title:
The Different Effect of Electron-Electron Interaction on the Spectrum of
  Atoms and Quantum Dots

abstract:
  The electron-electron scattering rate of single particle excitations in atoms
is estimated and compared with the corresponding rate in quantum dots. It is
found that in alkali atoms single particle excitations do not acquire a width
due to electron-electron interaction, while in complex atoms they may. This
width is typically smaller than the single particle level spacing, and hence
does not affect the number of discrete single particle excitations resolved
below the ionization threshold. This situation is contrasted with that of
quantum dots where electron-electron interaction severely limits the number of
resolved excitations. Unlike the case of quantum dots, the scattering rate in
atoms is found to decrease with increasing excitation energy. The different
effect of electron-electron interaction on the spectrum of quantum dots and
atoms is traced to the different confining potentials in the two systems.


title: Three-phase point in a binary hard-core lattice model?

abstract:
  Using Monte Carlo simulation, Van Duijneveldt and Lekkerkerker [Phys. Rev.
Lett. 71, 4264 (1993)] found gas-liquid-solid behaviour in a simple
two-dimensional lattice model with two types of hard particles. The same model
is studied here by means of numerical transfer matrix calculations, focusing on
the finite size scaling of the gaps between the largest few eigenvalues. No
evidence for a gas-liquid transition is found. We discuss the relation of the
model with a solvable RSOS model of which the states obey the same exclusion
rules. Finally, a detailed analysis of the relation with the dilute three-state
Potts model strongly supports the tricritical point rather than a three-phase
point.


title: Ordered Phase in the Fermionized Heisenberg Antiferromagnet

abstract:
  Thermal properties of the ordered phase of the spin 1/2 isotropic Heisenberg
Antiferromagnet on a d-dimensional hypercubical lattice are studied within the
fermionic representation when the constraint of single occupancy condition is
taken into account by the method suggested by Popov and Fedotov. Using saddle
point approximation in path integral approach we discuss not only the leading
order but also the fluctuations around the saddle point at one-loop level. The
influence of taking into account the single occupancy condition is discussed at
all steps.


title:
Integrability of the critical point of the Kagom\'e three-state Potts
  mode

abstract:
  The vicinity of the critical point of the three-state Potts model on a
Kagom\'e lattice is studied by mean of Random Matrix Theory. Strong evidence
that the critical point is integrable is given.


title:
Crossover between ballistic and diffusive regime of the spin-conductance
  and CPP-GMR in magnetic multilayered nanostructures

abstract:
  We analyze the interplay between disorder and band structure in current
perpendicular to the planes (CPP) giant magnetoresistance (GMR). We consider
finite magnetic multilayers attached to pure crystalline leads, described by a
tight-binding simple cubic two-band model (s-d). Several models of disorder are
considered, including random on-site potentials, lattice distortions,
impurities, vacancies, and cross-section fluctuations. Magneto-transport
properties are calculated in the zero-temperature zero-bias limit, within the
Landauer-Buttiker formalism. Using a very efficient numerical scattering
technique, we are able to perform simulations, over large length scales, and to
investigate spin-transport in the ballistic, diffusive and localized regimes,
as well as the crossover between them. The competition between disorder-induced
mean free path reduction and disorder-induced spin asymmetry enhancement of the
conductance highlights several different regimes of GMR.


title:
Modeling branching and chiral colonial patterning of lubricating
  bacteria

abstract:
  In nature, microorganisms must often cope with hostile environmental
conditions. To do so they have developed sophisticated cooperative behavior and
intricate communication capabilities, such as: direct cell-cell physical
interactions via extra-membrane polymers, collective production of
extracellular "wetting" fluid for movement on hard surfaces, long range
chemical signaling such as quorum sensing and chemotactic (bias of movement
according to gradient of chemical agent) signaling, collective activation and
deactivation of genes and even exchange of genetic material. Utilizing these
capabilities, the colonies develop complex spatio-temporal patterns in response
to adverse growth conditions. We present a wealth of branching and chiral
patterns formed during colonial development of lubricating bacteria (bacteria
which produce a wetting layer of fluid for their movement). Invoking ideas from
pattern formation in non-living systems and using ``generic'' modeling we are
able to reveal novel survival strategies which account for the salient features
of the evolved patterns. Using the models, we demonstrate how communication
leads to self-organization via cooperative behavior of the cells. In this
regard, pattern formation in microorganisms can be viewed as the result of the
exchange of information between the micro-level (the individual cells) and the
macro-level (the colony). We mainly review known results, but include a new
model of chiral growth, which enables us to study the effect of chemotactic
signaling on the chiral growth. We also introduce a measure for weak chirality
and use this measure to compare the results of model simulations with
experimental observations.

categories: cond-mat physics.bio-ph q-bio.CB q-bio.QM


title: Critical fields and devil's staircase in superconducting ladders

abstract:
  We have determined the ground state for both a ladder array of Josephson
junctions and a ladder of thin superconducting wires. We find that the
repulsive interaction between vortices falls off exponentially with separation.
The fact that the interaction is short-range leads to novel phenomena. The
ground state vortex density exhibits a complete devil's staircase as the
applied magnetic field is increased, each step producing a pair of
metal-insulator transitions. The critical fields in the staircase are all
calculated analytically and depend only on the connectivity of the ladder and
the area of the elementary plaquette. In particular the normal square ladder
contains no vortices at all until the flux per plaquette reaches 0.5/sqrt{3}
flux quanta.


title: Non Linear Effects in Resonant Tunneling Throught a Quantum Dot

abstract:
  We study non-linear phenomena in quantum dots. Non linearities are reflected
in the I-V characteristic curve as bistabilities, instabilities and time
dependent oscillations of the currents. The nature of the non-linear behavior
depends upon the parameters that define the system.


title: The dilute Potts model on random surfaces

abstract:
  We present a new solution of the asymmetric two-matrix model in the large $N$
limit which only involves a saddle point analysis. The model can be interpreted
as Ising in the presence of a magnetic field, on random dynamical lattices with
the topology of the sphere (resp. the disk) for closed (resp. open) surfaces;
we elaborate on the resulting phase diagram. The method can be equally well
applied to a more general $(Q+1)$-matrix model which represents the dilute
Potts model on random dynamical lattices. We discuss in particular duality of
boundary conditions for open random surfaces.


title:
Mesoscopic sensitivity of speckles in disordered nonlinear media to
  changes of disordered potential

abstract:
  We show that the sensitivity of wave speckle patterns in disordered nonlinear
media to changes of scattering potential increases with sample size. For large
enough sample size this quantity diverges, which implies that at given coherent
wave incident on a sample there are multiple solutions for the spatial
distribution of the wave's density. The number of solutions increases
exponentially with the sample size.


title: Three-Dimensional Quantum Percolation Studied by Level Statistics

abstract:
  Three-dimensional quantum percolation problems are studied by analyzing
energy level statistics of electrons on maximally connected percolating
clusters. The quantum percolation threshold $\pq$, which is larger than the
classical percolation threshold $\pc$, becomes smaller when magnetic fields are
applied, i.e., $\pq(B=0)>\pq(B\ne 0)>\pc$. The critical exponents are found to
be consistent with the recently obtained values of the Anderson model,
supporting the conjecture that the quantum percolation is classified onto the
same universality classes of the Anderson transition. Novel critical level
statistics at the percolation threshold is also reported.


title: Transverse-field Ising spin chain with inhomogeneous disorder

abstract:
  We consider the critical and off-critical properties at the boundary of the
random transverse-field Ising spin chain when the distribution of the couplings
and/or transverse fields, at a distance $l$ from the surface, deviates from its
uniform bulk value by terms of order $l^{-\kappa}$ with an amplitude $A$. Exact
results are obtained using a correspondence between the surface magnetization
of the model and the surviving probability of a random walk with time-dependent
absorbing boundary conditions. For slow enough decay, $\kappa<1/2$, the
inhomogeneity is relevant: Either the surface stays ordered at the bulk
critical point or the average surface magnetization displays an essential
singularity, depending on the sign of $A$. In the marginal situation,
$\kappa=1/2$, the average surface magnetization decays as a power law with a
continuously varying, $A$-dependent, critical exponent which is obtained
analytically. The behavior of the critical and off-critical autocorrelation
functions as well as the scaling form of the probability distributions for the
surface magnetization and the first gaps are determined through a
phenomenological scaling theory. In the Griffiths phase, the properties of the
Griffiths-McCoy singularities are not affected by the inhomogeneity. The
various results are checked using numerical methods based on a mapping to free
fermions.


title:
Localized and Extended States in One-Dimensional Disordered System:
  Random-Mass Dirac Fermions

abstract:
  System of Dirac fermions with random-varying mass is studied in detail. We
reformulate the system by transfer-matrix formalism.
  Eigenvalues and wave functions are obtained numerically for various
configurations of random telegraphic mass m(x).
  Localized and extended states are identified. For quasi-periodic m(x),
low-energy wave functions are also quasi-periodic and extended, though we are
not imposing the periodic boundary condition on wave function. On increasing
the randomness of the varying mass, states lose periodicity and most of them
tend to localize. At the band center or the low-energy limit, there exist
extended states which have more than one peak spatially separate with each
other comparatively large distance.
  Numerical calculations of the density of states and ensemble averaged Green's
functions are explicitly given. They are in good agreement with analytical
calculations by using the supersymmetric methods and exact form of the
zero-energy wave functions.


title: Thermal Noise Can Facilitate Energy Conversion by Ratchet System?

abstract:
  Molecular motors in biological systems are expected to use ambient
fluctuation. In a recent Letter [Phys. Rev. Lett. {\bf 80}, 5251 (1998)], it
was showed that the following question was unsolved, ``Can thermal noise
facilitate energy conversion by ratchet system?'' We consider it using
stochastic energetics, and show that there exist systems where thermal noise
helps the energy conversion.


title:
Domain-Wall Free-Energy of Spin Glass Models:Numerical Method and
  Boundary Conditions

abstract:
  An efficient Monte Carlo method is extended to evaluate directly domain-wall
free-energy for randomly frustrated spin systems. Using the method, critical
phenomena of spin-glass phase transition is investigated in 4d +/-J Ising model
under the replica boundary condition. Our values of the critical temperature
and exponent, obtained by finite-size scaling, are in good agreement with those
of the standard MC and the series expansion studies. In addition, two
exponents, the stiffness exponent and the fractal dimension of the domain wall,
which characterize the ordered phase, are obtained. The latter value is larger
than d-1, indicating that the domain wall is really rough in the 4d Ising spin
glass phase.


