title: Quadratic short-range order corrections to the mean-field free energy

abstract:
  A method for calculating the short-range order part of the free energy of
order-disorder systems is proposed. The method is based on the apllication of
the cumulant expansion to the exact configurational entropy. Second-order
correlation corrections to the mean-field approximation for the free energy are
calculated for arbitrary thermodynamic phase and type of interactions. The
resulting quadratic approximation for the correlation entropy leads to
substantially better values of transition temperatures for the
nearest-neighbour cubic Ising ferromagnets.


title: Stochastic PDEs: domain formation in dynamic transitions

abstract:
  Spatiotemporal evolution in the real Ginzburg-Landau equation is studied with
space-time noise and a slowly increasing critical parameter. Analytical
estimates for the characteristic size of the domains formed in a slow sweep
through the critical point agree with the results of finite difference solution
of the stochastic PDEs.


title: Cellular Structures for Computation in the Quantum Regime

abstract:
  We present a new cellular data processing scheme, a hybrid of existing
cellular automata (CA) and gate array architectures, which is optimized for
realization at the quantum scale. For conventional computing, the CA-like
external clocking avoids the time-scale problems associated with ground-state
relaxation schemes. For quantum computing, the architecture constitutes a novel
paradigm whereby the algorithm is embedded in spatial, as opposed to temporal,
structure. The architecture can be exploited to produce highly efficient
algorithms: for example, a list of length N can be searched in time of order
cube root N.


title: Unified View of Scaling Laws for River Networks

abstract:
  Scaling laws that describe the structure of river networks are shown to
follow from three simple assumptions. These assumptions are: (1) river networks
are structurally self-similar, (2) single channels are self-affine, and (3)
overland flow into channels occurs over a characteristic distance (drainage
density is uniform). We obtain a complete set of scaling relations connecting
the exponents of these scaling laws and find that only two of these exponents
are independent. We further demonstrate that the two predominant descriptions
of network structure (Tokunaga's law and Horton's laws) are equivalent in the
case of landscapes with uniform drainage density. The results are tested with
data from both real landscapes and a special class of random networks.


title:
Circulating and persistent currents induced by a current magnification
  and Aharonov-Casher phase

abstract:
  We considered the circulating current induced by the current magnification
and the persistent current induced by Aharonov-Casher flux. The persistent
currents have directional dependence on the direct current flow, but the
circulating currents have no directional dependence. Hence in the equilibrium,
only the persistent current can survives on the ring. For the charge current,
the persistent charge current cancelled between spin up and down states,
because of the time reversal symmetry of the Hamiltonian on the ring. So there
are only circulating charge currents on the ring for electrons with unpolarized
spin in the nonequilibrium. However, only the persistent spin currents
contributes to the spin currents for electrons with unpolarized spin.


title: Pairing of Fermions with Arbitrary Spin

abstract:
  Motivated by the recent success of optical trapping of alkali Bose
condensate, we have studied the superfluid state of optically trapped alkali
fermions, which can have Cooper pairs with total spin $J\geq 2$. In this paper,
we shall discuss the general structure of these large spin Cooper pairs and
their close relation with singlet Cooper pairs with non-zero orbital angular
momentum. We also present the exact solution for the J=2 pairing which shows a
surprising change of ground state as the spin $f$ of the constituent fermion
increases.


title: Critical properties of Toom cellular automata

abstract:
  This paper is the continuation of our earlier considerations on cellular
automata with Toom local rule (TCA) as the alternative to kinetic Ising
systems. The arguments for TCA stationary states not being the equilibrium
states are found in simulations.


title: Statistical Models of the Polaronic Phase Transition in Manganites

abstract:
  We propose two statistical models for description the metal-insulator phase
transition coupled with paramagnetic-ferromagnetic phase transition in
manganites of the type La_{1-x}Sr_xMnO_3. The first one based on the
competition of small polarons and delocalized carriers. In the second one the
conductivity appears as a result of overlapping of large polarons. We compare
our models with the experimental data.


title: Is the nonlinear Meissner effect unobservable?

abstract:
  We examine the effects of nonlocal electrodynamics for a d-wave
superconductor on the field dependence of the magnetic penetration depth. The
linear field dependence predicted in the local limit, commonly known as the
nonlinear Meissner effect, is instead found to be quadratic, $\delta\lambda\sim
H^2$ for fields below a crossover scale $H^*$. This crossover is shown to be
geometry dependent and for most orientations of the screening currents is of
the same order as or greater than $H_{c1}$, implying that the nonlinear
Meissner effect can not be observed. For special orientations where the current
flows along the nodal directions, however, the nonlinear Meissner effect may be
recovered.


