title: Linear theory of unstable growth on rough surfaces

abstract:
  Unstable homoepitaxy on rough substrates is treated within a linear continuum
theory. The time dependence of the surface width $W(t)$ is governed by three
length scales: The characteristic scale $l_0$ of the substrate roughness, the
terrace size $l_D$ and the Ehrlich-Schwoebel length $l_{ES}$. If $l_{ES} \ll
l_D$ (weak step edge barriers) and $l_0 \ll l_m \sim l_D \sqrt{l_D/l_{ES}}$,
then $W(t)$ displays a minimum at a coverage $\theta_{\rm min} \sim
(l_D/l_{ES})^2$, where the initial surface width is reduced by a factor
$l_0/l_m$. The r\^{o}le of deposition and diffusion noise is analyzed. The
results are applied to recent experiments on the growth of InAs buffer layers
[M.F. Gyure {\em et al.}, Phys. Rev. Lett. {\bf 81}, 4931 (1998)]. The overall
features of the observed roughness evolution are captured by the linear theory,
but the detailed time dependence shows distinct deviations which suggest a
significant influence of nonlinearities.


title: Electronic structure and resistivity of the double exchange model

abstract:
  The double exchange (DE) model with quantum local spins S is studied; an
equation of motion approach is used and decoupling approximations analogous to
Hubbard's are made. Our approximate one-electron Green function G is exact in
the atomic limit of zero bandwidth for all S and band filling n, and as n->0
reduces to a dynamical coherent potential approximation (CPA) due to Kubo; we
regard our approximation as a many-body generalisation of Kubo's CPA. G is
calculated self-consistently for general S in the paramagnetic state and for
S=1/2 in a state of arbitrary magnetization. The electronic structure is
investigated and four bands per spin are obtained centred on the atomic limit
peaks of the spectral function. A resistivity formula appropriate to the model
is derived from the Kubo formula and the paramagnetic state resistivity rho is
calculated; insulating states are correctly obtained at n=0 and n=1 for strong
Hund coupling. Our prediction for rho is much too small to be consistent with
experiments on manganites so we agree with Millis et al that the bare DE model
is inadequate. We show that the agreement with experiment obtained by Furukawa
is due to his use of an unphysical density of states.


title: Collectivity in the Brain Sensory Response

abstract:
  A question of cooperative effects in auditory brain processing on various
space- and time-scales is addressed. The experimental part of our study is
based on Multichannel Magnetoencephalography recordings in normal human
subjects. Left, right and binaural stimulations were used, in separate runs,
for each subject. The resulting time-series representing left and right
auditory cortex activity provide a clear evidence for two levels of neuronal
cooperation. One is the local hemispheric collective response, termed M100 for
its maximum at around 100ms after a stimulus onset. Its only global
characteristics turn out to be time-locked to a stimulus, however, which means
that the detailed neuronal evolution is largely stochastic. This, together with
the $1/f$ character of the corresponding power spectrum indicates
self-organized criticality as an underlying mechanism. The second level is
communication between the two hemispheres with clear laterality effects: as a
rule, the contralateral hemisphere leads by $\sim$10ms. Mutual information
analysis points to a direct information transport as a way of this
communication.


title: Hall potentiometer in the ballistic regime

abstract:
  We demonstrate theoretically how a two-dimensional electron gas can be used
to probe local potential profiles using the Hall effect. For small magnetic
fields, the Hall resistance is inversely proportional to the average potential
profile in the Hall cross and is independent of the shape and the position of
this profile in the junction. The bend resistance, on the other hand, is much
more sensitive on the exact details of the local potential profile in the cross
junction.


title: Stokes parameters for light scattering from a Faraday-active sphere

abstract:
  We present an exact calculation for the scattering of light from a single
sphere made of Faraday-active material, to first order in the external magnetic
field. We use a recent expression for the T-matrix of a Mie scatterer in a
magnetic field to compute the Stokes parameters in single scattering that
describe completely flux and polarization of the scattered light.


title: Quantum Computers and Quantum Coherence

abstract:
  If the states of spins in solids can be created, manipulated, and measured at
the single-quantum level, an entirely new form of information processing,
quantum computing, will be possible. We first give an overview of quantum
information processing, showing that the famous Shor speedup of integer
factoring is just one of a host of important applications for qubits, including
cryptography, counterfeit protection, channel capacity enhancement, distributed
computing, and others. We review our proposed spin-quantum dot architecture for
a quantum computer, and we indicate a variety of first generation materials,
optical, and electrical measurements which should be considered. We analyze the
efficiency of a two-dot device as a transmitter of quantum information via the
ballistic propagation of carriers in a Fermi sea.


title:
Freezing in polyampholyte globules: Influence of the long-range nature
  of the interaction

abstract:
  In random heteropolymer globules with short-range interactions between the
monomers, freezing takes place at the microscopic length scale only, and can be
described by a 1-step replica symmetry breaking. The fact that the long-range
Coulomb interaction has no intrinsic length scale suggests that freezing in
random polyampholyte globules might take place at all length scales,
corresponding to an overlap parameter q(x) that increases continuously from
zero to its maximum value. Study of the polyampholyte globule within the
independent interaction approximation seems to confirm this scenario. However,
the independent interaction model has an important deficiency: it cannot
account for self-screening, and we show that the model is only reliable at
length scales shorter than the self-screening length. Using the more realistic
sequence model we prove that in the general case of a random heteropolymer
globule containing two types of monomers such that unlike monomers attract each
other, freezing at arbitrarily large length scales is not possible. For
polyampholyte globules this implies that beyond the self-screening length, the
freezing behavior is qualitatively the same as in the case of short-range
interactions. We find that if the polyampholyte globule is not maximally
compact, the degree of frustration is insufficient to obtain freezing.


title: Optics of a Faraday-active Mie Sphere

abstract:
  We present an exact calculation for the scattering of light from a single
sphere made of a Faraday-active material, into first order of the external
magnetic field. When the size of the sphere is small compared to the wavelength
the known T-matrix for a magneto-active Rayleigh scatterer is found. We address
the issue whether or not there is a so called \em Photonic Hall Effect - \em a
magneto-transverse anisotropy in light scattering - for one Mie scatterer. In
the limit of geometrical optics, we compare our results to the Faraday effect
in a Fabry-Perot etalon.


title: Multiple Andreev reflections in diffusive SNS structures

abstract:
  We report new measurements on sup-gap energy structure originating from
multiple Andreev reflections in mesoscopic SNS junctions. The junctions were
fabricated in a planar geometry with high transparency superconducting contacts
of Al deposited on highly diffusive and surface d-doped n++-GaAs. For samples
with a normal GaAs region of active length 0.3um the Josephson effect with a
maximal supercurrent Ic=3mA at T=237mK was observed. The sub-gap structure was
observed as a series of local minima in the differential resistance at dc bias
voltages V=2D/ne with n=1,2,4 i.e. only the even sub-gap positions. While at
V=2D/e (n=1) only one dip is observed, the n=2, and the n=4 sub-gap structures
each consists of two separate dips in the differential resistance. The mutual
spacing of these two dips is independent of temperature, and the mutual spacing
of the n=4 dips is half of the spacing of the n=2 dips. The voltage bias
positions of the sub-gap differential resistance minima coincide with the
maxima in the oscillation amplitude when a magnetic field is applied in an
interferometer configuration, where one of the superconducting electrodes has
been replaced by a flux sensitive open loop.


title:
Spin dynamics in the generalized ferromagnetic Kondo model for
  manganites

abstract:
  Dynamical spin susceptibility is calculated for the generalized ferromagnetic
Kondo model which describes itinerant $e_{g}$ electrons interacting with
localized $t_{2g}$ electrons with antiferromagnetic coupling. The calculations
done in the mean field approximation show that the spin-wave spectrum of the
system in ferromagnetic state has two branches, acoustic and optic ones.
Self-energy corrections to the spectrum are considered and the acoustic
spin-wave damping is evaluated.


title: A Potts/Ising Correspondence on Thin Graphs

abstract:
  We note that it is possible to construct a bond vertex model that displays
q-state Potts criticality on an ensemble of phi3 random graphs of arbitrary
topology, which we denote as ``thin'' random graphs in contrast to the fat
graphs of the planar diagram expansion.
  Since the four vertex model in question also serves to describe the critical
behaviour of the Ising model in field, the formulation reveals an isomorphism
between the Potts and Ising models on thin random graphs. On planar graphs a
similar correspondence is present only for q=1, the value associated with
percolation.


title:
The modified group expansions for construction of solutions to the BBGKY
  hierarchy

abstract:
  A solution to the BBGKY hierarchy for nonequilibrium distribution functions
is obtained within modified boundary conditions. The boundary conditions take
into account explicitly both the nonequilibrium one-particle distribution
function as well as local conservation laws. As a result, modified group
expansions are proposed. On the basis of these expansions, a generalized
kinetic equation for hard spheres and a generalized Bogolubov-Lenard-Balescu
kinetic equation for a dense electron gas are derived within the polarization
approximation.


title:
Transport in multilevel quantum dots: from the Kondo effect to the
  Coulomb blockade regime

abstract:
  A new theoretical method is introduced to study coherent electron transport
in an interacting multilevel quantum dot. The method yields the correct
behavior both in the limit of weak and strong coupling to the leads, giving a
unified description of Coulomb blockade and the Kondo effect. Results for the
density of states and the temperature dependent conductance for a two- level
dot are presented. The relevance of these results in connection to recent
experiments on the Kondo effect in semiconducting quantum dots is discussed.


title:
Calculation of The Band Gap Energy and Study of Cross Luminescence in
  Alkaline-Earth Dihalide Crystals

abstract:
  The band gap energy as well as the possibility of cross luminescence
processes in alkaline-earth dihalide crystals have been calculated using the ab
initio Perturbed-Ion (PI) model. The gap is calculated in several ways: as a
difference between one-electron energy eigenvalues and as a difference between
total energies of appropriate electronic states of the crystal, both at the HF
level and with inclusion of Coulomb correlation effects. In order to study the
possibility of ocurrence of cross luminescence in these materials, the energy
difference between the valence band and the upmost core band for some
representative crystals has been calculated. Both calculated band gap energies
and cross luminescence predictions compare very well with the available
experimental results.


title:
The mutual information of a stochastic binary channel: validity of the
  Replica Symmetry Ansatz

abstract:
  We calculate the mutual information (MI) of a two-layered neural network with
noiseless, continuous inputs and binary, stochastic outputs under several
assumptions on the synaptic efficiencies. The interesting regime corresponds to
the limit where the number of both input and output units is large but their
ratio is kept fixed at a value $\alpha$. We first present a solution for the MI
using the replica technique with a replica symmetric (RS) ansatz. Then we find
an exact solution for this quantity valid in a neighborhood of $\alpha = 0$. An
analysis of this solution shows that the system must have a phase transition at
some finite value of $\alpha$. This transition shows a singularity in the third
derivative of the MI. As the RS solution turns out to be infinitely
differentiable, it could be regarded as a smooth approximation to the MI. This
is checked numerically in the validity domain of the exact solution.


