigated. The change of $\sigma_{xy}$ due to
randomness is focused on. It is a quantum phase transition where the {\it sum
rule} of $\sigma_{xy}$ plays an important role. By the {\it string} (anyon)
gauge, numerical study becomes possible in sufficiently weak magnetic field
regime which is essential to discuss the floating scenario in the continuum
model. Topological objects in the Bloch wavefunctions, charged vortices, are
obtained explicitly. The anomalous plateau transitions ($\Delta \sigma_{xy}=
2,3,... >1$) and the trajectory of delocalized states are discussed.


title: Monte Carlo algorithms based on the number of potential moves

abstract:
  We discuss Monte Carlo dynamics based on <N(sigma, Delta E)>_E, the
(microcanonical) average number of potential moves which increase the energy by
Delta E in a single spin flip. The microcanonical average can be sampled using
Monte Carlo dynamics of a single spin flip with a transition rate min(1,
<N(sigma', E-E')>_E' / <N(sigma, E'-E) >_E) from energy E to E'. A cumulative
average (over Monte Carlo steps) can be used as a first approximation to the
exact microcanonical average in the flip rate. The associated histogram is a
constant independent of the energy. The canonical distribution of energy can be
obtained from the transition matrix Monte Carlo dynamics. This second dynamics
has fast relaxation time - at the critical temperature the relaxation time is
proportional to specific heat. The dynamics are useful in connection with
reweighting methods for computing thermodynamic quantities.


title: Resonant Raman scattering of quantum wire in strong magnetic field

abstract:
  The resonant Raman scattering of a quantum wire in a strong magnetic field is
studied, focused on the effect of long range Coulomb interaction and the
spin-charge separation. The energy-momentum dispersions of charge and spin
excitation obtained from Raman cross-section show the characteristc cross-over
behaviour induced by inter-edge Coulomb interaction. The "SPE" peak near
resonance in polarized spectra becomes broad due to the momentum dependence of
charge velocity. The broad peak in the depolarized spectra is shown to
originate from the disparity between charge and spin excitation velocity.


title:
Optical conductivity in doped manganites with planar x$^2$-y$^2$ orbital
  order

abstract:
  We investigate a planar model for the ferromagnetic (FM) phase of manganites,
which develops orbital order of $e_g$ electrons with x$^2$-y$^2$-symmetry at
low temperature. The dynamic structure factor of orbital excitations and the
optical conductivity $\sigma(\omega)$ are studied with help of a
finite-temperature diagonalization method. Our calculations provide a
theoretical prediction for $\sigma(\omega)$ for the 2D FM state and are of
possible relevance for the recently found A-type phase of manganites at high
doping which consists of FM layers coupled antiferromagnetically. In the
x$^2$-y$^2$ ordered regime $\sigma(\omega)$ shows both a Drude peak and a
gapped incoherent absorption due to a gap in the orbital excitations.


title: Mechanism of CDW-SDW Transition in One Dimension

abstract:
  The phase transition between charge- and spin-density-wave (CDW, SDW) phases
is studied in the one-dimensional extended Hubbard model at half-filling. We
discuss whether the transition can be described by the Gaussian and the
spin-gap transitions under charge-spin separation, or by a direct CDW-SDW
transition. We determine these phase boundaries by level crossings of
excitation spectra which are identified according to discrete symmetries of
wave functions. We conclude that the Gaussian and the spin-gap transitions take
place separately from weak- to intermediate-coupling region. This means that
the third phase exists between the CDW and the SDW states. Our results are also
consistent with those of the strong-coupling perturbative expansion and of the
direct evaluation of order parameters.


title: Minority game with arbitrary cutoffs

abstract:
  We study a model of a competing population of N adaptive agents, with similar
capabilities, repeatedly deciding whether to attend a bar with an arbitrary
cutoff L. Decisions are based upon past outcomes. The agents are only told
whether the actual attendance is above or below L. For L-> N/2, the game
reproduces the main features of Challet and Zhang's minority game. As L is
lowered, however, the mean attendances in different runs tend to divide into
two groups. The corresponding standard deviations for these two groups are very
different. This grouping effect results from the dynamical feedback governing
the game's time-evolution, and is not reproduced if the agents are fed a random
history.