title: Orientation Correlation in Simplified Models of Polymer Melts

abstract:
  We investigate mutual local chain order in systems of fully flexible polymer
melts in a simple generic bead-spring model. The excluded-volume interaction
together with the connectivity leads to local ordering effects which are
independent of chain length between 25 and 700 monomers, i.e. in the Rouse as
well as in the reptation regime. These ordering phenomena extend to a distance
of about 3 to 4 monomer sizes and decay to zero afterwards.


title:
Observation of the Metal-Insulator Transition in Two-Dimensional n-type
  GaAs

abstract:
  The observation of a carrier-density driven metal-insulator transition in
n-type GaAs-based heterostructure is reported. Although weaker than in
comparable-quality p-type GaAs samples, the main features of the transition are
rather similar.


title:
Metal-insulator crossover in the Boson-Fermion model in infinite
  dimensions

abstract:
  The Boson-Fermion model, describing a mixture of tightly bound electron pairs
and quasi-free electrons hybridized with each other via a charge exchange term,
is studied in the limit of infinite dimensions, using the Non-Crossing
Approximation within the Dynamical Mean Field Theory. It is shown that a
metal-insulator crossover, driven by strong pair fluctuations, takes place as
the temperature is lowered. It manifests itself in the opening of a pseudogap
in the electron density of states, accompanied by a corresponding effect in the
optical and dc conductivity.


title:
Phase Diagram of the Extended Hubbard Model with Correlated Hopping
  Interaction

abstract:
  A one-dimensional model of interacting electrons with on-site $U$,
nearest-neighbor $V$, and correlated-hopping interaction $T^{\ast}$ is studied
at half-filling using the continuum-limit field theory approach. The ground
state phase diagram is obtained for a wide range of coupling constants. In
addition to the insulating spin- and charge-density wave phases for large $U$
and $V$, respectively, we identify bond-located ordered phases corresponding to
an enhanced Peierls instability in the system for $T^\ast>0$,
$|U-2V|<8T^\ast/\pi$ and to a staggered magnetization located on bonds between
sites for $T^\ast<0$, $|U-2V|<8|T^\ast|/\pi$. The general ground state phase
diagram including insulating, metallic, and superconducting phases is
discussed.


title:
Dynamics of a Vortex in Two-Dimensional Superfluid He3-A: Force Caused
  by the l-Texture

abstract:
  Based on the Landau-Ginzburg Lagrangian, the dynamics of a vortex is studied
for superfluid He3-A characterized by the l-texture. The resultant equation of
motion for a vortex leads to the Magnus-type force caused by the l-texture. The
force is explicitly written in terms of the mapping degree from the
compactified 2-dimensional plane to the space of l-vector, which reflects the
quantitative differences of vortex configurations, especially the Mermin-Ho and
Anderson-Toulouse vortices. The formulation is applied to anisotropic
superconductors in which the Hall current is shown to incorporate changes
between vortex configurations.


title:
Coadsorption of CO and O on Ru(0001): A structural analysis by density
  functional theory

abstract:
  Knowledge of the atomic geometry of a surface is a prerequisite for any
detailed understanding of the surface's electronic structure and chemical
properties. Previous studies have convincingly demonstrated that density
functional theory (DFT) yields accurate surface atomic geometries and that
reliable predictions concerning stable and metastable phases can be made on the
basis of the calculated energetics. In the present work we use DFT to
investigate the atomic structure of four ordered coadsorbate phases of carbon
monoxide and oxygen on Ru(0001). All of the structures have a (2x2) periodicity
with differing concentrations of CO molecules and O atoms. For two of these
phases dynamical low-energy electron diffraction (LEED) intensity analyses have
been performed and the agreement between our DFT- and the LEED-determined
structures is found to be very good. We predict the atomic geometry of the
third phase for which no structural determination based on experiments has been
made to date. We also predict the stability of a new ordered mixed phase.


title: Reactions at surfaces studied by ab initio dynamics calculations

abstract:
  Due to the development of efficient algorithms and the improvement of
computer power it is now possible to map out potential energy surfaces (PES) of
reactions at surfaces in great detail. This achievement has been accompanied by
an increased effort in the dynamical simulation of processes on surfaces. The
paradigm for simple reactions at surfaces -- the dissociation of hydrogen on
metal surfaces -- can now be treated fully quantum dynamically in the molecular
degrees of freedom from first principles, i.e., without invoking any adjustable
parameters. This relatively new field of ab initio dynamics simulations of
reactions at surfaces will be reviewed. Mainly the dissociation of hydrogen on
clean and adsorbate covered metal surfaces and on semiconductor surfaces will
be discussed. In addition, the ab initio molecular dynamics treatment of
reactions of hydrogen atoms with hydrogen-passivated semiconductor surfaces and
recent achievements in the ab initio description of laser-induced desorption
and further developments will be addressed.