title:
Extended and localized phonons, free electrons, and diffusive states in
  disordered lattice models

abstract:
  In this paper we propose that phonons, free diffusive electrons, and
diffusion in two-and three--dimensional disordered lattice problems belong to a
different class of localization than bonded electrons. This is manifested by
three effects that can be observed numerically. First, there are extended
states even at two dimensions, whereas there are no extended states in the
usual electronic models. Second, the correlation length does not diverge at the
mobility edges in three dimensions, and finally, the participation ratio of the
extended states, decays to zero at this edge. This indicates zero electronic
conductivity, in the extended region near the mobility edge. We show that low
energy modes for these models can either have diverging localization lengths or
are extended.


title:
Piezomagnetism and Stress Induced Paramagnetic Meissner Effect in
  Mechanically Loaded High-T_c Granular Superconductors

abstract:
  Two novel phenomena in a weakly coupled granular superconductor under an
applied stress are predicted which are based on recently suggested piezophase
effect (a macroscopic quantum analog of the piezoelectric effect) in
mechanically loaded grain boundary Josephson junctions. Namely, we consider the
existence of stress induced paramagnetic moment in zero applied magnetic field
(piezomagnetism) and its influence on a low-field magnetization (leading to a
mechanically induced paramagnetic Meissner effect). The conditions under which
these two effects can be experimentally measured in high-T_$ granular
superconductors are discussed.


title: Cold and Warm Denaturation of Hydrophobic Polymers

abstract:
  We introduce a polymer model where the transition from swollen to compact
configurations is due to interactions between the monomers and the solvent.
These interactions are the origin of the effective attractive interactions
between hydrophobic amminoacids in proteins. We find that in the low and high
temperature phases polymers are swollen, and there is an intermediate phase
where the most favorable configurations are compact. We argue that such a model
captures in a single framework both the cold and the warm denaturation
experimentally detected for proteins. Some consequences for protein folding are
discussed.


title:
The pseudogap in Bi2212 single crystals from tunneling measurements: a
  possible evidence for the Cooper pairs above Tc

abstract:
  We present electron-tunneling spectroscopy of slightly overdoped Bi2212
single crystals with Tc = 87 - 90 K in a temperature range between 14 K and 290
K using a break-junction technique. The pseudogap which has been detected above
Tc appears at T* = 280 K. The analysis of the spectra shows that there is a
contribution to the pseudogap above Tc, which disappears approximately at 110 -
115 K. We associate this contribution with the presence of incoherent Cooper
pairs.


title:
Canted antiferromagnetic phase in a double quantum well in a tilted
  quantizing magnetic field

abstract:
  We investigate the double-layer electron system in a parabolic quantum well
at filling factor $\nu=2$ in a tilted magnetic field using capacitance
spectroscopy. The competition between two ground states is found at the Zeeman
splitting appreciably smaller than the symmetric-antisymmetric splitting.
Although at the transition point the system breaks up into domains of the two
competing states, the activation energy turns out to be finite, signaling the
occurrence of a new insulator-insulator quantum phase transition. We interpret
the obtained results in terms of a predicted canted antiferromagnetic phase.


title:
Resistive Transition and Upper Critical Field in Underdoped
  YBa_2Cu_3O_{6+x} Single Crystals

abstract:
  A superconducting transition in the temperature dependence of the ab-plane
resistivity of underdoped YBa_2Cu_3O_{6+x} crystals in the range T_c<30 K has
been investigated. Unlike the case of samples with the optimal level of doping,
the transition width increased insignificantly with magnetic field, and in the
range T_c<13 K it decreased with increasing magnetic field. The transition
point T_c(B) was determined by analyzing the fluctuation conductivity. The
curves of B_{c2}(T) measured in the region T/T_c>0.1 did not show a tendency to
saturation and had a positive second derivative everywhere, including the
immediate neighborhood of T_c. The only difference among the curves of
B_{c2}(T) for different crystal states is the scales of T and B, so they can be
described in terms of a universal function, which fairly closely follows
Alexandrov's model of boson superconductivity.


title:
Surface effects in nanoparticles: application to maghemite
  $\gamma$-Fe_{2}O_{3}

abstract:
  We present a microscopic model for nanoparticles, of the maghemite ($\gamma
$% -Fe$_{2}$O$_{3}$) type, and perform classical Monte Carlo simulations of
their magnetic properties. On account of M\"{o}ssbauer spectroscopy and
high-field magnetisation results, we consider a particle as composed of a core
and a surface shell of constant thickness. The magnetic state in the particle
is described by the anisotropic classical Dirac-Heisenberg model including
exchange and dipolar interactions and bulk and surface anisotropy. We consider
the case of ellipsoidal (or spherical) particles with free boundaries at the
surface. Using a surface shell of constant thickness ($\sim 0.35$ nm) we vary
the particle size and study the effect of surface magnetic disorder on the
thermal and spatial behaviors of the net magnetisation of the particle. We
study the shift in the surface ``critical region'' for different
surface-to-core ratios of the exchange coupling constants. It is also shown
that the profile of the local magnetisation exhibits strong temperature
dependence, and that surface anisotropy is reponsible for the non saturation of
the magnetisation at low temperatures.


title: Density-induced BCS to Bose-Einstein crossover

abstract:
  We investigate the zero-temperature BCS to Bose-Einstein crossover at the
mean-field level, by driving it with the attractive potential and the particle
density.We emphasize specifically the role played by the particle density in
this crossover.Three different interparticle potentials are considered for the
continuum model in three spatial dimensions, while both s- and d-wave solutions
are analyzed for the attractive (extended) Hubbard model on a two-dimensional
square lattice. For this model the peculiar behavior of the crossover for the
d-wave solution is discussed.In particular, in the strong-coupling limit when
approaching half filling we evidence the occurrence of strong correlations
among antiparallel-spin fermions belonging to different composite bosons, which
give rise to a quasi-long-range antiferromagnetic order in this limit.


title: A Model for Persistent Levy Motion

abstract:
  We propose the model, which allows us to approximate fractional Levy noise
and fractional Levy motion. Our model is based (i) on the Gnedenko limit
theorem for an attraction basin of stable probability law, and (ii) on
regarding fractional noise as the result of fractional
integration/differentiation of a white Levy noise. We investigate self - affine
properties of the approximation and conclude that it is suitable for modeling
persistent Levy motion with the Levy index between 1 and 2.


title: Spinless Two-Band Model in Infinite Dimensions

abstract:
  A spinless two-band model is studied in infinite dimension limit. Starting
from the atomic limit, the formal exact solution of the model is obtained by
means a perturbative treatment of the hopping and hybridisation terms. The
model is solved in closed form in high dimensions assuming no local spin
fluctuations. The non-Fermi liquid properties appearing in the metallic phase
are analysed through the behaviour of the density of states and the self-energy
near the Fermi level.


title: Novel Mechanism for Discrete Scale Invariance in Sandpile Models

abstract:
  Numerical simulations and a mean-field analysis of a sandpile model of
earthquake aftershocks in 1d, 2d and 3d euclidean lattices determine that the
average stress decays in a punctuated fashion after a main shock, with events
occurring at characteristic times increasing as a geometrical series with a
well-defined multiplicative factor which is a function of the stress corrosion
exponent, the stress drop ratio and the degree of dissipation. These results
are independent of the discrete nature of the lattice and stem from the
interplay between the threshold dynamics and the power law stress relaxation.


title: The Role of Heat Bath and Pointer Modes in Quantum Measurement

abstract:
  We present an exact derivation of a process in which a microscopic measured
system interacts with "heat bath" and pointer modes of a measuring device, via
a linear coupling involving Hermitian operator $\Lambda$ of the system. In the
limit of strong interaction with these modes, over a small time interval, we
show that the measured system and the "pointer" part of the measuring device
evolve into a statistical mixture of direct-product states such that the system
is in each eigenstate of $\Lambda$ with the correct quantum-mechanical
probability, whereas the expectation values of pointer-space operators retain
amplified information of the system's eigenstate.

categories: cond-mat.stat-mech cond-mat.mes-hall cond-mat.soft math-ph math.MP physics.atom-ph quant-ph


title: Metamagnetism of antiferromagnetic XXZ quantum spin chains

abstract:
  The magnetization process of the one-dimensional antiferromagnetic Heisenberg
model with the Ising-like anisotropic exchange interaction is studied by the
exact diagonalization technique. It results in the evidence of the first-order
spin flop transition with a finite magnetization jump in the N\'eel ordered
phase for $S\geq 1$. It implies that the S=1/2 chain is an exceptional case
where the metamagnetic transition becomes second-order due to large quantum
fluctuations.


title: Anisotropic Transport of Two-Dimensional Holes in High Landau Levels

abstract:
  Magnetoresistance data taken along $[\bar{2}33]$ and $[01\bar{1}]$ directions
in a GaAs/AlGaAs two-dimensional hole sample with van der Pauw geometry exhibit
significant anisotropy at half-integer filling factors. The anisotropy appears
to depend on both the density and symmetry of the hole charge distribution.


title:
A systematic approach to bicontinuous cubic phases in ternary
  amphiphilic systems

abstract:
  The Fourier approach and theories of space groups and color symmetries are
used to systematically generate and compare bicontinuous cubic structures in
the framework of a Ginzburg-Landau model for ternary amphiphilic systems. Both
single and double structures are investigated; they correspond to systems with
one or two monolayers in a unit cell, respectively. We show how and why single
structures can be made to approach triply periodic minimal surfaces very
closely, and give improved nodal approximations for G, D, I-WP and P surfaces.
We demonstrate that the relative stability of the single structures can be
calculated from the geometrical properties of their interfaces only. The single
gyroid G turns out to be the most stable bicontinuous cubic phase since it has
the smallest porosity. The representations are used to calculate distributions
of the Gaussian curvature and 2H-NMR bandshapes for C(P), C(D), S, C(Y) and
F-RD surfaces.


title:
Evidence for an angular dependent contribution from columnar defects to
  the equilibrium magnetization of YBa_2Cu_3O_{7-d}

abstract:
  We have measured an angle-dependent contribution to the equilibrium
magnetization of a YBa_2Cu_3O_{7-d} single crystal with columnar defects
created by 5.8GeV Pb ions. This contribution manifests itself as a jump in the
equilibrium torque signal, when the magnetic field crosses that of the defects.
The magnitude of the signal, which is observed in a narrow temperature interval
less than 2K and for fields up to about twice the irradiation field, is used to
estimate the the energy gained by vortex pinning on the defects. The vanishing
of the effective pinning energy at a temperature below Tc is attributed to its
renormalization by thermal fluctuations.


title: Quantizing the Lotka-Volterra equations: Effects of discretization

abstract: This paper has been withdrawn by the authors.


