title:
Pairing symmetry and long range pair potential in a weak coupling theory
  of superconductivity

abstract:
  We study the superconducting phase with two component order parameter
scenario, such as, $d_{x^2-y^2} + e^{i\theta}s_{\alpha}$, where $\alpha = xy,
x^2+y^2$. We show, that in absence of orthorhombocity, the usual $d_{x^2-y^2}$
does not mix with usual $s_{x^2+y^2}$ symmetry gap in an anisotropic band
structure. But the $s_{xy}$ symmetry does mix with the usual d-wave for $\theta
=0$. The d-wave symmetry with higher harmonics present in it also mixes with
higher order extended $s$ wave symmetry. The required pair potential to obtain
higher anisotropic $d_{x^2-y^2}$ and extended s-wave symmetries, is derived by
considering longer ranged two-body attractive potential in the spirit of tight
binding lattice. We demonstrate that the dominant pairing symmetry changes
drastically from $d$ to $s$ like as the attractive pair potential is obtained
from longer ranged interaction. More specifically, a typical length scale of
interaction $\xi$, which could be even/odd multiples of lattice spacing leads
to predominant $s/d$ wave symmetry. The role of long range interaction on
pairing symmetry has further been emphasized by studying the typical interplay
in the temperature dependencies of these higher order $d$ and $s$ wave pairing
symmetries.


title: Approximate scaling relation for the anharmonic electron-phonon problem

abstract:
  An approximate scaling relation is found for the transition temperature to a
charge-density-wave instability in the anharmonic electron-phonon problem,
which maps a wide range of interaction strengths, anharmonicities, and phonon
frequencies onto a common functional form. The relation employs the
wave-function renormalization parameter and is valid even for systems that are
not Fermi liquids.


title: Bose-Einstein condensates with vortices in rotating traps

abstract:
  We investigate minimal energy solutions with vortices for an interacting
Bose-Einstein condensate in a rotating trap. The atoms are strongly confined
along the axis of rotation z, leading to an effective 2D situation in the x-y
plane. We first use a simple numerical algorithm converging to local minima of
energy. Inspired by the numerical results we present a variational Ansatz in
the regime where the interaction energy per particle is stronger than the
quantum of vibration in the harmonic trap in the x-y plane, the so-called
Thomas-Fermi regime. This Ansatz allows an easy calculation of the energy of
the vortices as function of the rotation frequency of the trap; it gives a
physical understanding of the stabilisation of vortices by rotation of the trap
and of the spatial arrangement of vortex cores. We also present analytical
results concerning the possibility of detecting vortices by a time-of-flight
measurement or by interference effects. In the final section we give numerical
results for a 3D configuration.


title:
Magnetic Roughness and Domain Correlations in Antiferromagnetically
  Coupled Multilayers

abstract:
  The in-plane correlation lengths and magnetic disorder of magnetic domains in
a transition metal multilayer have been studied using neutron scattering
techniques. A new theoretical framework is presented connecting the observed
scattering to the in-plane correlation length and the dispersion of the local
magnetization vector about the mean macroscopic direction. The results
unambiguously show the highly correlated nature of the antiferromagnetically
coupled domain structure vertically throughout the multilayer. We are easily
able to relate the neutron determined magnetic dispersion and domain
correlations to magnetization and magnetotransport experiments.


title: The critical exponents of fracture precursors

abstract:
  The acoustic emission of fracture precursors is measured in heterogeneous
materials. The statistical behaviour of these precursors is studied as a
function of the load features and the geometry. We find that the time interval
between events (precursors) and events energies are power law distributed and
that the exponents of these power laws depend on the load history and on the
material. In contrast, the cumulated acoustic energy presents a critical
divergency near the breaking time. The critical exponent is independent, within
error bars, by all the experimental parameters.


title: Spin-singlet hierarchy in the fractional quantum Hall effect

abstract:
  We show that the so-called permanent quantum Hall states are formed by the
integer quantum Hall effects on the Haldane-Rezayi quantum Hall state. Novel
conformal field theory description along with this picture is deduced. The odd
denominator plateaux observed around $\nu=5/2$ are the permanent states if the
$\nu=5/2$ plateau is the Haldane-Rezayi state. We point out that there is no
such hierarchy on other candidate states for $\nu=5/2$. We propose experiments
to test our prediction.


title:
Effects of annealing and strain on La_{1-x}Ca_{x}MnO_{3} thin films: a
  new phase diagram in the ferromagnetic region

abstract:
  Oriented, single phase thin films of La_{1-x}Ca_{x}MnO_{3} have been
deposited onto (100)-oriented LaAlO_{3} (0.1<x<0.5) substrates using the Pulsed
Laser Deposition technique. While for some compositions the physical properties
(transport and magnetization) of the as-grown films are higher than the bulk
values, for other calcium contents the optimized properties are obtained only
after annealing under oxygen. These data can be partly explained by changes in
oxygen content, resulting in cationic vacancies and thus self-doping effects -
accompanying structural changes, may be the cause of properties beyond the
phase diagram. We propose a new phase diagram for
(La_{1-x}Ca_{x})_{1-y}\square_{y}MnO_{3} (0.1<x<0.5) thin films.


title: Three-parameter model of a sand pile

abstract:
  The theory of a flux steady-state (avalanche) formation is presented for the
simplest model of a real sand pile within the framework of Lorenz approach. The
stationary values of sand velocity and sand pile slope are derived as functions
of controlling parameter (externally driven sandpile slope). The additive
noises of above values are taken into account to build the phase diagram, where
the noise intensities determine a domain of the avalanche appearance. This
domain shows to be crucial to the noise intensity of vertical component of sand
velocity.


title: Luttinger liquid behavior in single wall nanotubes

abstract:
  Transport properties of metallic single-wall nanotubes are examined based on
the Luttinger liquid theory. Focusing on a nanotube transistor setup, the
linear conductance is computed from the Kubo formula using perturbation theory
in the lead-tube tunnel conductances. For sufficiently long nanotubes and high
temperature, phonon backscattering should lead to an anomalous temperature
dependence of the resistivity.


title: Hall-like effect induced by spin-orbit interaction

abstract:
  The effect of spin-orbit interaction on electron transport properties of a
cross-junction structure is studied. It is shown that it results in spin
polarization of left and right outgoing electron waves. Consequently, incoming
electron wave of a proper polarization induces voltage drop perpendicularly to
the direct current flow between source and drain of the considered
four-terminal cross-structure. The resulting Hall-like resistance is estimated
to be of the order of 10^-3 - 10^-2 h/e^2 for technologically available
structures. The effect becomes more pronounced in the vicinity of resonances
where Hall-like resistance changes its sign as function of the Fermi energy.


title: Spinless impurities in high Tc cuprates: Kondo-like behavior

abstract:
  We compare the effects of in-plane non magnetic Li$^{+}$ and Zn$^{2+}$
impurities on the normal state of high-T$_{c}$ cuprates. Y NMR shows that the
extra hole introduced by Li is not localized in its vicinity. The Tc depression
and induced moments on near neighbour Cu sites of Zn or Li are found identical.
These universal effects of spinless impurities establish the major influence of
the spin perturbation with respect to the charge defect. The susceptibility of
the induced moment measured by Li NMR displays a 1/(T+Theta) behavior. Theta
increases with doping up to about 200 K in the overdoped regime. We attribute
this to a "Kondo like" effect.


title:
Three-Body and One-Body Channels of the Auger Core-Valence-Valence
  decay: Simplified Approach

abstract:
  We propose a computationally simple model of Auger and APECS line shapes from
open-band solids. Part of the intensity comes from the decay of unscreened
core-holes and is obtained by the two-body Green's function $G^{(2)}_{\omega}$,
as in the case of filled bands. The rest of the intensity arises from screened
core-holes and is derived using a variational description of the relaxed ground
state; this involves the two-holes-one-electron propagator $G_{\omega}$, which
also contains one-hole contributions. For many transition metals, the two-hole
Green's function $G^{(2)}_{\omega}$ can be well described by the Ladder
Approximation, but the three-body Green's function poses serious further
problems. To calculate $G_{\omega}$, treating electrons and holes on equal
footing, we propose a practical approach to sum the series to all orders. We
achieve that by formally rewriting the problem in terms of a fictitious
three-body interaction. Our method grants non-negative densities of states,
explains the apparent negative-U behavior of the spectra of early transition
metals and interpolates well between weak and strong coupling, as we
demonstrate by test model calculations.


title: Dimer statistics on the M\"obius strip and the Klein bottle

abstract:
  Closed-form expressions are obtained for the generating function of
close-packed dimers on a $2M \times 2N$ simple quartic lattice embedded on a
M\"obius strip and a Klein bottle. Finite-size corrections are also analyzed
and compared with those under cylindrical and free boundary conditions.
Particularly, it is found that, for large lattices of the same size and with a
square symmetry, the number of dimer configurations on a M\"obius strip is
70.2% of that on a cylinder. We also establish two identities relating dimer
generating functions for M\"obius strips and cylinders.


title:
A Maximum Entropy Method of Obtaining Thermodynamic Properties from
  Quantum Monte Carlo Simulations

abstract:
  We describe a novel method to obtain thermodynamic properties of quantum
systems using Baysian Inference -- Maximum Entropy techniques. The method is
applicable to energy values sampled at a discrete set of temperatures from
Quantum Monte Carlo Simulations. The internal energy and the specific heat of
the system are easily obtained as are errorbars on these quantities. The
entropy and the free energy are also obtainable. No assumptions as to the
specific functional form of the energy are made. The use of a priori
information, such as a sum rule on the entropy, is built into the method. As a
non-trivial example of the method, we obtain the specific heat of the
three-dimensional Periodic Anderson Model.


title: Reducing vortex density in superconductors using the ratchet effect

abstract:
  A serious obstacle that impedes the application of low and high temperature
superconductor (SC) devices is the presence of trapped flux. Flux lines or
vortices are induced by fields as small as the Earth's magnetic field. Once
present, vortices dissipate energy and generate internal noise, limiting the
operation of numerous superconducting devices. Methods used to overcome this
difficulty include the pinning of vortices by the incorporation of impurities
and defects, the construction of flux dams, slots and holes and magnetic
shields which block the penetration of new flux lines in the bulk of the SC or
reduce the magnetic field in the immediate vicinity of the superconducting
device. Naturally, the most desirable would be to remove the vortices from the
bulk of the SC. There is no known phenomenon, however, that could form the
basis for such a process. Here we show that the application of an ac current to
a SC that is patterned with an asymmetric pinning potential can induce vortex
motion whose direction is determined only by the asymmetry of the pattern. The
mechanism responsible for this phenomenon is the so called ratchet effect, and
its working principle applies to both low and high temperature SCs. As a first
step here we demonstrate that with an appropriate choice of the pinning
potential the ratchet effect can be used to remove vortices from low
temperature SCs in the parameter range required for various applications.