title:
Poisoning of Hydrogen Dissociation at Pd (100) by Adsorbed Sulfur
  Studied by ab initio Quantum Dynamics and ab initio Molecular Dynamics

abstract:
  We report calculations of the dissociative adsorption of H_2 at Pd (100)
covered with 1/4 monolayer of sulfur using quantum dynamics as well as
molecular dynamics and taking all six degrees of freedom of the two H atoms
fully into account. The ab initio potential-energy surface (PES) is found to be
very strongly corrugated. In particular we discuss the influence of tunneling,
zero-point vibrations, localization of the nuclei's wave function when narrow
valleys of the PES are passed, steering of the approaching H_2 molecules
towards low energy barrier configurations, and the time scales of the center of
mass motion and the other degrees of freedom. Several ``established'' concepts,
which were derived from low-dimensional dynamical studies, are shown to be not
valid.


title: Coefficient of restitution for elastic disks

abstract:
  We calculate the coefficient of restitution, $\epsilon$, starting from a
microscopic model of elastic disks. The theory is shown to agree with the
approach of Hertz in the quasistatic limit, but predicts inelastic collisions
for finite relative velocities of two approaching disks. The velocity
dependence of $\epsilon$ is calculated numerically for a wide range of
velocities. The coefficient of restitution furthermore depends on the elastic
constants of the material via Poisson's number. The elastic vibrations absorb
kinetic energy more effectively for materials with low values of the shear
modulus.


title:
Hydrogen dissociation on metal surfaces - a model system for reactions
  on surfaces

abstract:
  Reactions on surfaces play an important role in many technological
applications. Since these processes are often rather complex, one tries to
understand single steps of these complicated reactions by investigating simpler
system. In particular the hydrogen dissociation on surfaces serves as such a
model system. There has been much progress in recent years in the theoretical
description of reactions on surfaces by high-dimensional dynamics simulations
on potential energy surfaces which are derived from ab initio total energy
calculations. In this brief review I will focus on the hydrogen dissociation on
the clean and sulfur-covered Pd(100) surface. These calculations established
the importance of dynamical concepts like the steering effect. The electronic
structure calculations allow furthermore the determination of the factors that
determine the reactivity of a particular surface. This will be demonstrated for
the poisoning of hydrogen dissociation by sulfur adsorption on the Pd(100)
surface. In addition, quantum effects in the dynamics can be assessed by
comparing the results of classical with quantum dynamical calculations on the
same potential energy surface.


title: Ising spin glass with arbitrary spin beyond the mean field theory

abstract:
  We consider the Ising spin glass for the arbitrary spin S with the short-
ranged interaction using the Bethe- Peierls approximation previously formulated
by Serva and Paladin for the same system but limited to S=1/2. Results obtained
by us for arbitrary S are not a simple generalization of those for S=1/2. In
this paper we mainly concentrate our studies on the calcutation of the citical
temperature and the linear susceptibility in the paramagnetic phase as
functions of the dimension of the system and spin number S. These dependences
are illustrated by corresponding plots.


title: Theory of magnetic excitations in the Kondo lattice

abstract:
  We present a theory for the spin excitations of the Kondo lattice. We derive
an effective Hamiltonian, which describes Fermionic spin 1/2 charge
fluctuations interacting with Bosonic triplet spin fluctuations. We show that
already the simplest approximation possible, i.e. replacing the magnon
self-energy by the free polarization bubble for the charge excitation Greens
function, gives a quantitative description of the `spin gap' and `charge gap'
as obtained in numerical calculations for the Kondo insulator.


title: Beyond implied volatility

abstract:
  After a brief review of option pricing theory, we introduce various methods
proposed for extracting the statistical information implicit in options prices.
We discuss the advantages and drawbacks of each method, the interpretation of
their results in economic terms, their theoretical consequences and their
relevance for applications.


title: Universality in sandpiles

abstract:
  We perform extensive numerical simulations of different versions of the
sandpile model. We find that previous claims about universality classes are
unfounded, since the method previously employed to analyze the data suffered a
systematic bias. We identify the correct scaling behavior and conclude that
sandpiles with stochastic and deterministic toppling rules belong to the same
universality class.


title: Parent-bilinear Sea-boson Correspondence with Spin

abstract:
  This is a sort of tutorial where we present many of the tedious details
involved in deriving the correspondence between the number conserving product
of two fermi/bose fields with spin and the corresponding sea/condensate
displacements also with spin. This time we include finite temperature effects
differently by coupling the sea-bosons to an inhomogeneous chemical potential.
It is shown that many of the salient features of the free theory are reproduced
properly by this approach. An attempt is made to be as detailed as possible,
this includes providing formulas for the sea-bosons as well as computing the
finite temperature Fermi correlators.