title:
On the Asymptotic Convergence of the Transient and Steady State
  Fluctuation Theorems

abstract:
  Non-equilibrium molecular dynamics simulations are used to demonstrate the
asymptotic convergence of the Transient and Steady State forms of the
Fluctuation Theorem. In the case of planar Poiseuille flow, we find that the
Transient form, valid for all times, converges to the Steady State form on
microscopic time scales. Further, we find that the time of convergence for the
two Theorems coincides with the time required for satisfaction of the
asymptotic Steady State Fluctuation Theorem.
  PACS numbers: 05.20.-y, 05.70.Ln, 47.10.+g, 47.40.-n


title: Mean-field expansion for spin models with medium-range interactions

abstract:
  We study the critical crossover between the Gaussian and the Wilson-Fisher
fixed point for general O(N)-invariant spin models with medium-range
interactions. We perform a systematic expansion around the mean-field solution,
obtaining the universal crossover curves and their leading corrections. In
particular we show that, in three dimensions, the leading correction scales as
$R^{-3}, R$ being the range of the interactions. We compare our results with
the existing numerical ones obtained by Monte Carlo simulations and present a
critical discussion of other approaches.


title: On the properties of small-world network models

abstract:
  We study the small-world networks recently introduced by Watts and Strogatz
[Nature {\bf 393}, 440 (1998)], using analytical as well as numerical tools. We
characterize the geometrical properties resulting from the coexistence of a
local structure and random long-range connections, and we examine their
evolution with size and disorder strength. We show that any finite value of the
disorder is able to trigger a ``small-world'' behaviour as soon as the initial
lattice is big enough, and study the crossover between a regular lattice and a
``small-world'' one. These results are corroborated by the investigation of an
Ising model defined on the network, showing for every finite disorder fraction
a crossover from a high-temperature region dominated by the underlying
one-dimensional structure to a mean-field like low-temperature region. In
particular there exists a finite-temperature ferromagnetic phase transition as
soon as the disorder strength is finite.


abstract:
  We present here the special issue of Physica D in honor of Boris Chirikov. It
is based on the proceedings of the Conference "Classical Chaos and its Quantum
Manifestations" held in Toulouse in July 1998. This electronic version contains
the list of contributions, the introduction and the unformal conclusion. The
introduction represents "X Chirikov Chaos Commandments" and reviews Chirikov's
pioneering results in the field of classical and quantum chaos. The conclusion,
written by P.M.Koch, gives an outlook on the development of the field of chaos
associated with Chirikov, including the personal reminiscences of Professor
Andy Sessler. This electronic version of the special issue has certain
differences and extentions comparing to the journal.


title: Condensation and Metastability in the 2D Potts Model

abstract:
  For the first order transition of the Ising model below $T_c$, Isakov has
proven that the free energy possesses an essential singularity in the applied
field. Such a singularity in the control parameter, anticipated by condensation
theory, is believed to be a generic feature of first order transitions, but too
weak to be observable. We study these issues for the temperature driven
transition of the $q$ states 2D Potts model at $q>q_c=4$. Adapting the droplet
model to this case, we relate its parameters to the critical properties at
$q_c$ and confront the free energy to the many informations brought by previous
works. The essential singularity predicted at the transition temperature leads
to observable effects in numerical data. On a finite lattice, a metastability
domain of temperatures is identified, which shrinks to zero in the
thermodynamical limit. ~


title: Atom loss from Bose-Einstein condensates due to Feshbach resonance

abstract:
  In recent experiments on Na Bose-Einstein condensates [S. Inouye et al,
Nature 392, 151 (1998); J. Stenger et al, Phys. Rev. Lett. 82, 2422 (1999)],
large loss rates were observed when a time-varying magnetic field was used to
tune a molecular Feshbach resonance state near the state of pairs of atoms
belonging to the condensate many-body wavefunction. A mechanism is offered here
to account for the observed losses, based on the deactivation of the resonant
molecular state by interaction with a third condensate atom.


title: A thermal model for adaptive competition in a market

abstract:
  New continuous and stochastic extensions of the minority game, devised as a
fundamental model for a market of competitive agents, are introduced and
studied in the context of statistical physics. The new formulation reproduces
the key features of the original model, without the need for some of its
special assumptions and, most importantly, it demonstrates the crucial role of
stochastic decision-making. Furthermore, this formulation provides the exact
but novel non-linear equations for the dynamics of the system.


title:
Effects of Domain Wall on Electronic Transport Properties in Mesoscopic
  Wire of Metallic Ferromagnets

abstract:
  We study the effect of the domain wall on electronic transport properties in
wire of ferromagnetic 3$d$ transition metals based on the linear response
theory. We considered the exchange interaction between the conduction electron
and the magnetization, taking into account the scattering by impurities as
well. The effective electron-wall interaction is derived by use of a local
gauge transformation in the spin space. This interaction is treated
perturbatively to the second order. The conductivity contribution within the
classical (Boltzmann) transport theory turns out to be negligiblly small in
bulk magnets, due to a large thickness of the wall compared with the fermi
wavelength. It can be, however, significant in ballistic nanocontacts, as
indicated in recent experiments. We also discuss the quantum correction in
disordered case where the quantum coherence among electrons becomes important.
In such case of weak localization the wall can contribute to a decrease of
resistivity by causing dephasing. At lower temperature this effect grows and
can win over the classical contribution, in particular in wire of diameter
$L_{\perp}\lesssim \ell_{\phi}$, $\ell_{\phi}$ being the inelastic diffusion
length. Conductance change of the quantum origin caused by the motion of the
wall is also discussed.


title: Unlocking Transition for Modulated Surfaces and Quantum Hall Stripes

abstract:
  We develop a sine-Gordon model of layered systems of two-dimensional
modulated surfaces and one dimensional stripes, and demonstrate that these
systems can undergo a Kosterlitz-Thouless transition in which the modulations
unlock as a result of thermal or quantum fluctuations, respectively. The
unlocked phase is interpreted as an anisotropic crystal in which
soliton-antisoliton pairs proliferate. The properties of such a state for
modulated stripes in quantum Hall systems and its possible relevance to recent
anomalies in transport data are discussed.


title: Note on Two Theorems in Nonequilibrium Statistical Mechanics

abstract:
  An attempt is made to clarify the difference between a theorem derived by
Evans and Searles in 1994 on the statistics of trajectories in phase space and
a theorem proved by the authors in 1995 on the statistics of fluctuations on
phase space trajectory segments in a nonequilibrium stationary state.


title: Dynamical properties of liquid lithium above melting

abstract:
  The dynamical properties of liquid lithium at several thermodynamic states
near the triple point have been studied within the framework of the
mode-coupling theory. We present a self-consistent scheme which, starting from
the knowledge of the static structural properties of the liquid system, allows
the theoretical calculation of several single-particle and collective dynamical
properties. The study is complemented by performing Molecular Dynamics
simulations, and the obtained results are compared with the available
experimental data.


title: Formation of Liesegang patterns: A spinodal decomposition scenario

abstract:
  Spinodal decomposition in the presence of a moving particle source is
proposed as a mechanism for the formation of Liesegang bands. This mechanism
yields a sequence of band positions x_n that obeys the spacing law
x_n~Q(1+p)^n. The dependence of the parameters p and Q on the initial
concentration of the reagents is determined and we find that the functional
form of p is in agreement with the experimentally observed Matalon-Packter law.


title: SO(6)-Generalized Pseudogap Model of the Cuprates

abstract:
  The smooth evolution of the tunneling gap of Bi_2Sr_2CaCu_2O_8 with doping
from a pseudogap state in the underdoped cuprates to a superconducting state at
optimal and overdoping reflects an underlying SO(6) instability structure of
the (pi,0) saddle points. The pseudogap is probably not associated with
superconductivity, but is related to competing nesting instabilities, which are
responsible for the stripe phases. We earlier introduced a simple Ansatz of
this competition in terms of a pinned Balseiro-Falicov (pBF) model of competing
charge density wave and (s-wave) superconductivity. This model gives a good
description of the phase diagram and the tunneling and photoemission spectra.
Here, we briefly review these results, and discuss some recent developments:
experimental evidence for a non-superconducting component to the pseudogap; and
SO(6) generalizations of the pBF model, including flux phase and d-wave
superconductivity.


title: Stripe Disordering Transition

abstract:
  We have recently begun Monte Carlo simulations of the dynamics of stripe
phases in the cuprates. A simple model of spinodal decomposition of the holes
allows us to incorporate Coulomb repulsion and coherency strains. We find
evidence for a possible stripe disordering transition, at a temperature below
the pseudogap onset. Experimental searches for such a transition can provide
constraints for models of stripe formation.


title: Optimized Constant Pressure Stochastic Dynamics

abstract:
  A recently proposed method for computer simulations in the
isothermal-isobaric (NPT) ensemble, based on Langevin-type equations of motion
for the particle coordinates and the ``piston'' degree of freedom, is
re-derived by straightforward application of the standard Kramers-Moyal
formalism. An integration scheme is developed which reduces to a
time-reversible symplectic integrator in the limit of vanishing friction. This
algorithm is hence expected to be quite stable for small friction, allowing for
a large time step. We discuss the optimal choice of parameters, and present
some numerical test results.


title: Structure and energetics of the Si-SiO_2 interface

abstract:
  Silicon has long been synonymous with semiconductor technology. This unique
role is due largely to the remarkable properties of the Si-SiO_2 interface,
especially the (001)-oriented interface used in most devices. Although Si is
crystalline and the oxide is amorphous, the interface is essentially perfect,
with an extremely low density of dangling bonds or other electrically active
defects. With the continual decrease of device size, the nanoscale structure of
the silicon/oxide interface becomes more and more important. Yet despite its
essential role, the atomic structure of this interface is still unclear. Using
a novel Monte Carlo approach, we identify low-energy structures for the
interface. The optimal structure found consists of Si-O-Si "bridges" ordered in
a stripe pattern, with very low energy. This structure explains several
puzzling experimental observations.


title:
Theory of monolayers with boundaries: Exact results and Perturbative
  analysis

abstract:
  Domains and bubbles in tilted phases of Langmuir monolayers contain a class
of textures knows as boojums. The boundaries of such domains and bubbles may
display either cusp-like features or indentations. We derive analytic
expressions for the textures within domains and surrounding bubbles, and for
the shapes of the boundaries of these regions. The derivation is perturbative
in the deviation of the bounding curve from a circle. This method is not
expected to be accurate when the boundary suffers large distortions, but it
does provide important clues with regard to the influence of various energetic
terms on the order-parameter texture and the shape of the domain or bubble
bounding curve. We also look into the effects of thermal fluctuations, which
include a sample-size-dependent effective line tension.


