title: Persistent Edge Current In the Fractional Quantum Hall Effect

abstract:
  We study the persistent edge current in the fractional quantum Hall effect.
We give the grand partition functions for edge excitations of hierarchical
states coupled to an Aharanov-Bohm flux and derive the exact formula of the
persistent edge current. For $m$-th hierarchical states with $m>1$, it exhibits
anomalous oscillations in its flux dependence at low temperatures. The current
as a function of flux goes to the sawtooth function with period $\phi_0/m$ in
the zero temperature limit. This phenomenon provides a new evidence for exotic
condensation in the fractional quantum Hall effect. We propose experiments of
measuring the persistent edge current to confirm the existence of the
hierarchy.


title:
Real-axis direct solution of the d-wave Eliashberg equations and the
  tunneling density of states in optimally doped Bi2Sr2CaCu2O{8+x}

abstract:
  In this work we calculate the direct solution of the equations for the
retarded electron-boson interaction in the case of d-wave symmetry for the pair
wave function and in the real axis formulation. We use a spectral function
containing an isotropic part and an anisotropic one:
alpha^2(Omega,phi,phi')F(Omega)=[alpha^2]_s*F(Omega)+[alpha^2]_d*F(Omega)*cos(2
phi)*cos(2phi') and make the simple assumption:
[alpha^2]_d*F(Omega)=g_d*[alpha^2]_s*F(Omega) where g_d is a constant. For
appropriate values of the isotropic electron-boson coupling constant lambda_s
and the anisotropic one lambda_d, solutions are obtained with only d-wave
symmetry for the order parameter and only s-wave one for the renormalization
function. We have employed the real axis formulation in order to compare the
theoretical curves to the tunneling density of states of the optimally-doped
high-Tc superconductor Bi2Sr2CaCu2O{8+x}. The results of our numerical
simulations are able to fit very well the value of the gap, the critical
temperature and the shape of the density of states in the whole energy range,
as recently determined in our break-junction tunneling experiments. At T > Tc
the theoretical conductance still shows a broaden peak that disappears at T ~
140 K.


title:
Scattering by impurity-induced order parameter ``holes'' in d-wave
  superconductors

abstract:
  Nonmagnetic impurities in d-wave superconductors cause strong local
suppressions of the order parameter. We investigate the observable effects of
the scatterigng off such suppressions in bulk samples by treating the order
parameter "hole" as a pointlike off-diagonal scatterer treated within a
self-consistent t-matrix approximation. Strong scattering potentials lead to a
finite-energy spectral feature in the d-wave "impurity band", the observable
effects of which include enhanced low-temperature microwave power absorption
and a stronger sensitivity of the London penetration depth to disorder than
found previously in simpler ``dirty'' d-wave models.


title: Short Range Ising Spin Glasses: a critical exponent study

abstract:
  The critical properties of short-range Ising spin-glass models, defined on a
diamond hierarchical lattice of graph fractal dimension $d_{f}=2.58$, 3, and 4,
and scaling factor 2 are studied via a method based on the Migdal-Kadanoff
renormalization-group scheme. The order parameter critical exponent $\beta $ is
directly estimated from the data of the local Edwards- Anderson (EA) order
parameter, obtained through an exact recursion procedure. The scaling of the EA
order parameter, leading to estimates of the $\nu $ exponent of the correlation
length is also performed. Four distinct initial distributions of the quenched
coupling constants (Gaussian, bimodal, uniform and exponential) are considered.
Deviations from a universal behaviour are observed and analysed in the
framework of the renormalized flow in a two dimensional appropriate parameter
space.


title: Matrix Games, Mixed Strategies, and Statistical Mechanics

abstract:
  Matrix games constitute a fundamental problem of game theory and describe a
situation of two players with completely conflicting interests. We show how
methods from statistical mechanics can be used to investigate the statistical
properties of optimal mixed strategies of large matrix games with random payoff
matrices and derive analytical expressions for the value of the game and the
distribution of strategy strengths. In particular the fraction of pure
strategies not contributing to the optimal mixed strategy of a player is
calculated. Both independently distributed as well as correlated elements of
the payoff matrix are considered and the results compared with numerical
simulations.


title: Heisenberg Spins on an Elastic Torus Section

abstract:
  Classical Heisenberg spins in the continuum limit (i.e. the nonlinear
sigma-model) are studied on an elastic torus section with homogeneous boundary
conditions. The corresponding rigid model exhibits topological soliton
configurations with geometrical frustration due to the torus eccentricity.
Assuming small and smooth deformations allows to find shapes of the elastic
support by relaxing the rigidity constraint: an inhomogeneous Lam\'e equation
arises. Finally, this leads to a novel geometric effect: a global shrinking
with swellings.


title: Effective Action Studies of Quantum Hall Spin Textures

abstract:
  We report on analytic and numerical studies of spin textures in quantum Hall
systems using a long-wavelength effective action for the magnetic degrees of
freedom derived previously. The majority of our results concern skyrmions or
solitons of this action. We have constructed approximate analytic solutions for
skyrmions of arbitrary topological and electric charge and derived expressions
for their energies and charge and spin radii. We describe a combined
shooting/relaxational technique for numerical determination of the skyrmion
profiles and present results that compare favorably with the analytic treatment
as well as with Hartree-Fock studies of these objects. In addition, we describe
a treatment of textures at the edges of quantum Hall systems within this
approach and provide details not reported previously.


title: Computer Simulations of Supercooled Liquids and Glasses

abstract:
  After a brief introduction to the dynamics of supercooled liquids, we discuss
some of the advantages and drawbacks of computer simulations of such systems.
Subsequently we present the results of computer simulations in which the
dynamics of a fragile glass former, a binary Lennard-Jones system, is compared
to the one of a strong glass former, SiO_2. This comparison gives evidence that
the reason for the different temperature dependence of these two types of glass
formers lies in the transport mechanism for the particles in the vicinity of
T_c, the critical temperature of mode-coupling theory. Whereas the one of the
fragile glass former is described very well by the ideal version of
mode-coupling theory, the one for the strong glass former is dominated by
activated processes. In the last part of the article we review some simulations
of glass formers in which the dynamics below the glass transition temperature
was investigated. We show that such simulations might help to establish a
connection between systems with self generated disorder (e.g. structural
glasses) and quenched disorder (e.g. spin glasses).


title: Nontrivial stochastic resonance temperature for the kinetic Ising model

abstract:
  The kinetic Ising model in a weak oscillating magnetic field is studied in
the context of stochastic resonance. The signal-to-noise ratio calculated with
simulations is found to peak at a nontrivial resonance temperature above the
equilibrium critical temperature T_c. We argue that its appearance is closely
related to the vanishing of the kinetic coefficient at T_c. Comparisons with
various theoretical results in one and higher dimensions are made.


title: Compact support probability distributions in random matrix theory

abstract:
  We consider a generalization of the fixed and bounded trace ensembles
introduced by Bronk and Rosenzweig up to an arbitrary polynomial potential. In
the large-N limit we prove that the two are equivalent and that their
eigenvalue distribution coincides with that of the "canonical" ensemble with
measure exp[-$n$Tr V(M)]. The mapping of the corresponding phase boundaries is
illuminated in an explicit example. In the case of a Gaussian potential we are
able to derive exact expressions for the one- and two-point correlator for
finite $n$, having finite support.


title: Origin of Hall Anomaly in the Mixed State

abstract:
  There has been a corporative absence of understanding of Hall anomaly data in
the mixed state in terms of vortex many-body effect and pinning, because of the
dominant theoretical influence. Now D'Anna et al. [ Phys. Rev. Lett. 81, 2530
(1998) (cond-mat/9808164)] are brave enough to announce the prominent role
played by vortex many-body effect and pinning in their interpretation of their
own data. Here I wish to point out: (1) Indeed the data of D'Anna et al. can be
explained within an existing Hall anomaly theory based on vortex many-body
considerations; (2) It is not surprising that their data are not consistent
with available microscopic Hall anomaly theories, because those theories are
mathematically incorrect; and (3) The courage of D'Anna et al. should be
appreciated.


title: Negative differential resistance due to single-electron switching

abstract:
  We present the multilevel fabrication and measurement of a Coulomb-blockade
device displaying tunable negative differential resistance (NDR). Applications
for devices displaying NDR include amplification, logic, and memory circuits.
Our device consists of two Al/Al$_{x}$O$_{y}$ islands that are strongly coupled
by an overlap capacitor. Our measurements agree excellently with a model based
on the orthodox theory of single-electron transport.


title: The Contribution of the Smectic-Nematic Interface to the Surface Energy

abstract:
  The contribution of the smectic-nematic interface to the surface energy of a
nematic liquid crystal sample is analyzed. By means of a simple model it is
shown that the surface energy depends on the thickness of the region over which
the transition smectic-nematic takes place. For perfectly flat substrates this
thickness is of the order of the correlation length entering in the transition.
An estimate of this contribution shows that it is greater than the one arising
from the nematic-substrate interaction. Moreover, it is also shown that the
surface energy determined in this way presents a non-monotonic behavior with
the temperature.


title: Grassmann Algebra and Fermions at Finite Temperature

abstract:
  For any d-dimensional self-interacting fermionic model, all coefficients in
the high-temperature expansion of its grand canonical partition function can be
put in terms of multivariable Grassmann integrals. A new approach to calculate
such coefficients, based on direct exploitation of the grassmannian nature of
fermionic operators, is presented. We apply the method to the soluble
Hatsugai-Kohmoto model, reobtaining well-known results.


title: Synchronization and Maximum Lyapunov Exponents of Cellular Automata

abstract:
  We study the synchronization of totalistic one dimensional cellular automata
(CA). The CA with a non zero synchronization threshold exhibit complex non
periodic space time patterns and conversely. This synchronization transition is
related to directed percolation.
  We study also the maximum Lyapunov exponent for CA, defined in analogy with
continuous dynamical systems as the exponential rate of expansion of the linear
map induced by the evolution rule of CA, constructed with the aid of the
Boolean derivatives. The synchronization threshold is strongly correlated to
the maximum Lyapunov exponent and we propose approximate relations between
these quantities. The value of this threshold can be used to parametrize the
space time complexity of CA.

