title:
Screening of a macroion by multivalent ions: A new boundary condition
  for Poisson-Boltzmann equation and charge inversion

abstract:
  Screening of a macroion by multivalent counterions is considered. It is shown
that ions form strongly correlated liquid at the macroion surface. Cohesive
energy of this liquid leads to strong additional attraction of counterions to
the surface. Away from the surface this attraction is taken into account by a
new boundary condition for the Poisson-Boltzmann equation. This equation is
solved with the new boundary condition for a charged flat surface and a long
cylinder. For a cylinder Onsager-Manning theory looses its universality so that
apparent charge of the cylinder is smaller than their theory predicts and
depends on its bare charge. It can also vanish or change sign.


title: Phase-Field Model of Dendritic Sidebranching with Thermal Noise

abstract:
  We investigate dendritic sidebranching during crystal growth in an
undercooled melt by simulation of a phase-field model which incorporates
thermal noise of microscopic origin. As a non-trivial quantitative test of this
model, we first show that the simulated fluctuation spectrum of a
one-dimensional interface in thermal equilibrium agrees with the exact
sharp-interface spectrum up to an irrelevant short-wavelength cutoff comparable
to the interface thickness. Simulations of dendritic growth are then carried
out in two dimensions to compute sidebranching characteristics
(root-mean-square amplitude and mean spacing between sidebranches) as a
function of distance behind the tip. These quantities are found to be in good
overall quantitative agreement with the predictions of the existing linear WKB
theory of noise amplification. The extension of this study to three dimensions
remains needed to determine the origin of noise in experiments.


title: Modeling interest rate dynamics: an infinite-dimensional approach

abstract:
  We present a family of models for the term structure of interest rates which
describe the interest rate curve as a stochastic process in a Hilbert space. We
start by decomposing the deformations of the term structure into the variations
of the short rate, the long rate and the fluctuations of the curve around its
average shape. This fluctuation is then described as a solution of a stochastic
evolution equation in an infinite dimensional space. In the case where
deformations are local in maturity, this equation reduces to a stochastic PDE,
of which we give the simplest example. We discuss the properties of the
solutions and show that they capture in a parsimonious manner the essential
features of yield curve dynamics: imperfect correlation between maturities,
mean reversion of interest rates and the structure of principal components of
term structure deformations. Finally, we discuss calibration issues and show
that the model parameters have a natural interpretation in terms of empirically
observed quantities.

categories: cond-mat.stat-mech cond-mat.dis-nn math.PR q-fin.PR


title: Reply to Comment on "Theory of Spinodal Decomposition"

abstract:
  Rutenberg [cond-mat/9411016] claimed erroneously that I incorrectly apply
only global conservation constraint to systems with local conservation embodied
by continuity equation [PRL vol.72, p.1850 (1994)]. I reply that my theory
describes the order parameter (OP) evolution of all three types of systems:
without conservation law, with global one and with local one. I explain that
continuity equation is a differential form of global conservation law, taking
into account the absence of OP production. It is a supplementary equation, but
it contains an unknown variable - OP flux. In my theory OP flux can be always
found from OP evolution equation and continuity one.


title:
Characterization of transport and magnetic properties in thin film
  La(0.67)(Ca(x)Sr(1-x))(0.33)MnO(3) mixtures

abstract:
  We have grown thin films of (100) oriented
La_{0.67}(Ca_{x}Sr_{1-x})_{0.33}MnO_{3} on (100) NdGaO_{3} substrates by
off-axis sputtering. We have looked at the changes in the resistivity and
magnetoresistance of the samples as the Ca/Sr ratio was varied. We find that as
the calcium fraction is decreased, the lattice match to the substrate
decreases, and the films become more disordered, as observed in transport
measurements and the variation in Curie and peak resistance temperatures. We
find a correlation between the temperature independent and T^2 terms to the low
temperature resistivity. The room temperature magnetoresistance exhibits a
maximum as the peak temperature is increased by the substitution of Sr for Ca,
and a change in the field dependence to the resistivity at room temperature is
observed.


title: The Fluctuation Theorem and Green-Kubo Relations

abstract:
  Green-Kubo and Einstein expressions for the transport coefficients of a fluid
in a nonequilibrium steady state can be derived using the Fluctuation Theorem
and by assuming the probability distribution of the time-averaged dissipative
flux is Gaussian. These expressions are consistent with those obtained using
linear response theory and are valid in the linear regime. It is shown that
these expressions are however, not valid in the nonlinear regime where the
fluid is driven far from equilibrium. We advance an argument for why these
expression are only valid in the linear response, zero field limit.


title: Loops in One Dimensional Random Walks

abstract:
  Distribution of loops in a one-dimensional random walk (RW), or,
equivalently, neutral segments in a sequence of positive and negative charges
is important for understanding the low energy states of randomly charged
polymers. We investigate numerically and analytically loops in several types of
RWs, including RWs with continuous step-length distribution. We show that for
long walks the probability density of the longest loop becomes independent of
the details of the walks and definition of the loops. We investigate crossovers
and convergence of probability densities to the limiting behavior, and obtain
some of the analytical properties of the universal probability density.


title:
Statistical reconstruction of three-dimensional porous media from
  two-dimensional images

abstract:
  A method of modelling the three-dimensional microstructure of random
isotropic two-phase materials is proposed. The information required to
implement the technique can be obtained from two-dimensional images of the
microstructure. The reconstructed models share two-point correlation and
chord-distribution functions with the original composite. The method is
designed to produce models for computationally and theoretically predicting the
effective macroscopic properties of random materials (such as electrical and
thermal conductivity, permeability and elastic moduli). To test the method we
reconstruct the morphology and predict the conductivity of the well known
overlapping sphere model. The results are in very good agreement with data for
the original model.


title:
Thermodynamics of the quantum easy-plane antiferromagnet on the
  triangular lattice

abstract:
  The classical XXZ triangular-lattice antiferromagnet (TAF) shows both an
Ising and a BKT transition, related to the chirality and the in-plane spin
components, respectively. In this paper the quantum effects on the
thermodynamic quantities are evaluated by means of the pure-quantum
self-consistent harmonic approximation (PQSCHA), that allows one to deal with
any spin value through classical MC simulations. We report the internal energy,
the specific heat, and the in-plane correlation length of the quantum XX0 TAF,
for S=1/2, 1, 5/2. The quantum transition temperatures turn out to be smaller
the smaller the spin, and agree with the few available theoretical and
numerical estimates.


title: Quantum Conductors in a Plane

abstract:
  When electrons are confined to move in a plane, strange things happen. For
example, under normal circumstances, they are not expected to conduct
electricity at low temperatures. However, recent experiments on a dilute
electron gas confined to move at the interface between two semiconductors pose
a distinct counterexample to the standard view. Transport measurements reveal
that as the temperature is lowered, the resistivity drops without any signature
of the anticipated up-turn as required by the standard account. It is the
possible existence of a new conducting state in two dimensions that is the
primary focus of this session.


title: Instabilities at [110] Surfaces of d_{x^2-y^2} Superconductors

abstract:
  We compare different scenarios for the low temperature splitting of the
zero-energy peak in the local density of states at (110) surfaces of
d_{x^2-y^2}-wave superconductors, observed by Covington et al.
(Phys.Rev.Lett.79 (1997), 277). Using a tight binding model in the
Bogolyubov-de Gennes treatment we find a surface phase transition towards a
time-reversal symmetry breaking surface state carrying spontaneous currents and
an s+id-wave state. Alternatively, we show that electron correlation leads to a
surface phase transition towards a magnetic state corresponding to a local spin
density wave state.


title:
Initial-Value Problem for Inhomogeneous Condensate: Gaussian
  Approximation and Beyond

abstract:
  Using many-body theory we develop a set of formally exact kinetic equations
for inhomogeneous condensate and one-body observables. The method is illutrated
for phi^4 field theory in 1+1 dimensions. These equations, when computed with
the help of time-dependent projection technique, lead to a systematic
mean-field expansion. The lowest and the higher order terms correspond to,
respectively, the gaussian approximation and the dynamical correlation effect.


title: Edge Logarithmic Corrections probed by Impurity NMR

abstract:
  Semi-infinite quantum spin chains display spin autocorrelations near the
boundary with power-law exponents that are given by boundary conformal field
theories. We show that NMR measurements on spinless impurities that break a
quantum spin chain lead to a spin-lattice relaxation rate 1/T_1^edge that has a
temperature dependence which is a direct probe of the anomalous boundary
exponents. For the antiferromagnetic S=1/2 spin chain, we show that 1/T_1^edge
behaves as T (log T)^2 instead of (log T)^1/2 for a bulk measurement. We show
that, in the case of a one-dimensional conductor described by a Luttinger
liquid, a similar measurement leads to a relaxation rate 1/T_1^{edge} behaving
as T, independent of the anomalous exponent K_rho.


title: Re-entrant Melting in Polydisperse Hard Spheres

abstract:
  The effect of polydispersity on the freezing transition of hard spheres is
examined within a moment description. At low polydispersities a single
fluid-to-crystal transition is recovered. With increasing polydispersity we
find a density above which the crystal melts back into an amorphous phase. The
range of densities over which the crystalline phase is stable shrinks with
increasing polydispersity until, at a certain level of polydispersity, the
crystal disappears completely from the equilibrium phase diagram. The two
transitions converge to a single point which we identify as the polydisperse
analogue of a point of equal concentration. At this point, the freezing
transition is continuous in a thermodynamic sense.


title: Phaseseparation in overdoped Y_{1-0.8}Ca_{0-0.2}Ba_2Cu_3O_{6.96-6.98}

abstract:
  The dimpling in the CuO_2 planes of overdoped Y_{1-y}Ca_yBa_2Cu_3
O_{6.96-6.98} (y=0.02-0.2) has been measured by x-ray absorption-fine-
structure spectroscopy (Y-K EXAFS). A step-like decrease around 12% Ca
indicates a percolation threshold for distorted sites of 5 cells, and thus
phase segregation. We conclude the charge carriers added by substitution of
Y{3+} by Ca{2+} to be trapped at the Ca sites and their nn environment.


title: Skyrme crystal or Skyrme liquid?

abstract:
  We address the quantum melting phase transition of the Skyrme crystal. Based
on generic sum rules for two-dimensional, isotropic electron quantum liquids in
the lowest Landau level, we propose analytic expressions for the pair
distribution functions of spin-polarized and spin-unpolarized liquid phases at
filling factors $2/3\leq\nu\leq 1$. From the pair distribution functions we
calculate the energy of such liquid phases and compare with the energy of the
solid phase. The comparison suggests that the quantum melting phase transition
may lie much closer to $\nu=1$ than ever expected.