title:
Nonasymptotic Effects in Critical Sound Propagation Associated with
  Spin-Lattice Relaxation

abstract:
  The nonasymptotic critical behavior of sound attenuation coefficient has been
studied in an elastically isotropic Ising system above the critical point on
the basis of a complete stochastic model including both spin-energy and
lattice-energy modes linearly coupled to the longitudinal sound mode. The
effect of spin-lattice relaxation on the ultrasonic attenuation is
investigated. The crossover between weak-singularity behavior $t^{-2 \alpha}$
and strong singularity behavior $t^{-(z \nu +\alpha)}$ is studied. A new
high-frequency regime with singularity of the type $t^{-z \nu +\alpha}$ is
discovered in the magnetic systems. This new regime corresponds to an adiabatic
sound propagation and is very similar to the ones in binary mixture and liquid
helium. A new frequency-dependent specific-heat being the harmonic average of
the bare lattice and critical spin specific-heats is introduced. It was shown
that such specific-heat descibes the process of equilibrization between spin
and lattice subsystems and includes the most important features of critical
sound attenuation. In some regions of coupling constants the acoustic
self-energy can be very well approximated solely by this quantity.


title:
Site-diluted three dimensional Ising Model with long-range correlated
  disorder

abstract:
  We study two different versions of the site-diluted Ising model in three
dimensions with long-range spatially correlated disorder by Monte Carlo means.
We use finite-size scaling techniques to compute the critical exponents of
these systems, taking into account the strong scaling-corrections. We find a
$\nu$ value that is compatible with the analytical predictions.


title:
Circuit Effect On The Current-Voltage Characteristics Of Ultrasmall
  Tunnel Junctions

abstract:
  We have used the method of generating functional in imaginary time to derive
the current-voltage characteristics of a tunnel junction with arbitrary
tunneling conductance, connected in series with an external impedance and a
voltage source. We have shown that via the renormalized charging energy and the
renormalized environment conductance, our nonperturbative expressions of the
total action can be mapped onto the corresponding perturbative formulas. This
provides a straightforward way to go beyond the perturbation theory. For the
impedance being a pure resistance, we have calculated the conductance for
various voltages and temperatures, and the results agree very well with
experiments.


title:
Evidence for a Two-stage Melting Transition of the Vortex Matter in
  Bi2Sr2Ca1Cu2O8+d Single Crystals obtained by Muon Spin Rotation

abstract:
  From muon spin rotation measurements on under- to overdoped Bi-2212 crystals
we obtain evidence for a two-stage transition of the vortex matter as a
function of temperature. The first transition is well known and related to the
irreversibility line (IL). The second one is located below the IL and has not
been previously observed. It occurs for all three sets of crystals and is
unrelated to the vortex mobility. Our data are consistent with a two-stage
melting scenario where the intra-planar melting of the vortex lattice and the
inter-planar decoupling of the vortex lines occur independently.


title: Ground State Entropy of the Potts Antiferromagnet on Cyclic Strip Graphs

abstract:
  We present exact calculations of the zero-temperature partition function
(chromatic polynomial) and the (exponent of the) ground-state entropy $S_0$ for
the $q$-state Potts antiferromagnet on families of cyclic and twisted cyclic
(M\"obius) strip graphs composed of $p$-sided polygons. Our results suggest a
general rule concerning the maximal region in the complex $q$ plane to which
one can analytically continue from the physical interval where $S_0 > 0$. The
chromatic zeros and their accumulation set ${\cal B}$ exhibit the rather
unusual property of including support for $Re(q) < 0$ and provide further
evidence for a relevant conjecture.


title:
Interplay of spin density wave and superconductivity with different
  pairing symmetry

abstract:
  A model study for the coexistence of the spin density wave and
superconductivity is presented. With reference to the recent angle resolved
photo emmission experimental data in high T_c cuprates, presence of the nested
pieces of bands is assumed. The single band Hubbard model, therefore, when
treated within the Hatree-Fock mean field theory leads to a spin density wave
(SDW) ground state. The superconductivity (SC) is assumed to be due to a
generalised attractive potential with a separable form without specifying to
any particular origin. It therefore allows a comparative study of the
coexistence of superconductivity of different order parameter symmetry with the
spin density wave state. We find that the phase diagram, comprising of the
amplitudes of the respective gaps (SC and SDW) Vs. band filling resembles to
that of the high T_c cuprates only when the order parameter of the
superconducting phase has d-wave symmetry. Thermal variation of different order
parameters (e.g, SC and SDW) also show interesting coexistence and reentrance
behaviors that are consistent with experimental observations, specially for the
borocarbides.


title:
From the Liouville Equation to the Generalized Boltzmann Equation for
  Magnetotransport in the 2D Lorentz Model

abstract:
  We consider a system of non-interacting charged particles moving in two
dimensions among fixed hard scatterers, and acted upon by a perpendicular
magnetic field. Recollisions between charged particles and scatterers are
unavoidable in this case. We derive from the Liouville equation for this system
a generalized Boltzmann equation with infinitely long memory, but which still
is analytically solvable. This kinetic equation has been earlier written down
from intuitive arguments.