categories: cond-mat.stat-mech chao-dyn comp-gas nlin.CD nlin.CG


title: Asymptotic Expansion for the Magnetoconductance Autocorrelation Function

abstract:
  We complement a recent calculation (P.B. Gossiaux and the present authors,
Ann. Phys. (N.Y.) in press) of the autocorrelation function of the conductance
versus magnetic field strength for ballistic electron transport through
microstructures with the shape of a classically chaotic billiard coupled to
ideal leads. The function depends on the total number M of channels and the
parameter t which measures the difference in magnetic field strengths. We
determine the leading terms in an asymptotic expansion for large t at fixed M,
and for large M at fixed t/M. We compare our results and the ones obtained in
the previous paper with the squared Lorentzian suggested by semiclassical
theory.


title: Cooperative Transport of Brownian Particles

abstract:
  We consider the collective motion of finite-sized, overdamped Brownian
particles (e.g., motor proteins) in a periodic potential. Simulations of our
model have revealed a number of novel cooperative transport phenomena,
including (i) the reversal of direction of the net current as the particle
density is increased and (ii) a very strong and complex dependence of the
average velocity on both the size and the average distance of the particles.


title: Theory of pairing in the Cu-O Plane: Three-Band Hubbard Model and Beyond

abstract:
  We calculate the effective interaction $W_{eff}$ between two holes added to
the ground state of the repulsive three-band Hubbard model. To make contact
with Cooper theory and with earlier Hubbard model cluster studies, we first use
a perturbative canonical transformation, to generate a two-body Hamiltonian.
Then, we extend the results to all orders. The approach is exact in principle,
and we obtain a close analytic expression including explicitly the effects of
all virtual transitions to 4-body intermediate states. Our scheme naturally
lends itself to embody off-site, inter-planar, phonon-mediated and other
interactions which are not considered in the Hubbard model but may well be
important. The result depends qualitatively on the symmetry of the two-hole
state: $^{1}B_{2}$ and $ ^{1}A_{2}$ pairs are special, because the bare holes
do not interact by the on-site repulsion (W=0 pairs). The effective interaction
in these channels is attractive and leads to a Cooper-like instability of the
Fermi liquid; however $W_{eff}$ is repulsive for Triplet pairs. Bound two-hole
states of the same nature were reported earlier in small cluster calculations
by exact diagonalisation methods; only symmetric clusters are good models of
the plane. Once $W_{eff}$ is known, the pair eigenfunction is determined by an
integral equation. We present numerical estimates of the binding energy
$|\Delta|$ of the pairs, which is in the physically interesting range of tens
of meV if unscreened on-site repulsion parameters are used.


title: Critical Currents of Josephson-Coupled Wire Arrays

abstract:
  We calculate the current-voltage characteristics and critical current
I_c^{array} of an array of Josephson-coupled superconducting wires. The array
has two layers, each consisting of a set of parallel wires, arranged at right
angles, such that an overdamped resistively-shunted junction forms wherever two
wires cross. A uniform magnetic field equal to f flux quanta per plaquette is
applied perpendicular to the layers. If f = p/q, where p and q are mutually
prime integers, I_c^{array}(f) is found to have sharp peaks when q is a small
integer. To an excellent approximation, it is found in a square array of n^2
plaquettes, that I_c^{array}(f) \propto (n/q)^{1/2} for sufficiently large n.
This result is interpreted in terms of the commensurability between the array
and the assumed q \times q unit cell of the ground state vortex lattice.


title:
Extensive Simulations for Longest Common Subsequences: Finite Size
  Scaling, a Cavity Solution, and Configuration Space properties

abstract:
  The Longest Common Subsequence (LCS) Problem asks for the longest sequence of
(non-contiguous) matches between two given strings of characters. Using
extensive Monte Carlo simulations, we find a finite size scaling law of the
form E(L)/N =C + A/(N^1/2 ln N)+... for the mean LCS length of two random
strings of size N over S letters. We provide precise estimates of C for S
between 2 and 15. We consider also a related Bernoulli Matching model where the
different entries of an N times M array are independently occupied with
probability 1/S. In that case we find the expression of the limit of L(N,M)/N
as N grows to infinity, as a function of r=M/N. This expression provides a very
good approximation for the Random String model, which gets more and more
accurate as S increases. The question of the ``universality class'' of the LCS
problem is also considered. For the Bernoulli Matching model we find very good
agreement with recent scaling predictions of Hwa and Lassig for
Needleman-Wunsch sequence alignment. We find however that the variance of the
LCS length has a different scaling different in the Random String model,
suggesting that long-ranged correlations among the matches are relevant in this
model. We finally study the ``ground state'' properties of this problem. We
find in particular that the number of solutions typically grows exponentially
with N, i.e. this system has a residual entropy at T=0. Also the overlap
between two LCSs chosen at random is found to be self averaging and to aproach
a definite value q(S)<1 as N grows.


title:
Influence of Nearest-Neighbor Coulomb Interactions on the Phase Diagram
  of the Ferromagnetic Kondo Model

abstract:
  The influence of a nearest-neighbor Coulomb repulsion of strength V on the
properties of the Ferromagnetic Kondo model is analyzed using computational
techniques. The Hamiltonian studied here is defined on a chain using localized
S=1/2 spins, and one orbital per site. Special emphasis is given to the
influence of the Coulomb repulsion on the regions of phase separation recently
discovered in this family of models, as well as on the double-exchange-induced
ferromagnetic ground state. When phase separation dominates at V=0, the Coulomb
interaction breaks the large domains of the two competing phases into small
``islands'' of one phase embedded into the other. This is in agreement with
several experimental results, as discussed in the text. Vestiges of the
original phase separation regime are found in the spin structure-factor as
incommensurate peaks, even at large values of V. In the ferromagnetic regime
close to density $n=0.5$, the Coulomb interaction induces tendencies to charge
ordering without altering the fully polarized character of the state. This
regime of ``charge- ordered ferromagnetism'' may be related with experimental
observations of a similar phase by C. H. Chen and S-W. Cheong (Phys. Rev. Lett.
76, 4042 (1996)). Our results reinforce the recently introduction notion (see
e.g., S. Yunoki et al., Phys. Rev. Lett. 80, 845 (1998)) that in realistic
models for manganites analyzed with unbiased many-body techniques, the ground
state properties arise from a competition between ferromagnetism and
phase-separation/charge-ordering tendencies.


title:
First Principles Investigation of Ferromagnetism and Ferroelectricity in
  Bismuth Manganite

abstract:
  We present results of local spin density approximation (LSDA) pseudopotential
calculations for the perovskite structure oxide, bismuth manganite (BiMnO3).
The origin of the differences between bismuth manganite and other perovskite
manganites is determined by first calculating total energies and band
structures of the high symmetry cubic phase, then sequentially lowering the
magnetic and structural symmetry. Our results indicate that covalent bonding
between bismuth cations and oxygen anions stabilizes different magnetic and
structural phases compared with the rare earth manganites. This is consistent
with recent experimental results showing enhancement of charge ordering in
doped bismuth manganite.


title: Boson Mapping and Nonlinear Response of Type-II Superconductors

abstract:
  The vortices in a high-Tc superconductor with strong correlated pinning
centers have been studied numerically using the mapping to charged bosons in
two-dimensions(2D) and the Monte-Carlo algorithm. Considering the viscous
dissipation of moving vortices we derived a nonlinear voltage response
expression which describes different regimes and their crossover uniformly.
This equation accords with experimental results.


title: Quantum vortex creep :Hall and dissipative tunneling

abstract:
  Within the framework of the path-integral approach we study the quantum
vortex creep for the situation where both the Hall and the dissipative dynamics
are simultaneously present. We calculate the relaxation rate and the crossover
temperature separating the thermal activation and the quantum tunneling
processes for anisotropic or multilayer superconductors. The results are
compared with the available experimental data.


title: The Influence of Percolation in the generalized Chalker-Coddington Model

abstract:
  We numerically investigate the influence of classical percolation on the
quantum Hall localization-delocalization transition. This is accomplished
within the framework of the generalized Chalker--Coddington network model which
allows us to control the number of {\em classical} saddle points by setting the
width $W$ of the saddle point distribution. It is found that increasing this
width causes a new microscopic length scale to appear which depends on $W$ and
scales with the exponent $X\approx 1.36$ which indicates a close connection to
the classical percolation length $\xi$ and its exponent $\nu_p=4/3$.
Furthermore, the influence of an increase in $W$ on the spectral statistics of
the quasienergies of the network model is investigated. An effect similar to
the increase of the potential correlation length in the Landau model is seen.


title: The transverse magnetoresistance of the two-dimensional chiral metal

abstract:
  We consider the two-dimensional chiral metal, which exists at the surface of
a layered, three-dimensional sample exhibiting the integer quantum Hall effect.
We calculate its magnetoresistance in response to a component of magnetic field
perpendicular to the sample surface, in the low temperature, but macroscopic,
regime where inelastic scattering may be neglected. The magnetoresistance is
positive, following a Drude form with a field scale,
$B_0=\Phi_0/al_{\text{el}}$, given by the transverse field strength at which
one quantum of flux, $\Phi_0$, passes through a rectangle with sides set by the
layer-spacing, $a$, and the elastic mean free path, $l_{\text{el}}$.
Experimental measurement of this magnetoresistance may therefore provide a
direct determination of the elastic mean free path in the chiral metal.


title: Quantum Hall Dynamics on von Neumann Lattice

abstract:
  Quantum Hall Dynamics is formulated on von Neumann lattice representation
where electrons in Landau levels are defined on lattice sites and are treated
systematically like lattice fermions. We give a proof of the integer Hall
effect, namely the Hall conductance is the winding number of the propagator in
the momentum space and is quantized exactly as integer multiple of ${e^2}/h$ in
quantum Hall regime of the system of interactions and disorders. This shows
that a determination of the fine structure constant from integer quantum Hall
effect is in fact possible. We present also a unified mean field theory of the
fractional Hall effect for the in-compressible quantum liquid states based on
flux condensation and point out that the known Hofstadter butterfly spectrum of
the tight binding model has a deep connection with the fractional Hall effect
of the continuum electrons. Thus two of the most intriguing and important
physical phenomena of recent years, the integer Hall effect and fractional Hall
effect are studied and are solved partly by von Neumann lattice representation.


title: Perturbations of Noise: The origins of Isothermal Flows

abstract:
  We make a detailed analysis of both phenomenological and analytic background
for the "Brownian recoil principle" hypothesis (Phys. Rev. A 46, (1992), 4634).
A corresponding theory of the isothermal Brownian motion of particle ensembles
(Smoluchowski diffusion process approximation), gives account of the
environmental recoil effects due to locally induced tiny heat flows. By means
of local expectation values we elevate the individually negligible phenomena to
a non-negligible (accumulated) recoil effect on the ensemble average. The main
technical input is a consequent exploitation of the Hamilton-Jacobi equation as
a natural substitute for the local momentum conservation law. Together with the
continuity equation (alternatively, Fokker-Planck), it forms a closed system of
partial differential equations which uniquely determines an associated
Markovian diffusion process. The third Newton law in the mean is utilised to
generate diffusion-type processes which are either anomalous (enhanced), or
generically non-dispersive.