title: First-order transition in small-world networks

abstract:
  The small-world transition is a first-order transition at zero density $p$ of
shortcuts, whereby the normalized shortest-path distance undergoes a
discontinuity in the thermodynamic limit. On finite systems the apparent
transition is shifted by $\Delta p \sim L^{-d}$. Equivalently a ``persistence
size'' $L^* \sim p^{-1/d}$ can be defined in connection with finite-size
effects. Assuming $L^* \sim p^{-\tau}$, simple rescaling arguments imply that
$\tau=1/d$. We confirm this result by extensive numerical simulation in one to
four dimensions, and argue that $\tau=1/d$ implies that this transition is
first-order.


title:
Triple sign reversal of Hall effect in HgBa_{2}CaCu_{2}O_{6} thin films
  after heavy-ion irradiations

abstract:
  Triple sign reversal in the mixed-state Hall effect has been observed for the
first time in ion-irradiated HgBa_{2}CaCu_{2}O_{6} thin films. The negative dip
at the third sign reversal is more pronounced for higher fields, which is
opposite to the case of the first sign reversal near T_c in most high-T_c
superconductors. These observations can be explained by a recent prediction in
which the third sign reversal is attributed to the energy derivative of the
density of states and to a temperature-dependent function related to the
superconducting energy gap. These contributions prominently appear in cases
where the mean free path is significantly decreased, such as our case of
ion-irradiated thin films.


title:
Kinetics of Surfactant Adsorption at Fluid-Fluid Interfaces: Surfactant
  Mixtures

abstract:
  The adsorption at the interface between an aqueous solution of several
surface-active agents and another fluid (air or oil) phase is addressed
theoretically. We derive the kinetic equations from a variation of the
interfacial free energy, solve them numerically and provide an analytic
solution for the simple case of a linear adsorption isotherm. Calculating
asymptotic solutions analytically, we find the characteristic time scales of
the adsorption process and observe the behavior of the system at various
temporal stages. In particular, we relate the kinetic behavior of the mixture
to the properties of its individual constituents and find good agreement with
experiments. In the case of kinetically limited adsorption, the mixture
kinetics is found to be considerably different from that of the
single-surfactant solutions because of strong coupling between the species.


title: Thermal fluctuations and disorder effects in vortex lattices

abstract:
  We calculate using loop expansion the effect of fluctuations on the structure
function and magnetization of the vortex lattice and compare it with existing
MC results. In addition to renormalization of the height of the Bragg peaks of
the structure function, there appears a characteristic saddle shape ''halos''
around the peaks. The effect of disorder on magnetization is also calculated.
All the infrared divergencies related to soft shear cancel.


title: The half-filled Landau level - composite fermions and dipoles

abstract:
  The composite-fermion approach as formulated in the fermion Chern-Simons
theory has been very successful in describing the physics of the lowest Landau
level near Landau level filling factor 1/2. Recent work has emphasized the fact
that the true quasiparticles at these filling factors are electrically neutral
and carry an electric dipole moment. In a previous work, we discussed at length
two formulations in terms of dipolar quasiparticles. Here we briefly review one
approach - termed electron-centered quasiparticles - and show how it can be
extended from 1/2 to nearby filling factors where the quasiparticles carry both
an electric dipole moment and an overall charge.


title:
Crossover from Luttinger- to Fermi-liquid behavior in strongly
  anisotropic systems in large dimensions

abstract:
  We consider the low-energy region of an array of Luttinger liquids coupled by
a weak interchain hopping. The leading logarithmic divergences can be re-summed
to all orders within a self-consistent perturbative expansion in the hopping,
in the large-dimension limit. The anomalous exponent scales to zero below the
one-particle crossover temperature. As a consequence, coherent quasiparticles
with finite weight appear along the whole Fermi surface. Extending the
expansion self-consistently to all orders turns out to be crucial in order to
restore the correct Fermi-liquid behavior.


title:
Switching between different vortex states in 2-dimensional easy-plane
  magnets due to an ac magnetic field

abstract:
  Using a discrete model of 2-dimensional easy-plane classical ferromagnets, we
propose that a rotating magnetic field in the easy plane can switch a vortex
from one polarization to the opposite one if the amplitude exceeds a threshold
value, but the backward process does not occur. Such switches are indeed
observed in computer simulations.


title: Are citations of scientific papers a case of nonextensivity ?

abstract:
  The distribution $N(x)$ of citations of scientific papers has recently been
illustrated (on ISI and PRE data sets) and analyzed by Redner [Eur. Phys. J. B
{\bf 4}, 131 (1998)]. To fit the data, a stretched exponential ($N(x) \propto
\exp{-(x/x_0)^{\beta}}$) has been used with only partial success. The success
is not complete because the data exhibit, for large citation count $x$, a power
law (roughly $N(x) \propto x^{-3}$ for the ISI data), which, clearly, the
stretched exponential does not reproduce. This fact is then attributed to a
possibly different nature of rarely cited and largely cited papers. We show
here that, within a nonextensive thermostatistical formalism, the same data can
be quite satisfactorily fitted with a single curve (namely, $N(x) \propto
1/[1+(q-1) \lambda x]^{q/{q-1}}$ for the available values of $x$. This is
consistent with the connection recently established by Denisov [Phys. Lett. A
{\bf 235}, 447 (1997)] between this nonextensive formalism and the
Zipf-Mandelbrot law. What the present analysis ultimately suggests is that, in
contrast to Redner's conclusion, the phenomenon might essentially be one and
the same along the entire range of the citation number $x$.


title:
Gapless Singlet modes in the Kagome strips: A study through DMRG and
  strong coupling analysis

abstract:
  Recently Azaria et al have studied strips of the Kagome-lattice in the
weak-coupling limit, where they consist of two spin-half chains on the outside
weakly coupled to an array of half-integer spins in the middle. Using a number
of mappings they have arrived at the interesting result that in this system all
spin excitations are gapped but there are gapless spinless modes. Here we study
these Kagome strips in the limit where the interchain couplings are comparable
to the coupling to the middle spins by density matrix renormalization group and
by a strong coupling analysis. In the limit when the coupling to the
middle-spin dominates, the 5-spins of the unit-cell reduce to a single S=3/2
spin, and the overall system has well known gapless spin excitations. We study
the phase transition from this phase to the weak-coupling phase. We also carry
out a strong coupling analysis away from the S=3/2 limit, where the five-spin
blocks have four degenerate ground states with S=1/2, which can be thought of
as two spin and two pseudospin degrees of freedom. A numerical study of this
strong coupling model also suggests a finite spin-gap.


title:
The Effect of Spin Splitting on the Metallic Behavior of a
  Two-Dimensional System

abstract:
  Experiments on a constant-density two-dimensional hole system in a GaAs
quantum well reveal that the metallic behavior observed in the
zero-magnetic-field temperature dependence of the resistivity depends on the
symmetry of the confinement potential and the resulting spin-splitting of the
valence band.


title: Suppression of Kondo effect in a quantum dot by external irradiation

abstract:
  We demonstrate that the external irradiation brings decoherence in the spin
states of the quantum dot. This effect cuts off the Kondo anomaly in
conductance even at zero temperature. We evaluate the dependence of the DC
conductance in the Kondo regime on the power of irradiation, this dependence
being determined by the decoherence.


title:
Spin mapping, phase diagram, and collective modes in double layer
  quantum Hall systems at $\nu=2$

abstract:
  An exact spin mapping is identified to simplify the recently proposed
hard-core boson description (Demler and Das Sarma, Phys. Rev. Lett., to be
published) of the bilayer quantum Hall system at filling factor 2. The
effective spin model describes an easy-plane ferromagnet subject to an external
Zeeman field. The phase diagram of this effective model is determined exactly
and found to agree with the approximate calculation of Demler and Das Sarma,
while the Goldstone-mode spectrum, order parameter stiffness and
Kosterlitz-Thouless temperature in the canted antiferromagnetic phase are
computed approximately.


title: Anisotropic Coarsening: Grain Shapes and Nonuniversal Persistence

abstract:
  We solve a coarsening system with small but arbitrary anisotropic surface
tension and interface mobility. The resulting size-dependent growth shapes are
significantly different from equilibrium microcrystallites, and have a
distribution of grain sizes different from isotropic theories. As an
application of our results, we show that the persistence decay exponent depends
on anisotropy and hence is nonuniversal.


title:
Implementation of the LDA+U method using the full potential linearized
  augmented plane wave basis

abstract:
  We provide a straightforward and efficient procedure to combine LDA+U total
energy functional with the full potential linearized augmented plane wave
method. A detailed derivation of the LDA+U Kohn-Sham type equations is
presented for the augmented plane wave basis set, and a simple
``second-variation'' based procedure for self-consistent LDA+U calculations is
given. The method is applied to calculate electronic structure and magnetic
properties of NiO and Gd. The magnetic moments and band eigenvalues obtained
are in very good quantitative agreement with previous full potential LMTO
calculations. We point out that LDA+U reduces the total d charge on Ni by 0.1
in NiO.


title:
Generalized stacking fault energy surfaces and dislocation properties of
  aluminum

abstract:
  We have employed the semidiscrete variational generalized Peierls-Nabarro
model to study the dislocation core properties of aluminum. The generalized
stacking fault energy surfaces entering the model are calculated by using
first-principles Density Functional Theory (DFT) with pseudopotentials and the
embedded atom method (EAM). Various core properties, including the core width,
splitting behavior, energetics and Peierls stress for different dislocations
have been investigated. The correlation between the core energetics and
dislocation character has been explored. Our results reveal a simple
relationship between the Peierls stress and the ratio between the core width
and atomic spacing. The dependence of the core properties on the two methods
for calculating the total energy (DFT vs. EAM) has been examined. The EAM can
give gross trends for various dislocation properties but fails to predict the
finer core structures, which in turn can affect the Peierls stress
significantly (about one order of magnitude).


title:
Nonlocally-correlated disorder and delocalization in one dimension II:
  Localization length

abstract:
  In the previous paper (cond-mat/9809323), we calculated the density of staes
in the random-mass Dirac fermion system. In this paper, we obtain the mean
localization length of the single-fermion Greem's function by using the
supersymmetric methods. It is shown that the localization length is a
increasing function of the correation length of the disorders. This result is
in agreement with the density of states and the numerical studies
(cond-mat/9903389).