title: Novel metallic behavior in two dimensions

abstract:
  Experiments on a sufficiently disordered two-dimensional (2D) electron system
in silicon reveal a new and unexpected kind of metallic behavior, where the
conductivity decreases as \sigma (n_s,T)=\sigma (n_s,T=0)+A(n_s)T^2
(n_s-carrier density) to a non-zero value as temperature T->0. In 2D, the
existence of a metal with d\sigma/dT>0 is very surprising. In addition, a novel
type of a metal-insulator transition obtains, which is unlike any known quantum
phase transition in 2D.


title:
Giant Mutual Proximity Effects in Ferromagnetic/Superconducting
  Nanostructures

abstract:
  A strong mutual influence of superconductors (S) and ferromagnetic (F)
conductors in hybrid F/S (Ni/Al) nanostructures is observed. The magnitude of a
proximity-induced conductance on the F-side is more than two orders larger than
that predicted by theory. Re-entrance of the superconductors to the normal
state reciprocated by changes on the F-side has been found in low applied
magnetic fields with new peaks in the differential resistance as an effect of
the saturation magnetisation. An analysis has been developed providing a base
for a numerical description of the system. PACS numbers: 74.50.+r, 74.80.Fp,
85.30St


title: Metal-insulator transition in CMR materials

abstract:
  We report on resistivity measurements in La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ and
Nd$_{0.7}$Sr$_{0.3}$MnO$_{3}$ thin films in order to elucidate the underlying
mechanism for the CMR behavior. The experimental results are analyzed in terms
of quantum phase transition ideas to study the nature of the metal-insulator
transition in manganese oxides. Resistivity curves as functions of
magnetization for various temperatures show the absence of scaling behavior
expected in a continuous quantum phase transition, which leads us to conclude
that the observed metal-insulator transition is most likely a finite
temperature crossover phenomenon.


title: Theory of Interaction Effects in N-S Junctions out of Equilibrium

abstract:
  We consider a normal metal - superconductor (N-S) junction in the regime,
when electrons in the normal metal are driven out of equilibrium. We show that
the non-equilibrium fluctuations of the electron density in the N-layer cause
the fluctuations of the phase of the order parameter in the S-layer. As a
result, the density of states in the superconductor deviates from the BCS form,
most notably the density of states in the gap becomes finite. This effect can
be viewed as a result of the time reversal symmetry breaking due to the
non-equilibrium, and can be described in terms of a low energy collective mode
of the junction, which couples normal currents in N-layer and supercurrents.
This mode is analogous to the Schmid-Sch\"{o}n mode. To interpret their
measurements of the tunneling current, Pothier {\em et. al} [Phys. Rev. Lett.
{\bf 79}, 3490 (1997)] had to assume that the energy relaxation rate in the
normal metal is surprisingly high. The broadening of the BCS singularity of the
density of states in the S-layer manifest itself similarly to the broadening of
the distribution function. Mechanism suggested here can be a possible
explanation of this experimental puzzle. We also propose an independent
experiment to test our explanation.


title: How long does it take for the Kondo effect to develop?

abstract:
  The time-development of the Kondo effect is theoretically investigated by
studying a quantum dot suddenly shifted into the Kondo regime by a change of
voltage on a nearby gate. Using time-dependent versions of both the Anderson
and Kondo Hamiltonians, it is shown that after a time $t$ following the voltage
shift, the form of the Kondo resonance matches the {\it time-independent}
resonance at an effective temperature $T_{eff} = T/\tanh(\pi T t/2)$. Relevance
of the buildup of the Kondo resonance to the transport current through a
quantum dot is demonstrated.


title: Opening of the Haldane Gap in Anisotropic Two- and Four-Leg Spin Ladders

abstract:
  We study the opening of the Haldane gap in two-leg and four-leg anisotropic
spin ladders using bosonization and renormalization group methods, and we
determine the phase diagram as a function of the interchain coupling and the
relative anisotropy. It is found that the opening of the Haldane gap is
qualitatively different for the two cases considered. For the two-leg ladder
the Haldane gap opens for arbitrarily small interchain coupling, independent of
the relative anisotropy, and the Haldane phase exists in a large region of
parameter space. For the four-leg ladder the opening of the Haldane gap is
strongly dependent on both the interchain coupling as well as the relative
anisotropy, and the Haldane phase exists only in a narrow region about the
isotropic antiferromagnet.