title: Hall noise and transverse freezing in driven vortex lattices

abstract:
  We study driven vortices lattices in superconducting thin films. Above the
critical force $F_c$ we find two dynamical phase transitions at $F_p$ and
$F_t$, which could be observed in simultaneous noise measurements of the
longitudinal and the Hall voltage. At $F_p$ there is a transition from plastic
flow to smectic flow where the voltage noise is isotropic (Hall noise =
longitudinal noise) and there is a peak in the differential resistance. At
$F_t$ there is a sharp transition to a frozen transverse solid where the Hall
noise falls down abruptly and vortex motion is localized in the transverse
direction.


title: Threshold criterion for wetting at the triple point

abstract:
  Grand canonical simulations are used to calculate adsorption isotherms of
various classical gases on alkali metal and Mg surfaces. Ab initio adsorption
potentials and Lennard-Jones gas-gas interactions are used. Depending on the
system, the resulting behavior can be nonwetting for all temperatures studied,
complete wetting, or (in the intermediate case) exhibit a wetting transition.
An unusual variety of wetting transitions at the triple point is found in the
case of a specific adsorption potential of intermediate strength. The general
threshold for wetting near the triple point is found to be close to that
predicted with a heuristic model of Cheng et al. This same conclusion was drawn
in a recent experimental and simulation study of Ar on CO_2 by Mistura et al.
These results imply that a dimensionless wetting parameter w is useful for
predicting whether wetting behavior is present at and above the triple
temperature. The nonwetting/wetting crossover value found here is w circa 3.3.


title: Optical spectroscopy of pure and doped CuGeO3

abstract:
  We investigated in detail the optical properties of several
Cu(1-delta)Mg(delta)GeO3 (with delta=0,0.01), and CuGe(1-x)B(x)O3 with B=Si
(x=0,0.007,0.05,0.1), and Al (x=0,0.01) single crystals, in the frequency range
20-32000 cm^-1. We report temperature dependent reflectivity and transmission
measurements, performed with polarized light in order to probe the anisotropy
of the crystals along the b and c axes, and optical conductivity spectra
obtained by Kramers-Kronig transformation or direct inversion of the Fresnel
formula. Special emphasis is given to the far-infrared phonon spectra. The
temperature dependence of the phonon parameters is presented and discussed in
relation to the soft mode issue in CuGeO3. For T<Tsp we could detect zone
boundary folded modes activated by the spin-Peierls phase transition. Following
the temperature dependence of these modes, which shows the second order
character of the phase transition, we were able to study the effect of doping
on Tsp. Moreover, in transmission experiments we detected a direct
singlet-triplet excitation at 44 cm^-1, across the magnetic gap, which is not
understandable on the basis of the magnetic excitation spectrum so far assumed
for CuGeO3. The optical activity of this excitation and its polarization
dependence confirm the existence of a second (optical) magnetic branch,
recently suggested on the basis of inelastic neutron scattering data. The
anisotropy in the magnetic exchange constants along the b axis, necessary for
the optical triplet mode to gain a finite intensity, and the strong effect of
Si substitution on the phonon spectra are discussed in relation to the
alternative space group P2(1)2(1)2(1), recently proposed for CuGeO3 in the high
temperature uniform phase.


title: Spin Hall effect

abstract:
  It is proposed that when a charge current circulates in a paramagnetic metal
a transverse spin imbalance will be generated, giving rise to a 'spin Hall
voltage'. Similarly, that when a spin current circulates a transverse charge
imbalance will be generated, hence a Hall voltage, in the absence of charge
current and magnetic field. Based on these principles we propose an experiment
to generate and detect a spin current in a paramagnetic metal.


title:
Synchronization under periodic modulation of potential wells in a
  two-state stochastic system

abstract:
  We analyse the effect of synchronization between noise and periodic signal in
a two-state spatially extended system analytically. Resonance features are
demonstrated. To have the maximum cooperation between signal and noise, it is
shown that noise strength at resonance should increase linearly with the
frequency of the signal. The time scale of the process at resonance is also
shown to increase linearly with the period of the signal.


title: Memory in the aging of a polymer glass

abstract:
  Low frequency dielectric measurements on plexiglass (PMMA) show that cooling
and heating the sample at constant rate give an hysteretic dependence on
temperature of the dielectric constant $\epsilon$. A temporary stop of cooling
produces a downward relaxation of $\epsilon$. Two main features are observed i)
when cooling is resumed $\epsilon$ goes back to the values obtained without the
cooling stop (i.e. the low temperature state is independent of the cooling
history) ii) upon reheating $\epsilon$ keeps the memory of the aging history
({\it Memory}). The analogies and differences with similar experiments done in
spin glasses are discussed.


title:
Modeling Premartensitic effects in Ni2MnGa: A Mean Field and Monte Carlo
  Simulation Study

abstract:
  The degenerate Blume-Emery-Griffiths model for martensitic transformations is
extended by including both structural and magnetic degrees of freedom in order
to elucidate premartensitic effects. Special attention is paied to the effect
of the magnetoelastic coupling in Ni2MnGa. The microscopic model is constructed
and justified based on the analysis of the experimentally observed strain
variables and precursor phenomena. The description includes the (local)
tetragonal distortion, the amplitude of the plane-modulating strain, and the
magnetization. The model is solved by means of mean-field theory and Monte
Carlo simulations. The results show that a variety of premartensitic effects
may appear due to the magnetoelastic coupling. For large values of the
magnetoelastic coupling parameter we find a premartensitic first-order
transition line ending in a critical point. This critical point is responsible
for the existence of large premartensitic fluctuations which manifest as broad
peaks in the specific heat, not always associated with a true phase transition.
The main conclusion is that premartensitic effects result from the interplay
between the softness of the anomalous phonon driving the modulation and the
magnetoelastic coupling. In particular, the premartensitic transition occurs
when such coupling is strong enough to freeze the involved mode phonon.


title: An evolutionary model for simple ecosystems

abstract:
  In this review some simple models of asexual populations evolving on smooth
landscapes are studied. The basic model is based on a cellular automaton, which
is analyzed here in the spatial mean-field limit. Firstly, the evolution on a
fixed fitness landscape is considered. The correspondence between the time
evolution of the population and equilibrium properties of a statistical
mechanics system is investigated, finding the limits for which this mapping
holds. The mutational meltdown, Eigen's error threshold and Muller's ratchet
phenomena are studied in the framework of a simplified model. Finally, the
shape of a quasi-species and the condition of coexistence of multiple species
in a static fitness landscape are analyzed. In the second part, these results
are applied to the study of the coexistence of quasi-species in the presence of
competition, obtaining the conditions for a robust speciation effect in asexual
populations.


title: Output from an atom laser: theory vs. experiment

abstract:
  Atom lasers based on rf-outcoupling can be described by a set of coupled
generalized Gross-Pitaevskii equations (GPE). We compare the theoretical
predictions obtained by numerically integrating the time-dependent GPE of an
effective one-dimensional model with recently measured experimental data for
the F=2 and F=1 states of Rb-87. We conclude that the output of a rf-atom laser
can be well described by this model.


title:
The Crossover from the Bulk to the Few-Electron limit in Ultrasmall
  Metallic Grains

abstract:
  We study the properties of ultrasmall metallic grains with sizes in the range
of 20 up to 400 electrons. Using a particle-hole version of the DMRG method we
compute condensation energies, spectroscopic gaps, pairing parameters and
particle-hole probabilities of the ground state wave function. The results
presented in this paper confirm that the bulk superconducting regime (large
grains) and the fluctuation dominated regime (small grains) are qualitative
different, but show that the crossover between them is very smooth with no
signs of critical level spacings separating them. We compare our DMRG results
with the exact ones obtained with the Richardson solution finding complete
agreement. We also propose a simplified version of the DMRG wave function,
called the Particle-Hole BCS ansatz, which agrees qualitatively with the DMRG
solution and illustrates what is lacking in the PBCS wave function in order to
describe correctly the crossover. Finally we present a new recursive method to
compute norms and expectation values with the PBCS wave function.


title:
Screening of the Photon Field in Surface Photoemission from Simple
  Metals

abstract:
  A previous two-step hydrodynamic model calculation of angle-resolved
photoemission has been improved by modeling the surface with a soft analytic
function. The same ground-state potential is used to screen the photon field
and to calculate the electronic wave functions. The implemented code can be
used for arbitrary ground state densities along the perpendicular direction.
Mathematical and physical aspects of the model are discussed extensively. The
behavior of the main peak of the spectrum in response to the variation of some
parameters leads us to the conclusion that the peak cannot be interpreted as
the direct excitation of multipole modes. This conclusion was suggested in the
previous two-step model calculation.


title: Wetting films on chemically heterogeneous substrates

abstract:
  Based on a microscopic density functional theory we investigate the
morphology of thin liquidlike wetting films adsorbed on substrates endowed with
well-defined chemical heterogeneities. As paradigmatic cases we focus on a
single chemical step and on a single stripe. In view of applications in
microfluidics the accuracy of guiding liquids by chemical microchannels is
discussed. Finally we give a general prescription of how to investigate
theoretically the wetting properties of substrates with arbitrary chemical
structures.


title: Theoretical study of the optical conductivity of $\alpha'-NaV_2O_5$

abstract:
  Using finite temperature diagonalization techniques it is shown that the
quarter-filled t-J-V model on a trellis lattice structure provides a
quantitative explanation of the highly anisotropic optical conductivity of the
$\alpha'-NaV_{2}O_{5}$ compound. The combined effects of the short-range
Coulomb interaction and valence fluctuations of V-ions determine the main
absorption and the fundamental gap. Inter-ladder hopping is necessary for the
explanation of the anomalous in-gap absorption and generation of spectral
weight at high energy. The role of valence fluctuations is explained in terms
of the domain wall excitations of an anisotropic 2D Ising model in a transverse
field close to criticality.


title: Luttinger liquid behavior in multi-wall carbon nanotubes

abstract:
  The low-energy theory for multi-wall carbon nanotubes including the
long-ranged Coulomb interactions, internal screening effects, and
single-electron hopping between graphite shells is derived and analyzed by
bosonization methods. Characteristic Luttinger liquid power laws are found for
the tunneling density of states, with exponents approaching their Fermi liquid
value only very slowly as the number of conducting shells increases. With minor
modifications, the same conclusions apply to transport in ropes of single-wall
nanotubes.