title: Magnetic Bound States in Dimerized Quantum Spin Systems

abstract:
  Magnetic bound states are a general phenomenon in low dimensional
antiferromagnets with gapped singlet states. Using Raman scattering on three
compounds as dedicated examples we show how exchange topology, dimensionality,
defects and thermal fluctuations influence the properties and the spectral
weight of these states.


title:
Physical picture of the gapped excitation spectrum of the
  one-dimensional Hubbard model

abstract:
  A simple picture for the spectrum of the one-dimensional Hubbard model is
presented using a classification of the eigenstates based on an intuitive
bound-state Bethe-Ansatz approach. This approach allows us to prove a "string
hypothesis" for complex momenta and derive an exact formulation of the
Bethe-Ansatz equations including all states. Among other things we show that
all gapped eigenstates have the Bethe-Ansatz form, contrary to assertions in
the literature. The simplest excitations in the upper Hubbard band are
computed: we find an unusual dispersion close to half-filling.


title: Magnetization plateau in the spin ladder with the four-spin exchange

abstract:
  The magnetization process of the $S$=1/2 antiferromagnetic spin ladder with
the four-spin cyclic exchange interaction at T=0 is studied by the exact
diagonalization of finite clusters and size scaling analyses. It is found that
a magnetization plateau appears at half the saturation value if the ratio of
the four- and two-spin exchange coupling constants $J_4$ is larger than the
critical value $J_{4c}=0.05\pm$0.04. The phase transition with respect to $J_4$
at $J_{4c}$ is revealed to be the Kosterlitz-Thouless-type.


title:
Localization length of a soliton from a non-magnetic impurity in a
  general double-spin-chain model

abstract:
  A localization length of a free-spin soliton from a non-magnetic impurity is
deduced in a general double-spin-chain model ($J_0-J_1-J_2-J_3$ model). We have
solved a variational problem which employs the nearest-neighbor singlet-dimer
basis. The wave function of a soliton is expressed by the Airy function, and
the localization length $(\xi)$ is found to obey a power law of the
dimerization $(J_2-J_3)$ with an exponent -1/3; $\xi\sim (J_2-J_3)^{-1/3}$.
This explains why NaV_2O_5 does not show the antiferromagnetic order, while
CuGeO_3 does by impurity doping. When the gap exists by the bond-dimerization,
a soliton is localized and no order is expected. Contrary, there is a
possibility of the order when the gap is mainly due to frustration.


title: Visco-elastic spectra of a dilute Bose fluid

abstract:
  A recently developed local-density current functional formalism for confined
Bose-condensed superfluids requires visco-elastic spectra which are defined
through a finite-frequency extension of the dissipative coefficients entering
the linearized hydrodynamic equations of the two-fluid model. We evaluate these
spectra for a superfluid with contact interactions in the collisionless regime
at finite temperature, by working to first order beyond the Bogolubov
approximation. We find that within this approximation all the visco-elastic
spectra take the same value aside from simple multiplicative factors.


title:
Modular invariance, universality and crossover in the quantum Hall
  effect

abstract:
  An analytic form for the conductivity tensor in crossover between two quantum
Hall plateaux is derived, which appears to be in good agreement with existing
experimental data. The derivation relies on an assumed symmetry between quantum
Hall states, a generalisation of the law of corresponding states from rational
filling factors to complex conductivity, which has a mathematical expression in
terms of an action of the modular group on the upper-half complex conductivity
plane. This symmetry implies universality in quantum Hall crossovers. The
assumption that the $\beta$-function for the complex conductivity is a complex
analytic function, together with some experimental constraints, results in an
analytic expression for the crossover, as a function of the external magnetic
field.


title: Separation of Quasiparticle and Phononic Heat Currents in YBCO

abstract:
  Measurements of the transverse (k_{xy}) and longitudinal (k_{xx}) thermal
conductivity in high magnetic fields are used to separate the quasiparticle
thermal conductivity (k_{xx}^{el}) of the CuO_2-planes from the phononic
thermal conductivity in YBa_2Cu_3O_{7-\delta}. k_{xx}^{el} is found to display
a pronounced maximum below T_c. Our data analysis reveals distinct transport
(\tau) and Hall (\tau_H) relaxation times below T_c: Whereas \tau is strongly
enhanced, \tau_H follows the same temperature dependence as above T_c.


title: Kondo effect in AC transport through Quantum dots

abstract:
  A theory of the Kondo effect in quantum dots at zero temperature in the
presence of arbitrary intense AC potentials is presented.We generalize the
Friedel-Langreth sum rule to take care of charge conservation and propose a
consistent procedure to study a time dependent Anderson Hamiltonian. The effect
of the AC potential on both the quantum dot density of states and the linear
conductance shows the importance of using a theory which describes intradot
finite interaction and nonperturbative effects.


title:
Numerical Investigation of a Bifurcation Problem with free Boundaries
  Arising from the Physics of Josephson Junctions

abstract:
  A direct method for calculating the minimal length of ``one-dimensional''
Josephson junctions is proposed, in which the specific distribution of the
magnetic flux retains its stability. Since the length of the junctions is a
variable quantity, the corresponding nonlinear spectral problem as a problem
with free boundaries is interpreted. The obtained results give us warranty to
consider as ``long'', every Josephson junction in which there exists at least
one nontrivial stable distribution of the magnetic flux for fixed values of all
other parameters.


title: Electrodynamics of a Clean Vortex Lattice

abstract:
  We report on a microscopic evaluation of electrodynamic response for the
vortex lattice state of a model s-wave superconductor. Our calculation accounts
self-consistently for both quasiparticle and order parameter response and
establishes the collective nature of linear response in the clean limit. We
discuss the effects of homogeneous and inhomogeneous pinning on the optical
conductivity and the penetration depth, and comment on the relationship between
macroscopic and local penetration depths. We find unexpected relationships
between pinning arrangements and conductivity due to the strongly non-local
response.


title:
Scaling properties of the cluster distribution of a critical
  nonequilibrium model

abstract:
  A geometric approach to critical fluctuations of a nonequilibrium model is
reported. The two-dimensional majority vote model was investigated by Monte
Carlo simulations on square lattices of various sizes and a detailed scaling
analysis of cluster statistical and geometric properties was performed. The
cluster distribution exponents and fractal dimension were found to be the same
as those of the (two-dimensional) Ising model. This result, which cannot be
derived purely from the known bulk critical behaviour, widens our knowledge
about the range of validity of the Ising universality class for nonequilibrium
systems.


title: Disordered periodic systems at the upper critical dimension

abstract:
  The effects of weak point-like disorder on periodic systems at their upper
critical dimension D_c for disorder are studied. The systems studied range from
simple elastic systems with D_c=4 to systems with long range interactions with
D_c=2 and systems with D_c=3 such as the vortex lattice with dispersive elastic
constants. These problems are studied using the Gaussian Variational method and
the Functional Renormalisation Group. In all the cases studied we find a
typical ultra-slow loglog(x) growth of the asymptotic displacement correlation
function, resulting in nearly perfect translational order. Consequences for the
Bragg glass phase of vortex lattices are discussed.


title: Level Correlations And Persistent Currents In Mesoscopic Metals

abstract:
  We use the exact correlation function of the density of energy levels in the
magnetic field to evaluate persistent currents in mesoscopic metals. We also
analyze the perturbation theory limit of the correlation function vis-a-vis the
perturbation theory limit of the orbital response.


title: Anisotropic thermodynamics of d-wave superconductors in the vortex state

abstract:
  We show that the density of states and the thermodynamic properties of a 2D
d-wave superconductor in the vortex state with applied magnetic field $\bf H$
in the plane depend on the angle between $\bf H$ and the order parameter nodes.
Within a semiclassical treatment of the extended quasiparticle states, we
obtain fourfold oscillations of the specific heat, measurement of which
provides a simple probe of gap symmetry. The frequency dependence of the
density of states and the temperature dependence of thermodynamic properties
obey different power laws for field in the nodal and anti-nodal direction. The
fourfold pattern is changed to twofold when orthorhombicity is considered.


title:
Driven interfaces in disordered media: determination of universality
  classes from experimental data

abstract:
  While there have been important theoretical advances in understanding the
universality classes of interfaces moving in porous media, the developed tools
cannot be directly applied to experiments. Here we introduce a method that can
identify the universality class from snapshots of the interface profile. We
test the method on discrete models whose universality class is well known, and
use it to identify the universality class of interfaces obtained in experiments
on fluid flow in porous media.


title: Quantum absorption in small metal particles

abstract:
  We evaluate the electric dipole absorption in small metal particles in a
longitudinal electric field taking into account the Fermi-Thomas screening.
When either the level broadening or the frequency of the field are larger than
the mean energy-level spacing, the main contribution to absorption is
classical, with quantum corrections. When both the broadening and the frequency
are smaller than the mean level spacing, the absorption is manifestly quantum
and can be understood in terms of the two-level system.


title:
Comment on ``Electrical Transport in Junctions between Unconventional
  Superconductors: Application of the Green Function Formalism''

abstract:
  Effect of surface pair breaking, entirely neglected by M. Samanta and S.
Datta [Phys. Rev. B v. 57, 10972 (1998)], is quite important in considering
surface (or interface) quasiparticle bound states and associated
characteristics of junctions involving unconventional superconductors. The
whole class of bound states with nonzero energy is simply omitted within the
framework of the approach, using uniform spatial profile of the order parameter
up to the interface. The contribution of these bound states (as well as midgap
states) to current-voltage characteristics of the SIS' tunnel junctions were
studied for the first time in our earlier article. Dependence of midgap state
contribution to the Josephson critical current upon crystal to interface
orientations is shown as well to be fairly sensitive to the effect of surface
pair breaking.