title: Finite average lengths in critical loop models

abstract:
  A relation between the average length of loops and their free energy is
obtained for a variety of O(n)-type models on two-dimensional lattices, by
extending to finite temperatures a calculation due to Kast. We show that the
(number) averaged loop length L stays finite for all non-zero fugacities n, and
in particular it does not diverge upon entering the critical regime n -> 2+.
Fully packed loop (FPL) models with n=2 seem to obey the simple relation L = 3
L_min, where L_min is the smallest loop length allowed by the underlying
lattice. We demonstrate this analytically for the FPL model on the honeycomb
lattice and for the 4-state Potts model on the square lattice, and based on
numerical estimates obtained from a transfer matrix method we conjecture that
this is also true for the two-flavour FPL model on the square lattice. We
present in addition numerical results for the average loop length on the three
critical branches (compact, dense and dilute) of the O(n) model on the
honeycomb lattice, and discuss the limit n -> 0. Contact is made with the
predictions for the distribution of loop lengths obtained by conformal
invariance methods.


title: Spin-gap effect on resistivity in the t-J model

abstract:
  We calculate the spin-gap effect on dc resistivity in the t-J model of
high-$T_{\rm c}$ cuprates by using the Ginzburg-Landau theory coupled with a
gauge field as its effective field theory to get $\rho(T) \propto T \{1-c\:(T^*
-T)^d \}$, where $T^*$ is the spin-gap onset temperature. By taking the
compactness of massive gauge field into account, the exponent $d$ deviates from
its mean-field value 1/2 and becomes a nonuniversal $T$-dependent quantity,
which improves the correspondence with the experiments.


title: Studies of superconductivity in U(Pt1-xPdx)3 for x < 0.006

abstract:
  We report measurements of the specific heat and resistivity (T < 1 K) for
high quality polycrystals of U(Pt1-xPdx)3 with x < 0.006. The Tc-x phase
diagram can be constructed, and superconductivity is destroyed for x = 0.006;
this is approximately the same concentration above which the onset of
large-moment antiferromagnetism is observed to occur. The splitting of the
double superconducting transition increases smoothly with increasing Pd
content, and is large enough that for Pd concentrations 0.004 < x < 0.006 only
the superconducting A-phase will be present.


title: Absence of the Mott transition in infinite dimensions

abstract:
  It is shown that the half filled Hubbard model does not display the Mott
metal-insulator transition, in the limit of infinite dimensions. The proposed
transition on the Bethe lattice is proved to be an artifact. In the atomic
limit, the model is always in the antiferromagnetic phase, which can be
described by the Weiss mean field model.


title: On the thermal broadening of a quantum critical phase transition

abstract:
  The temperature dependence of an integer Quantum Hall effect transition is
studied in a sample where the disorder is dominated by short-ranged potential
scattering. At low temperatures the results are consistent with a
$(T/T_0)^{\kappa}$ scaling behaviour and at higher temperatures by a linear
dependence similar to that reported in other material systems. It is shown that
the linear behaviour results from thermal broadening produced by the
Fermi-Dirac distribution function and that the temperature dependence over the
whole range depends only on the scaling parameter T$_0^{\kappa}$.


title: Scaling critical behavior of superconductors at zero magnetic field

abstract:
  We consider the scaling behavior in the critical domain of superconductors at
zero external magnetic field. The first part of the paper is concerned with the
Ginzburg-Landau model in the zero magnetic field Meissner phase. We discuss the
scaling behavior of the superfluid density and we give an alternative proof of
Josephson's relation for a charged superfluid. This proof is obtained as a
consequence of an exact renormalization group equation for the photon mass. We
obtain Josephson's relation directly in the form $\rho_{s}\sim t^{\nu}$, that
is, we do not need to assume that the hyperscaling relation holds. Next, we
give an interpretation of a recent experiment performed in thin films of
$YBa_{2}Cu_{3}O_{7-\delta}$. We argue that the measured mean field like
behavior of the penetration depth exponent $\nu'$ is possibly associated with a
non-trivial critical behavior and we predict the exponents $\nu=1$ and
$\alpha=-1$ for the correlation lenght and specific heat, respectively. In the
second part of the paper we discuss the scaling behavior in the continuum dual
Ginzburg-Landau model. After reviewing lattice duality in the Ginzburg-Landau
model, we discuss the continuum dual version by considering a family of
scalings characterized by a parameter $\zeta$ introduced such that
$m_{h,0}^2\sim t^{\zeta}$, where $m_{h,0}$ is the bare mass of the magnetic
induction field. We discuss the difficulties in identifying the renormalized
magnetic induction mass with the photon mass. We show that the only way to have
a critical regime with $\nu'=\nu\approx 2/3$ is having $\zeta\approx 4/3$, that
is, with $m_{h,0}$ having the scaling behavior of the renormalized photon mass.