title: Fluctuation Effects in Steric Reaction-Diffusion Systems

abstract:
  We propose a simple model for reaction-diffusion systems with orientational
constraints on the reactivity of particles, and map it onto a field theory with
upper critical dimension d_c=2. To two-loop level the long-time particle
density N(t) is given by the same universal expression as for a nonsteric
system, with N(t)~t^{-d/2} for d<=2. For slow rotations of the particles we
find an intermediate regime where N(t)~t^{-d/4}, with the crossover to the
nonsteric asymptotics determined by the rates of rotations and reactions.
Consequences for experiments are discussed.


title:
Transition between two ferromagnetic states driven by orbital ordering
  in La_{0.88}Sr_{0.12}MnO_3

abstract:
  A lightly doped perovskite mangantite La_{0.88}Sr_{0.12}MnO_3 exhibits a
phase transition at T_{OO}=145 K from a ferromagnetic metal (T_C=172 K) to a
novel ferromagnetic insulator.We identify that the key parameter in the
transition is the orbital degree of freedom in e_g electrons. By utilizing the
resonant x-ray scattering technique, orbital ordering is directly detected
below T_{OO}, in spite of a significant diminution of the cooperative
Jahn-Teller distortion. The new experimental features are well described by a
theory treating the orbital degree of freedom under strong electron
correlation. The present experimental and theoretical studies uncover a crucial
role of the orbital degree in the metal-insulator transition in lightly doped
manganites.


title:
Simulations of interference effects in gated two-dimensional ballistic
  electron systems

abstract:
  We present detailed simulations addressing recent electronic interference
experiments, where a metallic gate is used to locally modify the Fermi
wave-length of the charge carriers. Our numerical calculations are based on a
solution of the one-particle Schroedinger equation for a realistic model of the
actual sample geometry, including a Poisson equation based determination of the
potential due to the gate. The conductance is determined with the multiprobe
Landauer-Buettiker formula, and in general we find conductance vs. gate voltage
characteristics which closely resemble the experimental traces. A detailed
examination based on quantum mechanical streamlines suggests that the simple
one-dimensional semiclassical model often used to describe the experiments has
only a limited range of validity, and that certain 'unexpected' periodicities
should not be assigned any particular significance, they arise due to the
complicated multiple scattering processes occurring in certain sample
geometries.


title: Observation of a Fifth of the Electron Charge

abstract:
  We report the observation of quasi-particles with a charge q=e/5 detected by
shot noise measurements in the 2/5 conducting channel (filling factor 2/5).
This is in agreement with previous measurements that showed that the current in
the lower, 1/3, channel is carried by quasiparticles with a charge q=e/3. These
results demonstrate that the actual fraction of the charge can be different
from the filling factor. Moreover, we show that there is no strong interaction
between the channels, which can be considered as conducting the current
independently.


title:
A Remark on Integrability of Stochastic Systems Solvable by Matrix
  Product Ansatz

abstract:
  Within the Matrix Product Formalism we have already introduced a multi-
species exclusion process in which different particles hop with different rates
and fast particles stochastically overtake slow ones. In this letter we show
that on an open chain, the master equation of this process can be exactly
solved via the coordinate Bethe ansatz. It is shown that the N-body S-matrix of
this process is factorized into a product of two-body S-matrices, which in turn
satisfy the quantum Yang-Baxter equation (QYBE). This solution is to our
knowledge, a new solution of QYBE.


title:
Comment on ``Adsorption of Polyelectrolyte onto a Colloid of Opposite
  Charge''

abstract:
  In a recent Letter, Gurovitch and Sens studied the adsorption of a weakly
charged polyelectrolyte chain onto an oppositely charged colloidal particle. By
using a variational technique they found that the colloidal particle can adsorb
a polymer of higher charge than its own, and thus be ``overcharged.'' I argue
that the observed overcharging by a factor of 16/5 is indeed an artifact of the
approximations involved in the study. Moreover, I show that the existence of
overcharging depends crucially on the choice of the trial wave function,
contrary to their claim.


title: Step-Bunching Transitions on Vicinal Surfaces and Quantum n-mers

abstract:
  We study vicinal crystal surfaces within the terrace-step-kink model on a
discrete lattice. Including both a short-ranged attractive interaction and a
long-ranged repulsive interaction arising from elastic forces, we discover a
series of phases in which steps coalesce into bunches of $n_b$ steps each. The
value of $n_b$ varies with temperature and the ratio of short to long range
interaction strengths. For bunches with large number of steps, we show that, at
T=0, our bunch phases correspond to the well known periodic groove structure
first predicted by Marchenko. An extension to $T>0$ is developed. We propose
that the bunch phases have been observed in very recent experiments on Si
surfaces, and further experiments are suggested. Within the context of a
mapping of the model to a system of bosons on a 1D lattice, the bunch phases
appear as quantum n-mers.


title: Strong coupling in the Kondo problem in the low-temperature region

abstract:
  The magnetic field dependence of the average spin of a localized electron
coupled to conduction electrons with an antiferromagnetic exchange interaction
is found for the ground state. In the magnetic field range $\mu H\sim 0.5 T_c$
($T_c$ is the Kondo temperature) there is an inflection point, and in the
strong magnetic field range $\mu H\gg T_c$, the correction to the average spin
is proportional to $(T_c/\mu H)^2$. In zero magnetic field, the interaction
with conduction electrons also leads to the splitting of doubly degenerate spin
impurity states.


title:
Numerical Renormalization Approach to Two-Dimensional Quantum
  Antiferromagnets with Valence-Bond-Solid Type Ground State

abstract:
  We study the ground-state properties of the two-dimensional quantum spin
systems having the valence-bond-solid (VBS) type ground states. The
``product-of-tensors'' form of the ground-state wavefunction of the system is
utilized to associate it with an equivalent classical lattice statistical model
which can be treated by the transfer-matrix method. For diagonalization of the
transfer matrix, we employ the product-wavefunction renormalization group
method which is a variant of the density-matrix renormalization group method.
We obtain the correlation length and the sublattice magnetization accurately.
For the anisotropically ``deformed'' S=3/2 VBS model on the honeycomb lattice,
we find that the correlation length as a function of the deformation parameter
behaves very much alike as that in the S=3/2 VBS chain.


title: Electronic and dynamic properties of a one-dimensional Thue-Morse chain

abstract:
  We study the electronic and dynamic properties of a one-dimensional
Thue-Morse chain within the framework of the transfer model. By means of direct
diagonalization of the Hamiltonian matrix we first show that the trace map of
the transfer matrices is exactly the same as that in the diagonal model. Then,
by calculating several quantities such as the wave function, the Lyapunov
exponent, and the Landauer resistivity, we show that all the electronic states
are extended despite the singular continuity of the energy spectrum. Our
results indicate that the electronic properties of the Thue-Morse chain is
independent of the kind of the model, which is contrary to the result of
Chakrabarti et al. [Phys. Rev. Lett. 74, 1403 (1995)]. To deepen our
understanding, we study the dynamics of an electronic wave packet and show that
the wave packet spreads superdiffusively for long times with dynamic indices
intermediate between periodic and quasiperiodic chains. Several features of
dynamics distinctive from the case of the Fibonacci chain are discussed. We
also study the effects of electron-phonon interaction on the dynamics of the
wave packet by taking into account a kind of nonlinear interaction. Degree of
dynamic localization is shown to be crucially dependent on the strength of the
hopping energy and the nonlinear interaction parameter.


title: Effects of the field modulation on the Hofstadter's spectrum

abstract:
  We study the effect of spatially modulated magnetic fields on the energy
spectrum of a two-dimensional (2D) Bloch electron. Taking into account four
kinds of modulated fields and using the method of direct diagonalization of the
Hamiltonian matrix, we calculate energy spectra with varying system parameters
(i.e., the kind of the modulation, the relative strength of the modulated field
to the uniform background field, and the period of the modulation) to elucidate
that the energy band structure sensitively depends on such parameters:
Inclusion of spatially modulated fields into a uniform field leads occurrence
of gap opening, gap closing, band crossing, and band broadening, resulting
distinctive energy band structure from the Hofstadter's spectrum. We also
discuss the effect of the field modulation on the symmetries appeared in the
Hofstadter's spectrum in detail.


title: Global Phase Diagram of a One-Dimensional Driven Lattice Gas

abstract:
  We investigate the non-equilibrium stationary state of a translationally
invariant one-dimensional driven lattice gas with short-range interactions. The
phase diagram is found to exhibit a line of continuous transitions from a
disordered phase to a phase with spontaneous symmetry breaking. At the phase
transition the correlation length is infinite and density correlations decay
algebraically. Depending on the parameters which define the dynamics, the
transition either belongs to the universality class of directed percolation or
to a universality class of a growth model which preserves the local minimal
height. Consequences of some mappings to other models, including a
parity-conserving branching-annihilation process are briefly discussed.


title:
Coherent Potential Approximation for `d - wave' Superconductivity in
  Disordered Systems

abstract:
  A Coherent Potential Approximation is developed for s-wave and d-wave
superconductivity in disordered systems. We show that the CPA formalism
reproduces the standard pair-breaking formula, the self-consistent Born
Approximation and the self-consistent T-matrix approximation in the appropriate
limits. We implement the theory and compute T_c for s-wave and d-wave pairing
using an attractive nearest neighbor Hubbard model featuring both binary alloy
disorder and a uniform distribution of scattering site potentials. We determine
the density of states and examine its consequences for low temperature heat
capacity. We find that our results are in qualitative agreement with
measurements on Zn doped YBCO superconductors.


title: Component separation in harmonically trapped boson-fermion mixtures

abstract:
  We present a numerical study of mixed boson-fermion systems at zero
temperature in isotropic and anisotropic harmonic traps. We investigate the
phenomenon of component separation as function of the strength of the
inter-particle interaction. While solving a Gross-Pitaevskii mean field
equation for the boson distribution in the trap, we utilize two different
methods to extract the density profile of the fermion component; a
semiclassical Thomas-Fermi approximation and a quantum mechanical Slater
determinant Schr\"{o}dinger equation.