title: A single defect approximation for localized states on random lattices

abstract:
  Geometrical disorder is present in many physical situations giving rise to
eigenvalue problems. The simplest case of diffusion on a random lattice with
fluctuating site connectivities is studied analytically and by exact numerical
diagonalizations. Localization of eigenmodes is shown to be induced by
geometrical defects, that is sites with abnormally low or large connectivities.
We expose a ``single defect approximation'' (SDA) scheme founded on this
mechanism that provides an accurate quantitative description of both extended
and localized regions of the spectrum. We then present a systematic
diagrammatic expansion allowing to use SDA for finite-dimensional problems,
e.g. to determine the localized harmonic modes of amorphous media.


title: Statistical properties of genealogical trees

abstract:
  We analyse the statistical properties of genealogical trees in a neutral
model of a closed population with sexual reproduction and non-overlapping
generations. By reconstructing the genealogy of an individual from the
population evolution, we measure the distribution of ancestors appearing more
than once in a given tree. After a transient time, the probability of
repetition follows, up to a rescaling, a stationary distribution which we
calculate both numerically and analytically. This distribution exhibits a
universal shape with a non-trivial power law which can be understood by an
exact, though simple, renormalization calculation. Some real data on human
genealogy illustrate the problem, which is relevant to the study of the real
degree of diversity in closed interbreeding communities.


title: Combinatorial Optimization by Iterative Partial Transcription

abstract:
  A procedure is presented which considerably improves the performance of local
search based heuristic algorithms for combinatorial optimization problems. It
increases the average `gain' of the individual local searches by merging pairs
of solutions: certain parts of either solution are transcribed by the related
parts of the respective other solution, corresponding to flipping clusters of a
spin glass. This iterative partial transcription acts as a local search in the
subspace spanned by the differing components of both solutions. Embedding it in
the simple multi-start-local-search algorithm and in the thermal-cycling
method, we demonstrate its effectiveness for several instances of the traveling
salesman problem. The obtained results indicate that, for this task, such
approaches are far superior to simulated annealing.


title:
A new formalism for the computation of RKKY interaction in aperiodic
  systems

abstract:
  A new formalism to investigate RKKY interaction in aperiodic materials is
presented.
  Particularly useful in the tight-binding scheme, it is used in this paper to
probe the role of local environment and long range quasiperiodic potential on
this mesoscopic coupling. Interesting features are revealed and discussed
within the context of anomalous localization and transport


title:
Effect of Local Magnetic Moments on the Metallic Behavior in Two
  Dimensions

abstract:
  The temperature dependence of conductivity $\sigma (T)$ in the metallic phase
of a two-dimensional electron system in silicon has been studied for different
concentrations of local magnetic moments. The local moments have been induced
by disorder, and their number was varied using substrate bias. The data suggest
that in the limit of $T\to 0$ the metallic behavior, as characterized by
$d\sigma/dT < 0$, is suppressed by an arbitrarily small amount of scattering by
local magnetic moments.


title:
Level statistics inside the vortex of a superconductor and symplectic
  random matrix theory in an external source

abstract:
  In the core of the vortex of a superconductor, energy levels appear inside
the gap. We discuss here through a random matrix approach how these levels are
broadened by impurities. It is first shown that the level statistics is
governed by an ensemble consisting of a symplectic random potential added to a
non-random matrix. A generalization of previous work on the unitary ensemble in
the presence of an external source (which relied on the Itzykson-Zuber
integral) is discussed for this symplectic case through the formalism
introduced by Harish-Chandra and Duistermaat-Heckman. This leads to explicit
formulae for the density of states and for the correlation functions, which
describe the cross-over from the clean to the dirty limits.


title: The new line of attack of analyses of NMR in 2D structures

abstract:
  There was shown that Fourier transform of the negative magnetoresistance
(NMR) which is due to interference correction to the conductivity contains the
information about the area distribution function of the closed paths and about
area dependence of the mean length of closed paths \bar{L}(S). Based on this
line of attack we suggest the method of analysis of NMR and use it for data
treatment of the NMR in 2D structure with doped barrier. There was shown that
in structure investigated \bar{L}(S)dependence is determined by the scattering
anisotropy.


title:
Escape rate of a biaxial nanospin system in a magnetic field : first-
  and second-order transition between quantum and classical regimes

abstract:
  We investigate the escape rate of the biaxial nanospin particle with a
magnetic field applied along the easy axis. The model studied here is described
by the Hamiltonian ${\cal H} = -AS_z^2 - BS_x^2 - HS_z, (A>B>0)$. By reducing
this Hamiltonian to a particle one, we derive, for the first time, an effective
particle potential for this model and find an analytical form of the phase
boundary line between first- and second-order transitions, from which a
complete phase diagram can be obtained. We also derive an analytical form of
the crossover temperature as a function of the applied field at the phase
boundary.


title:
Monte-Carlo simulation of the coherent backscattering of electrons in a
  ballistic system

abstract:
  We study weak localization effects in the ballistic regime as induced by
man-made scatterers. Specular reflection of the electrons off these scatterers
results into backscattered trajectories which interfere with their
time-reversed path resulting in weak localization corrections to the
resistance. Using a semi-classical theory, we calculate the change in
resistance due to these backscattered trajectories. We found that the inclusion
of the exact shape of the scatterers is very important in order to explain the
experimental results of Katine et al. [Superlattices and Microstructures 20,
337 (1996)].


title: Density Matrix Renormalization Group and Reaction-Diffusion Processes

abstract:
  The density matrix renormalization group (DMRG) is applied to some
one-dimensional reaction-diffusion models in the vicinity of and at their
critical point. The stochastic time evolution for these models is given in
terms of a non-symmetric ``quantum Hamiltonian'', which is diagonalized using
the DMRG method for open chains of moderate lengths (up to about 60 sites). The
numerical diagonalization methods for non-symmetric matrices are reviewed.
Different choices for an appropriate density matrix in the non-symmetric DMRG
are discussed. Accurate estimates of the steady-state critical points and
exponents can then be found from finite-size scaling through standard
finite-lattice extrapolation methods. This is exemplified by studying the
leading relaxation time and the density profiles of diffusion-annihilation and
of a branching-fusing model in the directed percolation universality class.


title:
Algorithm for obtaining the gradient expansion of the local density of
  states and the free energy of a superconductor

abstract:
  We present an efficient algorithm for obtaining the gauge-invariant gradient
expansion of the local density of states and the free energy of a clean
superconductor. Our method is based on a new mapping of the semiclassical
linearized Gorkov equations onto a pseudo-Schroedinger equation for a
three-component wave-function psi(x), where one component is directly related
to the local density of states. Because psi(x) satisfies a linear equation of
motion, successive terms in the gradient expansion can be obtained by simple
linear iteration. Our method works equally well for real and complex order
parameter, and in the presence of arbitrary external fields. We confirm a
recent calculation of the fourth order correction to the free energy by
Kosztin, Kos, Stone and Leggett [Phys. Rev. B 58, 9365 (1998)], who obtained a
discrepancy with an earlier result by Tewordt [Z. Phys. 180, 385 (1964)]. We
also give the fourth order correction to the local density of states, which has
not been published before.


title:
Properties of some mean-field like approximations for the triangular
  Ising antiferromagnet

abstract:
  Motivated by a recent proposal of a Bethe approximation for the triangular
Ising antiferromagnet [Phys. Rev. B {\bf 56}, 8241 (1997)], which seems to
predict a disordered phase at any temperature in zero field, we analyze in some
detail several mean-field like approximations for this model, namely the Bethe
approximation itself, the cluster variation method and the hard-spin mean-field
theory. We show: (i) that the disordered phase predicted by the Bethe
approximation is unphysical at low enough temperature because of a negative
entropy; (ii) how the results of the cluster variation method (namely, zero
temperature entropy and critical temperature of the spurious transition)
converge to the exact ones for increasing cluster size; (iii) that it is
possible to construct a cluster variation approximation which yields a
disordered phase which is stable down to zero temperature; (iv) a few, so far
unknown, zero temperature results (entropy and internal energy) of the
hard-spin mean-field theory.


title: How to reconcile Market Efficiency and Technical Analysis

abstract:
  Weak form of the Efficiency Market Hypothesis (EMH) excludes predictions of
future market movements from historical data and makes the technical analysis
(TA) out of law. However the technical analysis is widely used by traders and
speculators who steadely refuse to consider the market as a "fair game" and
survive with such believe. In the paper we make a conjecture that TA and EMH
correspond to different time regimes and show how both technical analysis
predictions for short times and realistic statistical data for larger times can
be obtained in a simple single stock model of Gauge Theory of Arbitrage.


title: Virtual Arbitrage Pricing Theory

abstract:
  We generalize the Arbitrage Pricing Theory (APT) to include the contribution
of virtual arbitrage opportunities. We model the arbitrage return by a
stochastic process. The latter is incorporated in the APT framework to
calculate the correction to the APT due to the virtual arbitrage opportunities.
The resulting relations reduce to the APT for an infinitely fast market
reaction or in the case where the virtual arbitrage is absent. Corrections to
the Capital Asset Pricing Model (CAPM) are also derived.


title: Derivative pricing with virtual arbitrage

abstract:
  In this paper we derive an effective equation for derivative pricing which
accounts for the presence of virtual arbitrage opportunities and their
elimination by the market. We model the arbitrage return by a stochastic
process and find an equation for the average derivative price. This is an
integro-differential equation which, in the absence of the virtual arbitrage or
for an infinitely fast market reaction, reduces to the Black-Scholes equation.
Explicit formulas are obtained for European call and put vanilla options.


title: How to account for virtual arbitrage in the standard derivative pricing

abstract:
  In this short note we show how virtual arbitrage opportunities can be
modelled and included in the standard derivative pricing without changing the
general framework.


title:
Electronic Structure of La2-xSrxCuO4 in the Vicinity of the
  Superconductor-Insulator Transition

abstract:
  We report on the result of angle-resolved photoemission (ARPES) study of
La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) from an optimally doped superconductor
($x=0.15$) to an antiferromagnetic insulator ($x=0$). Near the
superconductor-insulator transition (SIT) $x\sim 0.05$, spectral weight is
transferred with hole doping between two coexisting components, suggesting a
microscopic inhomogeneity of the doped-hole distribution. For the underdoped
LSCO ($x\leq 0.12$), the dispersive band crossing the Fermi level becomes
invisible in the $(0,0)-(\pi,\pi)$ direction unlike
Bi$_2$Sr$_2$CaCu$_2$O$_{8+y}$. These observations may be reconciled with the
evolution of holes in the insulator into fluctuating stripes in the
superconductor.