title:
Two-component theory of a droplet of electrons in half-filled Landau
  level

abstract:
  We have investigated low energy excitations of a disk of electrons in
half-filled Landau level using trail wave function and small-size exact
diagonalization approaches. We have constructed a set of many-body basis states
that describe correctly the low energy excitations. In this theory a droplet
consists of two types of composite fermion liquids, and suggests that a droplet
can support an edge magnetoplasmon and low energy droplet excitations. A
possibility of measuring these excitations in a quantum dot is discussed.


title:
Apparent Metallic Behavior at B = 0 of a two-dimensional electron system
  in AlAs

abstract:
  We report the observation of metallic-like behavior at low temperatures and
zero magnetic field in two dimensional (2D) electrons in an AlAs quantum well.
At high densities the resistance of the sample decreases with decreasing
temperature, but as the density is reduced the behavior changes to insulating,
with the resistance increasing as the temperature is decreased. The effect is
similar to that observed in 2D electrons in Si-MOSFETs, and in 2D holes in SiGe
and GaAs, and points to the generality of this phenomenon.


title: Evidence for superfluid B-phase of 3He in aerogel

abstract:
  We have made simultaneous torsional oscillator and transverse cw NMR (at 165
kHz) studies of the superfluid phase of 3He in aerogel glasses of 1% and 2% of
solid density. NMR occurs over a range of frequency extending from the Larmor
frequency to higher values, but strongly peaked at the Larmor value. This
behaviour together with the magnetic field independence of the effective
superfluid density provides convincing evidence for a B-phase state with an n
texture, in our spherical geometry, governed by the same energetic
considerations as for bulk superfluid 3He-B.


title:
Design of semiconductor heterostructures with preset electron
  reflectance by inverse scattering techniques

abstract:
  We present the application of the inverse scattering method to the design of
semiconductor heterostructures having a preset dependence of the (conduction)
electrons' reflectance on the energy. The electron dynamics are described by
either the effective mass Schr\"odinger, or by the (variable mass) BenDaniel
and Duke equations. The problem of phase (re)construction for the complex
transmission and reflection coefficients is solved by a combination of Pad\'e
approximant techniques, obtaining reference solutions with simple analytic
properties. Reflectance-preserving transformations allow bound state and
reflection resonance management. The inverse scattering problem for the
Schroedinger equation is solved using an algebraic approach due to Sabatier.
This solution can be mapped unitarily onto a family of BenDaniel and Duke type
equations. The boundary value problem for the nonlinear equation which
determines the mapping is discussed in some detail. The chemical concentration
profile of heterostructures whose self consistent potential yields the desired
reflectance is solved completely in the case of Schroedinger dynamics and
approximately for Ben-Daniel and Duke dynamics. The Appendix contains a brief
digest of results from scattering and inverse scattering theory for the
one-dimensional Schroedinger equation which are used in the paper.


title: Diffusion of two repulsive particles in a one-dimensional lattice

abstract:
  The problem of the lattice diffusion of two particles coupled by a contact
repulsive interaction is solved by finding analytical expressions of the
two-body probability characteristic function. The interaction induces anomalous
drift with a vanishing velocity, the average coordinate of each particle
growing at large times as $t^{1/2}$. The leading term of the mean square
dispersions displays normal diffusion, with a diffusion constant made smaller
by the interaction by the non-trivial factor $1-1/\pi$. Space continuous limit
taken from the lattice calculations allows to establish connection with the
standard problem of diffusion of a single fictitious particle constrained by a
totally reflecting wall. Comparison between lattice and continuous results
display marked differences for transient regimes, relevant with regards to high
time resolution experiments, and in addition show that, due to slowly
decreasing subdominant terms, lattice effects persist even at very large times.


title: Extended surface disorder in the quantum Ising chain

abstract:
  We consider random extended surface perturbations in the transverse field
Ising model decaying as a power of the distance from the surface towards a pure
bulk system. The decay may be linked either to the evolution of the couplings
or to their probabilities. Using scaling arguments, we develop a
relevance-irrelevance criterion for such perturbations. We study the
probability distribution of the surface magnetization, its average and typical
critical behaviour for marginal and relevant perturbations. According to
analytical results, the surface magnetization follows a log-normal distribution
and both the average and typical critical behaviours are characterized by
power-law singularities with continuously varying exponents in the marginal
case and essential singularities in the relevant case. For enhanced average
local couplings, the transition becomes first order with a nonvanishing
critical surface magnetization. This occurs above a positive threshold value of
the perturbation amplitude in the marginal case.


title: Stiffening Transition in Vicinal Surfaces with Adsorption

abstract:
  We study the vicinal surface with adsorption below the roughening
temperature, using the restricted solid-on-solid model coupled with the Ising
model. We calculate the step tension $\gamma$ and the step-interaction
coefficient $B$ by employing a variant of the density matrix algorithm. We find
a ``stiffening transition'' at a temperature $T_{\rm s}$ where $B$ vanishes. At
$T_{\rm s}$ surface free energy has the form $f(p)- f(0)= \gamma p +
({const.})p^5 + ...$ ($p$: surface gradient), which differs from the well-known
$p$-$p^3$ form.


title:
Spin dynamics in hole-doped two-dimensional S=1/2 Heisenberg
  antiferromagnets: ^{63}Cu NQR relaxation in La_{2-x}Sr_xCuO_4 for $x\leq
  0.04$

abstract:
  The effects on the correlated Cu^{2+} S = 1/2 spin dynamics in the
paramagnetic phase of La_{2-x}Sr_xCuO_4 (for $x \lesssim 0.04$) due to the
injection of holes are studied by means of ^{63}Cu NQR spin-lattice relaxation
time T_1 measurements. The results are discussed in the framework of the
connection between T_1 and the in-plane magnetic correlation length
$\xi_{2D}(x,T)$. It is found that at high temperatures the system remains in
the renormalized classical regime, with a spin stiffness constant $\rho_s(x)$
reduced by small doping to an extent larger than the one due to Zn doping. For
$x\gtrsim 0.02$ the effect of doping on $\rho_s(x)$ appears to level off. The
values for $\rho_s(x)$ derived from T_1 for $T\gtrsim 500$ K are much larger
than the ones estimated from the temperature behavior of sublattice
magnetization in the ordered phase ($T\leq T_N$). It is argued that these
features are consistent with the hypothesis of formation of stripes of
microsegregated holes.


title:
Spin dynamics and magnetic correlation length in two-dimensional quantum
  Heisenberg antiferromagnets

abstract:
  The correlated spin dynamics and the temperature dependence of the
correlation length $\xi(T)$ in two-dimensional quantum ($S=1/2$) Heisenberg
antiferromagnets (2DQHAF) on square lattice are discussed in the light of
experimental results of proton spin lattice relaxation in copper formiate
tetradeuterate (CFTD). In this compound the exchange constant is much smaller
than the one in recently studied 2DQHAF, such as La$_2$CuO$_4$ and
Sr$_2$CuO$_2$Cl$_2$. Thus the spin dynamics can be probed in detail over a
wider temperature range. The NMR relaxation rates turn out in excellent
agreement with a theoretical mode-coupling calculation. The deduced temperature
behavior of $\xi(T)$ is in agreement with high-temperature expansions, quantum
Monte Carlo simulations and the pure quantum self-consistent harmonic
approximation. Contrary to the predictions of the theories based on the
Non-Linear $\sigma$ Model, no evidence of crossover between different quantum
regimes is observed.


title:
Entropy and dynamic properties of water below the homogeneous nucleation
  temperature

abstract:
  Controversy exists regarding the possible existence of a transition between
the liquid and glassy states of water. Here we use experimental measurements of
the entropy, specific heat, and enthalpy of both liquid and glassy water to
construct thermodynamically-plausible forms of the entropy in the
difficult-to-probe region between 150 K and 236 K. We assume there is no
discontinuity in the entropy of the liquid in this temperature range, and use
the Adam-Gibbs theory -- which relates configurational entropy to dynamic
behavior -- to predict that dynamic quantities such as the diffusion constant
and the viscosity pass through an inflection where the liquid behavior changes
from that of an extremely ``fragile'' liquid to that of a ``strong'' liquid.


title:
Evidence for a Purely Double-Exchange Mechanism for the Anisotropic
  Conductivities in Layered La-Sr-Mn-O Single Crystals Below TC

abstract:
  Experimental evidence supports the double-exchange (DE) mechanism for both
in-plane and c-axis conductivity in the colossal-magnetoresistive (CMR) layered
manganite La1.4Sr1.6Mn2O7. Below TC, the data determine both the DE and
antiferromagnetic (AF) superexchange between bilayers. These agree with recent
spin-wave data, such that the ratio of intra- to inter-bilayer DE constants is
very close to the spin-independent conductance anisotropy of ~300. The
conductivity is shown to be proportional to the square of the measured
magnetization over a wide range of fields and for temperatures below the
in-plane ferromagnetic ordering temperature, TC. This dependence is shown to be
consistent with DE coupling and earlier zero-field studies of La1.2Sr1.8Mn2O7
near TC, which used neutron scattering for the local magnetization. A mixed AF
and spin-flop state (similar to the intermediate state of type-I
superconductors) is a rigorous prediction of the data and modeling.


title: Charge Segregation and Antiferromagnetism in High-Tc Superconductors

abstract:
  Local antiferromagnetism coexists with superconductivity in the cuprates.
Charge segregation provides a way to reconcile these properties. Direct
evidence for modulated spin and charge densities has been found in neutron and
X-ray scattering studies of Nd-doped La(2-x)Sr(x)CuO(4). Here we discuss the
nature of the modulation, and present some new results for a Zn-doped sample.
Some of the open questions concerning the connections between segregation and
superconductivity are described.


title: Torsional oscillator studies of the superfluidity of 3He in aerogel

abstract:
  We have made simultaneous torsional oscillator and transverse NMR
measurements (at 165 kHz) on 3He contained within aerogels with nominal
densities of 1% and 2% of solid glass. The superfluid transition is seen
simultaneously by both techniques and occurs at a temperature which agrees
semi-quantitatively with that expected for homogeneous isotropic pair-breaking
scattering of 3He atoms by strands of silica. Values obtained for the
superfluid density rho_s in the 2% sample are in reasonable agreement with
those observed previously. Coupling of the torsional mode to a parasitic
resonance prevented accurate determination of rho_s for the 1% aerogel. We have
identified other resonances coupling to the torsional oscillations as sound
modes within the helium/aerogel medium.