title:
Controlled Wavefunction Mixing in a Strongly Coupled One-Dimensional
  Wire

abstract:
  We investigate the transport properties of two strongly coupled ballistic
one-dimensional (1D) wires, defined by a split-gate structure deposited on a
GaAs/AlGaAs double quantum well. Matching the widths and electron densities of
the two wires brings them into resonance, forming symmetric and antisymmetric
1D subbands separated by energy gaps that are measured to be larger than their
two-dimensional counterpart. Applying a magnetic field parallel to the wire
axes enhances wavefunction mixing at low fields, whereas at high fields the
wires become completely decoupled.


title:
The Boson Peak in Amorphous Silica: Results from Molecular Dynamics
  Computer Simulations

abstract:
  We investigate a prominent vibrational feature in amorphous silica, the
so-called boson peak, by means of molecular dynamics computer simulations. The
dynamic structure factor S(q,nu) in the liquid, as well as in the glass state,
scales roughly with temperature, in agreement with the harmonic approximation.
By varying the size of the system and the masses of silicon and oxygen we show
that the excitations giving rise to the boson peak are due to the coupling to
transverse acoustic modes.


title:
Rotational quantum friction in superfluids: Radiation from object
  rotating in superfluid vacuum

abstract:
  We discuss the friction experienced by the body rotating in superfluid liquid
at T=0. The effect is analogous to the amplification of electromagnetic
radiation and spontaneous emission by the body or black hole rotating in
quantum vacuum, first discussed by Zel'dovich and Starobinsky. The friction is
caused by the interaction of the part of the liquid, which is rigidly connected
with the rotating body and thus represents the comoving detector, with the
"Minkowski" vacuum outside the body. The emission process is the quantum
tunneling of quasiparticles from the detector to the ergoregion, where the
energy of quasiparticles is negative in the rotating frame. This quantum
rotational friction caused by the emission of quasiparticles is estimated for
phonons and rotons in superfluid 4He and for Bogoliubov fermions in superfluid
3He.


title:
Dynamics of Holes and Universality Class of the Antiferromagnetic
  Transition in the Two Dimensional Hubbard Model

abstract:
  The dynamics of a single hole (or electron) in the two dimensional Hubbard
model is investigated. The antiferromagnetic background is described by a
N\`eel state, and the hopping of the carrier is analyzed within a configuration
interaction approach. Results are in agreement with other methods and with
experimental data when available. All data are compatible with the opening of a
mean field gap in a Fermi liquid of spin polarons, the so called Slater type of
transition. In particular, this hypothesis explains the unusual dispersion
relation of the quasiparticle bands near the transition. Recent photoemission
data for Ca$_2$CuO$_2$Cl$_2$ are analyzed within this context.


title:
Distribution of the area enclosed by a 2D random walk in a disordered
  medium

abstract:
  The asymptotic probability distribution for a Brownian particle wandering in
a 2D plane with random traps to enclose the algebraic area A by time t is
calculated using the instanton technique.


title: Thermal noise and the branching threshold in brittle fracture

abstract:
  Many studies have confirmed that cracks in brittle materials branch when the
crack speed exceeds a certain threshold velocity, but the value of that
threshold is not understood. Almost all theoretical calculations overestimate
the threshold by factors of two or more. We show that thermal noise can reduce
the threshold by a substantial amount, and we propose that this effect can
account for the discrepancy.


title: Kondo effect in a quantum critical ferromagnet

abstract:
  We study the Heisenberg ferromagnetic spin chain coupled with a boundary
impurity. Via Bethe ansatz solution, it is found that (i) for J>0, the impurity
spin behaves as a diamagnetic center and is completely screened by 2S bulk
spins in the ground state, no matter how large the impurity spin is; (ii) The
specific heat of the local composite (impurity plus 2S bulk spins which form
bound state with it) shows a simple power law $C_{loc}\sim T^{3/2}$. (iii)For
J<0, the impurity is locked into the critical behavior of the bulk. Possible
phenomena in higher dimensions are discussed.


title: Exact solution of a spin-ladder model

abstract:
  An integrable spin-ladder model with nearest-neighbor exchanges and
biquadratic interactions is proposed. With the Bethe ansatz solutions of the
model hamiltonian, it is found that there are three possible phases in the
ground state, i.e., a rung-dimerized phase with a spin gap, and two massless
phases. The possible fixed points of the system and the quantum critical
behavior at the critical point $J=J_+^c$ are discussed.


title:
Current self-oscillations, spikes and crossover between charge monopole
  and dipole waves in semiconductor superlattices

abstract:
  Self-sustained current oscillations in weakly-coupled superlattices are
studied by means of a self-consistent microscopic model of sequential tunneling
including boundary conditions naturally. Well-to-well hopping and recycling of
charge monopole domain walls produce current spikes (high frequency modulation)
superimposed on the oscillation. For highly doped injecting contacts, the
self-oscillations are due to dynamics of monopoles. As the contact doping
decreases, a lower-frequency oscillatory mode due to recycling and motion of
charge dipoles is predicted. For low contact doping, this mode dominates and
monopole oscillations disappear. At intermediate doping, both oscillation modes
coexist as stable solutions and hysteresis between them is possible.


title: Spin relaxation of conduction electrons

abstract:
  Prospect of building electronic devices in which electron spins store and
transport information has revived interest in the spin relaxation of conduction
electrons. Since spin-polarized currents cannot flow indefinitely, basic
spin-electronic devices must be smaller than the distance electrons diffuse
without losing its spin memory. Some recent experimental and theoretical effort
has been devoted to the issue of modulating the spin relaxation. It has been
shown, for example, that in certain materials doping, alloying, or changing
dimensionality can reduce or enhance the spin relaxation by several orders of
magnitude. This brief review presents these efforts in the perspective of the
current understanding of the spin relaxation of conduction electrons in
nonmagnetic semiconductors and metals.


title: Avalanches in Breakdown and Fracture Processes

abstract:
  We investigate the breakdown of disordered networks under the action of an
increasing external---mechanical or electrical---force. We perform a mean-field
analysis and estimate scaling exponents for the approach to the instability. By
simulating two-dimensional models of electric breakdown and fracture we observe
that the breakdown is preceded by avalanche events. The avalanches can be
described by scaling laws, and the estimated values of the exponents are
consistent with those found in mean-field theory. The breakdown point is
characterized by a discontinuity in the macroscopic properties of the material,
such as conductivity or elasticity, indicative of a first order transition. The
scaling laws suggest an analogy with the behavior expected in spinodal
nucleation.


title:
Theory of excitations of the condensate and non-condensate at finite
  temperatures

abstract:
  We give an overview of the current theory of collective modes in trapped
atomic gases at finite temperatures, when the dynamics of the condensate and
non-condensate must both be considered. A simple introduction is given to the
quantum field formulation of the dynamics of an interacting Bose-condensed
system, based on equations of motion for the condensate wavefunction and
single-particle Green's functions for the non-condensate atoms. We discuss the
nature of excitations in the mean-field collisionless region, including the
Beliaev second-order approximation for the self-energies. We also sketch the
derivation of coupled two-fluid hydrodynamic equations using a simple kinetic
equation which includes collisions between condensate and non-condensate atoms.


title: Structural Critical Scattering Study of Mg-Doped CuGeO3

abstract:
  We report a synchrotron x-ray scattering study of the diluted spin-Peierls
(SP) material Cu_(1-x)Mg_xGeO_3. We find that for x>0 the temperature T_m at
which the spin gap is established is significantly higher than the temperature
T_s at which the SP dimerization attains long-range order. The latter is
observed only for x<x_c =0.021, while for x>x_c the SP correlation length
quickly decreases with increasing x. We argue that impurity-induced competing
interactions play a central role in these phenomena.


title: Molecular Chirality and Chiral Parameters

abstract:
  The fundamental issues of symmetry related to chirality are discussed and
applied to simple situations relevant to liquid crystals. We show that any
chiral measure of a geometric object is a pseudoscalar (invariant under proper
rotations but changing sign under improper rotations) and must involve
three-point correlations which only come into play when the molecule has at
least four atoms. In general, a molecule is characterized by an infinite set of
chiral parameters. We illustrate the fact that these parameters can have
differing signs and can vanish at different points as a molecule is
continuously deformed into its mirror image. From this it is concluded that
handedness is not an absolute concept but depends on the property being
observed. Within a simplified model of classical interactions, we identify the
chiral parameter of the constituent molecules which determines the macroscopic
pitch of cholesterics.

categories: cond-mat.stat-mech cond-mat.soft physics.atm-clus physics.chem-ph


title:
Asymptotic temperature dependence of the superfluid density in liquid
  $^4$He

abstract:
  In a modified ideal Bose gas model we derive an expression for the
temperature dependence of the superfluid fraction in liquid $^4$He. This
expression leads to a fit formula for the asymptotic temperature dependence
that reproduces the data significantly better than comparable formulas.


title:
The Critical Behaviour of the Spin-3/2 Blume-Capel Model in Two
  Dimensions

abstract:
  The phase diagram of the spin-3/2 Blume-Capel model in two dimensions is
explored by conventional finite-size scaling, conformal invariance and Monte
Carlo simulations. The model in its $\tau$-continuum Hamiltonian version is
also considered and compared with others spin-3/2 quantum chains. Our results
indicate that differently from the standard spin-1 Blume-Capel model there is
no multicritical point along the order-disorder transition line. This is in
qualitative agreement with mean field prediction but in disagreement with
previous approximate renormalization group calculations. We also presented new
results for the spin-1 Blume-Capel model.


title: The surfactant effect in semiconductor thin film growth

abstract:
  The theoretical and experimental status of surfactant mediated semiconductor
epitaxial growth is reviewed. We discuss homoepitaxy as well as heteroepitaxy,
and emphasize in particular issues related to the mechanism by which
surfactants suppress growth of three dimensional islands in heteroepitaxy. We
argue that the dominant mechanism is passivation of island edges, which leads
to suppression of attachment and detachment of atoms to and from island edges.


title: Damping of Growth Oscillations

abstract:
  Computer simulations and scaling theory are used to investigate the damping
of oscillations during epitaxial growth on high-symmetry surfaces. The
crossover from smooth to rough growth takes place after the deposition of
(D/F)^\delta monolayers, where D and F are the surface diffusion constant and
the deposition rate, respectively, and the exponent \delta=2/3 on a
two-dimensional surface. At the transition, layer-by-layer growth becomes
desynchronized on distances larger than a layer coherence length proportional
l^2, where l is a typical distance between two-dimensional islands in the
submonolayer region of growth.