title:
Theory of Lipid Polymorphism: Application to Phosphatidylethanolamine
  and Phosphatidylserine

abstract:
  We introduce a microscopic model of a lipid with a charged headgroup and
flexible hydrophobic tails, a neutral solvent, and counter ions. Short-ranged
interactions between hydrophilic and hydrophobic moieties are included as are
the Coulomb interactions between charges. Further, we include a short-ranged
interaction between charges and neutral solvent, which mimics the short-ranged,
thermally averaged interaction between charges and water dipoles. We show that
the model of the uncharged lipid displays the usual lyotropic phases as a
function of the relative volume fraction of the headgroup. Choosing model
parameters appropriate to dioleoylphosphatidylethanolamine in water, we obtain
phase behavior which agrees well with experiment. Finally we choose a solvent
concentration and temperature at which the uncharged lipid exhibits an inverted
hexagonal phase and turn on the headgroup charge. The lipid system makes a
transition from the inverted hexagonal to the lamellar phase which is related
to the increased waters of hydration correlated with the increased headgroup
charge via the charge-solvent interaction. The polymorphism displayed upon
variation of pH mimics that of the behavior of phosphatidylserine.


title: Electron-drag effects in coupled electron systems

abstract:
  The advancement of fabrication and lithography techniques of semiconductors
have made it possible to study bi-layer systems made of two electronic layers
separated by distances of several hundred Angstroms. In this situation the
electrons in layer 1 are distinguishable from those in layer 2, and can
communicate through the direct inter-layer Coulomb interaction. In particular,
if a current is applied to one of the layers, the electrons in th e second will
be dragged giving rise to a transresistance $\rho_D$. In this article we review
recent theoretical and experimental developments in the understanding of this
effect. At very low temperatures it turns out that phonons dominate the
transresistance. The direct Coulomb interaction and plasmon excitations are
important at temperatures $T>0.1T_F$, with $T_F$ the Fermi temperature. If a
magnetic field is applied the transresistance is increased, in a very
interesting interplay between $\rho_D$ and Landau quantization. The
non-dissipative drag is also reviewed.


title: Ising spins on the labyrinth

abstract:
  We consider a zero-field Ising model defined on a quasiperiodic graph, the
so-called Labyrinth tiling. Exact information about the critical behaviour is
obtained from duality arguments and the subclass of models which yield
commuting transfer matrices. For the latter, the magnetization is independent
of the position and the phase transition between ordered and disordered phase
belongs to the Onsager universality class. In order to obtain information about
the generic case, we calculate the magnetization for a series of couplings by
standard Monte-Carlo methods.


title: Lee-Yang zeros for substitutional systems

abstract:
  Qualitative and quantitative information about critical phenomena is provided
by the distribution of zeros of the partition function in the complex plane. We
apply this idea to Ising models on non-periodic systems based on substitution.
In 1D we consider the Thue-Morse chain and show that the magnetic field zeros
are filling a Cantor subset of the unit circle, the gaps being related to a
general gap labeling theorem. In 2D we study the temperature zeros of the Ising
model on the Ammann-Beenker tiling. The use of corner transfer matrices allows
an efficient calculation of the partition function for rather large patches
which results in a reasonable estimate of the critical temperature.


title:
Ground State Spin Structure of Strongly Interacting Disordered 1D
  Hubbard Model

abstract:
  We study the influence of on-site disorder on the magnetic properties of the
ground state of the infinite U 1D Hubbard model. We find that the ground state
is not ferromagnetic. This is analyzed in terms of the algebraic structure of
the spin dependence of the Hamiltonian. A simple explanation is derived for the
1/N periodicity in the persistent current for this model.


title:
Charge Relaxation Resistances and Charge Fluctuations in Mesoscopic
  Conductors

abstract:
  A brief overview is presented of recent work which investigates the
time-dependent relaxation of charge and its spontaneous fluctuations on
mesoscopic conductors in the proximity of gates. The leading terms of the low
frequency conductance are determined by a capacitive or inductive emittance and
a dissipative charge relaxation resistance. The charge relaxation resistance is
determined by the ratio of the mean square dwell time of the carriers in the
conductor and the square of the mean dwell time. The contribution of each
scattering channel is proportional to half a resistance quantum. We discuss the
charge relaxation resistance for mesoscopic capacitors, quantum point contacts,
chaotic cavities, ballistic wires and for transport along edge channels in the
quantized Hall regime. At equilibrium the charge relaxation resistance also
determines via the fluctuation-dissipation theorem the spontaneous fluctuations
of charge on the conductor. Of particular interest are the charge fluctuations
in the presence of transport in a regime where the conductor exhibits shot
noise. At low frequencies and voltages charge relaxation is determined by a
nonequilibrium charge relaxation resistance.


title: Metamagnetism in the XXZ model with next-to-nearest-neighbor coupling

abstract:
  We investigate groundstate energies and magnetization curves in the one
dimensional XXZ-model with next to nearest neighbour coupling $\alpha>0$ and
anisotropy $\Delta$ ($-1 \le \Delta \le 1$) at T=0. In between the familiar
ferro- and antiferromagnetic phase we find a transition region -- called
metamagnetic phase -- where the magnetization curve is discontinuous at a
critical field $B_c(\alpha,\Delta)$.


title: Feynman's ratchet and pawl: an exactly solvable model

abstract:
  We introduce a simple, discrete model of Feynman's ratchet and pawl,
operating between two heat reservoirs. We solve exactly for the steady-state
directed motion and heat flows produced, first in the absence and then in the
presence of an external load. We show that the model can act both as a heat
engine and as a refrigerator. We finally investigate the behavior of the system
near equilibrium, and use our model to confirm general predictions based on
linear response theory.


title:
An observation of spin-valve effects in a semiconductor field effect
  transistor: a novel spintronic device

abstract:
  We present the first spintronic semiconductor field effect transistor.
  The injector and collector contacts of this device were made from magnetic
permalloy thin films with different coercive fields so that they could be
magnetized either parallel or antiparallel to each other in different applied
magnetic fields. The conducting medium was a two dimensional electron gas
(2DEG) formed in an AlSb/InAs quantum well.
  Data from this device suggest that its resistance is controlled by two
different types of spin-valve effect: the first occurring at the
ferromagnet-2DEG interfaces; and the second occuring in direct propagation
between contacts.


title:
Quantum versus Semiclassical Description of Selftrapping: Anharmonic
  Effects

abstract:
  Selftrapping has been traditionally studied on the assumption that
quasiparticles interact with harmonic phonons and that this interaction is
linear in the displacement of the phonon. To complement recent semiclassical
studies of anharmonicity and nonlinearity in this context, we present below a
fully quantum mechanical analysis of a two-site system, where the oscillator is
described by a tunably anharmonic potential, with a square well with infinite
walls and the harmonic potential as its extreme limits, and wherein the
interaction is nonlinear in the oscillator displacement. We find that even
highly anharmonic polarons behave similar to their harmonic counterparts in
that selftrapping is preserved for long times in the limit of strong coupling,
and that the polaronic tunneling time scale depends exponentially on the
polaron binding energy. Further, in agreement, with earlier results related to
harmonic polarons, the semiclassical approximation agrees with the full quantum
result in the massive oscillator limit of small oscillator frequency and strong
quasiparticle-oscillator coupling.


title: Interactions Between Charged Spheres in Divalent Counterion Solution

abstract:
  We simulate model systems of charged spherical particles in their counterion
solution and measure the thermodynamic pressure and the pair distribution
function from which we derive effective potentials of mean force. For a system
with only electrostatic and hard core interactions, we investigate the
effective potential between two like-charged spheres in divalent counterion
solution as a function of concentration. We find a strong attractive
interaction for high concentration and a global repulsive effective interaction
for dilute systems. The results indicate a first order phase transition in
sphere-counterion density as a function of global concentration and the
effective sphere-sphere potentials in the dilute (solvated) regime suggest
significant density fluctuations due to short range local minima in the
effective energy surface. Our results arise from a minimal approach model of
several recent experiments on polystyrene latex particles in monovalent
counterion solution.


title:
Canonical Transformation Approach to the Ultrafast Non-linear Optical
  Dynamics of Semiconductors

abstract:
  We develop a theory describing the effects of many-particle Coulomb
correlations on the coherent ultrafast nonlinear optical response of
semiconductors and metals. Our approach is based on a mapping of the nonlinear
optical response of the ``bare'' system onto the linear response of a
``dressed'' system. The latter is characterized by effective time-dependent
optical transition matrix elements, electron/hole dispersions, and interaction
potentials, which in undoped semiconductors are determined by the
single-exciton and two-exciton Green functions in the absence of optical
fields. This mapping is achieved by eliminating the optically-induced charge
fluctuations from the Hamiltonian using a Van Vleck canonical transformation.
It takes into account all many-body contributions up to a given order in the
optical fields as well as important Coulomb-induced quantum dynamics to all
orders in the optical field. Our approach allows us to distinguish between
optical nonlinearities of different origins and provides a physically-intuitive
interpretation of their manifestations in ultrafast coherent nonlinear optical
spectroscopy.


title:
Non-fermi-liquid single particle lineshape of the quasi-one-dimensional
  non-CDW metal Li_{0.9}Mo_{6}O_{17} : comparison to the Luttinger liquid

abstract:
  We report the detailed non-Fermi liquid (NFL) lineshape of the dispersing
excitation which defines the Fermi surface (FS) for quasi-one-dimensional
Li_{0.9}Mo_{6}O_{17}. The properties of Li_{0.9}Mo_{6}O_{17} strongly suggest
that the NFL behavior has a purely electronic origin. Relative to the
theoretical Luttinger liquid lineshape, we identify significant similarities,
but also important differences.


title: The spin quantum Hall effect in unconventional superconductors

abstract:
  We study the properties of the "spin quantum Hall fluid" - a novel spin phase
with quantized spin Hall conductance that is potentially realizable in
superconducting systems with unconventional pairing symmetry. A simple
realization is provided by a $d_{x^2-y^2} + id_{xy}$ superconductor which we
argue has a dimensionless spin Hall conductance equal to two. A theory of the
edge states of the $d_{x^2-y^2}+id_{xy}$ superconductor is developed. The
properties of the transition to a phase with vanishing spin Hall conductance
induced by disorder are considered. We construct a description of this
transition in terms of a supersymmetric spin chain, and use it to numerically
determine universal properties of the transition. We discuss various possible
experimental probes of this quantum Hall physics.