title:
Theoretical modeling for quantum liquids from 1d to 2d dimensional
  crossover using quantum groups

abstract:
  Recent experimental and theoretical work in strongly correlated electron
system necessitates a formulation to deal with 1d to 2d dimensional crossover
and raises several interesting questions.
  A particularly interesting question is what happens to the 1d Luttinger
liquid, as we go from 1d to 2d? The main point of emphasis of the present note
is that the transition from 1d to 2d has an underlying quantum group symmetry.
This relationhip of 1d to 2d transition with quantum group symmetry ties in
nicely with our previous proposal to model superconductivity,
antiferromagnetism and related phases arising from strongly correlated electron
states with quantum groups. A simple model based on two interacting chains with
nearest neighbor and next to nearest neighbor interactions is suggested.
  It appears reasonable to assume that it is the interaction between the 1d
Luttinger liquids which leads to the quantum group symmetry and the transition
to 2d quantum liquid. The conditions under which this 2d quantum liquid
deviates away from the 2d Fermi liquid is clearly of main interest to us. One
of the prime motivations underlying our proposal is the experimental
observation of stripe structure [phase] in high T_c superconductivity [HTSC]
materials. In short a simple model of two coupled chains 1d chains is
considered to answer an important question, how does one couple two 1d chains
[Luttinger Liquid] to obtain non-Fermi liquid [NFL] or partial NFL behavior in
2d?


title:
The nematic-isotropic phase transition in linear fused hard-sphere chain
  fluids

abstract:
  We present a modification of the generalized Flory dimer theory to
investigate the nematic (N) to isotropic (I) phase transition in chain fluids.
We focus on rigid linear fused hard-sphere (LFHS) chain molecules in this
study. A generalized density functional theory is developed, which involves an
angular weighting of the dimer reference fluid as suggested by decoupling
theory, to accommodate nematic ordering in the system. A key ingredient of this
theory is the calculation of the exact excluded volume for a pair of molecules
in an arbitrary relative orientation, which extends the recent work by
Williamson and Jackson for linear tangent hard-sphere chain molecules to the
case of linear fused hard-sphere chains with arbitrary intramolecular
bondlength. The present results for the N-I transition are compared with
previous theories and with computer simulations. In comparison with previous
studies, the results show much better agreement with simulations for both the
coexistence densities and the nematic order parameter at the transition.


title: Lattice-dynamics of a Disordered solid-solid Interface

abstract:
  Generic properties of elastic phonon transport at a disordered interface are
studied. The results show that phonon transmittance is a strong function of
frequency and the disorder correlation length. At frequencies lower than the
van Hove singularity the transmittance at a given frequency increases as the
correlation length decreases. At low frequencies, this is reflected by
different power-laws for phonon conductance across correlated and uncorrelated
disordered interfaces which are in approximate agreement with perturbation
theory of an elastic continuum. These results can be understood in terms of
simple mosaic and two-colour models of the interface.


title: Experimental Evidence for Topological Doping in the Cuprates

abstract:
  Some recent experiments that provide support for the concept of topological
doping in cuprate superconductors are discussed. Consistent with the idea of
charge segregation, it is argued that the scattering associated with the
``resonance'' peak found in YBa(2)Cu(3)O(6+x) and Bi(2)Sr(2)CaCu(2)O(8+\delta)
comes from the Cu spins and not from the doped holes.


title: Recent Experiments with Bose-Condensed Gases at JILA

abstract:
  We consider a binary mixture of two overlapping Bose-Einstein condensates in
two different hyperfine states of \Rb87 with nearly identical magnetic moments.
Such a system has been simply realized through application of radiofrequency
and microwave radiation which drives a two-photon transition between the two
states. The nearly identical magnetic moments afford a high degree of spatial
overlap, permitting a variety of new experiments. We discuss some of the
conditions under which the magnetic moments are identical, with particular
emphasis placed on the requirements for a time-averaged orbiting potential
(TOP) magnetic trap.


title:
First principles calculation of uniaxial magnetic anisotropy and
  magnetostriction in strained CMR films

abstract:
  We performed first - principles relativistic full-potential linearized
augmented plane wave calculations for strained tetragonal ferromagnetic
La(Ba)MnO$_3$ with an assumed experimental structure of thin strained
tetragonal La$_{0.67}$Ca$_{0.33}$MnO$_3$ (LCMO) films grown on SrTiO$_3$[001]
and LaAlO$_3$[001] substrates. The calculated uniaxial magnetic anisotropy
energy (MAE) values, are in good quantitative agreement with experiment for
LCMO films on SrTiO$_3$ substrate. We also analyze the applicability of linear
magnetoelastic theory for describing the stain dependence of MAE, and estimate
magnetostriction coefficient $\lambda_{001}$.


title: Periodic orbit resonances in layered metals in tilted magnetic fields

abstract:
  The frequency dependence of the interlayer conductivity of a layered Fermi
liquid in a magnetic field which is tilted away from the normal to the layers
is considered. For both quasi-one- and quasi-two-dimensional systems resonances
occur when the frequency is a harmonic of the frequency at which the magnetic
field causes the electrons to oscillate on the Fermi surface within the layers.
The intensity of the different harmonic resonances varies significantly with
the direction of the field. The resonances occur for both coherent and weakly
incoherent interlayer transport and so their observation does not imply the
existence of a three-dimensional Fermi surface.


title: On the 590cm-1 B1g feature in underdoped Bi2Sr2CaCu2O8+delta

abstract:
  Raman scattering studies have been performed on underdoped
Bi2Sr2CaCu2O8+delta. In single crystals underdoped by oxygen removal, a 590
cm-1 peak is observed in the B1g spectrum. The feature is observed to soften in
frequency by 3.8% with isotopic exchange for 16-O by 18-O. In contrast, the
590cm-1 peak is not observed in crystals underdoped by Y substitution which
suggests that it correspond to a disorder induced vibrational mode. We have
also found that underdoping leads to a depletion of low energy spectral weight
from regions of the Fermi surface located near the Brillouin zone axes.


title:
Coherent control of macroscopic quantum states in a single-Cooper-pair
  box

abstract:
  A small superconducting electrode (a single-Cooper-pair box) connected to a
reservoir via a Josephson junction constitutes an artificial two-level system,
in which two charge states that differ by 2e are coupled by tunneling of Cooper
pairs. Despite its macroscopic nature involving a large number of electrons,
the two-level system shows coherent superposition of the two charge states, and
has been suggested as a candidate for a qubit, i.e. a basic component of a
quantum computer. Here we report on time-domain observation of the coherent
quantum-state evolution in the two-level system by applying a short voltage
pulse that modifies the energies of the two levels nonadiabatically to control
the coherent evolution. The resulting state was probed by a tunneling current
through an additional probe junction. Our results demonstrate coherent
operation and measurement of a quantum state of a single two-level system, i.e.
a qubit, in a solid-state electronic device.


title: Electronic structure of spinel-type LiV_2O_4

abstract:
  The band structure of the cubic spinel compound LiV_2O_4, which has been
reported recently to show heavy Fermion behavior, has been calculated within
the local-density approximation using a full-potential version of the linear
augmented-plane-wave method. The results show that partially-filled V 3d bands
are located about 1.9 eV above the O 2p bands and the V 3d bands are split into
a lower partially-filled t_{2g} complex and an upper unoccupied e_{g} manifold.
The fact that the conduction electrons originate solely from the t_{2g} bands
suggests that the mechanism for the mass enhancement in this system is
different from that in the 4f heavy Fermion systems, where these effects are
attributed to the hybridization between the localized 4f levels and itinerant
spd bands.


title:
Magnetic Order-Disorder Transition in the Two-Dimensional Spatially
  Anisotropic Heisenberg Model at Zero Temperature

abstract:
  The ground-state properties of the spin-1/2 antiferromagnetic Heisenberg
model with spatially anisotropic couplings on a square lattice are investigated
by a spin-rotation-invariant Green's-function approach and by Lanczos
diagonalizations on lattices up to 36 sites supplemented by finite-size
scaling. We focus on the anisotropy-driven transition from the N\'eel state to
a paramagnetic state with antiferromagnetic short-range order and on the
spatial dependence of spin correlation functions. Our principal result is that
a rather sharp crossover in the magnetic behavior occurs at the coupling ratio
$R_0\simeq 0.2$ ($R=J_y/J_x$).


title: Critical Ising modes in low-dimensional Kondo insulators

abstract:
  We present an Ising-like intermediate phase for one-dimensional Kondo
insulator systems. Resulting from a spinon splitting, its low-energy
excitations are critical Ising modes, whereas the triplet sector has a spectral
gap. It should occur as long as the RKKY oscillation amplitude dominates over
any direct exchange between localized spins. The chiral fixed point, however,
becomes unstable in the far Infra-Red limit due to prevalent fluctuations among
localized spins which induce gapless triplet excitations in the spectrum. Based
on previous numerical results, we obtain a paramagnetic disordered state ruled
by the correlation length of the single impurity Kondo model.


title: Output coupling of a Bose-Einstein condensate formed in a TOP trap

abstract:
  Two distinct mechanisms are investigated for transferring a pure 87Rb
Bose-Einstein condensate in the F = 2, mF = 2 state into a mixture of
condensates in all the mF states within the F = 2 manifold. Some of these
condensates remain trapped whilst others are output coupled in the form of an
elementary pulsed atom laser. Here we present details of the condensate
preparation and results of the two condensate output coupling schemes. The
first scheme is a radio frequency technique which allows controllable transfer
into available mF states, and the second makes use of Majorana spin flips to
equally populate all the manifold sub-states.


title: Exact phase diagrams for an Ising model on a two-layer Bethe lattice

abstract:
  Using an iteration technique, we obtain exact expressions for the free energy
and the magnetization of an Ising model on a two - layer Bethe lattice with
intralayer coupling constants J1 and J2 for the first and the second layer,
respectively, and interlayer coupling constant J3 between the two layers; the
Ising spins also couple with external magnetic fields, which are different in
the two layers. We obtain exact phase diagrams for the system.


title:
Comparative study of spanning cluster distributions in different
  dimensions

abstract:
  The probability distributions of the masses of the clusters spanning from top
to bottom of a percolating lattice at the percolation threshold are obtained in
all dimensions from two to five. The first two cumulants and the exponents for
the universal scaling functions are shown to have simple power law variations
with the dimensionality. The cases where multiple spanning clusters occur are
discussed separately and compared.


title: Boundary Effects in Chiral Polymer Hexatics

abstract:
  Boundary effects in liquid-crystalline phases can be large due to long-ranged
orientational correlations. We show that the chiral hexatic phase can be locked
into an apparent three-dimensional N+6 phase via such effects. Simple numerical
estimates suggest that the recently discovered "polymer hexatic" may actually
be this locked phase.