title: Weight decay induced phase transitions in multilayer neural networks

abstract:
  We investigate layered neural networks with differentiable activation
function and student vectors without normalization constraint by means of
equilibrium statistical physics. We consider the learning of perfectly
realizable rules and find that the length of student vectors becomes infinite,
unless a proper weight decay term is added to the energy. Then, the system
undergoes a first order phase transition between states with very long student
vectors and states where the lengths are comparable to those of the teacher
vectors. Additionally in both configurations there is a phase transition
between a specialized and an unspecialized phase. An anti-specialized phase
with long student vectors exists in networks with a small number of hidden
units.


title: Nucleation Theory for Capillary Condensation

abstract:
  This paper is devoted to the thermally activated dynamics of the capillary
condensation. We present a simple model which enables us to identify the
critical nucleus involved in the transition mechanism. This simple model is
then applied to calculate the nucleation barrier from which we can obtain
informations on the nucleation time. These results are compared to the
numerical simulation of a Landau-Ginzburg model for the liquid-vapor interface
combined with a Langevin dynamics.


title: The effect of temperature jumps during polymer crystallization

abstract:
  Temperature changes during the growth of lamellar polymer crystals give rise
to steps on the surface of the crystals. It has recently been suggested that
these steps could provide important insights into the mechanism of polymer
crystallization. In particular, a characterization of the profiles of these
steps might reveal the fixed-point attractor that underlies a recently proposed
crystallization mechanism. Here we examine this hypothesis by performing
simulations of such temperature jumps using the Sadler-Gilmer model. We find
that for this model the step profiles do reveal the fixed-point attractor.
However, for temperature decreases they also reflect the rounding of the
crystal edge that occurs in this model and for temperature increases they also
reflect the fluctuations in the thickness present in the crystal. We discuss
the implications of these results for the interpretation of experimental step
profiles.


title: Sequence randomness and polymer collapse transitions

abstract:
  Contrary to expectations based on Harris' criterion, chain disorder with
frustration can modify the universality class of scaling at the theta
transition of heteropolymers. This is shown for a model with random two-body
potentials in 2D on the basis of exact enumeration and accurate Monte Carlo
results. When frustration grows beyond a certain finite threshold, the
temperature below which disorder becomes relevant coincides with the theta one
and scaling exponents definitely start deviating from those valid for
homopolymers.


title:
The Extended Coupled Cluster Treatment of Correlations in Quantum
  Magnets

abstract:
  The spin-half XXZ model on the linear chain and the square lattice are
examined with the extended coupled cluster method (ECCM) of quantum many-body
theory. We are able to describe both the Ising-Heisenberg phase and the
XY-Heisenberg phase, starting from known wave functions in the Ising limit and
at the phase transition point between the XY-Heisenberg and ferromagnetic
phases, respectively, and by systematically incorporating correlations on top
of them. The ECCM yields good numerical results via a diagrammatic approach,
which makes the numerical implementation of higher-order truncation schemes
feasible. In particular, the best non-extrapolated coupled cluster result for
the sublattice magnetization is obtained, which indicates the employment of an
improved wave function. Furthermore, the ECCM finds the expected qualitatively
different behaviours of the linear chain and the square lattice cases.


title:
Scaling limit of the one-dimensional XXZ Heisenberg chain with easy axis
  anisotropy

abstract:
  We construct the scaling limit of the easy axis XXZ chain. This limit is a
subtle combination of approaching the isotropic point, and letting the lattice
spacing to zero to obtain a continuous model with a finite mass gap. We give
the energy difference between the two lowest energy states (the two `vacua')
and analyze the structure of the excitation spectrum of the limiting model. We
find, that the excitations form two sets corresponding to the two vacua. In
both sets the dressed particles are described by Bethe Ansatz like equations
(higher level Bethe Ansatz), and the two sets can be distinguished through a
parameter entering into these secular equations. The degenerations in the
spectrum can be interpreted as originating from an SU(2) symmetry of the
dressed particles. The two particle scattering matrices obtained from the
secular equations are consistent with this symmetry, and they differ in an
overall sign in the two sectors.


title:
Statistical mechanics of permanent random atomic and molecular networks:
  Structure and heterogeneity of the amorphous solid state

abstract:
  Under sufficient permanent random covalent bonding, a fluid of atoms or small
molecules is transformed into an amorphous solid network. Being amorphous,
local structural properties in such networks vary across the sample. A natural
order parameter, resulting from a statistical-mechanical approach, captures
information concerning this heterogeneity via a certain joint probability
distribution. This joint probability distribution describes the variations in
the positional and orientational localization of the particles, reflecting the
random environments experienced by them, as well as further information
characterizing the thermal motion of particles. A complete solution, valid in
the vicinity of the amorphous solidification transition, is constructed
essentially analytically for the amorphous solid order parameter, in the
context of the random network model and approach introduced by Goldbart and
Zippelius [Europhys. Lett. 27, 599 (1994)]. Knowledge of this order parameter
allows us to draw certain conclusions about the stucture and heterogeneity of
randomly covalently bonded atomic or molecular network solids in the vicinity
of the amorphous solidification transition. Inter alia, the positional aspects
of particle localization are established to have precisely the structure
obtained perviously in the context of vulcanized media, and results are found
for the analogue of the spin glass order parameter describing the orientational
freezing of the bonds between particles.


title:
From the magnetic-field-driven transitions to the zero-field transition
  in two-dimensions

abstract:
  For more than a decade it was widely accepted that two-dimensional electrons
are insulating at zero temperature and at zero magnetic-field. Experimentally
it was demonstrated that, when placed in a strong perpendicular magnetic field,
the insulating phase turns into a quantum-Hall state. While this transition was
in accordance with existing theoretical models (KLZ), the density driven
metal-insulator transition at zero magnetic-field, recently observed in
high-quality two-dimensional systems, was unforeseen and, despite considerable
amount of effort, its origins are still unknown. In order to improve our
understanding of the zero magnetic-field transition, we conducted a study of
the insulator to quantum-Hall transition in low-density, two-dimensional, hole
system in GaAs that exhibits the zero magnetic-field metal-insulator
transition. We found that, in the low field insulating phase, upon increasing
the carrier density towards the metal-insulator transition, the critical
magnetic-field of the insulator to quantum-Hall transition decreases and
converges to the zero magnetic-field metal-insulator transition. This implies a
common origin for both the finite magnetic-field and the zero magnetic-field
transitions.


title: Metamagnets in uniform and random fields

abstract:
  We study a two-sublattice Ising metamagnet with nearest and
next-nearest-neighbor interactions, in both uniform and random fields. Using a
mean-field approximation, we show that the qualitative features of the phase
diagrams are significantly dependent on the distribution of the random fields.
In particular, for a Gaussian distribution of random fields, the behavior of
the model is qualitatively similar to a dilute Ising metamagnet in a uniform
field.


title: On Coupled Directed Percolation Processes: A Unifying View

abstract:
  It is shown that the universal critical properties of two recently introduced
coupled directed percolation processes can be described by a single rapidity
reversal invariant stochastic reaction-diffusion model. It is demonstrated that
all renormalizations needed for the calculation of the universal scaling
behavior near the multicritical point can be gained from the Gribov process
(Reggeon field theory). Consequently the crossover exponent describing the
scaling of the linear coupling parameter is given by Phi = 1 to all orders of
the perturbation expansion.


title:
X-ray photoemission characterization of
  La_{0.67}(Ca_{x}Sr_{1-x})_{0.33}MnO_{3} films

abstract:
  The Curie temperature and x-ray photoemission spectra of thin films of
La_{0.67}(Ca_{x}Sr_{1-x})_{0.33}MnO_{3} (LCSMO) have been studied as a function
of the Ca/Sr ratio. The films were grown by off-axis cosputtering from
individual targets of La_{0.67}Ca_{0.33}MnO_{3} (LCMO) and
La_{0.67}Sr_{0.33}MnO_{3} (LSMO) onto (100) oriented NdGaO_{3} substrates. The
films grow with a (100) orientation, with no other orientations observed by
x-ray diffraction. For the alloy mixtures, the Curie temperature, T_C, varies
slowly as the Ca/Sr is decreased, remaining $\approx$ 300 K, while for the LCMO
and LSMO films T_C is 260 and 330 K, respectively. The Mn-O valence structure
is composed of two dominant peaks, whose positions undergo a change as the Ca
fraction is decreased. The core lines behave as linear combinations of lines
from pure LCMO and LSMO.


title: Equivalent Linear Two-Body Equations for Many-Body Systems

abstract:
  A method has been developed for obtaining equivalent linear two-body
equations (ELTBE) for the system of many ($N$) bosons using the variational
principle. The method has been applied to the one-dimensional N-body problem
with pair-wise contact interactions (McGurie-Yang N-body problem) and to the
dilute Bose-Einstein condensation (BEC) of atoms in anisotropic harmonic traps
at zero temperature. For both cases, it is shown that the method gives
excellent results for large N.


title: Quantum Lifshitz Point

abstract:
  I study a quantum Lifshitz point in a three-dimensional itinerant
antiferromagnet, in particular the scaling of the N\'{e}el temperature, the
correlation length, the staggered susceptibility, the specific heat coefficient
and the resistivity. At low temperatures, the model is shown to have the
inverse staggered susceptibility and the resistivity varying as T$^{5/4}$, and
the specific heat coefficient varying as T$^{1/4}$.


title: A Solvable Model of Interacting Fermions in Two Dimensions

abstract:
  We introduce and study an exactly solvable model of several species of
fermions in which particles interact pairwise through a mutual magnetic field;
the interaction operates only between particles belonging to different species.
After an unitary transformation, the model reduces to one in which each
particle sees a magnetic field which depends on the total numbers of particles
of all the other species; this may be viewed as the mean-field model for a
class of anyonic theories. Our model is invariant under charge conjugation C
and the product PT (parity and time reversal). For the special case of two
species, we examine various properties of this system, such as the Hall
conductivity, the wave function overlap arising from the transfer of one
particle from one species to another, and the one-particle off-diagonal density
matrix. Our model is a generalization of a recently introduced solvable model
in one dimension.


title: An Unsettled Issue in the Theory of the Half-Filled Landau Level

abstract:
  The purpose of this paper is to identify an unsettled issue in the theory of
the half-filled Landau level, and state our point of view.


title:
Superconductivity of Quasi-Two-Dimensional Tight-Binding Electrons in a
  Strong Magnetic Field

abstract:
  We have investigated the transition temperature $T_{\rm c}(H)$ of
superconductiv ity in quasi-two-dimensional (Q2D) tight-binding electrons in a
strong magnetic field. When the magnetic field is parallel to 2D conducting
plane, $T_{\rm c}(H) $ of the Q2D superconductor is shown to increase in an
oscillatory manner as the magnetic field becomes large and to reach $T_{\rm
c}(0)$ in a strong magnetic f ield limit for the spin-triplet superconductor.
We consider the cases of on-site and nearest sites attractive interaction, and
calculate the magnetic field depe ndences of the transition temperature for
various types of symmetry. The first o rder transition from $p_y$-wave to
$p_x$-wave is shown to occur at $H\sim 35$T w hen the magnetic field is
parallel to the $y$ direction, which will be observed in a triplet
superconductor, Sr$_2$RuO$_4$.