title: Induced spin polarisation in a ferromagnetic gadolinium-yttrium alloy

abstract:
  The first direct evidence of an induced spin moment in Gd(62.4)Y(37.6) is
presented. This additional moment, of 0.16 +/- 0.03 Bohr magnetons, arises from
polarisation of Y electrons in the alloy. The moment was detected in a Compton
scattering experiment via the measurement of the one dimensional projection of
the momentum space electron spin density in Gd and in the alloy. The result is
consistent with theoretical predictions calculated using the LMTO method within
the local spin density approximation.


title:
The thermodynamical liquid-glass transition in a Lennard-Jones binary
  mixture

abstract:
  We use the results derived in the framework of the replica approach to study
the liquid-glass thermodynamic transition. The main results are rederived
without using replicas and applied to the study of the Lennard-Jones binary
mixture introduced by Kob and Andersen. We find that there is a phase
transition due to the entropy crisis. We compute both analytically and
numerically the value of the phase transition point $T_{K}$ and the specific
heat in the low temperature phase.


title: Functions of linear operators: Parameter differentiation

abstract:
  We derive a useful expression for the matrix elements $[\frac{\partial
f[A(t)]}{\partial t}]_{i j}$ of the derivative of a function $f[A(t)]$ of a
diagonalizable linear operator $A(t)$ with respect to the parameter $t$. The
function $f[A(t)]$ is supposed to be an operator acting on the same space as
the operator $A(t)$. We use the basis which diagonalizes A(t), i.e., $A_{i
j}=\lambda_i \delta_{i j}$, and obtain $[\frac{\partial f[A(t)]}{\partial
t}]_{i j}=[\frac{\partial A}{\partial t}]_ {i j}\frac{f(\lambda_j) -
f(\lambda_i)} {\lambda_j - \lambda_i}$. In addition to this, we show that
further elaboration on the (not necessarily simple) integral expressions given
by Wilcox 1967 (who basically considered $f[A(t)]$ of the exponential type) and
generalized by Rajagopal 1998 (who extended Wilcox results by considering
$f[A(t)]$ of the $q$-exponential type where $\exp_q(x) \equiv
[1+(1-q)x]^{1/(1-q)}$ with $q \in {\cal {R}}$; hence, $\exp_1 (x)=\exp(x))$
yields this same expression. Some of the lemmas first established by the above
authors are easily recovered.


title: Dynamics of Nonequilibrium Deposition

abstract:
  In this work we survey selected theoretical developments for models of
deposition of extended particles, with and without surface diffusion, on linear
and planar substrates, of interest in colloid, polymer, and certain biological
systems.


title:
The Loop-Gas Approach to Bose-Einstein Condensation for Trapped
  Particles

abstract:
  We examine Bose-Einstein condensation (BEC) for particles trapped in a
harmonic potential by considering it as a transition in the length of
permutation cycles that arise from wave-function symmetry. This ``loop-gas''
approach was originally developed by Feynman in his path-integral study of BEC
for an homogeneous gas in a box. For the harmonic oscillator potential it is
possible to treat the ideal gas exactly so that one can easily see how standard
approximations become more accurate in the thermodynamic limit (TDL). One
clearly sees that the condensate is made up of very long permutation loops
whose length fluctuates ever more wildly as the number of particles increases.
In the TDL, the WKB approximation, equivalent to the standard approach to BEC,
becomes precise for the non-condensate; however, this approximation neglects
completely the long cycles that make up the condensate. We examine the exact
form for the density matrix for the system and show how it describes the
condensate and behaves in the TDL.


title: Critical exponents for random knots

abstract:
  The size of a zero thickness (no excluded volume) polymer ring is shown to
scale with chain length $N$ in the same way as the size of the excluded volume
(self-avoiding) linear polymer, as $N^{\nu}$, where $\nu \approx 0.588$. The
consequences of that fact are examined, including sizes of trivial and
non-trivial knots.


title:
Mesoscopic coherence effect in superconductor-metallic ferromagnet
  structures

abstract: The boundary condition in Eq.6 is invalid.


title: Quantum ballistic transport in constrictions of n-PbTe

abstract:
  Conductance of submicron constrictions of PbTe:Bi was studied up to 8T and
between 4.2K and 50mK. The structures were fabricated by electron beam
lithography and chemical etching of high--electron mobility films grown by MBE
on BaF_2. In the moderately strong magnetic fields perpendicular to the
current, B>1T, the conductance shows accurate quantization in the units of
1e^2/h as a function of the side-gate voltage. In the absence of the field, a
temperature-independent step structure, with an average step height approx.
1e^2/h, is observed. It is suggested that such a quantization may reflect the
lifting of the Kramers degeneracy by the exchange interaction among the
electrons, effective despite a large dielectric constant of bulk PbTe.


title: Electron spectroscopy of the shot noise reduction effect

abstract:
  A general formula for current noise in a two-terminal ballistic nondegenerate
conductor under the action of long-range Coulomb correlations has been derived.
The noise reduction factor (in respect to the uncorrelated value) is obtained
for biases ranging from thermal to shot noise limits, and it is related to
spatial variations in transport characteristics. The contributions of different
energy groups of carriers to the noise are found, that leads us to suggest an
electron energy spectroscopy experiment to probe the Coulomb correlations in
ballistic conductors.


title:
Pair Excitations, Collective Modes and Gauge Invariance in the BCS --
  Bose-Einstein Crossover Scenario

abstract:
  In this paper we study the BCS Bose Einstein condensation (BEC) crossover
scenario within the superconducting state, using a T-matrix approach which
yields the ground state proposed by Leggett. Here we extend this ground state
analysis to finite temperatures T and interpret the resulting physics. We find
two types of bosonic-like excitations of the system: long lived, incoherent
pair excitations and collective modes of the superconducting order parameter,
which have different dynamics. Using a gauge invariant formalism, this paper
addresses their contrasting behavior as a function of T and superconducting
coupling constant g. At a more physical level, our paper emphasizes how, at
finite T, BCS-BEC approaches introduce an important parameter \Delta^2_{pg} =
\Delta^2 - \Delta_{sc}^2 into the description of superconductivity. This
parameter is governed by the pair excitations and is associated with
particle-hole asymmetry effects which are important for sufficiently large g.
In the fermionic regime, \Delta_{pg}^2 represents the difference between the
square of the excitation gap \Delta^2 and that of the superconducting order
parameter \Delta_{sc}^2. The parameter \Delta_{pg}^2, which is necessarily zero
in the BCS (mean field) limit increases monotonically with the strength of the
attractive interaction g. It follows that there is a significant physical
distinction between this BCS-BEC crossover approach (in which g is the
essential variable which determines \Delta_{pg}) and the widely discussed
(Coulomb-modulated) phase fluctuation scenario in which the plasma frequency is
the tuning parameter.


title:
Simulation and Experimental Investigation of Cellular Material Breakage
  Using the Pulsed Electric Field Treatment

abstract:
  We consider the simplified dielectric breakage model used for simulation of
the kinetics of cellular material breakage under the pulsed electric field
(PEF) treatment. The model is based on an effective media approximation, which
includes equations with the same morphology parameters as in percolation
theory. The probability of a whole cell breakage by the pulse with $t_{i}$
duration is estimated on the basis of electroporation theory. We account for
the bridging effect resulting from the deviations of the local conductivity
near the selected cell from the average effective media conductivity. The most
important feature of the proposed model is the existence of the ``jamming''
behaviour occurring sometimes in experimental observations of the biological
tissue breakage. The different transitions corresponding to the ``jamming''
steps are identified. The experimental results are obtained for thin apple
slices treated with electric pulses at field strengths $E=0.2-2.2$ kV
cm$^{-1}$, pulse durations $t_{i}=10-100$ $\mu$s, pulse repetition times
$t=10-100$ ms and the number of pulses $N=1-100000$. The model gives results
consistent in general with the experimental observations. We discuss the
correlation between the degree of cellular material destruction, field
strength, time of PEF treatment and power consumption.


title: Vortex State Structure of a Bose Condensate in an Asymmetric Trap

abstract:
  Based on an analytic solution of the Gross-Pitaevskii equation in the
large-condensate (Thomas-Fermi) limit we determine the structure of a
stationary vortex in a Bose-Einstein condensate in a nonaxisymmetric rotating
trap. The condensate velocity field has cylindrical symmetry only near the
vortex core and becomes intrinsically anisotropic near the condensate
boundaries. Rotating the anisotropic trap induces an additional irrotational
velocity field even for a vortex-free condensate.


title: Where are we in the theory of High Temperature Superconductors ?

abstract:
  In this talk I will briefly review our present theoretical understanding of
some of the important issues in the high Tc cuprates. In view of its success,
at a qualitative level and some times quantitative level, the theory initiated
by Anderson and developed further by him and collaborators will be discussed.
Many issues that still challenges us will be pointed out.


title:
K S Krishnan and the early experimental evidences for the Jahn-Teller
  Theorem

abstract:
  Jahn-Teller theorem, proposed in 1937, predicts a distortional instability
for a molecule that has symmetry based degenerate electronic states. In 1939
Krishnan emphasized the importance of this theorem for the arrangement of water
molecules around the transition metal or rare earth ions in aqueous solutions
and hydrated saltes, in a short and interesting paper published in Nature by
pointing out atleast four existing experimental results in support of the
theorem. This paper of Krishnan has remained essentially unknown to the
practitioners of Jahn-Teller effect, eventhough it pointed to the best
experimental results that were available, in the 30's and 40's, in support of
Jahn-Teller theorem. Some of the modern day experiments are also in conformity
with some specific suggestions of Krishnan.


title:
Effects of dilute Zn impurities on the uniform magnetic susceptibility
  of YBa2Cu3O{7-delta}

abstract:
  The effects of dilute Zn impurities on the uniform magnetic susceptibility
are calculated in the normal metallic state for a model of the spin
fluctuations of the layered cuprates. It is shown that scatterings from
extended impurity potentials can lead to a coupling of the q~(pi,pi) and the
q~0 components of the magnetic susceptibility chi(q). Within the presence of
antiferromagnetic correlations, this coupling can enhance the uniform
susceptibility. The implications of this result for the experimental data on Zn
substituted YBa2Cu3O{7-delta} are discussed.