title:
Corrections to Scaling in the Phase-Ordering Dynamics of a Vector Order
  Parameter

abstract:
  Corrections to scaling, associated with deviations of the order parameter
from the scaling morphology in the initial state, are studied for systems with
O(n) symmetry at zero temperature in phase-ordering kinetics. Including
corrections to scaling, the equal-time pair correlation function has the form
C(r,t) = f_0(r/L) + L^{-omega} f_1(r/L) + ..., where L is the coarsening length
scale. The correction-to-scaling exponent, omega, and the correction-to-scaling
function, f_1(x), are calculated for both nonconserved and conserved order
parameter systems using the approximate Gaussian closure theory of Mazenko. In
general, omega is a non-trivial exponent which depends on both the
dimensionality, d, of the system and the number of components, n, of the order
parameter. Corrections to scaling are also calculated for the nonconserved 1-d
XY model, where an exact solution is possible.


title:
Penetration and transformations of vortices in bulk current-carrying
  superconductors

abstract:
  The equations of viscous evolution of 3D arbitrarily shaped vortices in an
isotropic type II superconductor and necessary boundary conditions are
formulated in the frame of London approximation. The theory is applied to
analyse characteristic scenaria of vortex penetration into current-carrying
thick plate or bulk samples with another geometry. It is shown that regarding
of the surface transport current value a vortex penetrates either as "flexible
stick" or as "elastic thread". The latter regime is accompanied by giant
stretching of the vortex core along the current-induced surface magnetic field.
This geometrical transformation leads to decrease of viscous friction and large
increase of the vortex drift velocity as compared with the stick-like regime.
As a result, the vortex first winds round the sample cross-section and forms a
ring-like curve, and only later begins to move deep into the sample interior.
The analytical estimates of the vortex shape and stretching and its velocity
are obtained.


title: Realistic model of correlated disorder and Anderson localization

abstract:
  A conducting 1D chain or 2D film inside (or on the surface of) an insulator
is considered. Impurities displace the charges inside the insulator. This
results in a long-range fluctuating electric field acting on the conducting
line (plane). This field can be modeled by that of randomly distributed
electric dipoles. This model provides a random correlated potential with
<U(r)U(r+k)> \propto 1/k. In the 1D case such correlations may essentially
influence the localization length but do not destroy Anderson localization.


title: A class of Heisenberg models with the orthogonal dimer ground states

abstract:
  Extensions of the Shastry-Sutherland model are possible in various ways. In
particular, it is possible to construct a natural model in three dimensions
which has the exact dimer ground state. Recently found spin gap system
SrCu_2(BO_3)_2 has this structure. The exchange constants between the layers is
expected to be smaller than the intra-layer couplings. However, the exactness
of the dimer state for the three dimensional structure is important to
understand why magnetic properties of SrCu_2(BO_3)_2 are described well by the
two dimensional model.


title: Reentrant charge ordering caused by polaron formation

abstract:
  Based on a two-dimensional extended Hubbard model with electron-phonon
interaction, we have studied the effect of polaron formation on the charge
ordering (CO) transition. It is found that for fully ferromagnetically ordered
spins the CO state may go through a process of appearance, collapse and
reappearance with decreasing temperature. This is entirely due to a
emperature-dependent polaron bandwidth. On the other hand, when a paramagnetic
spin state is considered, only a simple reentrant behavior of the CO transition
is found, which is only partly due to polaron effect. This model is proposed as
an explanation of the observed reentrant behavior of the CO transition in the
layered manganite LaSr$_2$Mn$_2$O$_7$.


title: Superconducting Proximity Effect and Universal Conductance Fluctuations

abstract:
  We examine universal conductance fluctuations (UCFs) in mesoscopic
normal-superconducting-normal (N-S-N) structures using a numerical solution of
the Bogoliubov - de Gennes equation. We discuss two cases depending on the
presence (``open'' structure) or absence (``closed'' structure) of
quasiparticle transmission. In contrast to N-S structures, where the onset of
superconductivity increases fluctuations, we find that UCFs are suppressed by
superconductivity for N-S-N structures. We demonstrate that the fluctuations in
``open'' and ``closed'' structures exhibit distinct responses to an applied
magnetic field and to an imposed phase variation of the superconducting order
parameter.


title:
Composite fermions in the half-filled lowest Landau level: a macroscopic
  justification

abstract:
  An effective Hamiltonian for spinless electrons in the lowest Landau level
(LLL) close to half filling is derived. As opposed to the treatment in standard
Chern-Simons theories (CS) we first project to the LLL and only then apply a
CS-transformation on the Hamiltonian. The transformed field operators act in
the lowest Landau level only {\it and} have fermionic commutation relations for
small wavenumbers ignoring gauge field fluctuations. When acting on the
Hamiltonian at half filling the {\it gauge transformation removes the monopole
term in the interaction and does not eliminate the magnetic field.


title: Elastic Rod Model of a Supercoiled DNA Molecule

abstract:
  We study the elastic behaviour of a supercoiled DNA molecule. The simplest
model is that of a rod like chain, involving two elastic constants, the bending
and the twist rigidities. We show that this model is singular and needs a small
distance cut-off, which is a natural length scale giving the limit of validity
of the model, of the order of the double helix pitch. The rod like chain in
presence of the cutoff is able to reproduce quantitatively the experimentally
observed effects of supercoiling on the elongation-force characteristics, in
the small supercoiling regime. An exact solution of the model, using both
transfer matrix techniques and its mapping to a quantum mechanics problem,
allows to extract, from the experimental data,the value of the twist rigidity.
We also analyse the variation of the torque and the writhe to twist ratio
versus supercoiling, showing analytically the existence of a rather sharp
crossover regime which can be related to the excitation of plectonemic-like
structures. Finally we study the extension fluctuations of a stretched and
supercoiled DNA molecule, both at fixed torque and at fixed supercoiling angle,
and we compare the theoretical predictions to some preliminary experimental
data.


title: Exact Quantum Solutions of Extraordinary N-body Problems

abstract:
  The wave functions of Boson and Fermion gases are known even when the
particles have harmonic interactions. Here we generalise these results by
solving exactly the N-body Schrodinger equation for potentials V that can be
any function of the sum of the squares of the distances of the particles from
one another in 3 dimensions. For the harmonic case that function is linear in
r^2. Explicit N-body solutions are given when U(r) = -2M \hbar^{-2} V(r) =
\zeta r^{-1} - \zeta_2 r^{-2}. Here M is the sum of the masses and r^2 = 1/2
M^{-2} Sigma Sigma m_I m_J ({\bf x}_I - {\bf x}_J)^2. For general U(r) the
solution is given in terms of the one or two body problem with potential U(r)
in 3 dimensions. The degeneracies of the levels are derived for distinguishable
particles, for Bosons of spin zero and for spin 1/2 Fermions. The latter
involve significant combinatorial analysis which may have application to the
shell model of atomic nuclei. For large N the Fermionic ground state gives the
binding energy of a degenerate white dwarf star treated as a giant atom with an
N-body wave function. The N-body forces involved in these extraordinary N-body
problems are not the usual sums of two body interactions, but nor are forces
between quarks or molecules. Bose-Einstein condensation of particles in 3
dimensions interacting via these strange potentials can be treated by this
method.

categories: cond-mat.stat-mech astro-ph physics.atm-clus quant-ph


title:
On the adiabatic properties of a stochastic adiabatic wall: Evolution,
  stationary non-equilibrium, and equilibrium states

abstract:
  The time evolution of the adiabatic piston problem and the consequences of
its stochastic motion are investigated. The model is a one dimensional piston
of mass $M$ separating two ideal fluids made of point particles with mass $m\ll
M$. For infinite systems it is shown that the piston evolves very rapidly
toward a stationary nonequilibrium state with non zero average velocity even if
the pressures are equal but the temperatures different on both sides of the
piston. For finite system it is shown that the evolution takes place in two
stages: first the system evolves rather rapidly and adiabatically toward a
metastable state where the pressures are equal but the temperatures different;
then the evolution proceeds extremely slowly toward the equilibrium state where
both the pressures and the temperatures are equal. Numerical simulations of the
model are presented. The results of the microscopical approach, the
thermodynamical equations and the simulations are shown to be qualitatively in
good agreement.


title:
Storage capabilities of a 4-junction single electron trap with an
  on-chip resistor

abstract:
  We report on the operation of a single electron trap comprising a chain of
four Al/AlOx/Al tunnel junctions attached, at one side, to a memory island and,
at the other side, to a miniature on-chip Cr resistor R=50 kOhm which served to
suppress cotunneling. At appropriate voltage bias the bi-stable states of the
trap, with the charges differing by the elementary charge e, were realized. At
low temperature, spontaneous switching between these states was found to be
infrequent. For instance, at T=70 mK the system was capable of holding an
electron for more than 2 hours, this time being limited by the time of the
measurement.


title: Dimer-Type Correlations and Band Crossings in Fibonacci Lattices

abstract:
  Fibonacci system with both diagonal and off-diagonal disorder is mapped to a
model with dimer type 'defects'. The model exhibit resonance transition
corresponding to zero reflectance condition resulting in extended states in the
system. These Bloch-type states reside on the line of inversion symetry of the
energy spectrum and include states at the band crossings. The resonant states
at the band crossings are related to the harmonics of the sine wave and have
transmission coefficient equal to unity in quasiperiodic limit. This is in
contrast to other resonant states where the transmission coeffiecient
oscillates. An exact renormalization scheme confirms the fact that all resonant
states are Bloch type waves as they are described by trivial attractors of the
renormalization flow.


title:
Nonlinear differential equations based on nonextensive Tsallis entropy
  and physical applications

abstract:
  A family of nonlinear ordinary differential equations with arbitrary order is
obtained by using nonextensive concepts related to the Tsallis entropy.
Applications of these equations are given here. In particular, a connection
between Tsallis entropy and the one-dimensional correlated anomalous diffusion
equation is established. It is also developed explicitly a WKB-like method for
second order equations and it is applied to solve approximately a class of
equations that contains as a special case the Thomas-Fermi equation for an
atom. It is expected that the present ideas can be useful in the discussion of
other nonlinear contexts.