title:
Phase transitions in anisotropic superconducting and magnetic systems
  with vector order parameters: Three-loop renormalization-group analysis

abstract:
  The critical behavior of a model with N-vector complex order parameter and
three quartic coupling constants that describes phase transitions in
unconventional superconductors, helical magnets, stacked triangular
antiferromagnets, superfluid helium-3, and zero-temperature transitions in
fully frustrated Josephson-junction arrays is studied within the field-
theoretical renormalization-group approach in three dimensions. To obtain
qualitatively and quantitatively correct results perturbative expansions for
\beta-functions and critical exponents are calculated up to three-loop order
and resummed by means of the generalized Pade-Borel procedure. Fixed-point
coordinates, critical exponent values, RG flows, etc. are found for the
physically interesting cases N = 2 and N = 3. Marginal values of N at which the
topology of the flow diagram changes are determined as well. In most cases the
systems mentioned are shown to undergo fluctuation-driven first-order phase
transitions. Continuous transitions are allowed in hexagonal d-wave
superconductors, in planar helical magnets (into sinusoidal linearly-polarized
phase), and in triangular antiferromagnets (into simple unfrustrated ordered
states) with critical exponents \gamma = 1.336, \nu = 0.677, \alpha = -0.030,
\beta = 0.347, \eta = 0.026, which are hardly believed to be experimentally
distinguishable from those of the 3D XY model. The chiral fixed point of RG
equations is found to exist and possess some domain of attraction provided N >
3. Thus, magnets with Heisenberg (N = 3) and XY-like (N = 2) spins should not
demonstrate chiral critical behavior with unusual critical exponents; they can
approach the chiral state only via first-order phase transitions.

categories: cond-mat.stat-mech cond-mat.soft cond-mat.supr-con hep-lat hep-th


title: "Superconductor-Insulator transition" in a single Josephson junction

abstract:
  VI-curves of resistively shunted single Josephson junctions with different
capacitances and tunneling resistances are found to display a crossover between
two types of VI-curves: one without and another with a resistance bump
(negative second derivative) at zero-bias. The crossover corresponds to the
dissipative phase transition (superconductor-insulator transition) at which
macroscopic quantum tunneling delocalizes the Josephson phase and destroys
superconductivity. Our measured phase diagram does not agree with the diagram
predicted by the original theory, but does coincide with a theory that takes
into account the accuracy of voltage measurements and thermal fluctuations.


title:
Elastic constant dishomogeneity and $Q^2$ dependence of the broadening
  of the dynamical structure factor in disordered systems

abstract:
  We propose an explanation for the quadratic dependence on the momentum $Q$,
of the broadening of the acoustic excitation peak recently found in the study
of the dynamic structure factor of many real and simulated glasses. We ascribe
the observed $Q^2$ law to the spatial fluctuations of the local wavelength of
the collective vibrational modes, in turn produced by the dishomegeneity of the
inter-particle elastic constants. This explanation is analitically shown to
hold for 1-dimensional disordered chains and satisfatorily numerically tested
in both 1 and 3 dimensions.


title:
Transport Properties of the One Dimensional Ferromagnetic Kondo Lattice
  Model : A Qualitative Approach to Oxide Manganites

abstract:
  The transport properties of the ferromagnetic Kondo lattice model in one
dimension are studied via bosonization methods. The antiferromagnetic
fluctuations, which normally appear because of the RKKY interactions, are
explicitly taken into account as a direct exchange between the ``core'' spins.
It is shown that in the paramagnetic regime with the local antiferromagnetic
fluctuations, the resistivity decays exponentially as the temperature increases
while in the ferromagnetic regime the system is an almost perfect conductor. %A
non-perturbative description of localized spin polarons %in the paramagnetic
region is obtained.
  The effect of a weak applied field is discussed to be reduced to the case of
the ferromagnetic state leading to band splitting. The qualitative relevance of
the results for the problem of the Oxide Manganites is emphasized.


title: Persistence exponent in a superantiferromagnetic quenching

abstract:
  We measure the persistence exponent in a phase separating two-dimensional
spin system with non-conserved dynamics quenched in a region with four
coexisting stripe phases. The system is an Ising model with nearest neighbor,
next-to-the-nearest neighbor and plaquette interactions. Due the particular
nature of the ground states, the order parameter is defined in terms of blocks
of spins. Our estimate of the persistence exponent, $\theta=0.42$, differs from
those of the two-dimensional Ising and four state Potts models. Our procedure
allows the study of persistence properties also at finite temperature $T$: our
results are compatible with the hypothesis that $\theta$ does not depend on $T$
below the critical point.


title: Fermi Surface and Band Dispersion in La2-xSrxCuO4

abstract:
  Using angle-resolved photoemission spectroscopy (ARPES), we have observed the
band structure, Fermi surface and their doping dependences in La2-xSrxCuO4. The
results reveal that the Fermi surface undergoes a dramatic change: it is
hole-like and centered at k=(pi,pi) in underdoped (x = 0.1) and optimally doped
(x = 0.15) samples as in other cuprates while it is electron-like and centered
at (0,0) in heavily overdoped (x = 0.3) ones. The peak in the ARPES spectra
near (pi/2,pi/2) is broad and weak unlike in other cuprates. In the underdoped
and optimally doped samples, a superconducting gap (Delta = 10 - 15 meV) is
observed on the Fermi surface near (pi,0).


title:
Selftrapping and Quantum Fluctuations in the Discrete Nonlinear
  Schrodinger Equation

abstract:
  We study the robustness of the selftrapping phenomenon exhibited by the
Discrete Nonlinear Schrodinger (DNLS) equation against the effects of
nonadiabaticity and quantum fluctuations in a two-site system (dimer). To test
for nonadiabatic effects (in a semiclassical context), we consider the dynamics
of an electron (or excitation) in a dimer system and coupled to the vibrational
degrees of freedom, modeled here as classical Einstein oscillators of mass M.
For relaxed (coherent state) oscillators initial condition, the DNLS
selftrapping transition persists for a wide range of M spanning 5 decades. When
undisplaced initial conditions are used, the selftrapping transition is
destroyed for masses greater than 0.02 . To test for the effects of quantum
fluctuations, we performed a first-principles numerical calculation for the
fully quantum version of the above system: the two-site Holstein model. We
computed the long-time averaged probability for finding the electron at the
initial site as a function of the asymmetry and nonlinearity parameters,
defined in terms of the electron-phonon coupling strength and the oscillator
frequency. Substantial departures from the usual DNLS system are found: A
complex landscape in asymmetry-nonlinearity phase space, which is crisscrossed
with narrow "channels", where the average electronic probability on the initial
site remains very close to 1/2, being substantially larger outside. In the
adiabatic case, there are also low-probability "pockets" where the average
electronic probability is substantially smaller than 1/2.


title: Effective field approach to the Ising film in a transverse field

abstract:
  We study the phase transitions of the spin-1/2 Ising film in a transverse
field within the framework of the effective field theory. We evaluate the
critical temperature of the film as a function of the exchange interactions,
the transverse field and the film thickness. We find that, if the ratio of the
surface exchange interactions to the bulk ones R=Js/J is smaller than a
critical value Rc, the critical temperature Tc/J of the film is smaller than
the bulk critical temperature Tc^B/J and approaches Tc^B/J as R increases
further. On the other hand, if R>Rc, Tc/J is larger than both the bulk Tc^B/J
and the surface Tc^S/J critical temperatures of the corresponding semi-infinite
system, and approaches Tc^S as R increases further.


title: Analytic Scaling Functions Applicable to Dispersion Measurements

abstract:
  Scaling functions, $F_+(\omega/\omega_c^+)$ and $F_-(\omega/\omega_c^-)$ for
$\phi >\phi_c$ and $\phi <\phi_c$, respectively, are derived from an equation
for the complex conductivity of binary conductor-insulator composites. It is
shown that the real and imaginary parts of $F_{\pm}$ display most properties
required for the percolation scaling functions. One difference is that, for
$\omega /\omega_c<1$, $\Re F_-(\omega/\omega_c)$ has an $\omega $-dependence of
$(1+t)/t $ and not $\omega ^2$ as previously predicted, but never conclusively
observed. Experimental results on a Graphite-Boron Nitride system are given
which are in reasonable agreement with the $\omega ^{(1+t)/t}$ behaviour for
$\Re F_-$. Anomalies in the real dielectric constant just above $\phi_c$ are
also discussed.


title:
Casimir amplitudes in a quantum spherical model with long-range
  interaction

abstract:
  A $d$-dimensional quantum model system confined to a general hypercubical
geometry with linear spatial size $L$ and ``temporal size'' $1/T $ ($T$ -
temperature of the system) is considered in the spherical approximation under
periodic boundary conditions. For a film geometry in different space dimensions
$\frac 12\sigma <d<\frac 32\sigma $, where $0<\sigma \leq 2$ is a parameter
controlling the decay of the long-range interaction, the free energy and the
Casimir amplitudes are given. We have proven that, if $d=\sigma$, the Casimir
amplitude of the model, characterizing the leading temperature corrections to
its ground state, is $\Delta =-16\zeta(3)/[5\sigma(4\pi)^{\sigma/2}\Gamma
(\sigma /2)]$. The last implies that the universal constant $\tilde{c}=4/5$ of
the model remains the same for both short, as well as long-range interactions,
if one takes the normalization factor for the Gaussian model to be such that
$\tilde{c}=1$. This is a generalization to the case of long-range interaction
of the well known result due to Sachdev. That constant differs from the
corresponding one characterizing the leading finite-size corrections at zero
temperature which for $d=\sigma=1$ is $\tilde c=0.606$.


title: Pd/Cu Site Interchange and Non-Fermi-Liquid Behavior in UCu_4Pd

abstract:
  X-ray-absorption fine-structure measurements of the local structure in
UCu_4Pd are described which indicate a probable lattice-disorder origin for
non-Fermi-liquid behavior in this material. Short Pd-Cu distances are observed,
consistent with 24 +/- 3% of the Pd atoms occupying nominally Cu sites. A
"Kondo disorder" model, based on the effect on the local Kondo temperature T_K
of this interchange and some additional bond-length disorder, agrees
quantitatively with previous experimental susceptibility data, and therefore
also with specific heat and magnetic resonance experiments.


title:
Numerical studies of the phase diagram of layered type II
  superconductors in a magnetic field

abstract:
  We report on simulations of layered superconductors using the
Lawrence-Doniach model in the framework of the lowest Landau level
approximation. We find a first order phase transition with a $B(T)$ dependence
which agrees very well with the experimental ``melting'' line in YBaCuO. The
transition is not associated with vortex lattice melting, but separates two
vortex liquid states characterised by different degrees of short-range
crystalline order and different length scales of correlations between vortices
in different layers. The transition line ends at a critical end-point at low
fields. We find the magnetization discontinuity and the location of the lower
critical magnetic field to be in good agreement with experiments in YBaCuO.
Length scales of order parameter correlations parallel and perpendicular to the
magnetic field increase exponentially as 1/T at low temperatures. The dominant
relaxation time scales grow roughly exponentially with these correlation
lengths. We find that the first order phase transition persists in the presence
of weak random point disorder but can be suppressed entirely by strong
disorder. No vortex glass or Bragg glass state is found in the presence of
disorder. The consistency of our numerical results with various experimental
features in YBaCuO, including the dependence on anisotropy, and the temperature
dependence of the structure factor at the Bragg peaks in neutron scattering
experiments is demonstrated.