title:
Ground State and Spin Glass Phase of the Large N Infinite Range Spin
  Glass Via Supersymmetry

abstract:
  The large N infinite range spin glass is considered, in particular the number
of spin components k needed to form the ground state and the sample-to-sample
fluctuations in the Lagrange multiplier field on each site. The physical
significance of k for the correlation functions is discussed. The difference
between the large N and spherical spin glass is emphasized; a slight difference
between the average Lagrange multiplier of the large N and spherical spin
glasses is derived, leading to a slight increase in the energy of the ground
state compared to the naive expectation. Further, there is a change in the low
energy density of excitations in the large N system. A form of level repulsion,
similar to that found in random matrix theory, is found to exist in this
system, surviving interactions. Even though the system is an interacting one, a
supersymmetric formalism is developed to deal with the problem of averaging
over disorder.


title:
Systematics of c-axis Phonons in the Thallium and Bismuth Based Cuprate
  Superconductors

abstract:
  We present grazing incidence reflectivity measurements in the far infrared
region at temperatures above and below Tc for a series of thallium (Tl2Ba2CuO6,
Tl2Ba2CaCu2O8) and bismuth (Bi2Sr2CuO6, Bi2Sr2CaCu2O8, and
Bi(2-x)Pb(x)Sr2CaCu2O8) based cuprate superconductors. From the spectra, which
are dominated by the c-axis phonons, longitudinal frequencies (LO) are directly
obtained. The reflectivity curves are well fitted by a series of Lorentz
oscillators. In this way the transverse (TO) phonon frequencies were accurately
determined. On the basis of the comparative study of the Bi and Tl based
cuprates with different number of CuO2 layers per unit cell, we suggest
modifications of the assignment of the main oxygen modes. We compare the LO
frequencies in Bi2Sr2CaCu2O8 and Tl2Ba2Ca2Cu3O10 obtained from intrinsic
Josephson junction characteristics with our measurements, and explain the
discrepancy in LO frequencies obtained by the two different methods.


title: Imaging in turbid media using quasi-ballistic photons

abstract:
  We study by means of experiments and Monte Carlo simulations, the scattering
of light in random media, to determine the distance upto which photons travel
along almost undeviated paths within a scattering medium, and are therefore
capable of casting a shadow of an opaque inclusion embedded within the medium.
Such photons are isolated by polarisation discrimination wherein the plane of
linear polarisation of the input light is continuously rotated and the
polarisation preserving component of the emerging light is extracted by means
of a Fourier transform. This technique is a software implementation of lock-in
detection. We find that images may be recovered to a depth far in excess of
what is predicted by the diffusion theory of photon propagation. To understand
our experimental results, we perform Monte Carlo simulations to model the
random walk behaviour of the multiply scattered photons. We present a new
definition of a diffusing photon in terms of the memory of its initial
direction of propagation, which we then quantify in terms of an angular
correlation function. This redefinition yields the penetration depth of the
polarisation preserving photons. Based on these results, we have formulated a
model to understand shadow formation in a turbid medium, the predictions of
which are in good agreement with our experimental results.


title: Universal property of single topological singularity dynamics

abstract:
  Using the random matrix theory we demonstrate explicitly the insensitivity of
the transverse force on a moving vortex to non-intrinsic and non-magnetic
impurities in a superconductor.


title:
Glass transition temperature variation, cross-linking and structure in
  network glasses

abstract:
  We give general topological rules which very accurately predict the chemical
trends in glass transition temperature $T_g$ variation as a function of
cross-linking. In multicomponent glasses, these chemical trends permit to
distinguish homogeneous compositions (random network) from inhomogeneous ones
(local phase separation). The stochastic origin of the Gibbs-Di Marzio equation
is predicted at low connectivity and the analytical expression of its parameter
emerges naturally from the calculation.


title:
Edge State Transport of Separately Contacted Bilayer Systems in the
  Fractional Quantum Hall Regime

abstract:
  Hall and diagonal resistances of bilayer fractional quantum Hall systems are
discussed theoretically. The bilayers have electrodes attached separately to
each layer. They are assumed to be coupled weakly by interlayer tunneling,
while the interlayer Coulomb interaction is negligibly small. It is shown that
source-drain voltage dependence of the resistances reflects the Luttinger
liquid parameter of the edge state.


title:
Excitonic photoluminescence in symmetric coupled double quantum wells
  subject to an external electric field

abstract:
  The effect of an external electric field F on the excitonic photoluminescence
(PL) spectra of a symmetric coupled double quantum well (DQW) is investigated
both theoretically and experimentally. We show that the variational method in a
two-particle electron-hole wave function approximation gives a good agreement
with measurements of PL on a narrow DQW in a wide interval of F including
flat-band regime. The experimental data are presented for an MBE-grown DQW
consisting of two 5 nm wide GaAs wells, separated by a 4 monolayers (MLs) wide
pure AlAs central barrier, and sandwiched between Ga_{0.7}Al_{0.3}As layers.
The bias voltage is applied along the growth direction. Spatially direct and
indirect excitonic transitions are identified, and the radius of the exciton
and squeezing of the exciton in the growth direction are evaluated
variationally. The excitonic binding energies, recombination energies,
oscillator strengths, and relative intensities of the transitions as functions
of the applied field are calculated. Our analysis demonstrates that this simple
model is applicable in case of narrow DQWs not just for a qualitative
description of the PL peak positions but also for the estimation of their
individual shapes and intensities.


title:
$\Gamma(2)$ modular symmetry, renormalization, group flow and the
  quantum Hall effect

abstract:
  We construct a family of holomorphic $\beta$-functions whose RG flow
preserves the $\Gamma(2)$ modular symmetry and reproduces the observed
stability of the Hall plateaus. The semi-circle law relating the longitudinal
and Hall conductivities that has been observed experimentally is obtained from
the integration of the RG equations for any permitted transition which can be
identified from the selection rules encoded in the flow diagram. The generic
scale dependance of the conductivities is found to agree qualitatively with the
present experimental data. The existence of a crossing point occuring in the
crossover of the permitted transitions is discussed.


title: Electronic structure and magnetic interactions in LiV2O4

abstract:
  We present results of all-electron electronic structure calculations for the
recently discovered d electron heavy fermion compound LiV_2O_4. The augmented
spherical wave calculations are based on density functional theory within the
local density approximation. The electronic properties near the Fermi energy
originate almost exclusively from V 3d t_{2g} states, which fall into two
equally occupied subbands: While sigma-type metal-metal bonding leads to rather
broad bands, small pi-type p-d overlap causes a narrow peak at E_F. Without the
geometric frustration inherent in the crystal structure, spin-polarized
calculations reveal an antiferromagnetic ground state and ferromagnetic order
at slightly higher energy. Since direct d-d exchange interaction plays only a
minor role, ordering of the localized vanadium moments can be attributed
exclusively to a rather weak superexchange interaction. With the magnetic order
suppressed by the geometric frustration, the remaining spin fluctuations
suggest an explanation of the low temperature behaviour of the specific heat.


title:
Effect of Dilution on First Order Transitions: The Three Dimensional
  Three States Potts Model

abstract:
  We have studied numerically the effect of quenched site dilution on a first
order phase transition in three dimensions. We have simulated the site diluted
three states Potts model studying in detail the second order region of its
phase diagram. We have found that the $\nu$ exponent is compatible with the one
of the three dimensional diluted Ising model whereas the $\eta$ exponent is
definitely different.


title:
Stochastic relaxational dynamics applied to finance: towards
  non-equilibrium option pricing theory

abstract:
  Non-equilibrium phenomena occur not only in physical world, but also in
finance. In this work, stochastic relaxational dynamics (together with path
integrals) is applied to option pricing theory. A recently proposed model (by
Ilinski et al.) considers fluctuations around this equilibrium state by
introducing a relaxational dynamics with random noise for intermediate
deviations called ``virtual'' arbitrage returns. In this work, the model is
incorporated within a martingale pricing method for derivatives on securities
(e.g. stocks) in incomplete markets using a mapping to option pricing theory
with stochastic interest rates. Using a famous result by Merton and with some
help from the path integral method, exact pricing formulas for European call
and put options under the influence of virtual arbitrage returns (or
intermediate deviations from economic equilibrium) are derived where only the
final integration over initial arbitrage returns needs to be performed
numerically. This result is complemented by a discussion of the hedging
strategy associated to a derivative, which replicates the final payoff but
turns out to be not self-financing in the real world, but self-financing {\it
when summed over the derivative's remaining life time}. Numerical examples are
given which underline the fact that an additional positive risk premium (with
respect to the Black-Scholes values) is found reflecting extra hedging costs
due to intermediate deviations from economic equilibrium.


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 to
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.


title:
Breakdown of the lattice polaron picture in La0.7Ca0.3MnO3 single
  crystals

abstract:
  When heated through the magnetic transition at Tc, La0.7Ca0.3MnO3 changes
from a band metal to a polaronic insulator. The Hall constant R_H, through its
activated behavior and sign anomaly, provides key evidence for polaronic
behavior. We use R_H and the Hall mobility to demonstrate the breakdown of the
polaron phase. Above 1.4Tc, the polaron picture holds in detail, while below,
the activation energies of both R_H and the mobility deviate strongly from
their polaronic values. These changes reflect the presence of metallic,
ferromagnetic fluctuations, in the volume of which the Hall effect develops
additional contributions tied to quantal phases.


title:
Nature of Decoupling in the Mixed Phase of Extremely Type-II Layered
  Superconductors

abstract:
  The uniformly frustrated layered XY model is analyzed in its Villain form. A
decouple pancake vortex liquid phase is identified. It is bounded by both
first-order and second-order decoupling lines in the magnetic field versus
temperature plane. These transitions, respectively, can account for the
flux-lattice melting and for the flux-lattice depinning observed in the mixed
phase of clean high-temperature superconductors.


title: Quantum cavitation in liquid $^3$He: dissipation effects

abstract:
  We have investigated the effect that dissipation may have on the cavitation
process in normal liquid $^3$He. Our results indicate that a rather small
dissipation decreases sizeably the quantum-to-thermal crossover temperature
$T^*$ for cavitation in normal liquid $^3$He. This is a possible explanation
why recent experiments have not yet found clear evidence of quantum cavitation
at temperatures below the $T^*$ predicted by calculations which neglect
dissipation.


title:
Correlations between hidden units in multilayer neural networks and
  replica symmetry breaking

abstract:
  We consider feed-forward neural networks with one hidden layer, tree
architecture and a fixed hidden-to-output Boolean function. Focusing on the
saturation limit of the storage problem the influence of replica symmetry
breaking on the distribution of local fields at the hidden units is
investigated. These field distributions determine the probability for finding a
specific activation pattern of the hidden units as well as the corresponding
correlation coefficients and therefore quantify the division of labor among the
hidden units. We find that although modifying the storage capacity and the
distribution of local fields markedly replica symmetry breaking has only a
minor effect on the correlation coefficients. Detailed numerical results are
provided for the PARITY, COMMITTEE and AND machines with K=3 hidden units and
nonoverlapping receptive fields.