title: Pauli principle and chaos in a magnetized disk

abstract:
  We present results of a detailed quantum mechanical study of a gas of $N$
noninteracting electrons confined to a circular boundary and subject to
homogeneous dc plus ac magnetic fields $(B=B_{dc}+B_{ac}f(t)$, with
$f(t+2\pi/\omega_0)=f(t)$). We earlier found a one-particle {\it classical}
phase diagram of the (scaled) Larmor frequency
$\tilde\omega_c=omega_c/\omega_0$ {\rm vs} $\epsilon=B_{ac}/B_{dc}$ that
separates regular from chaotic regimes. We also showed that the quantum
spectrum statistics changed from Poisson to Gaussian orthogonal ensembles in
the transition from classically integrable to chaotic dynamics. Here we find
that, as a function of $N$ and $(\epsilon,\tilde\omega_c)$, there are clear
quantum signatures in the magnetic response, when going from the
single-particle classically regular to chaotic regimes. In the quasi-integrable
regime the magnetization non-monotonically oscillates between diamagnetic and
paramagnetic as a function of $N$. We quantitatively understand this behavior
from a perturbation theory analysis. In the chaotic regime, however, we find
that the magnetization oscillates as a function of $N$ but it is {\it always}
diamagnetic. Equivalent results are also presented for the orbital currents. We
also find that the time-averaged energy grows like $N^2$ in the
quasi-integrable regime but changes to a linear $N$ dependence in the chaotic
regime. In contrast, the results with Bose statistics are akin to the
single-particle case and thus different from the fermionic case. We also give
an estimate of possible experimental parameters were our results may be seen in
semiconductor quantum dot billiards.


title: Critical dynamics of a uniaxial and dipolar ferromagnet

abstract:
  We study the critical dynamics of three-dimensional ferromagnets with
uniaxial anisotropy by taking into account exchange and dipole-dipole
interaction. The dynamic spin correlation functions and the transport
coefficients are calculated within a mode coupling theory. It is found that the
crossover scenario is determined by the subtle interplay between three length
scales: the correlation length, the dipolar and uniaxial wave vector. We
compare our theoretical findings with hyperfine interaction experiments on Gd
and find quantitative agreement. This analysis allows us to identify the
universality class for Gd. It also turns out that the $\mu$SR relaxation rate
can be best fitted if it is assumed that muons occupy octahedral interstitials
sites within the Gd lattice.


title:
Radio-frequency impedance measurements using a tunnel-diode oscillator
  (TDO) technique

abstract:
  A resonant method based on a tunnel-diode oscillator (TDO) for precision
measurements of relative impedance changes in materials, is described. The
system consists of an effective self-resonant LC-tank circuit driven by a
forward-biased tunnel diode operating in its negative resistance region.
Samples under investigation are placed in the core of an inductive coil and
impedance changes are determined directly from the measured shift in resonance
frequency. A customized low temperature insert is used to integrate this
experiment with a commercial Model 6000 Physical Property Measurement System
(Quantum Design). Test measurements on a manganese-based perovskite sample
exhibiting colossal magneto-resistance (CMR) indicate that this method is well
suited to study the magneto-impedance in these materials.


title: Transition Temperature of a Uniform Imperfect Bose Gas

abstract:
  We calculate the transition temperature of a uniform dilute Bose gas with
repulsive interactions, using a known virial expansion of the equation of
state. We find that the transition temperature is higher than that of an ideal
gas, with a fractional increase K_0(na^3)^{1/6}, where n is the density and a
is the S-wave scattering length, and K_0 is a constant given in the paper. This
disagrees with all existing results, analytical or numerical. It agrees exactly
in magnitude with a result due to Toyoda, but has the opposite sign.


title: Dynamic Phase Transition and Hysteresis in Kinetic Ising Models

abstract:
  We briefly introduce hysteresis in spatially extended systems and the dynamic
phase transition observed as the frequency of the oscillating field increases
beyond a critical value. Hysteresis and the decay of metastable phases are
closely related phenomena, and a dynamic phase transition can occur only for
field amplitudes, temperatures, and system sizes at which the metastable phase
decays through nucleation and growth of many droplets. We present preliminary
results from extensive Monte Carlo simulations of a two-dimensional kinetic
Ising model in a square-wave oscillating field and estimate critical exponents
by finite-size scaling techniques adapted from equilibrium critical phenomena.
The estimates are consistent with the universality class of the two-dimensional
equilibrium Ising model and inconsistent with two-dimensional random
percolation. However, we are not aware of any theoretical arguments indicating
why this should be so. Thus, the question of the universality class of this
nonequilibrium critical phenomenon remains open.

categories: cond-mat.stat-mech chao-dyn cond-mat.mtrl-sci nlin.CD


title:
Simulation of a cusped bubble rising in a viscoelastic fluid with a new
  numerical method

abstract:
  We developed a new lattice Boltzmann method that allows the simulation of
two-phase flow of viscoelastic liquid mixtures. We used this new method to
simulate a bubble rising in a viscoelastic fluid and were able to reproduce the
experimentally observed cusp at the trailing end of the bubble.


title: Indications of Spin-Charge Separation in the 2D Extended t-J Model

abstract:
  The 2D extended $\rm t-J$ model is studied computationally in a broad region
of parameter space, motivated by recent photoemission experiments for the
undoped cuprate $\rm Ca_2 Cu O_2 Cl_2$ (F. Ronning et al., Science {\bf 282},
2067 (1998)). The one-hole ground state is shown to develop robust
antiferromagnetic (AF) correlations between spins separated by the mobile hole
(i.e. across the hole). This effect tends to decouple charge from spin, and the
quasiparticle weight becomes negligible, particularly at momenta
$(0,\pi)-(\pi,0)$. Studies with more holes show precursors of metallic stripe
formation, with holes sharing their individual spin arrangements, and
AF-correlations generated across the stripe.


title: Thermally activated reorientation of di-interstitial defects in silicon

abstract:
  We propose a di-interstitial model for the P6 center commonly observed in ion
implanted silicon. The di-interstitial structure and transition paths between
different defect orientations can explain the thermally activated transition of
the P6 center from low-temperature C1h to room-temperature D2d symmetry. The
activation energy for the defect reorientation determined by ab initio
calculations is 0.5 eV in agreement with the experiment. Our di-interstitial
model establishes a link between point defects and extended defects,
di-interstitials providing the nuclei for the growth.


title: Atom holography

abstract:
  We study the conditions under which atomic condensates can be used as a
recording media and then suggest a reading scheme which allows to reconstruct
an object with atomic reading beam. We show that good recording can be achieved
for flat condensate profiles and for negative detunings between atomic Bohr
frequency and optical field frequency. The resolution of recording dramatically
depends on the relation between the healing length of the condensate and the
spatial frequency contents of the optical fields involved.


title: Phase Separation under Shear in Two-dimensional Binary Fluids

abstract:
  We use lattice Boltzmann simulations to study the effect of shear on the
phase ordering of a two-dimensional binary fluid. The shear is imposed by
generalising the lattice Boltzmann algorithm to include Lees-Edwards boundary
conditions. We show how the interplay between the ordering effects of the
spinodal decomposition and the disordering tendencies of the shear, which
depends on the shear rate and the fluid viscosity, can lead to a state of
dynamic equilibrium where domains are continually broken up and re-formed.


title: Making, probing and understanding Bose-Einstein condensates

abstract:
  Contribution to the proceedings of the 1998 Enrico Fermi summer school on
Bose-Einstein condensation in Varenna, Italy.


title:
Heisenberg Spin-One Chain in Staggered Magnetic Field : A Density Matrix
  Renormalization Group Study

abstract:
  Using the density matrix renormalization group technique, we calculate
numerically the low energy excitation spectrum and magnetization curve of the
spin-1 antiferromagnetic chain in a staggered magnetic field, which is expected
to describe the physics of $R_2 Ba Ni O_5 (R \neq Y) $ family below the N\'eel
temperature of the magnetic rare-earth ($R$) sublattice. These results are
valid in the entire range of the staggered field, and agree with those given by
the non-linear $\sigma$ model study for small fields, but differ from the
latter for large fields. They are consistent with the available experimental
data. The correlation functions for this model are also calculated. The
transverse correlations display the anticipated exponential decay with shorter
correlation length, while the longitudinal correlations show explicitly the
induced staggered magnetization.


title:
Exact correlation functions of Bethe lattice spin models in external
  fields

abstract:
  We develop a transfer matrix method to compute exactly the spin-spin
correlation functions of Bethe lattice spin models in the external magnetic
field h and for any temperature T. We first compute the correlation function
for the most general spin - S Ising model, which contains all possible
single-ion and nearest-neighbor pair interactions. This general spin - S Ising
model includes the spin-1/2 simple Ising model and the Blume-Emery-Griffiths
(BEG) model as special cases. From the spin-spin correlation functions, we
obtain functions of correlation length for the simple Ising model and BEG
model, which show interesting scaling and divergent behavior as T approaches
the critical temperature. Our method to compute exact spin-spin correlation
functions may be applied to other Ising-type models on Bethe and Bethe-like
lattices.


title: Electron transport through a metal-molecule-metal junction

abstract:
  Molecules of bisthiolterthiophene have been adsorbed on the two facing gold
electrodes of a mechanically controllable break junction in order to form
metal-molecule(s)-metal junctions. Current-voltage (I-V) characteristics have
been recorded at room temperature. Zero bias conductances were measured in the
10-100 nS range and different kinds of non-linear I-V curves with step-like
features were reproducibly obtained. Switching between different kinds of I-V
curves could be induced by varying the distance between the two metallic
electrodes. The experimental results are discussed within the framework of
tunneling transport models explicitly taking into account the discrete nature
of the electronic spectrum of the molecule.


title:
Universal Amplitude Ratios in the Critical Two-Dimensional Ising Model
  on a Torus

abstract:
  Using results from conformal field theory, we compute several universal
amplitude ratios for the two-dimensional Ising model at criticality on a
symmetric torus. These include the correlation-length ratio x^\star =
\lim_{L\to\infty} \xi(L)/L and the first four magnetization moment ratios
V_{2n} = <{\cal M}^{2n}>/<{\cal M}^2>^n. As a corollary we get the first four
renormalized 2n-point coupling constants for the massless theory on a symmetric
torus, G_{2n}^*. We confirm these predictions by a high-precision Monte Carlo
simulation.


title:
Adsorbate-induced substrate relaxation and the adsorbate-adsorbate
  interaction

abstract:
  We formulate the theory of the perturbation caused by an adsorbate upon the
substrate lattice in terms of a local modification of the interatomic potential
energy around the adsorption site, which leads to the relaxation of substrate
atoms. We apply the approach to CO chemisorption on close-packed metal
surfaces, and show that the adsorbate-adsorbate interaction and a variety of
other properties can be well described by a simple model.


title:
Exact Solution of a Vertex Model with Unlimited Number of States Per
  Bond

abstract:
  The exact solution is obtained for the eigenvalues and eigenvectors of the
row-to-row transfer matrix of a two-dimensional vertex model with unlimited
number of states per bond. This model is a classical counterpart of a quantum
spin chain with an unlimited value of spin. This quantum chain is studied using
general predictions of conformal field theory. The long-distance behaviour of
some ground-state correlation functions is derived from a finite-size analysis
of the gapless excitations.