title: Exact exponents for the spin quantum Hall transition

abstract:
  We consider the spin quantum Hall transition which may occur in disordered
superconductors with unbroken SU(2) spin-rotation symmetry but broken
time-reversal symmetry. Using supersymmetry, we map a model for this transition
onto the two-dimensional percolation problem. The anisotropic limit is an
sl(2|1) supersymmetric spin chain. The mapping gives exact values for critical
exponents associated with disorder-averages of several observables in good
agreement with recent numerical results.

categories: cond-mat.mes-hall cond-mat.dis-nn cond-mat.supr-con hep-th


title: Neutron scattering and molecular correlations in a supercooled liquid

abstract:
  We show that the intermediate scattering function $S_n(q,t)$ for neutron
scattering (ns) can be expanded naturely with respect to a set of molecular
correlation functions that give a complete description of the translational and
orientational two-point correlations in the liquid. The general properties of
this expansion are discussed with special focus on the $q$-dependence and hints
for a (partial) determination of the molecular correlation functions from
neutron scattering results are given. The resulting representation of the
static structure factor $S_n(q)$ is studied in detail for a model system using
data from a molecular dynamics simulation of a supercooled liquid of rigid
diatomic molecules. The comparison between the exact result for $S_n(q)$ and
different approximations that result from a truncation of the series
representation demonstrates its good convergence for the given model system. On
the other hand it shows explicitly that the coupling between translational
(TDOF) and orientational degrees of freedom (ODOF) of each molecule and
rotational motion of different molecules can not be neglected in the
supercooled regime.Further we report the existence of a prepeak in the
ns-static structure factor of the examined fragile glassformer, demonstrating
that prepeaks can occur even in the most simple molecular liquids. Besides
examining the dependence of the prepeak on the scattering length and the
temperature we use the expansion of $S_n(q)$ into molecular correlation
functions to point out intermediate range orientational order as its principle
origin.


title: Collisionless modes of a trapped Bose gas

abstract:
  We calculate the excitation frequencies of the low-lying modes of a trapped
Bose-condensed gas at nonzero temperatures. We include in our calculation the
dynamics of the noncondensed cloud, and find agreement with experimental
results if we assume that in the experiment both the in-phase and out-of-phase
monopole modes are excited simultaneously. In order to explore whether this is
indeed the correct explanation, we also calculate how strongly the modes couple
to a perturbation of the external trapping potential.


title:
Quantum-Classical Phase Transition of Escape rate in Biaxial Spin
  Particles

abstract:
  The escape rates of the biaxial single domain spin particles with and without
an applied magnetic field are investigated. Using the strict potential field
description of spin systems developed by Ulyanov and Zaslavskii we obtain new
effective Hamiltonians which are considered to be in exact spin-coordinate
correspondence unlike the well studied effective Hamiltonians with the
approximate correspondence. The sharp first-order transition is found in both
cases. The phase diagram of the transitions depending on the anisotropy
constant and the external field is also given.

categories: cond-mat.soft astro-ph cond-mat.mes-hall hep-th


title: Spin dynamics in high-$T_C$ superconductors

abstract:
  Key features of antiferromagnetic dynamical correlations in high-$T_C$
superconductors cuprates are discussed. In underdoped regime, the sharp
resonance peak, occuring exclusively in the SC state, is accompanied by a
broader contribution located around $\sim$ 30 meV which remains above $T_C$.
Their interplay may induce incommensurate structure in the superconducting
state.


title: Stochastic multiplicative processes with reset events

abstract:
  We study a stochastic multiplicative process with reset events. It is shown
that the model develops a stationary power-law probability distribution for the
relevant variable, whose exponent depends on the model parameters. Two
qualitatively different regimes are observed, corresponding to intermittent and
regular behaviour. In the boundary between them, the mean value of the relevant
variable is time-independent, and the exponent of the stationary distribution
equals -2. The addition of diffusion to the system modifies in a non-trivial
way the profile of the stationary distribution. Numerical and analytical
results are presented.


title:
Attractive forces between anisotropic inclusions in the membrane of a
  vesicle

abstract:
  The fluctuation-induced interaction between two rod-like, rigid inclusions in
a fluid vesicle is studied by means of canonical ensemble Monte Carlo
simulations. The vesicle membrane is represented by a triangulated network of
hard spheres. Five rigidly connected hard spheres form rod-like inclusions that
can leap between sites of the triangular network. Their effective interaction
potential is computed as a function of mutual distance and angle of the
inclusions. On account of the hard-core potential among these, the nature of
the potential is purely entropic. Special precaution is taken to reduce lattice
artifacts and the influence of finite-size effects due to the spherical
geometry. Our results show that the effective potential is attractive and
short-range compared with the rod length L. Its well depth is of the order of
\kappa/10, where \kappa is the bending modulus.


title: Wetting at Non-Planar Substrates: Unbending & Unbinding

abstract:
  We consider fluid wetting on a corrugated substrate using effective
interfacial Hamiltonian theory and show that breaking the translational
invariance along the wall can induce an 'unbending' phase transition in
addition to unbinding. Both first order and second order unbending transitions
can occur at and out of coexistence. Results for systems with short-ranged and
long-ranged forces establish that the unbending critical point is characterised
by hyperuniversal scaling behaviour. We show that, at bulk coexistence, the
adsorption at the unbending critical point is a universal multiple of the
adsorption for the correspondent planar system.


title: Structure-Property Correlations in Model Composite Materials

abstract:
  We investigate the effective properties (conductivity, diffusivity and
elastic moduli) of model random composite media derived from Gaussian random
fields and overlapping hollow spheres. The morphologies generated in the models
exhibit low percolation thresholds and give a realistic representation of the
complex microstructure observed in many classes of composites. The statistical
correlation functions of the models are derived and used to evaluate rigorous
bounds on each property. Simulation of the effective conductivity is used to
demonstrate the applicability of the bounds. The key morphological features
which effect composite properties are discussed.


title:
Experimental evidence of delocalized states in random dimer
  superlattices

abstract:
  We study the electronic properties of GaAs-AlGaAs superlattices with
intentional correlated disorder by means of photoluminescence and vertical dc
resistance. The results are compared to those obtained in ordered and
uncorrelated disordered superlattices. We report the first experimental
evidence that spatial correlations inhibit localization of states in disordered
low-dimensional systems, as our previous theoretical calculations suggested, in
contrast to the earlier belief that all eigenstates are localized.


title:
Comment on ``Dynamic Scaling in the Spatial Distribution of Persistent
  Sites''

abstract:
  In a recent paper (cond-mat/9901130), the spatial distribution of unvisited
sites in the one-dimensional single-species annihilation process was
investigated. It was claimed that this distribution is characterized by a new
length scale, and that the corresponding dynamical exponent depends upon the
initial concentration. We show numerically that this assertion is erroneous.
Regardless of the initial concentration, this spatial distribution is
characterized by the diffusive length scale.


title:
1/f Noise and Long Configuration Memory in Bak-Tang-Wiesenfeld Models on
  Narrow Stripes

abstract:
  We report our findings of an 1/f power spectrum for the total amount of sand
in directed and undirected Bak-Tang-Wiesenfeld models confined on narrow
stripes and driven locally. The underlying mechanism for the 1/f noise in these
systems is an exponentially long configuration memory giving rise to a very
broad distribution of time scales. Both models are solved analytically with the
help of an operator algebra to explicitly show the appearance of the long
configuration memory.


title: Buckling, Fluctuations and Collapse in Semiflexible Polyelectrolytes

abstract:
  We present a systematic statistical mechanical analysis of the conformational
properties of a stiff polyelectrolyte chain with intrachain attractions that
are due to counterion correlations. We show that the mean-field solution
corresponds to an Euler-like buckling instability. The effect of the
conformational fluctuations on the buckling instability is investigated, first,
qualitatively, within the harmonic (``semiclassical'') theory, then,
systematically, within a 1/d-expansion, where d denotes the dimension of
embedding space. Within the ``semiclassical'' approximation, we predict that
the effect of fluctuations is to renormalize the effective persistence length
to smaller values, but not to change the nature of the mean-field (i.e.,
buckling) behavior. Based on the 1/d-expansion we are, however, led to conclude
that thermal fluctuations are responsible for a change of the buckling behavior
which is turned into a polymer collapse. A phase diagram is constructed in
which a sequence of collapse transitions terminates at a buckling instability
that occurs at a place that varies with the magnitude of the bare persistence
length of the polymer chain, as well as with the strength and range of the
attractive potential.


title:
Quantum Creep in Y_{1-x}Pr_{x}Ba_{2}Cu_{3}O_{7}$ Crystals: Magnetic
  Relaxation and Transport

abstract:
  We report transport and magnetic relaxation measurements in the mixed state
of strongly underdoped Y_{1-x}Pr_{x}Ba_{2}Cu_{3}O_{7} crystals. A transition
from thermally activated flux creep to temperature independent quantum flux
creep is observed in both transport and magnetic relaxation at temperatures T *
5 K. Flux transformer measurements indicate that the crossover to quantum creep
is preceded by a coupling transition. Based on these observations we argue that
below the coupling transition the current is confined within a very narrow
layer beneath the current contacts.


title: Classical Phase Fluctuations in High Temperature Superconductors

abstract:
  Phase fluctuations of the superconducting order parameter play a larger role
in the cuprates than in conventional BCS superconductors because of the low
superfluid density of a doped insulator. In this paper, we analyze an XY model
of classical phase fluctuations in the high temperature superconductors using a
low-temperature expansion and Monte Carlo simulations. In agreement with
experiment, the value of the superfluid density at temperature T=0 is a quite
robust predictor of Tc, and the evolution of the superfluid density with T,
including its T-linear behavior at low temperature, is insensitive to
microscopic details.


title:
Phase transition between quantum and classical regimes for the escape
  rate of a biaxial spin system

abstract:
  Employing the method of mapping the spin problem onto a particle one, we have
derived the particle Hamiltonian for a biaxial spin system with a transverse or
longitudinal magnetic field. Using the Hamiltonian and introducing the
parameter $p (\equiv (U_{max}-E)/(U_{max}-U_{min}))$ where $U_{max}$ (U_{min})
corresponds to the top (bottom) of the potential and $E$ is the energy of the
particle, we have studied the first- or second-order transition around the
crossover temperature between thermal and quantum regimes for the escape rate,
depending on the anisotropy constant and the external magnetic field. It is
shown that the phase boundary separating the first- and second-order transition
and its crossover temperature are greatly influenced by the transverse
anisotropy constant as well as the transverse or longitudinal magnetic field.