title:
Temperature and density extrapolations in canonical ensemble Monte Carlo
  simulations

abstract:
  We show how to use the multiple histogram method to combine canonical
ensemble Monte Carlo simulations made at different temperatures and densities.
The method can be applied to study systems of particles with arbitrary
interaction potential and to compute the thermodynamic properties over a range
of temperatures and densities. The calculation of the Helmholtz free energy
relative to some thermodynamic reference state enables us to study phase
coexistence properties. We test the method on the Lennard-Jones fluids for
which many results are available.


title: Large transverse field tunnel splittings in the Fe_8 spin Hamiltonian

abstract:
  The spin Hamiltonian that describes the molecular magnet Fe$_8$ has biaxial
symmetry with mutually perpendicular easy, medium, and hard magnetic axes.
Previous calculations of the ground state tunnel splittings in the presence of
a magnetic field along the hard axis are extended, and the meaning of the
previously discovered oscillation of this splitting is further clarified.


title: What is the Entanglement Length in a Polymer Melt ?

abstract:
  We present results of molecular dynamics simulations of very long model
polymer chains analyzed by various experimentally relevant techniques. The
segment motion of the chains is found to be in very good agreement with the
repatation model. We also calculated the plateau-modulus G_N. The predicitions
of the entanglement length N_e from G_N and from the mean square displacements
of the chains segments disagree by a factor of about 2.2(2), indicating an
error in the prefactor in the standard formula for G_N. We show that recent
neutron spin echo measurements were carried out for chain lengths which are too
small for a correct determination of N_e.


title:
Tunneling Spectra and Superconducting Gap in Bi2Sr2CaCu2O8+d and
  Tl2Ba2CuO6+d

abstract:
  Tunneling spectra are reported for Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} (Bi-2212)
over a wide doping range using superconductor-insulator-superconductor (SIS)
break junctions. The energy gap inferred from the tunneling data displays a
remarkable monotonic dependence on doping, increasing to very large values in
the underdoped region even as T_c decreases. This leads to unphysically large
values of the strong coupling ratio ($\sim$20). The tunneling spectra are
qualitatively similar over the entire doping range even though the gap
parameter, $\Delta$, changes from 12 meV to 60 meV. Each spectrum exhibits dip
and hump features at high bias with characteristic energies that scale with the
superconducting gap. Tunneling spectra of near optimally-doped
Tl_{2}Ba_{2}CuO_{6+\delta} (Tl-2201) also display a weak dip feature in
superconductor-insulator-normal metal (SIN) junctions. Generated SIS spectra of
Tl-2201 are compared with measured spectra on Bi-2212 and it is concluded that
the dip and hump features are generic to high temperature superconductors.


title:
Ocular dominance patterns in mammalian visual cortex: A wire length
  minimization approach

abstract:
  We propose a theory for ocular dominance (OD) patterns in mammalian primary
visual cortex. This theory is based on the premise that OD pattern is an
adaptation to minimize the length of intra-cortical wiring. Thus we can
understand the existing OD patterns by solving a wire length minimization
problem. We divide all the neurons into two classes: left-eye dominated and
right-eye dominated. We find that segregation of neurons into monocular regions
reduces wire length if the number of connections with the neurons of the same
class differs from that with the other class. The shape of the regions depends
on the relative fraction of neurons in the two classes. If the numbers are
close we find that the optimal OD pattern consists of interdigitating stripes.
If one class is less numerous than the other, the optimal OD pattern consists
of patches of the first class neurons in the sea of the other class neurons. We
predict the transition from stripes to patches when the fraction of neurons
dominated by the ipsilateral eye is about 40%. This prediction agrees with the
data in macaque and Cebus monkeys. This theory can be applied to other binary
cortical systems.

categories: cond-mat.soft cond-mat.dis-nn cs.NE physics.bio-ph q-bio


title: Optimal sizes of dendritic and axonal arbors in a topographic projection

abstract:
  I consider a topographic projection between two neuronal layers with
different densities of neurons. Given the number of output neurons connected to
each input neuron (divergence) and the number of input neurons synapsing on
each output neuron (convergence) I determine the widths of axonal and dendritic
arbors which minimize the total volume of axons and dendrites. Analytical
results for one-dimensional and two-dimensional projections can be summarized
qualitatively in the following rule: neurons of the sparser layer should have
arbors wider than those of the denser layer. This agrees with the anatomical
data from retinal and cerebellar neurons whose morphology and connectivity are
known. The rule may be used to infer connectivity of neurons from their
morphology.

categories: cond-mat.dis-nn cond-mat.soft physics.bio-ph q-bio


title:
Binocular disparity can explain the orientation of ocular dominance
  stripes in primate V1

abstract:
  In the primate primary visual area (V1), the ocular dominance pattern
consists of alternating monocular stripes. Stripe orientation follows
systematic trends preserved across several species. I propose that these trends
result from minimizing the length of intra-cortical wiring needed to recombine
information from the two eyes in order to achieve the perception of depth. I
argue that the stripe orientation at any point of V1 should follow the
direction of binocular disparity in the corresponding point of the visual
field. The optimal pattern of stripes determined from this argument agrees with
the ocular dominance pattern of macaque and Cebus monkeys. This theory predicts
that for any point in the visual field the limits of depth perception are
greatest in the direction along the ocular dominance stripes at that point.


title:
Electronic Structure of Cu_(1-x)Ni_xRh_2S_4 and CuRh_2Se_4: Band
  Structure Calculations, X-ray Photoemission and Fluorescence Measurements

abstract:
  The electronic structure of spinel-type Cu_(1-x)Ni_xRh_2S_4 (x = 0.0, 0.1,
0.3, 0.5, 1.0) and CuRh_2Se_4 compounds has been studied by means of X-ray
photoelectron and fluorescent spectroscopy. Cu L_3, Ni L_3, S L_(2,3) and Se
M_(2,3) X-ray emission spectra (XES) were measured near thresholds at Beamline
8.0 of the Lawrence Berkeley Laboratory's Advanced Light Source. XES
measurements of the constituent atoms of these compounds, reduced to the same
binding energy scale, are found to be in excellent agreement with XPS valence
bands. The calculated XES spectra which include dipole matrix elements show
that the partial density of states reproduce experimental spectra quite well.
States near the Fermi level (E_F) have strong Rh d and S(Se) p character in all
compounds. In NiRh_2S_4 the Ni 3d states contribute strongly at E_F, whereas in
both Cu compounds the Cu 3d bands are only ~1 eV wide and centered ~2.5 eV
below E_F, leaving very little 3d character at E_F. The density of states at
the Fermi level is less in NiRh_2S_4 than in CuRh_2S_4. This difference may
contribute to the observed decrease, as a function of Ni concentration, in the
superconducting transition temperature in Cu_(1-x)Ni_xRh_2S_4. The density of
states of the ordered alloy Cu_(1/2)Ni_(1/2)Rh_2S_4 shows behavior that is more
``split-band''-like than ``rigid band''-like.


title:
Sub-Dominant Pairing Channels in Unconventional Superconductors:
  Ginzburg-Landau Theory

abstract:
  A Ginzburg-Landau theory is developed for unconventional superconductors with
the three relevant singlet pairing channels. Various consequences of the
sub-dominant channels (i.e., s- and d_{xy}-channels) are examined in detail.
(1) In the case of a d_{x^2-y^2}+is-wave superconductor, The structure of a
single vortex above and below T_{DS} is four-fold and two-fold symmetric,
respectively. (2) In the case of a d_{x^2-y^2}+id_{xy}-wave superconductor,
there is also a second order zero-field phase transition from the pure
d_{x^2-y^2}-phase to the Time-reversal-symmetry-breaking
d_{x^2-y^2}+id_{xy}-wave phase at the temperature T_{DD'}. But the subdominant
phase can (not) be induced by vortices above T_{DD'}. Below the time-reversal-
symmetry-breaking transition, the sub-dominant phase in the mixed state is
nontrivial: it survives at low fields, but may disappear above a field
(increasing with decreasing temperature) presumably via a first-order
transition. (3)By including the strong coupling effects, a
time-reversal-symmetry -breaking coupling term between the d_{x^2-y^2}- and
d_{xy}-waves is found to have significant effects on the low temperature
behavior of d_{x^2-y^2}+id_{xy} superconductors. In a magnetic field, a
d_{x^2-y^2}+id_{xy} state is always established, but the field-dependence of
d_{xy}-amplitude above T_{DD'} is different from that below T_{DD'}. Above but
not very close to T_{DD'}, the induced minimum gap Delta_0 proportional to
B/(T-T_{DD'}).


title: Vortex dynamics of a $d+is$-wave superconductor

abstract:
  The vortex dynamics of a d+is-wave superconductor is studied numerically by
simulating the time-dependent Ginzburg-Landau equations. The critical fields,
the free flux flow, and the flux flow in the presence of twin-boundaries are
discussed. The relaxation rate of the order parameter turns out to play an
important role in the flux flow. We also address briefly the intrinsic Hall
effect in d- and d+is-wave superconductors.


title: Phase diagram of the integer quantum Hall effect in p-type Germanium

abstract:
  We experimentally study the phase diagram of the integer quantized Hall
effect, as a function of density and magnetic field. We used a two dimensional
hole system confined in a Ge/SiGe quantum well, where all energy levels are
resolved, because the Zeeman splitting is comparable to the cyclotron energy.
At low fields and close to the quantum Hall liquid-to-insulator transition, we
observe the floating up of the lowest energy level, but NO FLOATING of any
higher levels, rather a merging of these levels into the insulating state. For
a given filling factor, only direct transitions between the insulating phase
and higher quantum Hall liquids are observed as a function of density. Finally,
we observe a peak in the critical resistivity around filling factor one.


title:
Molecular dynamics study of a classical two-dimensional electron system:
  Positional and orientational orders

abstract:
  Molecular dynamics simulation is used to investigate the crystallization of a
classical two-dimensional electron system, in which electrons interact with the
Coulomb repulsion. From the positional and the orientational correlation
functions, we have found an indication that the solid phase has a
quasi-long-range (power-law correlated) positional order and a long-range
orientational order. This implies that the long-range 1/r system shares the
absence of the true long-range crystalline order at finite temperatures with
short-range ones for which Mermin's theorem applies. We also discuss the
existence of the ``hexatic'' phase predicted by the
Kosterlitz-Thouless-Halperin-Nelson-Young theory.