title: The double-funnel energy landscape of the 38-atom Lennard-Jones cluster

abstract:
  The 38-atom Lennard-Jones cluster has a paradigmatic double-funnel energy
landscape. One funnel ends in the global minimum, a face-centred-cubic (fcc)
truncated octahedron. At the bottom of the other funnel is the second lowest
energy minimum which is an incomplete Mackay icosahedron. We characterize the
energy landscape in two ways. Firstly, from a large sample of minima and
transition states we construct a disconnectivity tree showing which minima are
connected below certain energy thresholds. Secondly we compute the free energy
as a function of a bond-order parameter. The free energy profile has two
minima, one which corresponds to the fcc funnel and the other which at low
temperature corresponds to the icosahedral funnel and at higher temperatures to
the liquid-like state. These two approaches show that the greater width of the
icosahedral funnel, and the greater structural similarity between the
icosahedral structures and those associated with the liquid-like state, are the
cause of the smaller free energy barrier for entering the icosahedral funnel
from the liquid-like state and therefore of the cluster's preferential entry
into this funnel on relaxation down the energy landscape. Furthermore, the
large free energy barrier between the fcc and icosahedral funnels, which is
energetic in origin, causes the cluster to be trapped in one of the funnels at
low temperature. These results explain in detail the link between the
double-funnel energy landscape and the difficulty of global optimization for
this cluster.


title: Faceting Transition in an Exactly Solvable Terrace-Ledge-Kink model

abstract:
  We solve exactly a Terrace-Ledge-Kink (TLK) model describing a crystal
surface at a microscopic level. We show that there is a faceting transition
driven either by temperature or by the chemical potential that controls the
slope of the surface. In the rough phase we investigate thermal fluctuations of
the surface using Conformal Field Theory.


title:
Single-Particle Excitations in a Two-Dimensional Strong-Coupling
  Superconductor

abstract:
  We present calculations of the single-particle excitation spectrum for a 2D
strong-coupling superconductor in a conserving approximation. Spectral weight
at low frequency is substantially reduced as the superconducting transition is
approached from the normal state. The suppression of low-energy excitations is
a consequence of Cooper-pair fluctuations that are described self-consistently
in the fluctuation exchange approximation. The static and uniform
electromagnetic response provides a measure of the super fluid density and a
fully self-consistent indication of the superconducting transition temperature.


title: Heterovalent and A-atom effects in A(B'B'')O3 perovskite alloys

abstract:
  Using first-principles supercell calculations, we have investigated
energetic, structural and dielectric properties of three different A(B'B'')O_3
perovskite alloys: Ba(Zn_{1/3}Nb_{2/3})O_3 (BZN), Pb(Zn_{1/3}Nb_{2/3})O_3
(PZN), and Pb(Zr_{1/3}Ti_{2/3})O_3 (PZT). In the homovalent alloy PZT, the
energetics are found to be mainly driven by atomic relaxations. In the
heterovalent alloys BZN and PZN, however, electrostatic interactions among B'
and B'' atoms are found to be very important. These electrostatic interactions
are responsible for the stabilization of the observed compositional long-range
order in BZN. On the other hand, cell relaxations and the formation of short
Pb--O bonds could lead to a destabilization of the same ordered structure in
PZN. Finally, comparing the dielectric properties of homovalent and
heterovalent alloys, the most dramatic difference arises in connection with the
effective charges of the B' atom. We find that the effective charge of Zr in
PZT is anomalous, while in BZN and PZN the effective charge of Zn is close to
its nominal ionic value.


title:
The Effect of Structural Distortions on the Electronic Structure of
  Carbon Nanotubes

abstract:
  We calculated the effects of structural distortions on the electronic
structure of carbon nanotubes. The key modification of the electronic structure
brought about by bending a nanotube involves an increased mixing of $\sigma$
and $\pi$-states. This mixing leads to an enhanced density-of-states in the
valence band near the Fermi energy region. While in a straight tube the states
accessible for electrical conduction are essentially pure C($2p_{\pi}$)-states,
they acquire significant C($2sp_{\sigma}$) character upon bending. Bending also
leads to a charge polarization of the C-C bonds in the deformed region
reminiscent of interface dipole formation. Scattering of conduction electrons
at the distorted regions may lead to electron localization at low temperatures.


title: Nonlinear Hydrodynamics of Disentangled Flux-Line Liquids

abstract:
  In this paper we use non-Gaussian hydrodynamics to study the magnetic
response of a flux-line liquid in the mixed state of a type-II superconductor.
Both the derivation of our model, which goes beyond conventional Gaussian flux
liquid hydrodynamics, and its relationship to other approaches used in the
literature are discussed. We focus on the response to a transverse tilting
field which is controlled by the tilt modulus, c44, of the flux array. We show
that interaction effects can enhance c44 even in infinitely thick clean
materials. This enhancement can be interpreted as the appearance of a
disentangled flux-liquid fraction. In contrast to earlier work, our theory
incorporates the nonlocality of the intervortex interaction in the field
direction. This nonlocality is crucial for obtaining a nonvanishing
renormalization of the tilt modulus in the thermodynamic limit of thick
samples.