title:
Strong Enhancement of Superconducting Correlation in a Two-Component
  Fermion Gas

abstract:
  We study high-density electron-hole (e-h) systems with the electron density
slightly larger than the hole density. We find a new superconducting phase, in
which the excess electrons form Cooper pairs moving in an e-h BCS phase. The
coexistence of the e-h and e-e orders is possible because e and h have opposite
charges, whereas analogous phases are impossible in the case of two fermion
species that have the same charge or are neutral. Most strikingly, the e-h
order enhances the superconducting e-h order parameter by more than one order
of magnitude as compared with that given by the BCS formula, for the same value
of the effective e-e attractive potential \lambda^{ee}. This new phase should
be observable in an e-h system created by photoexcitation in doped
semiconductors at low temperatures.


title: Tunneling spectroscopy for ferromagnet/superconductor junctions

abstract:
  In tunneling spectroscopy studies of ferromagnet/superconductor (F/S)
junctions, the effects of spin polarization, Fermi wavevector mismatch (FWM)
between the F and S regions, and interfacial resistance play a crucial role. We
study the low bias conductance spectrum of these junctions, governed by Andreev
reflection at the F/S interface. We consider both d- and s-wave superconductors
as well as mixed states of the $d+is$ form. We present results for a range of
values of the relevant parameters and find that a rich variety of features
appears, depending on pairing state and other conditions. We show that in the
presence of FWM, spin polarization can enhance Andreev reflection and give rise
to a zero bias conductance peak for an s-wave superconductor.


title: Kinetics of shape equilibration for two-dimensional islands

abstract:
  We study the relaxation to equilibrium of two dimensional islands containing
up to 20000 atoms by Kinetic Monte Carlo simulations. We find that the commonly
assumed relaxation mechanism - curvature-driven relaxation via atom diffusion -
cannot explain the results obtained at low temperatures, where the island edges
consist in large facets. Specifically, our simulations show that the exponent
characterizing the dependence of the equilibration time on the island size is
different at high and low temperatures, in contradiction with the above cited
assumptions. Instead, we propose that - at low temperatures - the relaxation is
limited by the nucleation of new atomic rows on the large facets : this allows
us to explain both the activation energy and the island size dependence of the
equilibration time.


title: Fully Packed O(n=1) Model on Random Eulerian Triangulations

abstract:
  We introduce a matrix model describing the fully-packed O(n) model on random
Eulerian triangulations (i.e. triangulations with all vertices of even
valency). For n=1 the model is mapped onto a particular gravitational 6-vertex
model with central charge c=1, hence displaying the expected shift c -> c+1
when going from ordinary random triangulations to Eulerian ones. The case of
arbitrary n is also discussed.


title:
\eta-Exponents in the one dimensional antiferromagnetic Heisenberg model
  with next to nearest neighbour coupling

abstract:
  We investigate the critical exponents $\eta_3(\alpha,M), \eta_1(\alpha,M)$
associated with the singularities in the longitudinal and transverse structure
factors of the one dimensional antiferromagnetic Heisenberg model with nearest
(J_1) and next to nearest (J_2) neighbour coupling of relative strength $\alpha
= {J_2}/{J_1}$ and an external field $B$ with magnetization $M(B)$.


title:
Y NMR imaging of the staggered magnetization in doped Haldane chain
  Y(2)BaNi(1-x)Mg(x)O(5)

abstract:
  We present a static, model-independent, experimental determination of the
spin-spin correlation length \xi in a quantum spin system. This is achieved in
doped Haldane (i.e., S=1 Heisenberg antiferromagnetic) chain
Y(2)BaNi(1-x)Mg(x)O(5) by Y NMR imaging of the staggered magnetization induced
around the Mg-impurities (i.e., chain boundaries) by magnetic field. The
magnitude of this magnetization is found to decay exponentially with \xi equal
to the theoretical prediction for an infinite S=1 chain, and the staggered
magnetic moment at the edge site showing the Curie behavior of an effective
S=1/2 spin.


title: Beyond Poisson-Boltzmann: Fluctuations and Correlations

abstract:
  We formulate the non-linear field theory for a fluctuating counter-ion
distribution in the presence of a fixed, arbitrary charge distribution. The
Poisson-Boltzmann equation is obtained as the saddle-point, and the effects of
fluctuations and correlations are included by a loop-wise expansion around this
saddle point. We show that the Poisson equation is obeyed at each order in the
loop expansion and explicitly give the expansion of the Gibbs potential up to
two loops. We then apply our formalism to the case of an impenetrable, charged
wall, and obtain the fluctuation corrections to the electrostatic potential and
counter-ion density to one-loop order without further approximations. The
relative importance of fluctuation corrections is controlled by a single
parameter, which is proportional to the cube of the counter-ion valency and to
the surface charge density. We also calculate effective interactions between
charged particles, which reflect counter-ion correlation effects.


title:
Charged magnetoexcitons in two dimensions: Magnetic translations and
  families of dark states

abstract:
  We show that optical transitions of charged excitons in semiconductor
heterostructures are governed in magnetic fields by a novel exact selection
rule, a manifestation of magnetic translations. It is shown that the
spin-triplet ground state of the quasi-two-dimensional charged exciton X- --- a
bound state of two electrons and one hole --- is optically inactive in
photoluminescence at finite magnetic fields. Internal bound-to-bound X- triplet
transition has a specific spectral position, below the electron cyclotron
resonance, and is strictly prohibited in a translationally-invariant system.
These results allow one to discriminate between localized and free charged
excitons.


title:
Tilt waves dynamics of the oxygen octahedra in La2CuO4 from anelastic
  and 139La NQR relaxation

abstract:
  The anharmonic vibrational dynamics in nearly stoichiometric La2CuO4+delta is
studied by means of anelastic and ^{139}La NQR relaxation. In the absorption
component of the elastic susceptibility as well as in the nuclear relaxation
rate a peak is detected as a function of temperature, and a relaxation time tau
= 1.7 10^{-12} exp [(2800 K)/T] s is derived. The relaxation processes are
attributed to tilt motion of the CuO6 octahedra in doublewell potentials, whose
cooperative character increases the effective energy barrier to the observed
value. The analysis of the relaxation mechanisms has been carried out by
reducing the dynamics of the interacting octahedra to a one-dimensional
equation of motion. The soliton-like solutions correspond to parallel walls
separating domains of different tilt patterns and give rise to pseudo-diffusive
modes which appear as a central component in the spectral density of the motion
of the octahedra. The tilt waves may be considered to correspond to the
dynamical lattice stripes observed in La-based and Bi-based high-Tc
superconductors.


title:
Isotope effects and the charge gap formation in the charge ordered phase
  of colossal magnetoresistance manganites

abstract:
  Giant oxygen isotope effects observed in colossal magnetoresistance
manganites are investigated by employing the combined model of the double
exchange and interacting lattice polaron mechanism. We have shown that the
isotope effects on $ T_C$ in the metallic phase and $ T_{CO}$ in the charge
ordered phase of manganites can be explained well in terms of the double
exchange and polaron narrowing factors with reasonable physical parameters.


title:
Effects of Disorder on the Competition between Antiferromagnetism and
  Superconductivity

abstract:
  Motivated by the observation of unusual magnetism in Ce_xCu_2Si_2 ($x\sim
1$), we study the effect of disorder, such as Ce vacancy, on the competition
between superconductivity (SC) and antiferromagnetism (AF) on the basis of the
phenomenological Ginzburg-Landau theory. Assuming that the AF-SC transition is
of first order in clean system, we show that a single impurity in the SC state
can induce staggered magnetization by suppressing the SC around it. For finite
concentration of impurities, the first-order AF-SC boundary in the clean case
is replaced by a finite region where the SC and the induced AF moments coexist
microscopically with spatially varying order parameters. We argue that spin
excitation spectrum in the coexistent state has a dual structure of SC gapped
mode and the low-energy spin-wave mode. In accordance with the emergence of AF
out of SC ground state, the spectral weight will be transferred from the former
mode to the latter, keeping the structure of both modes basically unchanged.


title: Quantum transport and momentum conserving dephasing

abstract:
  We study numerically the influence of momentum-conserving dephasing on the
transport in a disordered chain of scatterers. Loss of phase memory is caused
by coupling the transport channels to dephasing reservoirs. In contrast to
previously used models, the dephasing reservoirs are linked to the transport
channels between the scatterers, and momentum conserving dephasing can be
investigated. Our setup provides a model for nanosystems exhibiting conductance
quantization at higher temperatures in spite of the presence of phononic
interaction. We are able to confirm numerically some theoretical predictions.


title: The Kondo lattice model with correlated conduction electrons

abstract:
  We investigate a Kondo lattice model with correlated conduction electrons.
Within dynamical mean-field theory the model maps onto an impurity model where
the host has to be determined self-consistently. This impurity model can be
derived from an Anderson-Hubbard model both by equating the low-energy
excitations of the impurity and by a canonical transformation. On the level of
dynamical mean-field theory this establishes the connection of the two lattice
models. The impurity model is studied numerically by an extension of the
non-crossing approximation to a two-orbital impurity. We find that with
decreasing temperature the conduction electrons first form quasiparticles
unaffected by the presence of the lattice of localized spins. Then, reducing
the temperature further, the particle-hole symmetric model turns into an
insulator. The quasiparticle peak in the one-particle spectral density splits
and a gap opens. The size of the gap increases when the correlations of the
conduction electrons become stronger. These findings are similar to the
behavior of the Anderson-Hubbard model within dynamical mean-field theory and
are obtained with much less numerical effort.


title:
Coherent Dynamics of Vortex Formation in Trapped Bose-Einstein
  Condensates

abstract:
  Simulations of a rotationally stirred condensate show that a regime of simple
behaviour occurs in which a single vortex cycles in and out of the condensate.
We present a simple quantitative model of this behaviour, which accurately
describes the full vortex dynamics, including a critical angular speed of
stirring for vortex formation. A method for experimentally preparing a
condensate in a central vortex state is suggested.


title: The effects of weak disorders on Quantum Hall critical points

abstract:
  We study the consequences of random mass, random scalar potential and random
vector potential on the line of clean fixed points between integer/fractional
quantum Hall states and an insulator. This line of fixed points was first
identified in a clean Dirac fermion system with both Chern-Simon coupling and
Coulomb interaction in Phys. Rev. Lett. {\bf 80}, 5409 (1998). By performing a
Renormalization Group analysis in 1/N (N is the No. of species of Dirac
fermions) and the variances of three disorders $\Delta_{M}, \Delta_{V},
\Delta_{A}$, we find that $\Delta_{M}$ is irrelevant along this line, both
$\Delta_{A}$ and $\Delta_{V}$ are marginal. With the presence of all the three
disorders, the pure fixed line is unstable. Setting Chern-Simon interaction to
zero, we find one non-trivial line of fixed points in $(\Delta_{A}, w)$ plane
with dynamic exponent z=1 and continuously changing $\nu$, it is stable against
small $(\Delta_{M},\Delta_{V})$ in a small range of the line $1< w < 1.31$,
therefore it may be relevant to integer quantum Hall transition. Setting
$\Delta_{M} =0$, we find a fixed plane with z=1, the part of this plane with
$\nu > 1$ is stable against small $\Delta_{M}$, therefore it may be relevant to
fractional quantum Hall transition.