title:
Ground State Phase Diagram of the One Dimensional S=1/2 XXZ Model with
  Dimerization and Quadrumerization

abstract:
  The one dimensional S=1/2 XXZ model with dimerization (1-j) and
quadrumerization \delta is studied by the numerical exact diagonalization of
finite size systems. Using the conformal field theory and the level
spectroscopy method, we calculate the ground state phase diagram with XY-like
anisotropy \Delta (0<\Delta<1). The ground states of this model contain the
Haldane state, S=1 dimer state, S=1 large-D state and S=1/2 dimer state as
limiting cases. The \Delta and \delta-dependence of the critical exponent \nu
of the energy gap is determined from the conformal dimensions of excited
states.


title:
Wave transmission, phonon localization and heat conduction of 1D
  Frenkel-Kontorova chain

abstract:
  We study the transmission coefficient of a plane wave through a 1D finite
quasi-periodic system -- the Frenkel-Kontorova (FK) model -- embedding in an
infinite uniform harmonic chain. By varying the mass of atoms in the infinite
uniform chain, we obtain the transmission coefficients for {\it all}
eigenfrequencies. The phonon localization of the incommensurated FK chain is
also studied in terms of the transmission coefficients and the Thouless
exponents. Moreover, the heat conduction of Rubin-Greer-like model for FK chain
at low temperature is calculated. It is found that the stationary heat flux
$J(N)\sim N^{\alpha}$, and $\alpha$ depends on the strength of the external
potential.


title:
Dispersion of passive tracers in velocity field with non
  delta-correlated noise

abstract:
  The diffusive properties in velocity fields whose small scales are
parameterized by non $\delta$-correlated noise is investigated using multiscale
technique. The analytical expression of the eddy diffusivity tensor is found
for a 2D steady shear flow and it is an increasing function of the
characteristic noise decorrelation time $\tau$. In order to study a generic
flow ${\bm v}$, a small-$\tau$ expansion is performed and the first correction
${\cal O}(\tau)$ to the effective diffusion coefficients is evaluated. This is
done using two different approaches and it results that at the order $\tau$ the
problem with a colored noise is equivalent to the white-in-time case provided
by a renormalization of the velocity field ${\bm v} \mapsto \tilde{\bm v}$
depending on $\tau$. Two examples of 2D closed-streamlines velocity field are
considered and in both the cases an enhancement of the diffusion is found.


title: Stability and diffusion of surface clusters

abstract:
  Using kinetic Monte Carlo simulations and a bond-counting ansatz, thermal
stability and diffusion of an adatom island on a crystal surface are studied.
At low temperatures, the diffusion constant $D$ is found to decrease for a wide
range of island sizes like $D \propto N^{-\alpha}$, where $\alpha$ is close to
one, $N$ being the number of adatoms in the cluster. By heating up the surface,
the system undergoes a phase transition above which the island disappears.
Characteristics of that transition are discussed.


title: Stabilizing Role of Mesoscopic Fluctuations in Spin Systems

abstract:
  The occurrence of mesoscopic fluctuations in statistical systems implies,
from the point of view of dynamical theory, the existence of local
instabilities. However, the presence of such fluctuations can make a system, as
a whole, more stable from the thermodynamic point of view. Thus, in many cases,
a local dynamic instability is a requisite for the global thermodynamic
stability. This idea is illustrated by several spin models.


title:
Singularities of the renormalization group flow for random elastic
  manifolds

abstract:
  We consider the singularities of the zero temperature renormalization group
flow for random elastic manifolds. When starting from small scales, this flow
goes through two particular points $l^{*}$ and $l_{c}$, where the average value
of the random squared potential $<U^{2}>$ turnes negative ($l^{*}$) and where
the fourth derivative of the potential correlator becomes infinite at the
origin ($l_{c}$). The latter point sets the scale where simple perturbation
theory breaks down as a consequence of the competition between many metastable
states. We show that under physically well defined circumstances $l_{c}<l^{*}$
and thus the apparent renormalization of $<U^{2}>$ to negative values does not
take place.


title: Entangled Bell and GHZ states of excitons in coupled quantum dots

abstract:
  We show that excitons in coupled quantum dots are ideal candidates for
reliable preparation of entangled states in solid-state systems. An optically
controlled exciton transfer process is shown to lead to the generation of Bell
and GHZ states in systems comprising two and three coupled dots, respectively.
The strength and duration of selective light-pulses for producing maximally
entangled states are identified by both analytic, and full numerical, solution
of the quantum dynamical equations. Experimental requirements to build such
entangled states are discussed.


title: Algorithm for normal random numbers

abstract:
  We propose a simple algorithm for generating normally distributed pseudo
random numbers. The algorithm simulates N molecules that exchange energy among
themselves following a simple stochastic rule. We prove that the system is
ergodic, and that a Maxwell like distribution that may be used as a source of
normally distributed random deviates follows when N tends to infinity. The
algorithm passes various performance tests, including Monte Carlo simulation of
a finite 2D Ising model using Wolff's algorithm. It only requires four simple
lines of computer code, and is approximately ten times faster than the
Box-Muller algorithm.


title: Velocity statistics in excited granular media

abstract:
  We present an experimental study of velocity statistics for a partial layer
of inelastic colliding beads driven by a vertically oscillating boundary. Over
a wide range of parameters (accelerations 3-8 times the gravitational
acceleration), the probability distribution P(v) deviates measurably from a
Gaussian for the two horizontal velocity components. It can be described by
P(v) ~ exp(-|v/v_c|^1.5), in agreement with a recent theory. The characteristic
velocity v_c is proportional to the peak velocity of the boundary. The granular
temperature, defined as the mean square particle velocity, varies with particle
density and exhibits a maximum at intermediate densities. On the other hand,
for free cooling in the absence of excitation, we find an exponential velocity
distribution. Finally, we examine the sharing of energy between particles of
different mass. The more massive particles are found to have greater kinetic
energy.


title:
Scaled variational computation of the energy spectrum of a
  two-dimensional hydrogenic donor in a magnetic field of arbitrary strength

abstract:
  We compute the energy levels of a 2D Hydrogen atom when a constant magnetic
field is applied. With the help of a mixed-basis variational method and a
genera lization of virial theorem, which consists in scaling the wave function,
we calculate the binding energies of the 1S, $2P^-$ and $3D^-$ levels. We
compare the computed energy spectra with those obtained via a generalization of
the mesh point technique as well as the shifted 1/N method. We show that the
variational solutions present a very good behavior in the weak and strong
magnetic field regimes.


title: Andreev Scattering and the Kondo Effect

abstract:
  We examine the properties of an infinite-$U$ Anderson impurity coupled to
both normal and superconducting metals. Both the cases of a quantum dot and a
quantum point contact containing an impurity are considered; for the latter, we
study both one and two-channel impurities. Using a generalization of the
noncrossing approximation which incorporates multiple Andreev reflection, we
compute the impurity spectral function and the linear-response conductance of
these devices. We find generically that the Kondo resonance develops structure
at energies corresponding to the superconducting gap, and that the magnitude of
the resonance at the Fermi energy is altered. This leads to observable changes
in the zero-bias conductance as compared to the case with no superconductivity.


title:
Continuum elasticity theory of edge excitations in a two-dimensional
  electron liquid with finite range interactions

abstract:
  We make use of continuum elasticity theory to investigate the collective
modes that propagate along the edge of a two-dimensional electron liquid or
crystal in a magnetic field. An exact solution of the equations of motion is
obtained with the following simplifying assumptions: (i) The system is {\it
macroscopically} homogeneous and isotropic in the half-plane delimited by the
edge (ii) The electron-electron interaction is of finite range due to screening
by external electrodes (iii) The system is nearly incompressible. At
sufficiently small wave vector $q$ we find a universal dispersion curve $\omega
\sim q$ independent of the shear modulus. At larger wave vectors the dispersion
can change its form in a manner dependent on the comparison of various length
scales. We obtain analytical formulas for the dispersion and damping of the
modes in various physical regimes.


title:
Resistive Anomalies at Ferromagnetic Transitions Revisited: the case of
  SrRuO_3

abstract:
  We show that recent resistivity data on SrRuO_3 for T->T_c are consistent
with conventional theory when corrections to scaling are included and a small
shift in T_c is allowed.


title: Novel antiferromagnetic quantum phase transition in underdoped cuprates

abstract:
  We investigate a zero-temperature itinerant antiferromagnetic transition
where the fermions possess a d-wave gap. This problem pertains to both the
nodal liquid insulating phase and the d-wave superconducting phase of the
underdoped cuprates. We find that a non-trivial quantum phase transition
exists, and that the quantum critical point is dominated by a long-ranged
interaction ($|x-y|^{-2d}$) of the N\'{e}el order parameter, which is induced
by the Dirac-like fermions near gap nodes. We formulate a Ginzburg-Landau
functional and estimate the critical exponents via the large-N expansion
method.


title:
Possible Existence Of Topological Excitations In Quantum Spin Models In
  Low Dimensions

abstract:
  The possibility of existence of topological excitations in the anisotropic
quantum Heisenberg model in one and two spatial dimensions is studied using
coherent state method. It is found that a part of the Wess-Zumino term
contributes to the partition function, as a topological term for ferromagnets
in the long wavelength limit in both one and two dimensions. In particular, the
XY limit of the two-dimensional anisotropic ferromagnet is shown to retain the
topological excitations, as expected from the quantum Kosterlitz-Thouless
scenario.


title:
Lanczos exact diagonalization study of field-induced phase transition
  for Ising and Heisenberg antiferromagnets

abstract:
  Using an exact diagonalization treatment of Ising and Heisenberg model
Hamiltonians, we study field-induced phase transition for two-dimensional
antiferromagnets. For the system of Ising antiferromagnet the predicted
field-induced phase transition is of first order, while for the system of
Heisenberg antiferromagnet it is the second-order transition. We find from the
exact diagonalization calculations that the second-order phase transition
(metamagnetism) occurs through a spin-flop process as an intermediate step.