title: Force, charge, and conductance of an ideal metallic nanowire

abstract:
  The conducting and mechanical properties of a metallic nanowire formed at the
junction between two macroscopic metallic electrodes are investigated. Both
two- and three-dimensional wires with a W(ide)-N(arrow)-W(ide) geometry are
modelled in the free-electron approximation with hard-wall boundary conditions.
Tunneling and quantum-size effects are treated exactly using the scattering
matrix formalism. Oscillations of order E_F/lambda_F in the tensile force are
found when the wire is stretched to the breaking point, which are synchronized
with quantized jumps in the conductance. The force and conductance are shown to
be essentially independent of the width of the wide sections (electrodes). The
exact results are compared with an adiabatic approximation; the later is found
to overestimate the effects of tunneling, but still gives qualitatively
reasonable results for nanowires of length L>>lambda_F, even for this abrupt
geometry. In addition to the force and conductance, the net charge of the
nanowire is calculated and the effects of screening are included within linear
response theory. Mesoscopic charge fluctuations of order e are predicted which
are strongly correlated with the mesoscopic force fluctuations. The local
density of states at the Fermi energy exhibits nontrivial behavior which is
correlated with fine structure in the force and conductance, showing the
importance of treating the whole wire as a mesoscopic system rather than
treating only the narrow part.


title: Magnetic Force Exerted by the Aharonov-Bohm Line (II)

abstract:
  The controversial question of the transverse force exerted by the
Aharonov-Bohm (AB) magnetic flux line is reconsidered with the help of a new
paraxial solution to the AB-scattering problem. It is shown that despite the
left-right symmetry in the AB scattering cross-section, a beam of a finite
width is deflected by the AB-line as if by the ``Lorentz'' force. The asymmetry
and the magnetic force originate from the quantum interference in the forward
direction within the angular size of the incident wave. In the context of the
superfluid He-4 theory, the paper confirmes the Iordanskii force acting on the
vortex line.


title:
Deformed Fermi Surface Theory of Magneto-Acoustic Anomaly in Modulated
  Quantum Hall Systems Near $/nu=1/2$

abstract:
  We introduce a new generic model of a deformed Composite Fermion-Fermi
Surface (CF-FS) for the Fractional Quantum Hall Effect near $/nu=1/2$ in the
presence of a periodic density modulation. Our model permits us to explain
recent Surface Acoustic Wave observations of anisotropic anomalies [1,2] in
sound velocity and attenuation- appearance of peaks and anisotropy - which
originate from contributions to the conductivity tensor due to regions of the
CF-FS which are flattened by the applied modulation. The calculated magnetic
field and wave vector dependence of the CF conductivity,velocity shift and
attenuation agree with experiments.


title:
Numerical renormalization group approach to fluctuation exchange in the
  presence of electron-phonon coupling: Pairing in the Holstein-Hubbard model

abstract:
  The fluctuation exchange (FLEX) approximation is applied to study the
Holstein-Hubbard model. Due to the retarded nature of the phonon-mediated
electron-electron interaction, neither fast Fourier transform (FFT) nor
previously developed NRG methods for Hubbard-type purely electronic models are
applicable, while brute force solutions are limited by the demands on
computational time and storage which increase rapidly at low temperature $T$.
Here,we describe a new numerical renormalization group (NRG) technique to solve
the FLEX equations efficiently. Several orders of magnitude of CPU time and
storage can be saved at low $T$ ($\sim 80K$). To test our approach, we compare
our NRG results to brute force calculations on small lattices at elevated
temperatures. Both s-wave and d-wave superconducting phase diagrams are then
obtained by applying the NRG approach at low $T$. The isotope effect for s-wave
pairing is BCS-like in a realistic phonon frequency range, but vanishes at
unphysically large phonon frequency ($\sim $ band width). For d-wave pairing,
the isotope exponent is negative and small compared to the typical observed
values in non-optimally doped cuprates.


title: Kondo effect in optics of quantum dots

abstract:
  Anderson impurity model for semiconductor quantum dot is extended to take
into account both particle and hole branches of charge excitations. It is shown
that in dots with even number of electrons where the Kondo effect is absent in
the ground state, novel midgap excitonic states emerge in the energy spectrum
due to Kondo-type shake-up processes. The relevance of the model to rare earth
ions adsorbed on metallic surfaces is discussed.


title:
Nonlocally-Correlated Disorder and Delocalization in One Dimension:
  Density of States

abstract:
  We study delocalization transition in a one-dimensional electron system with
quenched disorder by using supersymmetric (SUSY) methods. Especially we focus
on effects of nonlocal correlation of disorder, for most of studies given so
far considered $\delta$-function type white noise disorder. We obtain wave
function of the "lowest-energy" state which dominates partition function in the
limit of large system size. Density of states is calculated in the scaling
region. The result shows that delocalization transition is stable against
nonlocal short-ranged correlation of disorder. Especially states near the band
center are enhanced by the correlation of disorder which partially suppresses
random fluctuation of disorder. Physical picture of the localization and the
delocalization transition is discussed.


title: Phase Diagram of Traffic States in the Presence of Inhomogeneities

abstract:
  We present a phase diagram of the different kinds of congested traffic that
are triggered by disturbances when passing ramps or other spatial
inhomogeneities of a freeway. The simulation results obtained by the nonlocal,
gas-kinetic-based traffic model are in good agreement with empirical findigs.
They allow to understand the observed trasitions between free and various kinds
of congested traffic, among them localized clusters, stop-and-go waves, and
different types of ``synchronized'' traffic. The proposed conditions for the
existence of these states suggest that the phase diagram is universal for a
class of different microscopic and macroscopic traffic models.


title: Evidence for distinct polymer chain orientations in KC60 and RbC60

abstract:
  The KC60 and RbC60 polymer phases exhibit contrasting electronic properties
while powder diffraction studies have revealed no definite structural
difference. We have performed single crystal X-ray diffraction and diffuse
scattering studies of these compounds. It is found that KC60 and RbC60 possess
different chain orientations about their axes, which are described by distinct
space groups Pmnn and I2/m, respectively. Such a structural difference will be
of great importance to a complete understanding of the physical properties.


title: Sluggish Kinetics in the Parking Lot Model

abstract:
  We investigate, both analytically and by computer simulation, the kinetics of
a microscopic model of hard rods adsorbing on a linear substrate. For a small,
but finite desorption rate, the system reaches the equilibrium state very
slowly, and the long-time kinetics display three successive regimes: an
algebraic one where the density varies as $1/t$, a logarithmic one where the
density varies as $1/ln(t)$, followed by a terminal exponential approach. A
mean-field approach fails to predict the relaxation rate associated with the
latter. We show that the correct answer can only be provided by using a
systematic description based on a gap-distribution approach.


title:
Polymer Chain Orientations in KC60 and RbC60 : Structural Analysis and
  Relation with Electronic Properties

abstract:
  First single crystal X-ray diffraction studies of polymerized KC60 and RbC60
are presented. While powder diffraction studies showed no definite structural
differences, the single crystal diffraction data analysis reveals different
relative chain orientations in the two compounds. They are discussed in
relation with semi-empirical energy calculations. The structural difference
between KC60 and RbC60 may be a key feature for the understanding of their
contrasted conducting and magnetic properties.


title:
Emergence of superconducting gap features in Bi2Sr2CaCu2O8+delta single
  crystals from intrinsic c-axis interlayer tunneling

abstract:
  Static and dynamic transport measurements on stacks of intrinsic Josephson
junctions on Bi2Sr2CaCu2O8+delta single crystals, near to optimal doping, are
presented. The zero bias minimum in dynamic conductance is consistent with a
normal state pseudo-gap. At Tc, a critical current develops simultaneously with
new features consistent with a gap in the superconducting density states at an
already well-established energy. The lack of thermal broadening and the small
scale of the features force us to conclude a high degree of coherent interlayer
tunneling in both the superconducting and normal states.


title: Anomalous roughening of wood fractured surfaces

abstract:
  Scaling properties of wood fractured surfaces are obtained from samples of
three different sizes. Two different woods are studied: Norway spruce and
Maritime pine. Fracture surfaces are shown to display an anomalous dynamic
scaling of the crack roughness. This anomalous scaling behavior involves the
existence of two different and independent roughness exponents. We determine
the local roughness exponents ${\zeta}_{loc}$ to be 0.87 for spruce and 0.88
for pine. These results are consistent with the conjecture of a universal local
roughness exponent. The global roughness exponent is different for both woods,
$\zeta$ = 1.60 for spruce and $\zeta$ = 1.35 for pine. We argue that the global
roughness exponent $\zeta$ is a good index for material characterization.


title: Directed polymers on a Cayley tree with spatially correlated disorder

abstract:
  In this paper we consider directed walks on a tree with a fixed branching
ratio K at a finite temperature T. We consider the case where each site (or
link) is assigned a random energy uncorrelated in time, but correlated in the
transverse direction i.e. within the shell. In this paper we take the
transverse distance to be the hierarchical ultrametric distance, but other
possibilities are discussed. We compute the free energy for the case of
quenched disorder and show that there is a fundamental difference between the
case of short range spatial correlations of the disorder which behaves
similarly to the non-correlated case considered previously by Derrida and Spohn
and the case of long range correlations which has a totally different overlap
distribution which approaches a single delta function about q=1 for large L,
where L is the length of the walk. In the latter case the free energy is not
extensive in L for the intermediate and also relevant range of L values,
although in the true thermodynamic limit extensivity is restored. We identify a
crossover temperature which grows with L, and whenever T<T_c(L) the system is
always in the low temperature phase. Thus in the case of long-ranged
correlation as opposed to the short-ranged case a phase transition is absent.