title:
Temperature-induced resonances and Landau damping of collective modes in
  Bose-Einstein condensed gases in spherical traps

abstract:
  Interaction between collective monopole oscillations of a trapped
Bose-Einstein condensate and thermal excitations is investigated by means of
perturbation theory. We assume spherical symmetry to calculate the matrix
elements by solving the linearized Gross-Pitaevskii equations. We use them to
study the resonances of the condensate induced by temperature when an external
perturbation of the trapping frequency is applied and to calculate the Landau
damping of the oscillations.


title:
Internal Spatiotemporal Stochastic Resonance in a Microscopic Surface
  Reaction Model

abstract:
  We show the existence of internal stochastic resonance in a microscopic
stochastic model for the oscillating CO oxidation on single crystal surfaces.
This stochastic resonance arises directly from the elementary reaction steps of
the system without any external input. The lattice gas model is investigated by
means of Monte Carlo simulations. It shows oscillation phenomena and mesoscopic
pattern formation. Stochastic resonance arises once homogeneous nucleation in
the individual surface phases (reconstructed and non-reconstructed) is added.
This nucleation is modelled as a noise process. As a result, synchronization of
the kinetic oscillations is obtained. Internal stochastic resonance may thus be
an internal regulation mechanism of extreme adaptability.


title:
Separation of quadrupolar and magnetic contributions to spin-lattice
  relaxation in the case of a single isotope

abstract:
  We present a NMR pulse double-irradiation method which allows one to separate
magnetic from quadrupolar contributions in the spin-lattice relaxation. The
pulse sequence fully saturates one transition while another is observed. In the
presence of a Delta m = 2 quadrupolar contribution, the intensity of the
observed line is altered compared to a standard spin-echo experiment. We
calculated analytically this intensity change for spins I=1, 3/2, 5/2, thus
providing a quantitative analysis of the experimental results. Since the pulse
sequence we used takes care of the absorbed radio-frequency power, no problems
due to heating arise. The method is especially suited when only one NMR
sensitive isotope is available. Different cross-checks were performed to prove
the reliability of the obtained results. The applicability of this method is
demonstrated by a study of the plane oxygen 17O (I = 5/2) in the
high-temperature superconductor YBa_2Cu_4O_8: the 17O spin-lattice relaxation
rate consists of magnetic as well as quadrupolar contributions.


title: Cluster Persistence: a Discriminating Probe of Soap Froth Dynamics

abstract:
  The persistent decay of bubble clusters in coarsening two-dimensional soap
froths is measured experimentally as a function of cluster volume fraction.
Dramatically stronger decay is observed in comparison to soap froth models and
to measurements and calculations of persistence in other systems. The fraction
of individual bubbles that contain any persistent area also decays, implying
significant bubble motion and suggesting that T1 processes play an important
role in froth persistence.


title: Do crossover functions depend on the shape of the interaction profile?

abstract:
  We examine the crossover from classical to non-classical critical behaviour
in two-dimensional systems with a one-component order parameter. Since the
degree of universality of the corresponding crossover functions is still
subject to debate, we try to induce non-universal effects by adding
interactions with a second length scale. Although the crossover functions
clearly depend on the range of the interactions, they turn out to be remarkably
robust against further variation of the interaction profile. In particular, we
find that the earlier observed non-monotonic crossover of the effective
susceptibility exponent occurs for several qualitatively different shapes of
this profile.


title: Monte Carlo simulation of spin models with long-range interactions

abstract:
  An efficient Monte Carlo algorithm for the simulation of spin models with
long-range interactions is discussed. Its central feature is that the number of
operations required to flip a spin is independent of the number of interactions
between this spin and the other spins in the system. In addition, critical
slowing down is strongly suppressed. In order to illustrate the range of
applicability of the algorithm, two specific examples are presented. First,
some aspects of the Kosterlitz--Thouless transition in the one-dimensional
Ising chain with inverse-square interactions are calculated. Secondly, the
crossover from Ising-like to classical critical behavior in two-dimensional
systems is studied for several different interaction profiles.


title: The Rotational Structure of Molecules In the Quantum He4 and He3 Liquids

abstract:
  It is shown that the drastic distinction in the rotational structure of
molecules in the He4 and He3 liquids observed in [1] is due to the difference
in the spectral density of excitations regardless of the hydrodynamic
properties. The large width of rotational levels in the He3 is determined by
particle-hole excitations whereas the small density of phonon excitations
results only in the small broadening of levels in the He4.


title:
Planar cyclotron motion in unidirectional superlattices defined by
  strong magnetic and electric fields: Traces of classical orbits in the energy
  spectrum

abstract:
  We compare the quantum and the classical description of the two-dimensional
motion of electrons subjected to a perpendicular magnetic field and a
one-dimensional lateral superlattice defined by spatially periodic magnetic and
electric fields of large amplitudes. We explain in detail the complicated
energy spectra, consisting of superimposed branches of strong and of weak
dispersion, by the correspondence between the respective eigenstates and the
``channeled'' and ``drifting'' orbits of the classical description.


title:
Thermodynamically consistent equilibrium properties of normal-liquid
  Helium-3

abstract:
  The high-precision data for the specific heat C_{V}(T,V) of normal-liquid
Helium-3 obtained by Greywall, taken together with the molar volume V(T_0,P) at
one temperature T_0, are shown to contain the complete thermodynamic
information about this phase in zero magnetic field. This enables us to
calculate the T and P dependence of all equilibrium properties of normal-liquid
Helium-3 in a thermodynamically consistent way for a wide range of parameters.
The results for the entropy S(T,P), specific heat at constant pressure
C_P(T,P), molar volume V(T,P), compressibility kappa(T,P), and thermal
expansion coefficient alpha(T,P) are collected in the form of figures and
tables. This provides the first complete set of thermodynamically consistent
values of the equilibrium quantities of normal-liquid Helium-3. We find, for
example, that alpha(T,P) has a surprisingly intricate pressure dependence at
low temperatures, and that the curves alpha(T,P) vs T do not cross at one
single temperature for all pressures, in contrast to the curves presented in
the comprehensive survey of helium by Wilks.
  Corrected in cond-mat/9906222v3: The sign of the coefficient d_0 was
misprinted in Table I of cond-mat/9906222v1 and v2. It now correctly reads
d_0=-7.1613436. All results in the paper were obtained with the correct value
of d_0. (We would like to thank for E. Collin, H. Godfrin, and Y. Bunkov for
finding this misprint.)


title:
Floating bonds and gap states in a-Si and a-Si:H from first principles
  calculations

abstract:
  We study in detail by means of ab-initio pseudopotential calculations the
electronic structure of five-fold coordinated (T_5) defects in a-Si and a-Si:H,
also during their formation and their evolution upon hydrogenation. The
atom-projected densities of states (DOS) and an accurate analysis of the
valence charge distribution clearly indicate the fundamental contribution of
T_5 defects in originating gap states through their nearest neighbors. The
interaction with hydrogen can reduce the DOS in the gap annihilating T_5
defects.


title: Density Matrices for a Chain of Oscillators

abstract:
  We consider chains with an optical phonon spectrum and study the reduced
density matrices which occur in density-matrix renormalization group (DMRG)
calculations. Both for one site and for half of the chain, these are found to
be exponentials of bosonic operators. Their spectra, which are correspondingly
exponential, are determined and discussed. The results for large systems are
obtained from the relation to a two-dimensional Gaussian model.


title:
Incorporation of a non-amphiphilic nematic liquid crystal into a host
  monolayer

abstract:
  Many nematic liquid crystals are not able to form stable monolayers at the
air/water interface because of the lack of a polar headgroup. A possible way to
obtain a monomolecular film with these compounds is to incorporate them into
host monolayers of amphiphilic compounds. Stable monolayers containing a high
fraction of the liquid crystal can be obtained. We have prepared stable and
transferable monolayers of MBBA (which is not an amphiphilic compound) using
octadecylmalonic acid (OMA) as host. The monolayers at the air/water interface
have been characterized by measurements of surface-pressure/area and
surface-potential/area isotherms. The monolayers deposited on quartz plates
have been characterized by determining the transfer ratio and by spectroscopic
measurements.


title: Skyrmion Physics Beyond the Lowest Landau Level Approximation

abstract:
  The effects of Landau level mixing and finite thickness of the
two-dimensional electron gas on the relative stability of skyrmion and single
spin-flip excitations at Landau level filling factor $\nu=1$ have been
investigated. Landau level mixing is studied by fixed-phase diffusion Monte
Carlo and finite thickness is included by modifying the effective Coulomb
interaction. Both Landau level mixing and finite thickness lower skyrmion
excitation energies and favor skyrmions with fewer spin flips. However, the two
effects do not work `coherently'. When finite thickness is included the effect
of Landau level mixing is strongly suppressed.


title:
The backscattering of polarised light from turbid media - An analysis of
  the azimuthal intensity variations and its implications for the position of
  the source of diffusing radiation

abstract:
  We study the azimuthal variation in the backscattered intensity that is seen
when polarised light is scattered by a turbid medium. We present experimental
observations of these intensity variations in colloidal suspensions over a wide
range of optical densities. For a medium composed of spherical scatterers, we
have developed using Mie scattering theory and Monte Carlo simulations of
photon transport, a model which calculates the constant intensity contours of
the backscattered intensity. Comparisons of calculated and experimentally
obtained contours show very good agreement. To our knowledge, this is the first
model to provide a quantitative comparison with experimental data. Close to the
exact backscattering direction, where we have made our intensity measuements,
we show that the patterns are formed by what we have called `reflected snake
photons'. These are photons that have been backscattered once and have
maintained their direction of propagation thereafter until they exit the
medium. We also find that the reflected snake photons originate within a depth
of about $4l^{*}$ from the point of entry of the incident beam, where $l^{*}$
is the photon transport mean free path. Further, in a novel approach, we have
used these patterns as a probe of the assumptions underlying the diffusion
approximation and present new results on the position of the apparent source of
diffusing radiation within the medium. Possible applications are also
discussed.


title: Superconducting ``metals'' and ``insulators''

abstract:
  We propose a characterization of zero temperature phases in disordered
superconductors on the basis of the nature of quasiparticle transport. In three
dimensional systems, there are two distinct phases in close analogy to the
distinction between normal metals and insulators: the superconducting "metal"
with delocalized quasiparticle excitations and the superconducting "insulator"
with localized quasiparticles. We describe experimental realizations of either
phase, and study their general properties theoretically. We suggest experiments
where it should be possible to tune from one superconducting phase to the
other, thereby probing a novel "metal-insulator" transition inside a
superconductor. We point out various implications of our results for the phase
transitions where the superconductor is destroyed at zero temperature to form
either a normal metal or a normal insulator.