title:
Neutron Scattering Studies on Magnetic Structure of the Double-Layered
  Manganite La_{2-2x}Sr_{1+2x}Mn_2O_7 ($0.30 \leq x \leq 0.50$)

abstract:
  Systematic powder diffraction studies have been carried out to establish the
magnetic phase diagram of La_{2-2x}Sr_{1+2x}Mn_2O_7 (LSMO327) in a wide hall
concentration range ($0.30 \leq x \leq 0.50$), using the HERMES diffractometer.
LSMO327 exhibits a planar ferromagnetic structure for $0.32\leq x\leq 0.38$ at
low temperatures. A finite canting angle between planar magnetic moments on
neighboring planes starts appearing around $x\sim 0.40$ and reaches
180$^{\circ}$ (A-type antiferromagnet) at x=0.48. At x=0.30, on the other hand,
the magnetic moments are aligned parallel to the c-axis.


title: Analysis of Natural Gradient Descent for Multilayer Neural Networks

abstract:
  Natural gradient descent is a principled method for adapting the parameters
of a statistical model on-line using an underlying Riemannian parameter space
to redefine the direction of steepest descent. The algorithm is examined via
methods of statistical physics which accurately characterize both transient and
asymptotic behavior. A solution of the learning dynamics is obtained for the
case of multilayer neural network training in the limit of large input
dimension. We find that natural gradient learning leads to optimal asymptotic
performance and outperforms gradient descent in the transient, significantly
shortening or even removing plateaus in the transient generalization
performance which typically hamper gradient descent training.


title:
Interplay between Mesoscopic and Microscopic Fluctuations in
  Ferromagnets

abstract:
  A model of a ferromagnet is considered, in which there arise mesoscopic
fluctuations of paramagnetic phase. The presence of these fluctuations
diminishes the magnetization of the ferromagnet, softens the spin-wave
spectrum, increases the spin-wave attenuation, shortens the magnon free path,
lowers the critical point, and can change the order of phase transition. A
special attention is paid to the interplay between these mesoscopic
paramagnetic fluctuations and microscopic fluctuations due to magnons. One of
the main results of this interplay is an essential extension of the region of
parameters where the ferromagnet-paramagnet phase transition is of first order.


title:
Exchange interactions and magnetic properties of the layered vanadates
  CaV2O5, MgV2O5, CaV3O7 and CaV4O9

abstract:
  We have performed ab-initio calculations of exchange couplings in the layered
vanadates CaV2O5, MgV2O5, CaV3O7 and CaV4O9. The uniform susceptibility of the
Heisenberg model with these exchange couplings is then calculated by quantum
Monte Carlo method; it agrees well with the experimental measurements. Based on
our results we naturally explain the unusual magnetic properties of these
materials, especially the huge difference in spin gap between CaV2O5 and
MgV2O5, the unusual long range order in CaV3O7 and the "plaquette resonating
valence bond (RVB)" spin gap in CaV4O9.


title: Protein folding using contact maps

abstract:
  We present the development of the idea to use dynamics in the space of
contact maps as a computational approach to the protein folding problem. We
first introduce two important technical ingredients, the reconstruction of a
three dimensional conformation from a contact map and the Monte Carlo dynamics
in contact map space. We then discuss two approximations to the free energy of
the contact maps and a method to derive energy parameters based on perceptron
learning. Finally we present results, first for predictions based on threading
and then for energy minimization of crambin and of a set of 6 immunoglobulins.
The main result is that we proved that the two simple approximations we studied
for the free energy are not suitable for protein folding. Perspectives are
discussed in the last section.


title:
Mean-Field-Theory for Polymers in Mixed Solvents Thermodynamic and
  Structural Properties

abstract:
  Theoretical aspects of polymers in mixed solvents are considered using the
Edwards Hamiltonian formalism. Thermodynamic and structural properties are
investigated and some predictions are made when the mixed solvent approaches
criticality. Both the single and the many chain problems are examined. When the
pure mixed solvent is near criticality, addition of a small amount of polymers
shifts the criticality towards either enhanced compatibility or induced phase
separation depending upon the value of the parameter describing the interaction
asymmetry of the solvents with respect to the polymer. The polymer-solvent
effective interaction parameter increases strongly when the solvent mixture
approaches criticality. Accordingly, the apparent excluded volume parameter
decreases and may vanish or even become negative. Consequently, the polymer
undergoes a phase transition from a swollen state to an unperturbed state or
even take a collapsed configuration. The effective potential acting on a test
chain in strong solutions is calculated and the concept of Edwards screening
discussed. Structural properties of ternary mixtures of polymers in mixed
solvents are investigated within the Edwards Hamiltonian model. It is shown
that the effective potential on a test chain in strong solutions could be
written as an infinite series expansion of terms describing interactions via
one chain, two chains etc. This summation can be performed following a similar
scheme as in the Ornstein-Zernike series expansion.


title: The Reaction Process A+A->O in Sinai Disorder

abstract:
  The single-species reaction-diffusion process $A+A\to O$ is examined in the
presence of an uncorrelated, quenched random velocity field. Utilising a
field-theoretic approach, we find that in two dimensions and below the density
decay is altered from the case of purely diffusing reactants. In two-dimensions
the density amplitude is reduced in the presence of weak disorder, yielding the
interesting result that Sinai disorder can cause reactions to occur at an {\it
increased} rate. This is in contrast to the case of long-range correlated
disorder, where it was shown that the reaction becomes sub-diffusion limited.
However, when written in terms of the microscopic diffusion constant it is seen
that increasing the disorder has the effect of reducing the rate of the
reaction. Below two dimensions, the effect of Sinai disorder is much more
severe and the reaction is shown to become sub-diffusion limited. Although
there is no universal amplitude for the time-dependence of the density, it is
universal when expressed in terms of the disorder-averaged diffusion length.
The appropriate amplitude is calculated to one-loop order.


title: Exact solution of the Lawrence-Doniach model in parallel magnetic fields

abstract:
  For the first time, we obtain the complete and exact analytical solution of
the Lawrence-Doniach model for layered superconductors in external parallel
magnetic fields. By solving a nontrivial mathematical problem of exact
minimization of the free-energy functional, we derive a closed, self-consistent
system of mean-field equations involving only two variables. Exact solutions to
these equations prove simultaneous penetration of Josephson vortices into all
the barriers, yield a completely new expression for the lower critical field,
refute the concept of a triangular Josephson vortex lattice and clarify the
physics of Fraunhofer oscillations of the total critical Josephson current.
  PACS numbers: 74.80.Dm, 74.20.De, 74.50.+r


title: Intermediate spin and quantum critical points, etc

abstract:
  Unlike that of SO(3) or SU(2), the Lie algebra for SO(2), which defines
intermediate spin, comprises only $S_{z}$ and implies $S^{\pm}$ commute. In
general, $S_{z}$ has a continuous spectrum. This intermediate spin scheme can
realized for the low energy excitations of a wide class of large spin magnets.
A magnetic field provides the necessary time reversal symmetry breaking and
controls the effective value of the spin $\tilde S$. Physical quantities are
periodic in the equilibrium magnetization component induced by this field. In
particular for one dimensional antiferromagnets there are periodic regions on
the field axis for which the model is quantum critical while in two or three
dimensions criticality is reduced to points.


title:
Influence of finite Hund rules and charge transfer on properties of
  Haldane systems

abstract:
  We consider the Kondo-Hubbard model with ferromagnetic exchange coupling $%
J_{H}$, showing that it is an approximate effective model for late transition
metal-O linear systems. We study the dependence of the charge and spin gaps
$\Delta_{C}$, $\Delta_{S}$, and several spin-spin correlation functions,
including the hidden order parameter $Z(\pi)$, as functions of $J_{H}/t$ and
$U/t$, by numerical diagonalization of finite systems. Except for $Z(\pi)$, all
properties converge slowly to the strong-coupling limit. When $J_{H}/t\sim 2$
and $U/t\sim 7$ (the effective parameters that we obtain for Y_2BaNiO_5),
$\Delta_{S}$ is roughly half of the value expected from a strong-coupling
expansion.


title:
Role of the intrinsic surface state in the decay of image states at a
  metal surface

abstract:
  The role of the intrinsic surface state ($n=0$) in the decay of the first
image state ($n=1$) at the (111) surface of copper is investigated. Inelastic
linewidths are evaluated from the knowledge of the imaginary part of the
electron self-energy, which we compute, within the GW approximation of
many-body theory, by going beyond a free-electron description of the metal
surface. Single-particle wave functions are obtained by solving the
Schr\"odinger equation with a realistic one-dimensional model potential, and
departure of the motion along the surface from free-electron behaviour is
considered through the introduction of the effective mass. The decay of the
first image state of Cu(111) into the intrinsic surface state is found to
result in a linewidth that represents a 40% of the total linewidth. The
dependence of linewidths on the momentum of the image state parallel to the
surface is also investigated.


title: On the robustness of scale invariance in SOC models

abstract:
  A random neighbor extremal stick-slip model is introduced. In the
thermodynamic limit, the distribution of states has a simple analytical form
and the mean avalanche size, as a function of the coupling parameter, is
exactly calculable. The system is critical only at a special point Jc in the
coupling parameter space. However, the critical region around this point, where
approximate scale invariance holds, is very large, suggesting a mechanism for
explaining the ubiquity of scale invariance in Nature.


title:
The Resistance of Feynman Diagrams and the Percolation Backbone
  Dimension

abstract:
  We present a new view of Feynman diagrams for the field theory of transport
on percolation clusters. The diagrams for random resistor networks are
interpreted as being resistor networks themselves. This simplifies the field
theory considerably as we demonstrate by calculating the fractal dimension
$D_B$ of the percolation backbone to three loop order. Using renormalization
group methods we obtain $D_B = 2 + \epsilon /21 - 172\epsilon^2 /9261 + 2
\epsilon^3 (- 74639 + 22680 \zeta (3))/4084101$, where $\epsilon = 6-d$ with
$d$ being the spatial dimension and $\zeta (3) = 1.202057..$.


title:
Bound pair states beyond the condensate for Fermi systems below T_c: the
  pseudogap as a necessary condition

abstract:
  As is known, the 1/q^2 theorem of Bogoliubov asserts that the mean density of
the fermion pair states with the total momentum q obeys the inequality n_q >
C/q^2 (q \to 0) in the case of the Fermi system taken at nonzero temperature
and in the superconducting state provided the interaction term of its
Hamiltonian is locally gauge invariant. With the principle of correlation
weakening it is proved in this paper that the reason for the mentioned singular
behaviour of n_q is the presence of the bound states of particle pairs with
nonzero total momenta. Thus, below the temperature of the superconducting phase
transition there always exist the bound states of the fermion couples beyond
the pair condensate. If the pseudogap observed in the normal phase of the
high-T_c superconductors is stipulated by the presence of the electron bound
pairs, then the derived result suggests, in a model-independent manner, that
the pseudogap survives below T_c.