title:
Hydrodynamic Limit of Brownian Particles Interacting with Short and Long
  Range Forces

abstract:
  We investigate the time evolution of a model system of interacting particles,
moving in a $d$-dimensional torus. The microscopic dynamics are first order in
time with velocities set equal to the negative gradient of a potential energy
term $\Psi$ plus independent Brownian motions: $\Psi$ is the sum of pair
potentials, $V(r)+\gamma^d J(\gamma r)$, the second term has the form of a Kac
potential with inverse range $\gamma$. Using diffusive hydrodynamical scaling
(spatial scale $\gamma^{-1}$, temporal scale $\gamma^{-2}$) we obtain, in the
limit $\gamma\downarrow 0$, a diffusive type integro-differential equation
describing the time evolution of the macroscopic density profile.


title:
Dynamical properties of the two-dimensional Holstein-Hubbard model in
  the normal state at zero temperature: A fluctuation-based effective cumulant
  approach

abstract:
  The two-dimensional many-body Holstein-Hubbard model in the T=0 normal state
is examined within the framework of the self-consistent coupling of charge
fluctuation correlations to the vibrational ones. The parameters of our model
are the adiabaticity, the electron concentration, as well as the
electron-phonon and the Coulomb interaction strengths. A fluctuation-based
effective cumulant approach is introduced to examine the T=0 normal-state
fluctuations and an analytic approximation to the true dynamical entangled
ground state is suggested. Our results for the effective charge-transfer
amplitude, the ground state energy, the fluctuations in the phonon population,
the phonon softening as well as the coupling constant renormalizations suggest
that, the recent numerical calculations of de Mello and Ranninger (Ref.5),
Berger, Valasek and von der Linden (Ref.2) and Marsiglio (Ref. 4 and 8) on
systems with finite degrees of freedom can be qualitatively extended to the
systems with large degrees of freedom.


title:
Quasiparticle relaxation dynamics in superconductors with different gap
  structures: theory and experiments on YBa_{2}Cu_{3}O_{7-\delta}

abstract:
  Photoexcited quasiparticle relaxation dynamics are investigated in a
$YBa_{2}Cu_{3}O_{7-\delta}$ superconductor as a function of doping $\delta$ and
temperature $T$ using ultrafast time-resolved optical spectroscopy. A model
calculation is presented which describes the temperature dependence of the
photoinduced quasiparticle population $n_{pe}$, photoinduced transmission
$\Delta{\cal T}/{\cal T}$ and relaxation time $\tau $ for three different
superconducting gaps: (i) a temperature-dependent collective gap such that
${\bf \Delta}(T)\to 0$ as $T\to T_{c}$, (ii) a temperature-independent gap,
which might arise for the case of a superconductor with pre-formed pairs and
(iii) an anisotropic (e.g. d-wave) gap with nodes. Comparison of the theory
with data of photoinduced transmission $| \Delta{\cal T}/{\cal T} |$,
reflection $| Delta{\cal R}/{\cal R}|$ and quasiparticle recombination time
$\tau$ in $YBa_{2}Cu_{3}O_{7-\delta}$ over a very wide range of doping ($0.1 <
\delta <0.48)$ is found to give good quantitative agreement with a
temperature-dependent BCS-like isotropic gap near optimum doping ($\delta
<0.1)$ and a temperature-independent isotropic gap in underdoped
$YBa_{2}Cu_{3}O_{7-\delta}$ ($0.15<\delta <0.48$). A pure d-wave gap was found
to be inconsistent with the data.


title: The $n\to\infty$ limit of O(n) model on graphs

abstract:
  Thirty years ago, Stanley showed that an O(n) spin model on a lattice tends
to a spherical model as $n\to\infty$. This means that at any temperature the
corresponding free energies coincide. This fundamental result, providing the
basis for more detailed studies of continuous symmetry spin models, is no
longer valid on more general discrete structures lacking of translation
invariance, i.e. on graphs. However only the singular parts of the free
energies determine the critical behavior of the two statistical models. Here we
show that such singular parts still coincide even on general graphs in the
thermodynamic limit. This implies that the critical exponents of O(n) models on
graphs for $n\to\infty$ tend to the spherical ones and therefore they only
depend on the graph spectral dimension.


title:
Molecular-Dynamics Simulation of a Glassy Polymer Melt: Incoherent
  Scattering Function

abstract:
  We present simulation results for a model polymer melt, consisting of short,
nonentangled chains, in the supercooled state. The analysis focuses on the
monomer dynamics, which is monitored by the incoherent intermediate scattering
function. The scattering function is recorded over six decades in time and for
many different wave-vectors. The lowest temperatures studied are slightly above
the critical temperature of mode-coupling theory (MCT), which was determined
from a quantitative analysis of the beta- and alpha-relaxations. We find
evidence for the space-time factorization theorem in the beta-relaxation
regime, and for the time-temperature superposition principle in the
alpha-regime, if the temperature is not too close to the critical temperature.
The wave-vector dependence of the nonergodicity parameter, of the critical
amplitude, and the alpha-relaxation time are in qualitative agreement with
calculations for hard spheres. For wave-vectors larger than the maximum of the
structure factor the alpha-relaxation time already agrees fairly well with the
asymptotic MCT-prediction. The behavior of the relaxation time at small
wave-vectors can be rationalized by the validity of the Gaussian approximation
and the value of the Kohlrausch stretching exponent.


title: Axial Particle Diffusion in Rotating Cylinders

abstract:
  We study the interface dynamics of a binary particle mixture in a rotating
cylinder numerically. By considering only the particle motion in axial
direction, it is shown that the initial dynamics can be well described by a
one-dimensional diffusion process. This allows us to calculate a macroscopic
diffusion constant and we study its dependence on the inter-particle friction
coefficient, the rotation speed of the cylinder and the density ratio of the
two components. It is found that radial segregation reduces the drift velocity
of the interface. We then perform a microscopic calculation of the diffusion
coefficient and investigate its dependence on the position along the cylinder
axis and the density ratio of the two particle components. The latter
dependence can be explained by looking at the different hydrostatic pressures
of the two particle components at the interface. We find that the
microscopically calculated diffusion coefficient agrees well with the value
from the macroscopic definition when taken in the middle of the cylinder.


title: Superinsulator Phase of Two-Dimensional Superconductors

abstract:
  Using path-integral Quantum Monte Carlo we study the low-temperature phase
diagram of a two-dimensional superconductor within a phenomenological model,
where vortices have a finite mass and move in a dissipative environment modeled
by a Caldeira-Leggett term. The quantum vortex liquid at high magnetic fields
exhibits superfluidity and thus corresponds to a {\em superinsulating} phase
which is characterized by a nonlinear voltage-current law for an infinite
system in the absence of pinning. This superinsulating phase is shifted to
higher magnetic fields in the presence of dissipation.


title: Immunization and Aging: a Learning Process in the Immune Network

abstract:
  The immune system can be thought as a complex network of different
interacting elements. A cellular automaton, defined in shape-space, was
recently shown to exhibit self-regulation and complex behavior and is,
therefore, a good candidate to model the immune system. Using this model to
simulate a real immune system we find good agreement with recent experiments on
mice. The model exhibits the experimentally observed refractory behavior of the
immune system under multiple antigen presentations as well as loss of its
plasticity caused by aging.


title: High-field noise in degenerate and mesoscopic systems

abstract:
  We analyse high-field current fluctuations in metallic systems by direct
mapping of the Fermi-liquid correlations to the semiclassical nonequilibrium
state. We give three applications. First, for bulk conductors, we show that
there is a unique nonequilibrium analogue to the fluctuation-dissipation
theorem for thermal noise. With it, we calculate suppression of the excess
hot-electron term by Pauli exclusion. Second, in the degenerate regime, we
argue that shot noise and thermal noise are incommensurate. They cannot be
connected by a smooth, universal interpolation formula. This follows from their
contrasting responses to Coulomb screening. We propose an experiment to test
this mismatch. Third, we carry out an exact model calculation of high-field
shot noise in narrow mesoscopic wires. We show that a distinctive mode of
suppression arises from the structure of the semiclassical Boltzmann equation
in two and three dimensions. In one dimension such a mode does not exist.


title: Spectral Properties of Three Dimensional Layered Quantum Hall Systems

abstract:
  We investigate the spectral statistics of a network model for a three
dimensional layered quantum Hall system numerically. The scaling of the
quantity $J_0={1/2}< s^2>$ is used to determine the critical exponent $\nu$ for
several interlayer coupling strengths. Furthermore, we determine the level
spacing distribution $P(s)$ as well as the spectral compressibility $\chi$ at
criticality. We show that the tail of $P(s)$ decays as $\exp(-\kappa s)$ with
$\kappa=1/(2\chi)$ and also numerically verify the equation
$\chi=(d-D_2)/(2d)$, where $D_2$ is the correlation dimension and $d=3$ the
spatial dimension.


title: Absence of anisotropic universal transport in YBCO

abstract:
  There exists significant in-plane anisotropy between $a$ and $b$ axis for
various properties in YBCO. However recent thermal conductivity measurement by
Chiao et al. which confirms previous microwave conductivity measurement by
Zhang et al., shows no obvious anisotropy in the context of universal
transport. We give a possible explanation of why the anisotropy is seen in most
properties but not seen in the universal transport.


title: Effective Potential and Quantum Dynamical Correlators

abstract:
  The approach to the calculation of quantum dynamical correlation functions is
presented in the framework of the Mori theory. An unified treatment of classic
and quantum dynamics is given in terms of Weyl representation of operators and
holomorphic variables. The range of validity of an approximate molucular
dynamics is discussed


title:
Effect of long range forces on the interfacial profiles in thin binary
  polymer films

abstract:
  We study the effect of surface fields on the interfacial properties of a
binary polymer melt confined between two parallel walls. Each wall attracts a
different component of the blend by a non-retarded van der Waals potential. An
interface which runs parallel to the surfaces is stabilized in the center of
the film. Using extensive Monte Carlo simulations we study the interfacial
properties as a function of the film thickness, the strength of the surface
forces and the lateral size over which the profiles across the film are
averaged. We find evidence for capillary wave broadening of the apparent
interfacial profiles. However, the apparent interfacial width cannot be
described quantitatively by a simple logarithmic dependence on the film
thickness. The Monte Carlo simulations reveal that the surface fields give rise
to an additional reduction of the intrinsic interfacial width and an increase
of the effective interfacial tension upon decreasing the film thickness. These
modifications of the intrinsic interfacial properties are confirmed by
self-consistent field calculations. Taking account of the thickness dependence
of the intrinsic interfacial properties and the capillary wave broadening, we
can describe our simulation results quantitatively.


title:
Study of the critical properties of the Quantum Hall Fluid in the
  framework of a dual statistical model

abstract:
  By using Renormalization Group methods we analyze the description of the
Quantum Hall Fluid in terms of a dual plasma with dyons as effective degrees of
freedom. The physical interpretation of the parameters of the model as the
longitudinal and Hall conductance togheter with their scaling properties allow
for the determination of the critical index. The universality of the critical
properties result as a consequence of the generalized duality symmetry of the
model.