title: Ivory Tower Universities and Competitive Business Firms

abstract:
  There is nowadays considerable interest on ways to quantify the dynamics of
research activities, in part due to recent changes in research and development
(R&D) funding. Here, we seek to quantify and analyze university research
activities, and compare their growth dynamics with those of business firms.
Specifically, we analyze five distinct databases, the largest of which is a
National Science Foundation database of the R&D expenditures for science and
engineering of 719 United States (US) universities for the 17-year period
1979--1995. We find that the distribution of growth rates displays a
``universal'' form that does not depend on the size of the university or on the
measure of size used, and that the width of this distribution decays with size
as a power law. Our findings are quantitatively similar to those independently
uncovered for business firms, and consistent with the hypothesis that the
growth dynamics of complex organizations may be governed by universal
mechanisms.


title: Examination of self interaction correction methods for Na clusters

abstract:
  We examine whether the self interaction correction method by Harrison, which
does not introduce the spherical single particle density approximation to
energy functional, can be applied to Na clusters. We show that it does not work
well, especially, for large clusters, though it works well for atomic systems.
We suggest that it is better to apply this method only to the Hartree term. We
also show that the effects of non-diagonal Lagrange multiplier originating from
the orthonormality of single particle orbitals are negligible.


title: Theory of the metamagnetic crossover in CeRu_2Si_2

abstract:
  Based on the periodic Anderson model, it is shown that the competition
between the quenching of magnetic moments by local quantum spin fluctuations
and a magnetic exchange interaction caused by the virtual exchange of pair
excitations of quasiparticles in spin channels is responsible for the
metamagnetic crossover in CeRu_2Si_2, cooperated with the electron-lattice
interaction. The strength of the exchange interaction is proportional to the
bandwidth of quasiparticles and its sign changes with increasing
magnetizations; it is antiferromagnetic in the absence of magnetizations,
whereas it is ferromagnetic in the metamagnetic crossover region. Experimental
results of static quantities are well reproduced.


title:
N-body Study of Anisotropic Membrane Inclusions: Membrane Mediated
  Interactions and Ordered Aggregation

abstract:
  We study the collective behavior of inclusions inducing local anisotropic
curvatures in a flexible fluid membrane. The N-body interaction energy for
general anisotropic inclusions is calculated explicitly, including multi-body
interactions. Long-range attractive interactions between inclusions are found
to be sufficiently strong to induce aggregation. Monte Carlo simulations show a
transition from compact clusters to aggregation on lines or circles. These
results might be relevant to proteins in biological membranes or colloidal
particles bound to surfactant membranes.


title: SO(5) theory of insulating vortex cores in high-$T_c$ materials

abstract:
  We study the fermionic states of the antiferromagnetically ordered vortex
cores predicted to exist in the superconducting phase of the newly proposed
SO(5) model of strongly correlated electrons. Our model calculation gives a
natural explanation of the recent STM measurements on BSCCO, which in
surprising contrast to YBCO revealed completely insulating vortex cores.


title: Multiscale analysis of blood pressure signals

abstract:
  We describe the multiresolution wavelet analysis of blood pressure waves in
vasovagal syncope-affected patients compared with those in healthy people,
using Haar and Gaussian bases. A comparison between scale-dependent and
scale-independent measures discriminating the two classes of subjects is made.
What emerges is a sort of equivalence between these two methodological
approaches, that is, both methods reach the same statistical significance of
separation between the two classes.


title:
Lyapounov exponent and density of states of a one-dimensional
  non-Hermitian Schroedinger equation

abstract:
  We calculate, using numerical methods, the Lyapounov exponent gamma(E) and
the density of states rho(E) at energy E of a one-dimensional non-Hermitian
Schroedinger equation with off-diagonal disorder. For the particular case we
consider, both gamma(E) and rho(E) depend only on the modulus of E. We find a
pronounced maximum of rho(|E|) at energy E=2/sqrt(3), which seems to be linked
to the fixed point structure of an associated random map. We show how the
density of states rho(E) can be expanded in powers of E. We find rho(|E|) =
1/pi^2 + 4/(3 pi^3) |E|^2 + ... This expansion, which seems to be asymptotic,
can be carried out to an arbitrarily high order.


title: Direct ab initio calculation of the dynamical matrix

abstract:
  In this paper there is presented method for ab initio calculation of the
phonon spectra. The method is based upon a direct calculation of the dynamical
matrix via second derivatives of the total energy. The pseudopotential
technique in plane-wave basis set was used to calculate the total energy within
the local density approximation (LDA). For the change of the electron density
there is derived the self-consistent equation which is solved analytically with
no use of iterations. In this paper the attention is paid only to non-metallic
systems.


title:
Thermodynamic properties of the SO(5) theory for the antiferromagnetism
  and d-wave superconductivity: a Monte Carlo study

abstract:
  The present approach takes into account thermal fluctuations both in the
rotation of SO(5) superspins between the AF and SC subspaces, and in the phase
variables of SC order parameters. Temperature vs. g-field phase diagrams for
null external magnetic field are presented, where the g field is conjugate with
the quadratic order parameters and breaks the SO(5) symmetry. The normal(N)/AF
and N/SC phase boundaries merge tangentially at the bicritical point into the
AF/SC phase boundary. Hysteresis phenomenon is observed at the AF/SC phase
transition. Enhancement of AF correlations is observed above the SC critical
temperature in systems with AF couplings stronger than SC ones. Its relation
with the spin-gap phenomenon is addressed. The SO(5) theory in an external
magnetic field is also investigated. At sufficiently large g fields the SC
order is established through a first-order freezing transition of the flux-line
lattice. Short-range AF fluctuations are larger at cores of flux lines than
elsewhere, and decrease continuously to zero with increasing g field. At
intermediate g fields, the flux-line lattice of long-range SC order and the
long-range AF order coexist. Superlattice spots surrounding the strong AF Bragg
peaks at Q=(\pm\pi,\pm\pi) are observed in the simulated structure factor, and
are identified with the modulation by the triangular flux-line lattice of SC.
The AF phase boundary associated with the continuous onset of long-range AF
order drops sharply to the g axis from a finite temperature in the temperature
vs. g-field phase diagram.


title: Franck-Condon Factors as Spectral Probes of Polaron Structure

abstract:
  We apply the Merrifield variational method to the Holstein molecular crystal
model in D dimensions to compute non-adiabatic polaron band energies and
Franck-Condon factors at general crystal momenta. We analyze these observable
properties to extract characteristic features related to polaron self-trapping
and potential experimental signatures. These results are combined with others
obtained by the Global-Local variational method in 1D to construct a polaron
phase diagram encompassing all degrees of adiabaticity and all electron-phonon
coupling strengths. The polaron phase diagram so constructed includes disjoint
regimes occupied by "small" polarons, "large" polarons, and a newly-defined
class of "compact" polarons, all mutually separated by an intermediate regime
occupied by transitional structures.


title:
Negative Magnetoresistance of Bi_2Sr_2CuO_x Single Crystals in a Strong
  Magnetic Fields

abstract:
  Magnetoresistance (MR) in the out-of-plane resistivity $\rho_c$ for the
normal state of the one-layer high-quality Bi_2Sr_2CuO_x single crystals under
various dc magnetic fields up to 28 T over the temperature region 6-100 K has
been measured. We observed the anomalously large negative longitudinal MR up to
60%. At low temperatures the normal-state MR in contrast to the MR in mixed
state is independent of the direction of the current relatively to the field
direction suggesting uniquely the spin dominated origin of that. The magnitude
of the MR is activated in magnetic field and temperature. We interpret the
activated form of $\rho_c$ and the negative MR in terms of 2D stacked
alternating metallic and dielectric layers assuming the tunneling between CuO_2
planes. If the main fluctuations inside CuO_2 planes have magnetic origin, the
magnetic field suppresses these fluctuations leading to the uniform spin
orientation. In this case the interlayer current will be enhanced well.


title:
Quantum Phase Transitions in Two-Dimensional Spin Systems with Ladder,
  Plaquette and Mixed-Spin Structures

abstract:
  Quantum phase transitions between the magnetically ordered and disordered
states are studied for the two-dimensional antiferromagnetic quantum spin
systems with ladder, plaquette, and mixed-spin structures. Starting with
properly chosen singlet-cluster configurations, we perform the series expansion
for the staggered magnetic susceptibility. The phase boundary is determined by
applying the Dlog and biased Pad\'e approximants to the staggered
susceptibility thus obtained. The resulting phase diagram allows us to discuss
the quantum phase transitions quantitatively, which agrees fairly well with the
quantum Monte Carlo results for several cases previously studied.


title: Perturbation Theory for Particle in a Box

abstract:
  Recently developed strong-coupling theory open up the possibility of treating
quantum-mechanical systems with hard-wall potentials via perturbation theory.
To test the power of this theory we study here the exactly solvable quantum
mechanics of a point particle in a one-dimensional box. Introducing an
auxiliary harmonic mass term $m$, the ground-state energy $E^{(0)$ can be
expanded perturbatively in powers of $1/md$, where $d$ is the box size. The
removal of the infrared cutoff $m$ requires the resummation of the series at
infinitely strong coupling. We show that strong-coupling theory yields a
fast-convergent sequence of approximations to the well-known quantum-mechanical
energy $E^{(0)= \pi ^2/2d^2$.


title:
Calculations of exchange interaction in impurity band of two-dimensional
  semiconductors with out of plane impurities

abstract:
  We calculate the singlet-triplet splitting for a couple of two-dimensional
electrons in the potential of two positively charged impurities which are
located out of plane. We consider different relations between vertical
distances of impurities $h_1$ and $h_2$ and their lateral distance $R$. Such a
system has never been studied in atomic physics but the methods, worked out for
regular two-atomic molecules and helium atom, have been found to be useful.
Analytical expressions for several different limiting configurations of
impurities are obtained an interpolated formula for intermediate range of
parameters is proposed. The $R$-dependence of the splitting is shown to become
weaker with increasing $h_1,h_2$.


title:
Perturbation Theory by Flow Equations: Dimerized and Frustrated S=1/2
  Chain

abstract:
  The flow equation method (Wegner 1994) is used as continuous unitary
transformation to construct perturbatively effective Hamiltonians. The method
is illustrated in detail for dimerized and frustrated antiferromagnetic S=1/2
chains. The effective Hamiltonians conserve the number of elementary
excitations which are S=1 magnons for the dimerized chains. The sectors of
different number of excitations are clearly separated. Easy-to-use results for
the gap, the dispersion and the ground state energies of the chains are
provided.