title:
The Effects of Finite Length on the Electronic Structure of Carbon
  Nanotubes

abstract:
  The electronic structure of finite-length armchair carbon nanotubes has been
studied using several ab-initio and semi-empirical quantum computational
techniques. The additional confinement of the electrons along the tube axis
leads to the opening of a band-gap in short armchair tubes. The value of the
band-gap decreases with increasing tube length, however, the decrease is not
monotonic but shows a well defined oscillation in short tubes. This oscillation
can be explained in terms of periodic changes in the bonding characteristics of
the HOMO and LUMO orbitals of the tubes. Finite size graphene sheets are also
found to have a finite band-gap, but no clear oscillation is observed. As the
length of the tube increases the density of states (DOS) spectrum evolves from
that characteristic of a zero-dimensional (0-D) system to that characteristic
of a delocalized one-dimensional (1-D) system. This transformation appears to
be complete already for tubes 5-10 nm long. The chemical stability of the
nanotubes, expressed by the binding energy of a carbon atom, increases in a
similar manner.


title:
Directed Ising type dynamic preroughening transition in one dimensional
  interfaces

abstract:
  We present a realization of directed Ising (DI) type dynamic absorbing state
phase transitions in the context of one-dimensional interfaces, such as the
relaxation of a step on a vicinal surface. Under the restriction that particle
deposition and evaporation can only take place near existing kinks, the
interface relaxes into one of three steady states: rough, perfectly ordered
flat (OF) without kinks, or disordered flat (DOF) with randomly placed kinks
but in perfect up-down alternating order. A DI type dynamic preroughening
transition takes place between the OF and DOF phases. At this critical point
the asymptotic time evolution is controlled not only by the DI exponents but
also by the initial condition. Information about the correlations in the
initial state persists and changes the critical exponents.


title:
Localization in non-chiral network models for two-dimensional disordered
  wave mechanical systems

abstract:
  Scattering theoretical network models for general coherent wave mechanical
systems with quenched disorder are investigated. We focus on universality
classes for two dimensional systems with no preferred orientation: Systems of
spinless waves undergoing scattering events with broken or unbroken time
reversal symmetry and systems of spin 1/2 waves with time reversal symmetric
scattering. The phase diagram in the parameter space of scattering strengths is
determined. The model breaking time reversal symmetry contains the critical
point of quantum Hall systems but, like the model with unbroken time reversal
symmetry, only one attractive fixed point, namely that of strong localization.
Multifractal exponents and quasi-one-dimensional localization lengths are
calculated numerically and found to be related by conformal invariance.
Furthermore, they agree quantitatively with theoretical predictions. For
non-vanishing spin scattering strength the spin 1/2 systems show
localization-delocalization transitions.


title: Photogeneration Dynamics of a Soliton Pair in Polyacetylene

abstract:
  Dynamical process of the formation of a soliton pair from a photogenerated
electron-hole pair in polyacetylene is studied numerically by adopting the SSH
Hamiltonian. A weak local disorder is introduced in order to trigger the
formation. Starting from an initial configuration with an electron at the
bottom of the conduction band and a hole at the top of the valence band,
separated by the Peierls gap, the time dependent Schr${\rm \ddot{o}}$ndinger
equation for the electron wave functions and the equation of motion for the
lattice displacements are solved numerically. After several uniform
oscillations of the lattice system at the early stage, a large distortion
corresponding to a pair of a soliton and an anti-soliton develops from a point
which is determined by the location and type of the disorder. In some cases,
two solitons run in opposite directions, leaving breather like oscillations
behind, and in other cases they form a bound state emitting acoustic lattice
vibrational modes.


title: P-wave Pairing and Colossal Magnetoresistance in Manganese Oxides

abstract:
  We point out that the existing experimental data of most manganese oxides
show the {\sl frustrated} p-wave superconducting condensation in the
ferromagnetic phase in the sense that the superconducting coherence is not long
enough to cover the whole system. The superconducting state is similar to the
$A_{1}$ state in superfluid He-3. The sharp drop of resistivity, the steep jump
of specific heat, and the gap opening in tunneling are well understood in terms
of the p-wave pairing. In addition, colossal magnetoresistance (CMR) is
naturally explained by the superconducting fluctuations with increasing
magnetic fields. The finite resistivity may be due to some magnetic
inhomogeneities. This study leads to the possibility of room temperature
superconductivity.