title:
Experimental Evidence for Coulomb Charging Effects in an Open Quantum
  Dot at Zero Magnetic Field

abstract:
  We have measured the low-temperature transport properties of an open quantum
dot formed in a clean one-dimensional channel. For the first time, at zero
magnetic field, continuous and periodic oscillations superimposed upon
ballistic conductance steps are observed when the conductance through the dot
$G$ exceeds $2e^2/h$. We ascribe the observed conductance oscillations to
evidence for Coulomb charging effects in an open dot. This is supported by the
evolution of the oscillating features for $G>2e^2/h$ as a function of both
temperature and barrier transparency. Our results strongly suggest that at zero
magnetic field, current theoretical and experimental understanding of Coulomb
charging effects overlooks charging in the presence of fully transmitted 1D
channels.


title: Dynamical response functions in models of vibrated granular media

abstract:
  In recently introduced schematic lattice gas models for vibrated dry granular
media, we study the dynamical response of the system to small perturbations of
shaking amplitudes and its relations with the characteristic fluctuations.
Strong off equilibrium features appear and a generalized version of the
fluctuation dissipation theorem is introduced. The relations with thermal
glassy systems and the role of Edwards' compactivity are discussed.


title:
Theory of the Spin Reorientation Transition of Ultra-Thin Ferromagnetic
  Films

abstract:
  The reorientation transition of the magnetization of ferromagnetic films is
studied on a microscopic basis within Heisenberg spin models. Analytic
expressions for the temperature dependent anisotropy are derived from which it
is seen that the reduced magnetization in the film surface at finite
temperatures plays a crucial role for this transition. Detailed phase diagrams
in the temperature-thickness plane are calculated.


title: Dynamics of polymeric manifolds in melts: Hartree approximation

abstract:
  The Martin-Siggia-Rose functional technique and the self-consistent Hartree
approximation is applied to the dynamics of a D-dimensional manifold in a melt
of similar manifolds.The generalized Rouse equation is derived and its static
and dynamic properties are studied. The static upper critical dimension
discriminate between Gaussian and non-Gaussian regimes, whereas its dynamic
counterpart discriminates between Rouse- and renormalized-Rouse behavior. The
dynamic exponents are calculated explicitly. The special case of linear chains
shows agreement with MD- and MC-simulations.


title: Excitation spectrum of the two-dimensional attractive Hubbard model

abstract:
  We calculate the one-particle spectral functions above the superconducting
transition temperature $T_{C}$, in the framework of a functional integral
approach. The coupling of the electronic self-energy to pair fluctuations,
which are treated by means of a time-dependent Ginzburg-Landau equation, yields
a double-peak structure, around the Fermi wavenumber. The peak separation is
essentially temperature-independent, but the structure sharpens when $T_{C}$ is
approached.


title: The Hartree approximation in dynamics of polymeric manifolds in the melt

abstract:
  The Martin-Siggia-Rose (MSR) functional integral technique is applied to the
dynamics of a D - dimensional manifold in a melt of similar manifolds. The
integration over the collective variables of the melt can be simply implemented
in the framework of the dynamical random phase approximation (RPA). The
resulting effective action functional of the test manifold is treated by making
use of the selfconsistent Hartree approximation. As an outcome the generalized
Rouse equation (GRE) of the test manifold is derived and its static and dynamic
properties are studied. It was found that the static upper critical dimension,
$d_{\rm uc}=2D/(2-D)$, discriminates between Gaussian (or screened) and
non-Gaussian regimes, whereas its dynamical counterpart, ${\tilde
d}_{uc}=2d_{\rm uc}$, distinguishes between the simple Rouse and the
renormalized Rouse behavior. We have argued that the Rouse mode correlation
function has a stretched exponential form. The subdiffusional exponents for
this regime are calculated explicitly. The special case of linear chains, D=1,
shows good agreement with MD- and MC-simulations.


title: Critical adsorption on curved objects

abstract:
  A systematic fieldtheoretic description of critical adsorption on curved
objects such as spherical or rodlike colloidal particles immersed in a fluid
near criticality is presented. The temperature dependence of the corresponding
order parameter profiles and of the excess adsorption are calculated
explicitly. Critical adsorption on elongated rods is substantially more
pronounced than on spherical particles. It turns out that, within the context
of critical phenomena in confined geometries, critical adsorption on a
microscopically thin `needle' represents a distinct universality class of its
own. Under favorable conditions the results are relevant for the flocculation
of colloidal particles.


title:
Subgap resonances and conduction channels in mesoscopic superconducting
  devices

abstract:
  Peaks associated to multiple Andreev reflections have been used to identify
the number of conduction channels, and their transmitivity in mesoscopic
superconducting junctions. We analyze the influence of the detailed shape of
each channel, and the number of weak links within it, on the final results.
Connexions with the statistical interpretation of conductance histograms is
also made.


title: Filling a silo with a mixture of grains: Friction-induced segregation

abstract:
  We study the filling process of a two-dimensional silo with inelastic
particles by simulation of a granular media lattice gas (GMLG) model. We
calculate the surface shape and flow profiles for a monodisperse system and we
introduce a novel generalization of the GMLG model for a binary mixture of
particles of different friction properties where, for the first time, we
measure the segregation process on the surface. The results are in good
agreement with a recent theory, and we explain the observed small deviations by
the nonuniform velocity profile.


title:
Early time behavior of the order parameter coupled to a conserved
  density: A study in a semi-infinite geometry

abstract:
  We study the short time behavior of the order parameter coupled to a
conserved field in semi-infinite geometry. The short time exponent, obtained by
solving the one loop differential equations for the conserved density and the
order parameter, agrees with the prediction from a scaling argument based on
short distance expansion. The scaling analysis further shows that this exponent
satisfies a scaling relation similar to that known in the case of a
nonconserved order parameter without any coupling.


title: Surface-induced layer formation in polyelectrolytes

abstract:
  We analyze, by means of an RPA calculation, the conditions under which a
mixture of oppositely charged polyelectrolytes can micro-segregate in the
neighborhood of a charged surface creating a layered structure. A number of
stable layers can be formed if the surface is sufficiently strongly charged
even at temperatures at which the bulk of the mixture is homogeneous.


title: Bose-Einstein condensation of interacting gases

abstract:
  We study the occurrence of a Bose-Einstein transition in a dilute gas with
repulsive interactions, starting from temperatures above the transition
temperature. The formalism, based on the use of Ursell operators, allows us to
evaluate the one-particle density operator with more flexibility than in
mean-field theories, since it does not necessarily coincide with that of an
ideal gas with adjustable parameters (chemical potential, etc.). In a first
step, a simple approximation is used (Ursell-Dyson approximation), which allow
us to recover results which are similar to those of the usual mean-field
theories. In a second step, a more precise treatment of the correlations and
velocity dependence of the populations in the system is elaborated. This
introduces new physical effects, such as a marked change of the velocity
profile just above the transition: low velocities are more populated than in an
ideal gas. A consequence of this distortion is an increase of the critical
temperature (at constant density) of the Bose gas, in agreement with those of
recent path integral Monte-Carlo calculations for hard spheres.


title:
Phase diagram and validity of one-parameter scaling near the
  two-dimensional metal-insulator transition

abstract:
  We explore the scaling description for a two-dimensional metal-insulator
transition (MIT) of electrons in silicon. Near the MIT, $\beta_{T}/p =
(-1/p)d(\ln g)/d(\ln T)$ is universal (with $p$, a sample dependent exponent,
determined separately; $g$--conductance, $T$--temperature). We obtain the
characteristic temperatures $T_0$ and $T_1$ demarking respectively the quantum
critical region and the regime of validity of single parameter scaling in the
metallic phase, and show that $T_1$ vanishes as the transition is approached.
For $T<T_1$, the scaling of the data requires a second parameter. Moreover, all
of the data can be described with two-parameter scaling at all densities --
even far from the transition.


title: Spiral surface growth without desorption

abstract:
  Spiral surface growth is well understood in the limit where the step motion
is controlled by the local supersaturation of adatoms near the spiral ridge. In
epitaxial thin-film growth, however, spirals can form in a step-flow regime
where desorption of adatoms is negligible and the ridge dynamics is governed by
the non-local diffusion field of adatoms on the whole surface. We investigate
this limit numerically using a phase-field formulation of the
Burton-Cabrera-Frank model, as well as analytically. Quantitative predictions,
which differ strikingly from those of the local limit, are made for the
selected step spacing as a function of the deposition flux, as well as for the
dependence of the relaxation time to steady-state growth on the screw
dislocation density.


title: Polarization and Strong Infra-Red Activity in Compressed Solid Hydrogen

abstract:
  Under a pressure of ~150 GPa solid molecular hydrogen undergoes a phase
transition accompanied by a dramatic rise in infra-red absorption in the vibron
frequency range. We use the Berry's phase approach to calculate the electric
polarization in several candidate structures finding large, anisotropic dynamic
charges and strongly IR-active vibron modes. The polarization is shown to be
greatly affected by the overlap between the molecules in the crystal, so that
the commonly used Clausius-Mossotti description in terms of polarizable,
non-overlapping molecular charge densities is inadequate already at low
pressures and even more so for the compressed solid.


title: The approach to thermal equilibrium in quantized chaotic systems

abstract:
  We consider many-body quantum systems that exhibit quantum chaos, in the
sense that the observables of interest act on energy eigenstates like banded
random matrices. We study the time-dependent expectation values of these
observables, assuming that the system is in a definite (but arbitrary) pure
quantum state. We induce a probability distribution for the expectation values
by treating the zero of time as a uniformly distributed random variable. We
show explicitly that if an observable has a nonequilibrium expectation value at
some particular moment, then it is overwhelmingly likely to move towards
equilibrium, both forwards and backwards in time. For deviations from
equilibrium that are not much larger than a typical quantum or thermal
fluctuation, we find that the time dependence of the move towards equilibrium
is given by the Kubo correlation function, in agreement with Onsager's
postulate. These results are independent of the details of the system's quantum
state.


title: Effective Mass and g-Factor of Four Flux Quanta Composite Fermions

abstract:
  We investigate the properties of composite fermions with four attached flux
quanta through tilted field experiments near Landau level filling factor nu =
3/4. The observed collapse of fractional quantum Hall gaps in the vicinity of
this quarter-filling state can be comprehensively understood in terms of
composite fermions with mass and spin. Remarkably, the effective mass and
g-factor of these four flux quanta composite fermions around nu = 3/4 are very
similar to those of two flux quanta composite fermions around nu = 3/2.