title:
Geometric (Berry) phases and statistical transmutation in the
  two-dimensional systems of strongly correlated electrons

abstract:
  Focusing on the hole-doped two-dimensional systems of strongly correlated
electrons, we examine geometric phases acquired by electronic wave functions as
a result of polaron transport around a closed loop. For this study we apply the
Lanczos exact diagonalization method to Holstein-Hubbard, Holstein-$tJ$, and
Holstein-$tJ_z$ models in order to reveal various aspects of geometric phases.
We demonstrate that transverse spin fluctuations are responsible for the
generation of nontrivial geometric phases. From the exchange symmetry of
polarons we find that the statistical transmutation of polarons occurs
depending on the strength of electron correlations.


title: The microwave power handling of a FIB generated weak link in a YBCO film

abstract:
  We have measured the power dependent microwave properties of a weak link in a
YBa2Cu3O7 thin film formed by writing a line of damage using a focused ion
beam. The measurement was made using a parallel plate resonator at 5.5 GHz with
the weak link written across the width of one of the plates. The ion induced
damage was characterized using a TRIM computer simulation and the dc properties
of similar weak links was measured. Using a 200 eV Si ion dose of 2e13 cm-2,
the Tc of the damaged region was reduced by 5.5 K and the normal resistivity
was doubled. Surprisingly, the microwave measurements did not show any
Josephson junction characteristics. Rather, the ion damaged region exhibited a
greatly increased microwave resistivity that was constant as a function of
microwave power up to rf fields of 20 mT at 21 K.


title: On the Reconstruction of Random Media using Monte Carlo Methods

abstract:
  A simulated annealing algorithm is applied to the reconstruction of
two-dimensional porous media with prescribed correlation functions. The
experimental correlation function of an isotropic sample of Fontainebleau
sandstone and a synthetic correlation function with damped oscillations are
used in the reconstructions. To reduce the numerical effort we follow a
proposal suggesting to evaluate the correlation functions only along certain
directions. The results show, that this simplification yields significantly
different micro-structures as compared to a full evaluation of the correlation
function. In particular we find that the simplified reconstruction method
introduces an artificial anisotropy that is originally not present.


title: Critical temperature of the superfluid transition in bose liquids

abstract:
  A phenomenological criterion for the superfluid transition is proposed, which
is similar to the Lindemann criterion for the crystal melting. Then we derive a
new formula for the critical temperature, relating $T_{\lambda}$ to the mean
kinetic energy per particle above the transition. The suppression of the
critical temperature in a sufficiently dense liquid is described as a result of
the quantum decoherence phenomenon. The theory can account for the observed
dependence of $T_{\lambda}$ on density in liquid helium and results in an
estimate $T_{\lambda} \sim 1.1$ K for molecular hydrogen.


title: Crossing of Specific Heat Curves in some correlated Fermion systems

abstract:
  Specific heat versus temperature curves for various pressures, or magnetic
fields (or some other external control parameter) have been seen to cross at a
point or in a very small range of temperatures in many correlated fermion
systems. We show that this behavior is related to the vicinity of a quantum
critical point in these systems which leads to a crossover at some temperature
T^* from quantum to classical fluctuation regime. The temperature at which the
curves cross turns out to be near T^*. We have discussed the case of the normal
phase of liquid Helium three and the heavy fermion systems CeAl3 and UBe13 in
detail within the spin fluctuation theory. When the crossover scale is any
homogeneous function of these control parameters there is always crossing at a
point.


title:
Density Modulations and Addition Spectra of Interacting Electrons in
  Disordered Quantum Dots

abstract:
  We analyse the ground state of spinless fermions on a lattice in a weakly
disordered potential, interacting via a nearest neighbour interaction, by
applying the self-consistent Hartree-Fock approximation. We find that charge
density modulations emerge progressively when r_s >1, even away from
half-filling, with only short-range density correlations. Classical geometry
dependent "magic numbers" can show up in the addition spectrum which are
remarkably robust against quantum fluctuations and disorder averaging.


title: Modeling Disordered Quantum Systems with Dynamical Networks

abstract:
  It is the purpose of the present article to show that so-called network
models, originally designed to describe static properties of disordered
electronic systems, can be easily generalized to quantum-{\em dynamical}
models, which then allow for an investigation of dynamical and spectral
aspects. This concept is exemplified by the Chalker-Coddington model for the
Quantum Hall effect and a three-dimensional generalization of it. We simulate
phase coherent diffusion of wave packets and consider spatial and spectral
correlations of network eigenstates as well as the distribution of
(quasi-)energy levels. Apart from that it is demonstrated how network models
can be used to determine two-point conductances. Our numerical calculations for
the three-dimensional model at the Metal-Insulator transition point delivers
among others an anomalous diffusion exponent of $\eta = 3 - D_2 = 1.7 \pm 0.1$.
The methods presented here in detail have been used partially in earlier work.


title:
Anomalous magnetoconductance due to weak localization in 2D systems with
  anisotropic scattering: computer simulation

abstract:
  The results of computer simulation of particle motion over the plane with
randomly distributed scatters are presented. They are used to analyse the
influence of scattering anisotropy on the negative magnetoresistance due to
weak localisation. It is shown that the magnetic field dependence of
magnetoresistance in this case can be described by the well known expression,
obtained in the diffusion limit for isotropic scatternig, but with the
prefactor less than unity and breaking-phase length which differs from the true
value.


title: On the center of mass of Ising vectors

abstract:
  We show that the center of mass of Ising vectors that obey some simple
constraints, is again an Ising vector.


title: Universality in an integer Quantum Hall transition

abstract:
  An integer Quantum Hall effect transition is studied in a modulation doped
p-SiGe sample. In contrast to most examples of such transitions the
longitudinal and Hall conductivities at the critical point are close to 0.5 and
1.5 (e^2/h), the theoretically expected values. This allows the extraction of a
scattering parameter, describing both conductivity components, which depends
exponentially on filling factor. The strong similarity of this functional form
to those observed for transitions into the Hall insulating state and for the
B=0 metal- insulator transition implies a universal quantum critical behaviour
for the transitions. The observation of this behaviour in the integer Quantum
Hall effect, for this particular sample, is attributed to the short-ranged
character of the potential associated with the dominant scatterers.


title:
Nonlinear sigma Model Treatment of Quantum Antiferromagnets in a
  Magnetic Field

abstract:
  We present a theoretical analysis of the properties of low-dimensional
quantum antiferromagnets in applied magnetic fields. In a nonlinear sigma model
description, we use a spin stiffness analysis, a 1/N expansion, and a
renormalization group approach to describe the broken-symmetry regimes of
finite magnetization, and, in cases of most interest, a low-field regime where
symmetry is restored by quantum fluctuations. We compute the magnetization,
critical fields, spin correlation functions, and decay exponents accessible by
nuclear magnetic resonance experiments. The model is relevant to many systems
exhibiting Haldane physics, and provides good agreement with data for the
two-chain spin ladder compound CuHpCl.


title: Quasi Markovian behavior in mixing maps

abstract:
  We consider the time dependent probability distribution of a coarse grained
observable Y whose evolution is governed by a discrete time map. If the map is
mixing, the time dependent one-step transition probabilities converge in the
long time limit to yield an ergodic stochastic matrix. The stationary
distribution of this matrix is identical to the asymptotic distribution of Y
under the exact dynamics. The nth time iterate of the baker map is explicitly
computed and used to compare the time evolution of the occupation probabilities
with those of the approximating Markov chain. The convergence is found to be at
least exponentially fast for all rectangular partitions with Lebesgue measure.
In particular, uniform rectangles form a Markov partition for which we find
exact agreement.


title: Anisotropic superexchange of a 90 degree Cu-O-Cu bond

abstract:
  The magnetic anisotropy af a rectangular Cu-O-Cu bond is investigated in
second order of the spin-orbit interaction. Such a bond is characteristic for
cuprates having edge sharing CuO_2 chains, and exists also in the Cu_3O_4 plane
or in ladder compounds. For a ferromagnetic coupling between the copper spins
an easy axis is found perpendicular to the copper oxygen plaquettes in
agreement with the experimental spin structure of Li_2CuO_2. In addition, a
pseudo-dipolar interaction is derived. Its estimation in the case of the
Cu_3O_4 plane (which is present for instance in Ba_2Cu_3O_4Cl_2 or
Sr_2Cu_3O_4Cl_2) gives a value which is however two orders of magnitude smaller
than the usual dipole-dipole interaction.


title: Recurrent Variational Approach to the Two-Leg Hubbard Ladder

abstract:
  We applied the Recurrent Variational Approach to the two-leg Hubbard ladder.
At half-filling, our variational Ansatz was a generalization of the resonating
valence bond state. At finite doping, hole pairs were allowed to move in the
resonating valence bond background. The results obtained by the Recurrent
Variational Approach were compared with results from Density Matrix
Renormalization Group.


title: Monte Carlo Simulation of Magnetic System in the Tsallis Statistics

abstract:
  We apply the Broad Histogram Method to an Ising system in the context of the
recently reformulated Generalized Thermostatistics, and we claim it to be a
very efficient simulation tool for this non-extensive statistics. Results are
obtained for the nearest-neighbour version of the Ising model for a range of
values of the $q$ parameter of Generalized Thermostatistics. We found an
evidence that the 2D-Ising model does not undergo phase transitions at finite
temperatures except for the extensive case $q=1$.


title:
Electronic states of metallic and semiconducting carbon nanotubes with
  bond and site disorder

abstract:
  Disorder effects on the density of states in carbon nanotubes are analyzed by
a tight binding model with Gaussian bond or site disorder. Metallic armchair
and semiconducting zigzag nanotubes are investigated. In the strong disorder
limit, the conduction and valence band states merge, and a finite density of
states appears at the Fermi energy in both of metallic and semiconducting
carbon nanotubes. The bond disorder gives rise to a huge density of states at
the Fermi energy differently from that of the site disorder case. Consequences
for experiments are discussed.