title: Efficiency in foreign exchange markets

abstract:
  A quantitative check of weak efficiency in US dollar/German mark exchange
rates is developed using high frequency data. We show the existence of long
term return anomalies. We introduce a technique to measure the available
information and show it can be profitable following a particular trading rule.


title: Phase Coexistence of Complex Fluids in Shear Flow

abstract:
  We present some results of recent calculations of rigid rod-like particles in
shear flow, based on the Doi model. This is an ideal model system for
exhibiting the generic behavior of shear-thinning fluids (polymer solutions,
wormlike micelles, surfactant solutions, liquid crystals) in shear flow. We
present calculations of phase coexistence under shear among weakly-aligned
(paranematic) and strongly-aligned phases, including alignment in the shear
plane and in the vorticity direction (log-rolling). Phase coexistence is
possible, in principle, under conditions of both common shear stress and common
strain rate, corresponding to different orientations of the interface between
phases. We discuss arguments for resolving this degeneracy. Calculation of
phase coexistence relies on the presence of inhomogeneous terms in the
dynamical equations of motion, which select the appropriate pair of coexisting
states. We cast this condition in terms of an equivalent dynamical system, and
explore some aspects of how this differs from equilibrium phase coexistence.


title: Dynamics and flow-induced phase separation in polymeric fluids

abstract:
  The past few years have seen many advances in our understanding of the
dynamics of polymeric fluids. These include improvements on the successful
reptation theory; an emerging molecular theory of semiflexible chain dynamics;
and an understanding of how to calculate and classify ``phase diagrams'' for
flow-induced transitions. Experimentalists have begun mapping out the phase
behavior of wormlike micelles, a ``living'' polymeric system, in flow: these
systems undergo transitions into shear-thinning or shear-thickening phases,
whose variety is remarkably rich and poorly understood. Polymeric ideas must be
extended to include the delicate charge and composition effects which conspire
to stabilize the micelles and are strongly influenced by flow.


title: Braided Rivers and Superconducting Vortex Avalanches

abstract:
  Magnetic vortices intermittently flow through preferred channels when they
are forced in or out of a superconductor. We study this behavior using a
cellular model, and find that the vortex flow can make braided rivers
strikingly similar to aerial photographs of braided fluvial rivers, such as the
Brahmaputra. By developing an analysis technique suitable for characterizing a
self-affine (multi)fractal, the scaling properties of the braided vortex rivers
in the model are compared with those of braided fluvial rivers. We suggest that
avalanche dynamics leads to braiding in both cases.

categories: cond-mat.stat-mech adap-org cond-mat.supr-con nlin.AO nlin.PS patt-sol


title: Orbital occupation, local spin and exchange interactions in V2O3

abstract:
  We present the results of an LDA and LDA+U band structure study of the
monoclinic and the corundum phases of V2O3 and argue that the most prominent
(spin 1/2) models used to describe the semiconductor metal transition are not
valid. Contrary to the generally accepted assumptions we find that the large on
site Coulomb and exchange interactions result in a total local spin of 1 rather
than 1/2 and especially an orbital occupation which removes the orbital
degeneracies and the freedom for orbital ordering. The calculated exchange
interaction parameters lead to a magnetic structure consistent with experiment
again without the need of orbital ordering. While the low-temperature
monoclinic distortion of the corundum crystal structure produces a very small
effect on electronic structure of v2o3, the change of magnetic order leads to
drastic differences in band widths and band gaps. The low temperature
monoclinic phase clearly favors the experimentally observed magnetic structure,
but calculations for corundum crystal structure gave two consistent sets of
exchange interaction parameters with nearly degenerate total energies
suggesting a kind of frustration in the paramagnetic phase. These results
strongly suggest that the phase transitions in V2O3 which is so often quoted as
the example of a S=1/2 Mott Hubbard system have a different origin. So back to
the drawing board!


title:
Superconducting Fluctuation Effects on the Spin-Lattice Relaxation Rate
  in YBa_2Cu_3O_{6.95}

abstract:
  We report 63Cu(2) spin-lattice relaxation rate measurements of
YBa_2Cu_3O_{6.95} in magnetic fields from 2.1 T to 27.3 T obtained from
17O(2,3) nuclear magnetic resonance spin-spin relaxation. For T < 120 K, the
spin-lattice rate increases with increasing magnetic field. We identify this
magnetic field dependence with the change in the low-energy spectral weight
originating from d-wave pairing fluctuation corrections to the density of
states.


title:
Superconducting Fluctuation Effects on the Electron Spin Susceptibility
  in YBa_2Cu_3O_{6.95}

abstract:
  The electronic spin susceptibility of YBa_2Cu_3O_{6.95} has been measured
with high precision up to 24 Tesla with 17O nuclear magnetic resonance. Its
temperature dependence can be accounted for by superconducting fluctuations
that result in a smooth crossover from the normal to the vortex liquid state. A
magnetic field-temperature phase diagram for this crossover has been
established having strong upward curvature.


title:
Comment on "Magnetic Field Independence of the Spin Gap in
  YBa_2Cu_3O_{7-d}" by K. Gorny et al., Phys. Rev. Lett. 82, 177 (1999)

abstract:
  In a recent Letter of Gorny et al. it was reported that the spin-lattice
relaxation rate in YBCO_{7-d} is magnetic field independent. These authors
point out that their results are inconsistent with prior reports of Borsa et
al., Carretta et al., and Mitrovic et al., which concluded that there is a
small but significant magnetic field dependence. In this comment we propose an
explanation for this discrepancy.


title:
Effects of Strong Magnetic Fields on Pairing Fluctuations in High
  Temperature Superconductors

abstract:
  We present the theory for the effects of superconducting pairing fluctuations
on the nuclear spin-lattice relaxation rate, 1/T_1, and the NMR Knight shift
for layered superconductors in high magnetic fields. These results can be used
to clarify the origin of the pseudogap in high-T_c cuprates, which has been
attributed to spin fluctuations as well as pairing fluctuations. We present
theoretical results for s-wave and d-wave pairing fluctuations and show that
recent experiments in optimally doped YBa_2Cu_3O_{7-d} are described by d-wave
pairing fluctuations. In addition, we show that the orthorhombic distortion in
YBa_2Cu_3O_{7-d} accounts for an experimentally observed discrepancy between
1/T_1 obtained by NQR and NMR at low field. We propose an NMR experiment to
distinguish a fluctuating s-wave order parameter from a fluctuating strongly
anisotropic order parameter, which may be applied to the system
Nd_{2-x}Ce_xCuO_{4-d} and possibly other layered superconductors.


title: The Effective Theory of Hole Doped Spin-1 Chain

abstract:
  An effective theory for the hole doped spin-1 antiferromagnetic chain is
proposed in this paper. The two branches of low energy quasipaticle excitation
is obtained by the diagrammic technique. In the large t limit(in which t is the
hole hopping term), the lower band is essentially the bound state of one hole
and one magnon and the other band is the sigle hole state. We find a critical
value of t, $t_c=0.21\Delta_H$(in which $\Delta_H$ is the Haldane gap).For
$t>t_c$, with the decrement of t, the mixing of these two bands become stronger
and stronger, and at the same time the effective band mass becomes larger and
larger. When $t<t_c$ the minimum of the lower band moves away from zero the
another point between zero and $\pi/2$. The spin strcture factor is also
calculated in this paper, and we find that for large t limit the main
contribution is from the inter-band transition which induce a resonant peak in
the Haldane gap. While for small t limit the main contribution is from the
intra-band transition which only cause a diffusion like broad bump in the
Haldan gap.


title:
Difference of optical conductivity between one- and two-dimensional
  doped nickelates

abstract:
  We study the optical conductivity in doped nickelates, and find the dramatic
difference of the spectrum in the gap ($\omega$$\alt$4 eV) between one- (1D)
and two-dimensional (2D) nickelates. The difference is shown to be caused by
the dependence of hopping integral on dimensionality. The theoretical results
explain consistently the experimental data in 1D and
  2D nickelates, Y$_{2-x}$Ca$_x$BaNiO$_5$ and La$_{2-x}$Sr$_x$NiO$_4$,
respectively. The relation between the spectrum in the X-ray aborption
experiments and the optical conductivity in La$_{2-x}$Sr$_x$NiO$_4$ is
discussed.


title:
Skyrmions and Quantum Hall Ferromagnets in Improved Composite-Boson
  Theory

abstract:
  An improved composite-boson theory of quantum Hall ferromagnets is proposed.
It is tightly related with the microscopic wave-function theory. The
characteristic feature is that the field operator describes solely the physical
degrees of freedom representing the deviation from the ground state. It
presents a powerful tool to analyze excited states within the \LLLd.
Excitations include a Goldstone mode and nonlocal topological solitons.
Solitons are vortices and Skyrmions carrying the U(1) and SU(2) topological
charges, respectively. Their classical configurations are derived from their
microscopic wave functions. The activation energy of one Skyrmion is estimated,
which explains experimental data remarkably well.


title:
Local chain ordering in amorphous polymer melts: Influence of chain
  stiffness

abstract:
  Molecular dynamics simulation of a generic polymer model is applied to study
melts of polymers with different types of intrinsic stiffness. Important static
observables of the single chain such as gyration radius or persistence length
are determined. Additionally we investigate the overall static melt structure
including pair correlation function, structure function and orientational
correlation function.


title: Anomalous BCS equation for a Luttinger superconductor

abstract:
  In the context of the Anderson theory of high T_c cuprates, we develop a BCS
theory for Luttinger liquids. If the Luttinger interaction is much stronger
than the BCS potential we find that the BCS equation is quite modified compared
to usual BCS equation for Fermi liquids. In particular T_c predicted by the BCS
equation for Luttinger liquids is quite higher than the usual T_c for Fermi
liquids.


title: Explanation of Observed Features of Self-Organization in Traffic Flow

abstract:
  Based on simulations with the ``intelligent driver model'', a microscopic
traffic model, we explain the recently discovered transition from free over
``synchronized'' traffic to stop-and-go patterns [B. S. Kerner, Phys. Rev.
Lett. 81, 3797 (1998)]. We obtain a nearly quantitative agreement with
empirical findings such as the ``pinch effect'', the flow-density diagram, the
emergence of stop-and-go waves from nonhomogeneous congested traffic, and the
dimensions of their wavelength.


title:
Derivation, Properties, and Simulation of a Gas-Kinetic-Based, Non-Local
  Traffic Model

abstract:
  We derive macroscopic traffic equations from specific gas-kinetic equations,
dropping some of the assumptions and approximations made in previous papers.
The resulting partial differential equations for the vehicle density and
average velocity contain a non-local interaction term which is very favorable
for a fast and robust numerical integration, so that several thousand freeway
kilometers can be simulated in real-time. The model parameters can be easily
calibrated by means of empirical data. They are directly related to the
quantities characterizing individual driver-vehicle behavior, and their optimal
values have the expected order of magnitude. Therefore, they allow to
investigate the influences of varying street and weather conditions or freeway
control measures. Simulation results for realistic model parameters are in good
agreement with the diverse non-linear dynamical phenomena observed in freeway
traffic.