title:
Metal-Insulator Transition in a Disordered Two-Dimensional Electron Gas
  in GaAs-AlGaAs at zero Magnetic Field

abstract:
  A metal-insulator transition in two-dimensional electron gases at B=0 is
found in Ga(Al)As heterostructures, where a high density of self-assembled InAs
quantum dots is incorporated just 3 nm below the heterointerface. The
transition occurs at resistances around h/e^2 and critical carrier densities of
1.2 10^11cm^-2. Effects of electron-electron interactions are expected to be
rather weak in our samples, while disorder plays a crucial role.


title: Collective effects in the 2D lattices of the magnetic nanoparticles

abstract:
  The work is devoted to investigation of the collective behavior of the
regular rectangular lattices of the NiFe nanoparticles caused by the
dipole-dipole interaction between them. The samples was prepared by the method
of the electron lithography and consist of about 100000 particles of
approximately 50 nm size. The magnetization curves were investigated by the
Hall magnetometry for the different orientation of the external magnetic field
at 4.2K and 77K. The results points on the collective behavior of the system.
The observed peculiarities we connect with the quasi-one-dimensional behavior
of the system and the formation of the solitons in the system.


title: Kinetics of ballistic annihilation and branching

abstract:
  We consider a one-dimensional model consisting of an assembly of two-velocity
particles moving freely between collisions. When two particles meet, they
instantaneously annihilate each other and disappear from the system. Moreover
each moving particle can spontaneously generate an offspring having the same
velocity as its mother with probability 1-q. This model is solved analytically
in mean-field approximation and studied by numerical simulations. It is found
that for q=1/2 the system exhibits a dynamical phase transition. For q<1/2, the
slow dynamics of the system is governed by the coarsening of clusters of
particles having the same velocities, while for q>1/2 the system relaxes
rapidly towards its stationary state characterized by a distribution of small
cluster sizes.


title:
Superconductivity in the Extended Hubbard Model with More Than
  Nearest-Neighbour Contributions

abstract:
  Superconducting phase diagram of the extended Hubbard model supplemented with
interaction and hopping terms exceeding nearest neighbour distance in range is
analysed systematically at different band-filling and temperature values in a
mean-field approximation. The obtained results clearly underline the importance
of next-nearest neighbour terms in developing the main superconducting
properties of the model system. In particular, the emergence of superconducting
phases of different symmetry at a given point of the phase diagram, critical
temperatures $T_c$, zero temperature gap amplitude values $\Delta_{0}$,
$\Delta_{0}/T_{c}$ ratios, doping and temperature dependences are all strongly
influenced by next-nearest neighbour contributions.


title:
Comment on "High-Temperature Series Analysis of the 2D Random-Bond Ising
  Ferromagnet"

abstract:
  A comment on PRL vol. 80, 4697 (1998) by Roder, Adler and Janke. Minior
correction included.


title: Fractional Populations in Sex-linked Inheritance

abstract:
  We study the fractional populations in chromosome inherited diseases. The
governing equations for the fractional populations are found and solved in the
presence of mutation and selection. The physical fixed points obtained are used
to discuss the cases of color blindness and hemophilia.


title:
General Green's function formalism for transport calculations with
  spd-Hamiltonians and giant magnetoresistance in Co and Ni based magnetic
  multilayers

abstract:
  A novel, general Green's function technique for elastic spin-dependent
transport calculations is presented, which (i) scales linearly with system size
and (ii) allows straightforward application to general tight-binding
Hamiltonians (spd in the present work). The method is applied to studies of
conductance and giant magnetoresistance (GMR) of magnetic multilayers in CPP
(current perpendicular to planes) geometry in the limit of large coherence
length. The magnetic materials considered are Co and Ni, with various
non-magnetic materials from the 3d, 4d, and 5d transition metal series.
Realistic tight-binding models for them have been constructed with the use of
density functional calculations. We have identified three qualitatively
different cases which depend on whether or not the bands (densities of states)
of a non-magnetic metal (i) form an almost perfect match with one of spin
sub-bands of the magnetic metal (as in Cu/Co spin valves); (ii) have almost
pure sp character at the Fermi level (e.g. Ag); (iii) have almost pure d
character at the Fermi energy (e.g. Pd, Pt). The key parameters which give rise
to a large GMR ratio turn out to be (i) a strong spin polarization of the
magnetic metal, (ii) a large energy offset between the conduction band of the
non-magnetic metal and one of spin sub-bands of the magnetic metal, and (iii)
strong interband scattering in one of spin sub-bands of a magnetic metal. The
present results show that GMR oscillates with variation of the thickness of
either non-magnetic or magnetic layers, as observed experimentally.


title:
Effective interactions and superconductivity in the t-J model in the
  large-N limit

abstract:
  The feasibility of a perturbation expansion for Green's functions of the t-J
model directly in terms of X-operators is demonstrated using the Baym- Kadanoff
functional method. As an application we derive explicit expressions for the
kernel of the linearized equation for the superconducting order parameter in
leading order of a 1/N expansion. The linearized equation is solved numerically
on a square lattice. We find that a reasonably strong instability occurs only
for even frequency pairing with d-wavelike symmetry. Results for the transition
temperature and the effective interaction are given as a function of doping.