title:
Exact solution of an exclusion process with three classes of particles
  and vacancies

abstract:
  We present an exact solution for an asymmetric exclusion process on a ring
with three classes of particles and vacancies. Using a matrix product Ansatz,
we find explicit expressions for the weights of the configurations in the
stationary state. The solution involves tensor products of quadratic algebras.


title: First-Order Lagrangians and Path-Integral Quantization in the t-J Model

abstract:
  By using the supersymmetric version of the Faddeev-Jackiw symplectic
formalism, a family of first-order constrained Lagrangians for the t-J model is
found. In this approach the Hubbard ${\hat X}$-operators are used as field
variables. In this framework, we first study the spinless fermion model which
satisfies the graded algebra spl(1,1). Later on, in order to satisfy the
Hubbard ${\hat X}$-operators commutation rules satisfiying the graded algebra
spl(2,1), the number and kind of constraints that must be included in a
classical first-order Lagrangian formalism for the t-J model are found. This
model is also analyzed in the context of the path- integral formalism, and so
the correlation generating functional and the effective Lagrangian are
constructed.


title: Effective temperatures out of equilibrium

abstract:
  We describe some interesting effects observed during the evolution of
nonequilibrium systems, using domain growth and glassy systems as examples. We
breafly discuss the analytical tools that have been recently used to study the
dynamics of these systems. We mainly concentrate on one of the results obtained
from this study, the violation of the fluctuation-dissipation theorem and we
discuss, in particular, its relation to the definition and measurement of
effective temperatures out of equilibrium.


title: Superconductivuty versus Tunneling in a Doped Antiferromagnetic Ladder

abstract:
  The low-energy charge excitations of a doped antiferromagnetic ladder are
modeled by a system of interacting spinless fermions that live on the same
ladder. A relatively large spin gap is assumed to ``freeze out'' all spin
fluctuations. We find that the formation of rung hole pairs coincides with the
opening of a single-particle gap for charge excitations along chains and with
the absence of coherent tunneling in between chains. We also find that such
hole pairs condense into either a crystalline or superconducting state as a
function of the binding energy.


title:
Resonant states and order-parameter suppression near point-like
  impurities in d-wave superconductors

abstract:
  We examine the role of order-parameter suppression in the development of
low-energy peaks (i.e., resonances) in the tunneling density of states near a
non-magnetic impurity in a d-wave superconductor. Without order-parameter
suppression, the zero-energy resonance appears only in the unitary (i.e.,
strong impurity) limit. However, suppression makes the resonance appear even
when the impurity is much weaker. To model this situation, we make the physical
hypothesis that the order parameter is reduced whenever one electron of a
Cooper pair encounters the impurity, a hypothesis that retains the exact
solvability of the problem. In this way, we determine that suppression of the
order parameter drives the effective strength of the impurity towards the
unitary limit. We determine the order-parameter reduction variationally, and
show that the ratios between the main energy scales--the band width and
superconducting gap--strongly affect this reduction and, in consequence, the
position and width of the resonance.


title:
There are Asymmetric Minimizers for the One-Dimensional Ginzburg-Landau
  Model of Superconductivity

abstract:
  We study a boundary value problem associated with a system of two second
order differential equations with cubic nonlinearity which model a film of
superconductor material subjected to a tangential magnetic field. We show that
for an appropriate range of parameters there are {\it asymmetric} solutions,
and only trivial {\it symmetric} solutions. We then show that the associated
energy function is negative for the asymmetric solutions, and zero for the
trivial symmetric solution. It follows that a global minimizer of the energy is
asymmetric.


title: Infrared properties of exotic superconductors

abstract:
  The infrared spectra of the non-traditional superconductors share certain
common features. The lack of a gap signature at $2\Delta$ and the residual
conductivity are the consequence of a d-wave order parameter. The high $T_c$
materials, the organic conductors and the heavy Fermion materials have a strong
mid-infrared absorption band which can be interpreted as strong coupling of the
carriers to electronic degrees of freedom which leads to a breakdown of the
Fermi liquid picture. The cuprates and the organic charge transfer salts are
unique in possessing an intrinsic low dimensionality. The charge transport
normal to the highly conducting direction is incoherent down to the lowest
temperatures and frequencies.