title: Moment of inertia of a trapped superfluid gas of atomic Fermions

abstract:
  The moment of inertia of a trapped superfluid gas of atomic Fermions (6Li) is
calculated as a function of two system parameters: temperature and deformation
of the trap. For moderate deformations at zero temperature the moment of
inertia takes on the irrotational flow value. Only for T very close to the
critical temperature rigid rotation is attained. For very strong trap
deformations the moment of inertia approaches its rigid body value even in the
superfluid state. It is proposed that future measurements of the rotational
energy will unambiguously reveal whether the system is in a superfluid state or
not.


title: Modelling Molecular Motors as Folding-Unfolding Cycles

abstract:
  We propose a model for motor proteins based on a hierarchical Hamiltonian
that we have previously introduced to describe protein folding. The proposed
motor model has high efficiency and is consistent with a linear load-velocity
response. The main improvement with respect to previous models is that this
description suggests a connection between folding and function of allosteric
proteins.


title: Toward a Theory of Marginally Efficient Markets

abstract:
  Empirical evidence suggests that even the most competitive markets are not
strictly efficient. Price histories can be used to predict near future returns
with a probability better than random chance. Many markets can be considered as
{\it favorable games}, in the sense that there is a small probabilistic edge
that smart speculators can exploit. We propose to identify this probability
using conditional entropy concept. A perfect random walk has this entropy
maximized, and departure from the maximal value represents a price history's
predictability. We propose that market participants should be divided into two
categories: producers and speculators. The former provides the negative entropy
into the price, upon which the latter feed. We show that the residual negative
entropy can never be arbitraged away: infinite arbitrage capital is needed to
make the price a perfect random walk.


title: Moment scaling at the sol - gel transition

abstract:
  Two standard models of sol-gel transition are revisited here from the point
of view of their fluctutations in various moments of both the mass-distribution
and the gel-mass. Bond-percolation model is an at-equilibrium system and
undergoes a static second-order phase transition, while Monte-Carlo
Smoluchowski model is an off-equilibrium one and shows a dynamical critical
phenomenon. We show that the macroscopic quantities can be splitted into the
three classes with different scaling properties of their fluctuations,
depending on wheather they correspond to : (i) non-critical quantities, (ii)
critical quantities or to (iii) an order parameter. All these three scaling
properties correspond to a single form : $<M>^{\delta} P(M) = \Phi
((M-<M>)/<M>^{\delta})$, with the values of $\delta$ respectively : =1/2
(regime (i)), \neq 1/2 and 1 (regime (ii)), and =1 (regime (iii)). These new
scalings are very robust and, in particular, they do not depend on the precise
form of an Hamiltonian.


title:
Magnetoresistance, Micromagnetism, and Domain Wall Scattering in
  Epitaxial hcp Co Films

abstract:
  Large negative magnetoresistance (MR) observed in transport measurements of
hcp Co films with stripe domains were recently reported and interpreted in
terms of a novel domain wall (DW) scattering mechanism. Here detailed MR
measurements, magnetic force microscopy, and micromagnetic calculations are
combined to elucidate the origin of MR in this material. The large negative
room temperature MR reported previously is shown to be due to ferromagnetic
resistivity anisotropy. Measurements of the resistivity for currents parallel
(CIW) and perpendicular to DWs (CPW) have been conducted as a function of
temperature. Low temperature results show that any intrinsic effect of DWs
scattering on MR of this material is very small compared to the anisotropic MR.


title:
Lattice dynamics of BaTiO3, PbTiO3 and PbZrO3: a comparative
  first-principles study

abstract:
  The full phonon dispersion relations of lead titanate and lead zirconate in
the cubic perovskite structure are computed using first-principles variational
density-functional perturbation theory, with ab initio pseudopotentials and a
plane-wave basis set. Comparison with the results previously obtained for
barium titanate shows that the change of a single constituent (Ba to Pb, Ti to
Zr) has profound effects on the character and dispersion of unstable modes,
with significant implications for the nature of the phase transitions and the
dielectric and piezoelectric responses of the compounds. Examination of the
interatomic force constants in real space, obtained by a transformation which
correctly treats the long-range dipolar contribution, shows that most are
strikingly similar, while it is the differences in a few key interactions which
produce the observed changes in the phonon dispersions. These trends suggest
the possibility of the transferability of force constants to predict the
lattice dynamics of perovskite solid solutions.


title: Second Low Temperature Phase Transition in Frustrated UNi_4B

abstract:
  Hexagonal UNi_4B is magnetically frustrated, yet it orders
antiferromagnetically at T_N = 20 K. However, one third of the U-spins remain
paramagnetic below this temperature. In order to track these spins to lower
temperature, we measured the specific heat C of \unib between 100 mK and 2 K,
and in applied fields up to 9 T. For zero field there is a sharp kink in C at
$T^\ast\approx$ 330 mK, which we interpret as an indication of a second phase
transition involving paramagnetic U. The rise in $\gamma = C/T$ between 7 K and
330 mK and the absence of a large entropy liberated at $T^\ast$ may be due to a
combination of Kondo screening effects and frustration that strongly modifies
the low T transition.


title: On the Generalized Kramers Problem with Oscillatory Memory Friction

abstract:
  The time-dependent transmission coefficient for the Kramers problem exhibits
different behaviors in different parameter regimes. In the high friction regime
it decays monotonically ("non-adiabatic"), and in the low friction regime it
decays in an oscillatory fashion ("energy-diffusion-limited"). The generalized
Kramers problem with an exponential memory friction exhibits an additional
oscillatory behavior in the high friction regime ("caging"). In this paper we
consider an oscillatory memory kernel, which can be associated with a model in
which the reaction coordinate is linearly coupled to a nonreactive coordinate,
which is in turn coupled to a heat bath. We recover the non-adiabatic and
energy-diffusion-limited behaviors of the transmission coefficient in
appropriate parameter regimes, and find that caging is not observed with an
oscillatory memory kernel. Most interestingly, we identify a new regime in
which the time-dependent transmission coefficient decays via a series of rather
sharp steps followed by plateaus ("stair-like"). We explain this regime and its
dependence on the various parameters of the system.


title: Magnetic skyrmions and their lattices in triplet superconductors

abstract:
  Complete topological classification of solutions in SO(3) symmetric
Ginzburg-Landau free energy has been performed and a new class of solutions in
weak external magnetic field carrying two units of magnetic flux has been
identified. These solutions, magnetic skyrmions, do not have singular core like
Abrikosov vortices and at low magnetic field become lighter for strongly type
II superconductors. As a consequence, the lower critical magnetic field Hc1 is
reduced by a factor of log(kappa). Magnetic skyrmions repel each other as 1/r
at distances much larger then magnetic penetration depth forming relatively
robust triangular lattice. Magnetic induction near Hc1 increases gradually as
(H-Hc1)^2. This agrees very well with experiments on heavy fermion
superconductor UPt3. Newly discovered Ru based compounds Sr2RuO4 and
Sr2YRu(1-x)Cu(x)O6 are other possible candidates to possess skyrmion lattices.
Deviations from exact SO(3) symmetry are also studied.


title:
Generalized Lotka-Volterra (GLV) Models and Generic Emergence of Scaling
  Laws in Stock Markets

abstract:
  This is a pedagogical review of the the Generalized Lotka-Volterra (GLV)
model: w_i(t+1) = lambda * w_i(t) + a * W (t) - c * W (t) * w_i(t) where i=1,
>......, N and W= (w_1 + w_2 + ...w_N)/N is the average of the w_i's. The GLV
models provide a generic method to simulate, analyze and understand a wide
class of phenomena which are characterized by (truncated) power-law probability
distributions: P(w) dw ~ w**(-1 -alpha) dw and (truncated) Levy flights
fluctuations L_alpha (W). The implications and the interpretation of the model
in the stock markets are discussed.


title: Fixed points of Hopfield type neural networks

abstract:
  The set of the fixed points of the Hopfield type network is under
investigation. The connection matrix of the network is constructed according
the Hebb rule from the set of memorized patterns which are treated as distorted
copies of the standard-vector. It is found that the dependence of the set of
the fixed points on the value of the distortion parameter can be described
analytically. The obtained results are interpreted in the terms of neural
networks and the Ising model at T=0.


title: Paramagnetic state in d-wave Superconductors

abstract:
  We study theoretically the paramagnetic state in d-wave superconductors. We
present the specific heat, the magnetization, superfluid density obtained
within the weak-coupling model. At low temperatures and for small magnetic
fields they exhibit simple power law behaviors, which should be accessible
experimentally in hole-doped high-T_c cuprates and \kappa-(ET)_2 salts in a
magnetic field within the conducting plane.


title: Half-Metallic Ferrimagnetism in Mn_2VAl

abstract:
  We show that Mn_2VAl is a compound for which the generalized gradient
approximation (GGA) to the exchange-correlation functional in density
functional theory makes a qualitative change in predicted behavior compared to
the usual local density approximation (LDA). Application of GGA leads to
prediction of Mn_2VAl being a half-metallic ferrimagnet, with the minority
channel being the conducting one. The electronic and magnetic structure is
analyzed and contrasted with the isostructural enhanced semimetal Fe_2VAl.


title: Vertically coupled double quantum dots in magnetic fields

abstract:
  Ground-state and excited-state properties of vertically coupled double
quantum dots are studied by exact diagonalization. Magic-number total angular
momenta that minimize the total energy are found to reflect a crossover between
electron configurations dominated by intra-layer correlation and ones dominated
by inter-layer correlation. The position of the crossover is governed by the
strength of the inter-layer electron tunneling and magnetic field. The magic
numbers should have an observable effect on the far infra-red optical
absorption spectrum, since Kohn's theorem does not hold when the confinement
potential is different for two dots. This is indeed confirmed here from a
numerical calculation that includes Landau level mixing. Our results take full
account of the effect of spin degrees of freedom. A key feature is that the
total spin, $S$, of the system and the magic-number angular momentum are
intimately linked because of strong electron correlation. Thus $S$ jumps hand
in hand with the total angular momentum as the magnetic field is varied. One
important consequence of this is that the spin blockade (an inhibition of
single-electron tunneling) should occur in some magnetic field regions because
of a spin selection rule. Owing to the flexibility arising from the presence of
both intra-layer and inter-layer correlations, the spin blockade is easier to
realize in double dots than in single dots.