title: Mean-Field Description of Phase String Effect in the $t-J$ Model

abstract:
  A mean-field treatment of the phase string effect in the $t-J$ model is
presented. Such a theory is able to unite the antiferromagnetic (AF) phase at
half-filling and metallic phase at finite doping within a single theoretical
framework. We find that the low-temperature occurrence of the AF long range
ordering (AFLRO) at half-filling and superconducting condensation in metallic
phase are all due to Bose condensations of spinons and holons, respectively, on
the top of a spin background described by bosonic resonating-valence-bond (RVB)
pairing. The fact that both spinon and holon here are bosonic objects, as the
result of the phase string effect, represents a crucial difference from the
conventional slave-boson and slave-fermion approaches. This theory also allows
an underdoped metallic regime where the Bose condensation of spinons can still
exist. Even though the AFLRO is gone here, such a regime corresponds to a
microscopic charge inhomogeneity with short-ranged spin ordering. We discuss
some characteristic experimental consequences for those different metallic
regimes. A perspective on broader issues based on the phase string theory is
also discussed.


title: Magnetic Incommensurability in Doped Mott Insulator

abstract:
  In this paper we explore the incommensurate spatial modulation of spin-spin
correlations as the intrinsic property of the doped Mott insulator, described
by the $t-J$ model. We show that such an incommensurability is a direct
manifestation of the phase string effect introduced by doped holes in both one-
and two-dimensional cases. The magnetic incommensurate peaks of dynamic spin
susceptibility in momentum space are in agreement with the neutron-scattering
measurement of cuprate superconductors in both position and doping dependence.
In particular, this incommensurate structure can naturally reconcile the
neutron-scattering and NMR experiments of cuprates.


title: Hidden spin-current conservation in 2d Fermi liquids

abstract:
  We report the existence of regimes of the two dimensional Fermi liquid that
show unusual conservation of the spin current and may be tuned by varying some
parameter like the density of fermions. We show that for reasonable models of
the effective interaction the spin current may be conserved in general in 2d,
not only for a particular regime. Low temperature spin waves propagate
distinctively in these regimes and entirely new ``spin-acoustic'' modes are
predicted for scattering-dominated temperature ranges. These new
high-temperature propagating spin waves provide a clear signature for the
experimental search of such regimes.


title: Spectra of large random matrices: A method of study

abstract:
  A formalism for study of spectral correlations in non-Gaussian, unitary
invariant ensembles of large random matrices with strong level confinement is
reviewed. It is based on the Shohat method in the theory of orthogonal
polynomials. The approach presented is equally suitable for description of both
local and global spectral characteristics, thereby providing an overall look at
the phenomenon of spectral universality in Random Matrix Theory.

categories: cond-mat.stat-mech chao-dyn cond-mat.dis-nn hep-th nlin.CD nlin.SI solv-int


title: Economic returns of research: the Pareto law and its implications

abstract:
  At what level should government or companies support research? This complex
multi-faceted question encompasses such qualitative bonus as satisfying natural
human curiosity, the quest for knowledge and the impact on education and
culture, but one of its most scrutinized component reduces to the assessment of
economic performance and wealth creation derived from research. In certain
areas such as biotechnology, semi-conductor physics, optical communications,
the impact of basic research is direct while, in other disciplines, the path
from discovery to applications is full of surprises. As a consequence, there
are persistent uncertainties in the quantification of the exact economic
returns of public expenditure on basic research. Here, we suggest that these
uncertainties have a fundamental origin to be found in the interplay between
the intrinsic ``fat tail'' power law nature of the distribution of economic
returns, characterized by a mathematically diverging variance, and the
stochastic character of discovery rates. In the regime where the cumulative
economic wealth derived from research is expected to exhibit a long-term
positive trend, we show that strong fluctuations blur out significantly the
short-time scales: a few major unpredictable innovations may provide a finite
fraction of the total creation of wealth. In such a scenario, any attempt to
assess the economic impact of research over a finite time horizon encompassing
only a small number of major discoveries is bound to be highly unreliable. New
tools, developed in the theory of self-similar and complex systems to tackle
similar extreme fluctuations in Nature can be adapted to measure the economic
benefits of research, which is intimately associated to this large variability.


title:
Reply to Comment on "Localization and Metal-Insulator Transition in
  Multilayer Quantum Hall Structures"

abstract:
  This is a Reply to a Comment by Tanaka and Machida. We provide some details
of the derivation of the effective field theory for integer quantum Hall
transitions using the non-Abelian chiral anomaly.


title:
Bruekner approach to the spin-wave gap critical index for the two-layer
  Heisenberg antiferromagnet

abstract:
  We consider the two-layer Heisenberg antiferromagnet near a zero temperature
quantum phase transition from a disordered dimer phase to a collinear Neel
state. At approaching the transition point the spin-wave gap vanishes as
$\Delta \propto (J_\perp-J_{\perp c})^{\nu}$. To account for strong
correlations between the S=1 elementary excitations we apply the Brueckner
diagram approach which gives the critical index $\nu\approx 0.5$. We
demonstrate also that the linearized in density Brueckner equations give the
mean field result $\nu=1$. Finally an expansion of the Brueckner equations in
powers of the density, combined with the scaling hypothesis, give $\nu\approx
0.67$. This value reasonably agrees with that of the nonlinear O(3)
$\sigma$-model. Our approach demonstrates that for other quantum spin models
the critical index can be different from that in the nonlinear $\sigma$-model.
We discuss the conditions for this to occur.


title:
Scaling the localisation lengths for two interacting particles in
  one-dimensional random potentials

abstract:
  Using a numerical decimation method, we compute the localisation length
$\lambda_{2}$ for two onsite interacting particles (TIP) in a one-dimensional
random potential. We show that an interaction $U>0$ does lead to $\lambda_2(U)
> \lambda_2(0)$ for not too large $U$ and test the validity of various proposed
fit functions for $\lambda_2(U)$. Finite-size scaling allows us to obtain
infinite sample size estimates $\xi_{2}(U)$ and we find that $ \xi_{2}(U) \sim
\xi_2(0)^{\alpha(U)} $ with $\alpha(U)$ varying between $\alpha(0)\approx 1$
and $\alpha(1) \approx 1.5$. We observe that all $\xi_2(U)$ data can be made to
coalesce onto a single scaling curve. We also present results for the problem
of TIP in two different random potentials corresponding to interacting
electron-hole pairs.


title: Application of random matrix theory to quasiperiodic systems

abstract:
  We study statistical properties of energy spectra of a tight-binding model on
the two-dimensional quasiperiodic Ammann-Beenker tiling. Taking into account
the symmetries of finite approximants, we find that the underlying universal
level-spacing distribution is given by the Gaussian orthogonal random matrix
ensemble, and thus differs from the critical level-spacing distribution
observed at the metal-insulator transition in the three-dimensional Anderson
model of disorder. Our data allow us to see the difference to the Wigner
surmise.


title:
Oscillation Frequencies for a Bose Condensate in a Triaxial Magnetic
  Trap

abstract:
  We investigate the dynamics of a Bose condensate, interacting with either
repulsive or attractive forces, confined in a fully anisotropic harmonic
potential. The (3+1)-dimensional Gross-Pitaevskii equation is integrated
numerically to derive the collective excitation frequencies, showing a good
agreement with those calculated with a variational technique.


title:
ESR Spectrometry of High-Temperature Superconductors with
  Temperature-Modulation-Based Thermoregulation

abstract:
  A light-beam-assisted temperature-control system operating within a
temperature range from 77 to 180 K is developed for investigating the
absorption of electromagnetic waves by high-temperature superconductors in the
vicinity of superconducting transition with an ESR spectrometer. The advantage
of this system is the feasibility of modulating the temperature of a sample
with a frequency of 80 Hz and amplitude of 0.01 -0.1 K. The rms temperature
instability over a 5-min time interval is within 0.06 K, the temperature
gradient in a sample is 0.01 K/mm for T=90 K, and the system relaxation time is
1 - 10 s. The microwave absorption peaks in the vicinity of the superconducting
transition of an Y-Ba-Cu-O, single crystal were first measured at (the
simultaneous application of magnetic and temperature modulations.


title:
Evolution of the nu = 1 Ground-State in Coupled Double Quantum Wells:
  Optical Evidence for Broken-Symmetry States

abstract:
  We present the first magneto-absorption studies of coupled electron double
layers in the quantum Hall regime. Optical absorption spectra in the vicinity
of total filling factor nu = 1 reveal intriguing behavior that have no analog
in the single electron layer nu = 1 state and demonstrate the interplay between
single-particle tunneling and inter-layer Coulomb effects. The spectra provide
direct evidence of a ground-state that evolves from a region dominated by
single-particle tunneling to a regime in which inter-layer Coulomb interactions
determine the nature of the ground-state. Moreover the spectra provide the
first direct evidence that the incompressible ground-state at nu = 1 in the
many-body regime is not fully pseudospin polarized and is sensitive to the
effects of quantum fluctuations in the pseudospin variable.


title: Effective Hamiltonian, Mori Product and Quantum Dynamics

abstract:
  An appropriate extension of the effective potential theory is presented that
permits the approximate calculation of the dynamical correlation functions for
quantum systems. These are obtained by evaluating the generating functionals of
the Mori products of quantities related to the relaxation functions in the
(PQSCHA) pure self consistent harmonic approximation.


title: Equilibrium Pure States and Nonequilibrium Chaos

abstract:
  We consider nonequilibrium systems such as the Edwards-Anderson Ising spin
glass at a temperature where, in equilibrium, there are presumed to be (two or
many) broken symmetry pure states. Following a deep quench, we argue that as
time goes to infinity, although the system is usually in some pure state
locally, either it never settles permanently on a fixed lengthscale into a
single pure state, or it does but then the pure state depends on both the
initial spin configuration and the realization of the stochastic dynamics. But
this latter case can occur only if there exists an uncountable number of pure
states (for each coupling realization) with almost every pair having zero
overlap. In both cases, almost no initial spin configuration is in the basin of
attraction of a single pure state; that is, the configuration space (resulting
from a deep quench) is all boundary (except for a set of measure zero). We
prove that the former case holds for deeply quenched two-dimensional
ferromagnets. Our results raise the possibility that even if more than one pure
state exists for an infinite system, time averages don't necessarily disagree
with Boltzmann averages.

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