categories: cond-mat physics.chem-ph physics.optics quant-ph


title: Dressed States of a two component Bose-Einstein Condensate

abstract:
  A condensate with two internal states coupled by external electromagnetic
radiation, is described by coupled Gross Pitaevskii equations, whose
eigenstates are analogous to the dressed states of quantum optics. We solve for
these eigenstates numerically in the case of one spatial dimension, and explore
their properties as a function of system parameters. In contrast to the quantum
optical case, the condensate dressed states exhibit spatial behaviour which
depends on the system parameters, and can be manipulated by changing the cw
external field.


title: Hysteresis and Avalanches in Two Dimensional Foam Rheology Simulations

abstract:
  Foams have unique rheological properties that range from solid-like to
fluid-like. We study two-dimensional non-coarsening foams of different disorder
under shear in a Monte Carlo simulation, using a driven large-Q Potts model. We
find that the dynamics of topological rearrangements depend sensitively on the
structural disorder.


title: Magnetic Properties of Zig-Zag Ladders

abstract:
  We analyze the phase diagram of a system of spin-1/2 Heisenberg
antiferromagnetic chains interacting through a zig-zag coupling, also called
zig-zag ladders. Using bosonization techniques we study how a spin-gap or more
generally plateaux in magnetization curves arise in different situations. While
for coupled XXZ chains, one has to deal with a recently discovered chiral
perturbation, the coupling term which is present for normal ladders is restored
by an external magnetic field, dimerization or the presence of charge carriers.
We then proceed with a numerical investigation of the phase diagram of two
coupled Heisenberg chains in the presence of a magnetic field. Unusual
behaviour is found for ferromagnetic coupled antiferromagnetic chains. Finally,
for three (and more) legs one can choose different inequivalent types of
coupling between the chains. We find that the three-leg ladder can exhibit a
spin-gap and/or non-trivial plateaux in the magnetization curve whose
appearance strongly depends on the choice of coupling.


title: Exact Results of Strongly Correlated Systems at Finite Temperature

abstract:
  Some rigorous conclusions of the Hubbard model, Kondo lattice model and
periodic Anderson model at finite temperature are acquired employing the
fluctuation-dissipation theorem and particle-hole transform. The main
conclusion states that for the three models, the expectation value of ${\bf
\tilde{S}}^2-{\bf \tilde{S}}^2_z$ will be of order $N_{\Lambda}$ at any finite
temperature.


title: Many-chain Generalization of Gauge-Coupled Fermionic Systems

abstract:
  We present a many chain generalization of a recent work of ours, wherein an
arbitrary number of fermionic chains are coupled via a Gauge interaction.
Central to this construction is the role of an antisymmetric tensor which
enters the couplings and helps define a pseudo-unitary transformation, that
maps the models on to ``free fermi'' models with twisted boundary conditions.


title: A note on the proof of magnetic flux quantization from ODLRO

abstract:
  It is noticed that the excellent proof of the connection of magnetic flux
quantization and off-diagonal long range order (ODLRO) presented recently by
Nieh, Su and Zhao suffers from an imperfection, namely, the f-factors in the
case of finite translation do not satisfy $f(a)f(b)=f(a+b)$, which was employed
in the proof. A corrected proof is proposed to remedy this point.


title: Spin liquid ground state in a two dimensional non-frustrated spin model

abstract:
  We consider an exchange model describing two isotropic spin-1/2 Heisenberg
antiferromagnets coupled by a quartic term on the square lattice. The model is
relevant for systems with orbital degeneracy and strong electron-vibron
coupling in the large Hubbard repulsion limit, and is known to show a
spin-Peierls-like dimerization in one dimension. In two dimensions we calculate
energy gaps, susceptibilities, and correlation functions with a Green's
Function Monte Carlo. We find a finite spin gap and no evidence of any kind of
order. We conclude that the ground state is, most likely, a spin liquid of
resonating valence bonds.


title: High-temperature superconductivity in doped antiferromagnets

abstract:
  In the context of an effective model for doped antiferromagnets, whereby the
charge carriers are treated as hard-core bosons, we demonstrate that the ground
state energy close to half-filling is an even periodic function of the external
magnetic flux threading the square lattice in an Aharonov-Bohm geometry. The
period is equal to the flux quantum $\Phi_{0}=2\pi\hbar c/q$ entering the
Peierls phase factor of the hopping matrix elements. Thus flux quantization and
a concomitant finite value of superfluid weight D_{s} occur along with metallic
antiferromagnetism. We argue that the charge q in the associated flux quantum
might be set equal to 2e. The superconducting transition temperature T_{c} is
related to D_{s} linearly, in accordance to the generic Kosterlitz-Thouless
type of transition in a two-dimensional system, signalling the coherence of the
phase fluctuations of the condensate. The calculated dependence of T_{c} on
hole concentration is qualitatively similar to that observed in the
high-temperature superconducting cuprates.


title: Electronic structure of CdTe nanocrystals: A tight-binding study

abstract:
  We present a symmetry-based calculation of the electronic structure of a
compound semiconductor quantum dot (QD) in the sp^3s* tight-binding model
including the spin-orbit interaction. The Hamiltonian matrix is diagonalized
exactly for CdTe QD sizes up to 60 {\AA}. The surface dangling bonds are
passivated by hydrogen through a careful analysis of the density of states and
wave functions. The calculated size dependence of the energy gap shows a
reasonable agreement with the available experimental data. Our symmetry
analysis indicates that, in contrast with a reported prediction of the
three-band effective-mass model, the fundamental interband transition remains
dipole-allowed in CdTe nanocrystals.


title: Mode Spectroscopy and Level Coupling in Ballistic Electron Waveguides

abstract:
  A tunable quantum point contact with modes occupied in both transverse
directions is studied by magnetotransport experiments. We use conductance
quantization of the one-dimensional subbands as a tool to determine the mode
spectrum. A magnetic field applied along the direction of the current flow
couples the modes. This can be described by an extension of the Darwin-Fock
model. Anticrossings are observed as a function of the magnetic field, but not
for zero field or perpendicular field directions, indicating coupling of the
subbands due to nonparabolicity in the electrical confinement.


title:
Ground-state clusters of two-, three- and four-dimensional +-J Ising
  spin glasses

abstract:
  A huge number of independent true ground-state configurations is calculated
for two-, three- and four-dimensional +- J spin-glass models. Using the genetic
cluster-exact approximation method, system sizes up to N=20^2,8^3,6^4 spins are
treated. A ``ballistic-search'' algorithm is applied which allows even for
large system sizes to identify clusters of ground states which are connected by
chains of zero-energy flips of spins. The number of clusters n_C diverges with
N going to infinity. For all dimensions considered here, an exponential
increase of n_C appears to be more likely than a growth with a power of N. The
number of different ground states is found to grow clearly exponentially with
N. A zero-temperature entropy per spin of s_0=0.078(5)k_B (2d), s_0=0.051(3)k_B
(3d) respectively s_0=0.027(5)k_B (4d) is obtained.


title: Dissipative Particle Dynamics with Energy Conservation: Heat Conduction

abstract:
  We study by means of numerical simulations the model of dissipative particle
dynamics with energy conservation for the simple case of thermal conduction. It
is shown that the model displays correct equilibrium fluctuations and
reproduces Fourier law. The connection between "mesoscopic coarse graining" and
"resolution" is clarified.


title:
The effect of the spin-orbit geometric phase on the spectrum of
  Aharonov-Bohm oscillations in a semiconductor mesoscopic ring

abstract:
  Taking into account the spin precession caused by the spin-orbit splitting of
the conduction band in semiconductor quantum wells, we have calculated the
Fourier spectra of conductance and state-density correlators in a 2D ring, in
order to investigate the structure of the main peak corresponding to
Aharonov-Bohm oscillations. In narrow rings the peak structure is determined by
the competition between the spin-orbit and the Zeeman couplings. The latter
leads to a peak broadening, and produces the peak splitting in the
state-density Fourier spectrum. We have found an oscillation of the peak
intensity as a function of the spin-orbit coupling constant, and this effect of
the quantum interference caused by the spin geometric phase is destroyed with
increasing Zeeman coupling.


title: Energy Functional and Fixed Points of a Neural Networks

abstract:
  It turned out that the set of the fixed points is not necessarily the same as
the set of the local minima of the energy functional. It depends on the
diagonal elements of the connection matrix. The simple method which allows to
cut off fictitious fixed points with high energies was found out. Especially
the method is effective if the connection matrix is the projection one.


title:
Why T_c is too high when antiferromagnetism is underestimated? --- An
  understanding based on the phase string effect

abstract:
  It is natural for a Mott antiferromagnetism in RVB description to become a
superconductor in doped metallic regime. But the issue of superconducting
transition temperature is highly nontrivial, as the AF fluctuations in the form
of RVB pair-breaking are crucial in determining the phase coherence of the
superconductivity. Underestimated AF fluctuations in a fermionic RVB state are
the essential reason causing an overestimate of T_c in the same system. We
point out that by starting with a {\it bosonic} RVB description where both the
long-range and short-range AF correlations can be accurately described, the AF
fluctuations can effectively reduce T_c to a reasonable value through the phase
string effect, by controlling the phase coherence of the superconducting order
parameter.


title: Andreev reflections in the pseudogap state of cuprate supercondcutors

abstract:
  We propose that, if the pseudogap state in the cuprate superconductors can be
described in terms of the phase-incoherent preformed pairs, there should exist
Andreev reflection from these pairs even above the superconducting transition
temperature, $T_c$. After giving qualitative arguments for this effect, we
present more quantitative calculations based on the Bogoliubov--de Gennes
equation. Experimental observations of the effects of Andreev reflections above
$T_c$---such as an enhanced tunneling conductance below the gap along the
copper oxide plane---could provide unambiguous evidence for the preformed pairs
in the pseudogap state.


title: Non-periodic Ising quantum chains and conformal invariance

abstract:
  A while ago, Luck (J. Stat. Phys. 72 (1993) 417) investigated the critical
behaviour of one-dimensional Ising quantum chains with couplings constants
modulated according to general non-periodic sequences. In this short note, we
take a closer look at the case where the sequences are obtained from
(two-letter) substitution rules and at the consequences of Luck's results at
criticality. They imply that only for a certain class of substitution rules the
long-distance behaviour is still described by the c=1/2 conformal field theory
of a free Majorana fermion as for the periodic Ising quantum chain, whereas the
general case does not lead to a conformally invariant scaling limit.