title:
Unconventional Transition from Metallic to Insulating Resistivity in the
  Spin-ladder Compound (Sr,Ca)$_{14}$Cu$_{24}$O$_{41}$

abstract:
  Spin-ladder compounds make interesting analogs of the high-temperature
superconductors, because they contain layers of nearly one-dimensional
"ladders" consisting of a square array of copper and oxygen atoms. Increasing
the number of legs in the ladders provides a step-wise approach toward the
two-dimensional copper-oxygen plane, that structure believed to be a key to
high temperature superconductivity. Short-range spin correlations in ladders
have been predicted to lead to formation of hole pairs favorable for
superconductivity, once enough holes are introduced onto the ladders by doping.
Indeed, superconductivity has been discovered in the two-leg ladder compound
(Sr,Ca)$_{14}$Cu$_{24}$O$_{41}$ under high pressure. Here we show that charge
transport in the non-superconducting state of (Sr,Ca)$_{14}$Cu$_{24}$O$_{41}$
shares three distinct regimes in common with high-temperature superconductors,
including an unexplained insulating behavior at low temperatures in which the
resistivity increases as the logarithm of the temperature. These observations
suggest that the logarithmic divergence arises from a new localization
mechanism common to the ladder compounds and the high-temperature
superconductors, which may arise from nearly one-dimensional charge transport
in the presence of a spin gap.


title:
Spin polarized tunneling in ferromagnet/unconventional superconductor
  junctions

abstract:
  We study tunneling in ferromagnet/unconventional superconductor (F/S)
junctions. We include the effects of spin polarization, interfacial resistance,
and Fermi wavevector mismatch (FWM) between the F and S regions. Andreev
reflection (AR) at the F/S interface, governing tunneling at low bias voltage,
is strongly modified by these parameters. The conductance exhibits a very wide
variety of features as a function of applied voltage.


title:
Orbital Ordering Induced Antiferromagnetic Spin Fluctuations in the
  Ferromagnetic State of 50 % Hole-doped Manganites: Pr{1/2}Sr{1/2}MnO3 and
  Nd{1/2}Sr{1/2}MnO3

abstract:
  We report inelastic neutron scattering results on the spin dynamics in the 50
% hole-doped manganites, Pr{1/2}Sr{1/2}MnO3 and Nd{1/2}Sr{1/2}MnO3. In the
paramagnetic phase, these systems exhibit a ridge-type diffuse scattering,
indicating a two-dimensional ferromagnetic (FM) correlation. With decreasing
temperature, the systems enter the FM state, but simultaneously they develop
A-type antiferromagnetic spin fluctuations. The spin wave dispersion in the FM
state is strongly anisotropic. These behaviors can be consistently interpreted
from a viewpoint of the underlying orbital ordering.


title: Thermodynamics of the harmonic oscillator using coherent states

abstract:
  The ongoing discussion whether thermodynamic properties can be extracted from
a (possibly approximate) quantum mechanical time evolution using time averages
is fed with an instructive example. It is shown for the harmonic oscillator how
the Hilbert space or an appropriately defined phase space must be populated in
terms of coherent states in order to obtain the quantum result respectively the
classical one.


title: ac driven sine-Gordon solitons: dynamics and stability

abstract:
  The ac driven sine-Gordon equation is studied analytically and numerically,
with the aim of providing a full description of how soliton solutions behave.
To date, there is much controversy about when ac driven dc motion is possible.
Our work shows that kink solitons exhibit dc or oscillatory motion depending on
the relation between their initial velocity and the force parameters. Such
motion is proven to be impossible in the presence of damping terms. For
breathers, the force amplitude range for which they exist when dissipation is
absent is found. All the analytical results are compared with numerical
simulations, which in addition exhibit no dc motion at all for breathers, and
an excellent agreement is found. In the conclusion, the generality of our
results and connections to others systems for which a similar phenomenology may
arise are discussed.

categories: cond-mat math-ph math.MP nlin.PS patt-sol


title:
Formation of Clusters of Interstitial Carbon Atoms in Graphite due to
  Deformation Interaction and their Spatial Arrangement

abstract:
  Formation of clusters of interlayer interstitial carbon atoms in graphite is
studied by means of molecular dynamics simulation. It is shown that the
deformation potential is attractive for interstitials located in one interlayer
region and repulsive for interstitials located in different interlayer regions.
As a result, relatively small interstitial clusters are formed which are
arranged in a checkerboard-like order.