title:
Proximity effects near the interface between d-wave superconductors and
  ferro/antiferromagnets

abstract:
  We study the proximity effects near the interface between a d-wave
superconductor (S) and a ferromagnet (F) or an antiferromagnet (AF). The S-side
(F and AF-side) is described by the attractive (repulsive) Hubbard model, and
the Bogoliubov de Gennes equation derived within the Hartree-Fock approximation
is solved numerically. The superconducting order parameter and the
magnetization (m) can coexist near the interface, and the spatial variation of
m induces the spin-triplet p-wave component in both F and AF cases. The local
density of states is also calculated and discussed.


title: Solvable Models of Random Hetero-Polymers at Finite Density: I. Statics

abstract:
  We introduce $\infty$-dimensional versions of three common models of random
hetero-polymers, in which both the polymer density and the density of the
polymer-solvent mixture are finite. These solvable models give valuable insight
into the problems related to the (quenched) average over the randomness in
statistical mechanical models of proteins, without having to deal with the hard
geometrical constraints occurring in finite dimensional models. Our exact
solution, which is specific to the $\infty$-dimensional case, is compared to
the results obtained by a saddle-point analysis and by the grand ensemble
approach, both of which canalso be applied to models of finite dimension. We
find, somewhat surprisingly, that the saddle-point analysis can lead to
qualitatively incorrect results.


title:
Fast-Convergent Resummation Algorithm and Critical Exponents of
  phi^4-Theory in Three Dimensions

abstract:
  We develop an efficient algorithm for evaluating divergent perturbation
expansions of field theories in the bare coupling constant g_B for which we
possess a finite number L of expansion coefficients plus two more informations:
The knowledge of the large-order behavior proportional to (- alpha)^kk!k^beta
g_B^k, with a known growth parameter alpha, and the knowledge of the approach
to scaling being of the type c+c'/g_B^ omega, with constants c,c' and a
critical exponent of approach omega . The latter information leads to an
increase in the speed of convergence and a high accuracy of the results. The
algorithm is applied to the six- and seven-loop expansions for the critical
exponents of O(N)-symmetric phi^4-theories, and the result for the critical
exponent alpha is compared with the recent satellite experiment.


title: Erratum: Small-world networks: Evidence for a crossover picture

abstract: We correct the value of the exponent \tau.


title: Voronoi-Delaunay analysis of normal modes in a simple model glass

abstract:
  We combine a conventional harmonic analysis of vibrations in a one-atomic
model glass of soft spheres with a Voronoi-Delaunay geometrical analysis of the
structure. ``Structure potentials'' (tetragonality, sphericity or perfectness)
are introduced to describe the shape of the local atomic configurations
(Delaunay simplices) as function of the atomic coordinates. Apart from the
highest and lowest frequencies the amplitude weighted ``structure potential''
varies only little with frequency. The movement of atoms in soft modes causes
transitions between different ``perfect'' realizations of local structure. As
for the potential energy a dynamic matrix can be defined for the ``structure
potential''. Its expectation value with respect to the vibrational modes
increases nearly linearly with frequency and shows a clear indication of the
boson peak. The structure eigenvectors of this dynamical matrix are strongly
correlated to the vibrational ones. Four subgroups of modes can be
distinguished.


title: Capital flow in a two-component dynamical system

abstract:
  A model of open economics composed of producers and speculators is
investigated by numerical simulations. The capital flows from the environment
to the producers and from them to the speculators. The price fluctuations are
suppressed by the speculators. When the aggressivity of the speculators grows,
there is a transition from the regime with almost sure profit to a very risky
regime in which very small fraction of speculators have stable gain. The
minimum of price fluctuations occurs close to the transition.

categories: cond-mat.stat-mech adap-org cond-mat.dis-nn nlin.AO q-fin.TR


title: Prospects for p-wave paired BCS states of fermionic atoms

abstract:
  We present theoretical prospects for creating p-wave paired BCS states of
magnetic trapped fermionic atoms. Based on our earlier proposal of using dc
electric fields to control both the strength and anisotropic characteristic of
atom-atom interaction and our recently completed multi-channel atomic collision
calculations we discover that p-wave pairing with $^{40}$K and $^{82,84,86}$Rb
in the low field seeking maximum spin polarized state represent excellent
choices for achieving superfluid BCS states; and may be realizable with current
technology in laser cooling, magnetic trapping, and evaporative/sympathetic
cooling, provided the required strong electric field can be applied. We also
comment on the prospects of similar p-wave paired BCS states in $^{6}$Li, and
more generally on creating other types exotic BCS states. Our study will open a
new area in the vigorous pursuit to create a quantum degenerate fermionic atom
vapor.


title:
Oxygen Ordering Superstructures and Structural Phase Diagram of
  YBa_2Cu_3O_{6+x} Studied by Hard X-ray Diffraction

abstract:
  High energy x-ray diffraction is used to investigate the bulk oxygen ordering
properties of YBa_2Cu_3O_{6+x}. Superstructures of Cu-O chains aligned along
the b axis and ordered with periodicity ma, along the a axis have been
observed. For $x < 0.62$ the only observed superstructure is ortho-II with m=2.
At room temperature we find ortho-III (m=3) for $0.72\le x\le 0.82$, ortho-V
(m=5) in a mixed state with ortho-II at x=0.62, and ortho-VIII (m=8) at x=0.67.
Ortho-II is a 3D ordered structural phase, the remaining ones are essentially
2D. None of the superstructures develops long range ordering. Studies of the
ortho-II ordering properties in samples prepared with x=0.5 but by different
methods show that finite size domains with internal thermodynamic equilibrium
are formed. The crystal perfection as well as the thermal annealing history
determine the domain size. The temperature dependence of the observed
superstructure ordering is investigated explicitly and a structural phase
diagram is presented.


title:
Giant Enhancement of Surface Second Harmonic Generation in BaTiO_3 due
  to Photorefractive Surface Wave Excitation

abstract:
  We report observation of strongly enhanced surface SHG in BaTiO_3 due to
excitation of a photorefractive surface electromagnetic wave. Surface SH
intensity may reach 10^{-2} of the incident fundamental light intensity.
Angular, crystal orientation and polarization dependencies of this SHG are
presented. Possible applications of this effect in nonlinear surface
spectroscopy are discussed.


title: A Non-Crossing Approximation for the Study of Intersite Correlations

abstract:
  We develop a Non-Crossing Approximation (NCA) for the effective cluster
problem of the recently developed Dynamical Cluster Approximation (DCA). The
DCA technique includes short-ranged correlations by mapping the lattice problem
onto a self-consistently embedded periodic cluster of size $N_c$. It is a fully
causal and systematic approximation to the full lattice problem, with
corrections ${\cal{O}}(1/N_c)$ in two dimensions. The NCA we develop is a
systematic approximation with corrections ${\cal{O}}(1/N_c^3)$. The method will
be discussed in detail and results for the one-particle properties of the
Hubbard model are shown. Near half filling, the spectra display pronounced
features including a pseudogap and non-Fermi-liquid behavior due to
short-ranged antiferromagnetic correlations.


title: Percolation of Superconductivity

abstract:
  In case of superconductors whose electrons attract each other only if they
are near certain centers, the question arises 'How many such centers are needed
to make the ground state superconducting?' We shall examine it in the context
of a random U Hubbard model. In short we study the case where U_i is -|U| and 0
with probability c and 1-c respectively on a lattice whose sites are labelled i
using the Gorkov decoupling and the Coherent Potential Approximation (CPA). We
argue that for this model there is a critical concentration c_0 below which the
system is not a superconductor.


title: Theory of transient spectroscopy of multiple quantum well structures

abstract:
  A theory of the transient spectroscopy of quantum well (QW) structures under
a large applied bias is presented. An analytical model of the initial part of
the transient current is proposed. The time constant of the transient current
depends not only on the emission rate from the QWs, as is usually assumed, but
also on the subsequent carrier transport across QWs. Numerical simulation was
used to confirm the validity of the proposed model, and to study the transient
current on a larger time scale. It is shown that the transient current is
influenced by the nonuniform distribution of the electric field and related
effects, which results in a step-like behavior of the current. A procedure of
extraction of the QW emission time from the transient spectroscopy experiments
is suggested.


title: Stripes, Carriers, Pseudogap, and Superconductivity in the Cuprates

abstract:
  The electronic structure of the high-T_c cuprates is studied on the basis of
both "large-U" and "small-U" orbitals. A striped structure is obtained, and
three types of carriers: polaron-like "stripons" carrying charge,
"quasielectrons'' carrying charge and spin, and "svivons" carrying spin and
lattice distortion. Anomalous properties of the cuprates and specifically their
transport properties are derived. Pairing is found to result from transitions
between pair states of quasielectrons and stripons through the exchange of
svivons. The pairing results in superconductivity when the stripons conduction
is coherent, and in a pseudogap phase when it is not.


title:
A novel superconducting glass state in disordered thin films in Clogston
  limit

abstract:
  A theory of mesoscopic fluctuations in disordered thin superconducting films
in a parallel magnetic field is developed. At zero temperature, the
superconducting state undergoes a phase transition into a state characterized
by superfluid densities of random signs, instead of a spin polarized disordered
Fermi liquid phase. Consequently, the ground state belongs to the same
universality class as the 2D XY spin glass. As the magnetic field increases
further, mesoscopic pairing states are nucleated in an otherwise homogeneous
spin polarized disordered Fermi liquid. The statistics of these pairing states
is universal depending on the sheet conductance of the 2D film.


title: Effective Linear Two-Body Method for Many-Body Problems

abstract:
  This paper reports a detailed description of the equivalent linear two-body
method for the many body problem, which is based on an approximate reduction of
the many-body Schroedinger equation by the use of a variational principle. To
test the accuracy of the method it 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
harmonic traps at zero temperature. For both cases, it is shown that the method
gives excellent results for large N.


title: Theory of Doped Excitonic Insulators

abstract:
  The theory of doped excitonic insulators is reinvestigated in light of recent
experiments on hexaborides. For the appropriate valley-degenerate X_3,X_3' band
structure, ``intra-valley'' condensation is energetically favored.
Ferromagnetism occurs upon doping due to the quenching of kinetic energy at the
otherwise direct first-order excitonic insulator-metal transition. The phase
diagram includes states of spatially inhomogeneous density and magnetization at
low temperatures.