title: Influence of Interaction on Weak Localization

abstract:
  We discuss the influence of the electromagnetic environment and the
electron-electron interaction on the weak localization correction to the
conductivity of a disordered metal. The theory of this phenomenon for
sufficiently high temperature, where the quantum nature of the interaction of
electrons with the electromagnetic field can be disregarded, has been
understood for some time. We consider the first order quantum correction to
this semiclassical description and work out the temperature range in which this
correction is small. No external low frequency cut-off is needed in our
calculation. We conclude that in the whole region of temperature where the weak
localization correction is much smaller than the Drude conductivity the
classical treatment of the interaction is valid.


title: Meron-Cluster Solution of Fermion Sign Problems

abstract:
  We present a general strategy to solve the notorious fermion sign problem
using cluster algorithms. The method applies to various systems in the Hubbard
model family as well as to relativistic fermions. Here it is illustrated for
non-relativistic lattice fermions. A configuration of fermion world-lines is
decomposed into clusters that contribute independently to the fermion
permutation sign. A cluster whose flip changes the sign is referred to as a
meron. Configurations containing meron-clusters contribute 0 to the path
integral, while all other configurations contribute 1. The cluster
representation describes the partition function as a gas of clusters in the
zero-meron sector.


title: Discovery of the Acoustic Faraday Effect in Superfluid 3He-B

abstract:
  We report the discovery of the acoustic Faraday effect in superfluid 3He-B.
The observation of this effect provides the first direct evidence for
propagating transverse acoustic waves in liquid 3He, a mode first predicted by
Landau in 1957. The Faraday rotation is large and observable because of
spontaneously broken spin-orbit symmetry in 3He-B. We compare the experimental
observations with a simulation of the transverse acoustic impedance that
includes the field-induced circular birefringence of transverse waves.


title:
Effect of magnetic field on over-doped HTc superconductors: Conflicting
  predictions of various HTc theories

abstract:
  Following the recent NMR experiments by Gorny et al., we discuss the effect
of a magnetic field on the superconducting T_c and the spin pseudo-gap T^*. As
a testable prediction, we argue that a spin pseudo-gap should also be observed
in over-doped samples in a magnetic fied (while normally there is no pseudo-gap
above T_c). We find that different theoretical approaches have marked
differences in their predictions for over-doped HTc cuprates.


title: Dilute Quantum Droplets

abstract:
  In the limit when the two-body scattering length $a$ is negative and much
larger than the effective two-body interaction radius the contribution to the
ground state energy due to the three-body correlations is given by the Efimov
effect. For particular values of the diluteness parameter $\rho |a|^3$ the
three-body contribution can become the dominant term of the energy density
functional. Under these conditions both Bose-Einstein and Fermi-Dirac systems
could become self-bound and either boson droplets or fermion ``designer
nuclei'' of various sizes and densities could be manufactured.


title:
Threshold electronic structure at the oxygen K edge of 3d transition
  metal oxides: a configuration interaction approach

abstract:
  It has been generally accepted that the threshold structure observed in the
oxygen K edge X-ray absorption spectrum in 3d transition metal oxides
represents the electronic structure of the 3d transition metal. There is,
however, no consensus about the correct description. We present an
interpretation, which includes both ground state hybridization and electron
correlation. It is based on a configuration interaction cluster calculation
using a MO6 cluster. The oxygen K edge spectrum is calculated by annihilating a
ligand hole in the ground state and is compared to calculations representing
inverse photoemission experiments in which a 3d transition metal electron is
added. Clear differences are observed related to the amount of ligand hole
created in the ground state. Two "rules" connected to this are discussed.
Comparison with experimental data of some early transition metal compounds is
made and shows that this simple cluster approach explains the experimental
features quite well.


title:
Bethe Ansatz Solution for a Defect Particle in the Asymmetric Exclusion
  Process

abstract:
  The asymmetric exclusion process on a ring in one-dimension is considered
with a single defect particle. The steady state has previously been solved by a
matrix product method. Here we use the Bethe ansatz to solve exactly for the
long time limit behaviour of the generating function of the distance travelled
by the defect particle. This allows us to recover steady state properties known
from the matrix approach such as the velocity, and obtain new results such as
the diffusion constant of the defect particle. In the case where the defect
particle is a second class particle we determine the large deviation function
and show that in a certain range the distribution of the distance travelled
about the mean is Gaussian. Moreover the variance (diffusion constant) grows as
L to the power 1/2 where is the system size. This behaviour can be related to
the superdiffusive spreading of excess mass fluctuations on an infinite system.
In the case where the defect particle produces a shock, our expressions for the
velocity and the diffusion constant coincide with those calculated previously
for an infinite system by Ferrari and Fontes.


title: Nonlocality in mesoscopic Josephson junctions with strip geometry

abstract:
  We study the current in a clean superconductor-normal-metal-superconductor
junction of length d and width w in the presence of an applied magnetic field
H. We show that both the geometrical pattern of the current density and the
critical current as a function of the total flux in the junction, depend on the
ratio of the Josephson vortex distance a_0 and the range r of the nonlocal
electrodynamics. In particular, the critical current has the periodicity of the
superconducting flux quantum only for r<a_0 and acquires, due to boundary
effects, the double (pseudo-) periodicity for strong nonlocality, r>a_0.
Comparing our results to recent experiments of Heida et al. [Phys. Rev. B 57,
R5618 (1998)] we find good agreement.


title: Punctuated equilibrium in an evolving bacterial population

abstract:
  Recently, Lenski et al have carried out an experiment on bacterial evolution.
Their findings support the theory of punctuated equilibrium in biological
evolution. We show that the M=2 Bak-Sneppen model can explain some of the
experimental results in a qualitative manner.


title: Truncated-cone SPM tips for surface-molecules interaction studies

abstract:
  A new tecnique for the production of special shaped SPM tips is reported.
Noble metal coated tips are produced with flat and circular tip front. The
unusual truncated-cone tip shape is obtained by modifying commercial tips. The
diameter of the tip front flat area can be varied continuously in the range
0.05 - 1 micrometer.


title: Some Results and a Conjecture for Manna's Stochastic Sandpile Model

abstract:
  We present some analytical results for the stochastic sandpile model, studied
earlier by Manna. In this model, the operators corresponding to particle
addition at different sites commute. The eigenvalues of operators satisfy a
system of coupled polynomial equations. For an L X L square, we construct a
nontrivial toppling invariant, and hence a ladder operator which acting on
eigenvectors of evolution operator gives new eigenvectors with different
eigenvalues. For periodic boundary conditions in one direction, one more
toppling invariant can be constructed. We show that there are many forbidden
subconfigurations, and only an exponentially small fraction of all stable
configurations are recurrent. We obtain rigorous lower and upper bounds for the
minimum number of particles in a recurrent configuration, and conjecture a
formula for its exact value for finite-size rectangles.


title:
Thermodynamics of the incommensurate state in Rb_2WO_4: on the Lifshitz
  point in A`A``BX_4 compounds

abstract:
  We consider the evolution of the phase transition from the parent hexagonal
phase $P6_{3}/mmc$ to the orthorhombic phase $Pmcn$ that occurs in several
compounds of $A'A''BX_{4}$ family as a function of the hcp lattice parameter
$c/a$. For compounds of $K_{2}SO_{4}$ type with $c/a$ larger than the threshold
value 1.26 the direct first-order transition $Pmcn-P6_{3}/mmc$ is characterized
by the large entropy jump $Rln2$. For compounds $Rb_{2}WO_{4}$, $K_{2}MoO_{4}$,
$K_{2}WO_{4}$ with $c/a<1.26$ this transition occurs via an intermediate
incommensurate $(Inc)$ phase. DSC measurements were performed in $Rb_{2}WO_{4}$
to characterize the thermodynamics of the $Pmcn-Inc-P6_{3}/mmc$ transitions. It
was found that both transitions are again of the first order with entropy jumps
$0.2Rln2 and $0.3Rln2$. Therefore, at $c/a ~ 1.26$ the $A'A''BX_{4}$ compounds
reveal an unusual Lifshitz point where three first order transition lines meet.
We propose the coupling of crystal elasticity with $BX_{4}$ tetrahedra
orientation as a possible source of the transitions discontinuity.


title: Non-Fermi Liquid Behavior in a Disordered Kondo Alloy Model

abstract:
  We study a mean-field model of a Kondo alloy using numerical techniques and
analytic approximations. In this model, randomly distributed magnetic
impurities interact with a band of conduction electrons and have a residual
RKKY coupling of strength $J$. This system has a quantum critical point at
$J=J_{c} \sim T_{K}^0$, the Kondo scale of the problem. The $T$ dependence of
the spin susceptibility near the quantum critical point is singular with
$\chi(0)-\chi(T) \propto T^{\gamma}$ and non-integer $\gamma$. At $J_{c}$,
$\gamma = 3/4$. For $J\lesssim J_{c}$ there are two crossovers with decreasing
$T$, first to $\gamma=3/2$ and then to $\gamma=2$, the Fermi-liquid value. The
dissipative part of the time-dependent susceptibility $\chi''(\omega)\propto
\omega$ as $\omega \to 0$ except at the quantum critical point where we find
$\chi''(\omega) \propto \sqrt{\omega}$. The characteristic spin-fluctuation
energy vanishes at the quantum critical point with $\omega_{\rm sf} \sim
(1-J/J_{c})$ for $J\lesssim J_{c}$, and $\omega_{\rm sf} \propto T^{3/2}$ at
the critical coupling.


title:
Barrier effects on the collective excitations of split Bose-Einstein
  condensates

abstract:
  We investigate the collective excitations of a single-species Bose gas at T=0
in a harmonic trap where the confinement undergoes some splitting along one
spatial direction. We mostly consider onedimensional potentials consisting of
two harmonic wells separated a distance 2 z_0, since they essentially contain
all the barrier effects that one may visualize in the 3D situation. We find,
within a hydrodynamic approximation, that regardless the dimensionality of the
system, pairs of levels in the excitation spectrum, corresponding to
neighbouring even and odd excitations, merge together as one increases the
barrier height up to the current value of the chemical potential. The
excitation spectra computed in the hydrodynamical or Thomas-Fermi limit are
compared with the results of exactly solving the time-dependent
Gross-Pitaevskii equation. We analyze as well the characteristics of the
spatial pattern of excitations of threedimensional boson systems according to
the amount of splitting of the condensate.


title: Directed Percolation and Generalized Friendly Walkers

abstract:
  We show that the problem of directed percolation on an arbitrary lattice is
equivalent to the problem of m directed random walkers with rather general
attractive interactions, when suitably continued to m=0. In 1+1 dimensions,
this is dual to a model of interacting steps on a vicinal surface. A similar
correspondence with interacting self-avoiding walks is constructed for
isotropic percolation.