title: Average persistence in random walks

abstract:
  We study the first passage time properties of an integrated Brownian curve
both in homogeneous and disordered environments. In a disordered medium we
relate the scaling properties of this center of mass persistence of a random
walker to the average persistence, the latter being the probability P_pr(t)
that the expectation value <x(t)> of the walker's position after time t has not
returned to the initial value. The average persistence is then connected to the
statistics of extreme events of homogeneous random walks which can be computed
exactly for moderate system sizes. As a result we obtain a logarithmic
dependence P_pr(t)~{ln(t)}^theta' with a new exponent theta'=0.191+/-0.002. We
note on a complete correspondence between the average persistence of random
walks and the magnetization autocorrelation function of the transverse-field
Ising chain, in the homogeneous and disordered case.


title: A Real-Space Discrete Inverse Renormalization Group Method

abstract:
  A numerical real-space version of the Inverse Renormalization Group proposed
by Gawedzky et al is developed. It has been tested to obtain the scaling
behaviour of the random-forced heat equation in the short scales limit.
Prospectives are described, and the most important target for the procedure is
fully developed turbulence.


title:
On the extended states in the Quantum Hall regime and in zero magnetic
  field

abstract:
  For a two dimensional electron system in a strong perpendicular magnetic
field B, there are extended states at each Landau level. We show that if the
inverse compressibility is negative, then the extended states float downward in
energy when B decreases. We set up a condition for the case where all extended
states are below the fermi energy in the low B limit. The condition may explain
why a conducting state has been observed in a high mobility Si-Mosfet but so
far not in high mobility n-GaAs.


title:
Compact, orthogonal, and complete basis sets for solving the Schrodinger
  equation

abstract:
  We present a new type of basis set which is local, compact, and orthogonal.
The basis functions, called orthlets, are centered at the sites of a lattice
and are specifically adapted to represent the system being studied. The
adaptability includes the ability to have singular behavior within an orthlet,
allowing a single orthlet to represent a function in the vicinity of a
singularity.


title:
Analytic computation of the energy levels of a two-dimensional
  hydrogenic donor in a constant magnetic field

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


title: Probability distribution of drawdowns in risky investments

abstract:
  We study the risk criterion for investments based on the drawdown from the
maximal value of the capital in the past. Depending on investor's risk
attitude, thus his risk exposure, we find that the distribution of these
drawdowns follows a general power law. In particular, if the risk exposure is
Kelly-optimal, the exponent of this power law has the borderline value of 2,
i.e. the average drawdown is just about to diverge


title:
Long-Range Order in a Quasi One-Dimensional Non-Equilibrium Three-State
  Lattice Gas

abstract:
  Biased diffusion of two species with conserved dynamics on a 2xL periodic
lattice is studied via Monte Carlo simulations. In contrast to its simple
one-dimensional version on a ring, this quasi one-dimensional model
surprisingly exhibits phase separation in its steady state, which is
characterized by one macroscopic particle cluster. We study the order parameter
and the cluster-size distributions as a function of the system size L, to
support the above picture.


title:
Weak localization of disordered quasiparticles in the mixed
  superconducting state

abstract:
  Starting from a random matrix model, we construct the low-energy effective
field theory for the noninteracting gas of quasiparticles of a disordered
superconductor in the mixed state. The theory is a nonlinear sigma model, with
the order parameter field being a supermatrix whose form is determined solely
on symmetry grounds. The weak localization correction to the field-axis thermal
conductivity is computed for a dilute array of s-wave vortices near the lower
critical field H_c1. We propose that weak localization effects, cut off at low
temperatures by the Zeeman splitting, are responsible for the field dependence
of the thermal conductivity seen in recent high-T_c experiments by Aubin et al.


title:
Evolution of the pairing pseudogap in the spectral function with
  interplane anisotropy

abstract:
  We study the pairing pseudogap in the spectral function as a function of
interplane coupling. The analytical expressions for the self-energy in the
critical regime are obtained for any degree of anisotropy. The frequency
dependence of the self-energy is found to be qualitatively different in two and
three dimensions, and the crossover from two to three dimensional behavior is
discussed. In particular, by considering the anisotropy of the Fermi velocity
and gap along the Fermi surface, we can qualitatively explain recent
photoemission experiments on high temperature superconductors concerning the
temperature dependent Fermi arcs seen in the pseudogap phase.


title:
Computer Simulations of the Wetting Properties of Neon on Heterogeneous
  Surfaces

abstract:
  We use the grand canonical Monte Carlo method to study the nature of wetting
transitions on a variety of heterogeneous surfaces. The model system we
explore, Ne adsorption on Mg, is one for which a prewetting transition has been
found in our previous simulations. We find that the first order transition
present on the flat surface is absent from the rough surface. Nevertheless, the
resulting isotherms are, in some cases, so close to being discontinuous that
the distinction would be difficult to discern in most experiments.


title:
Magnetization Plateaus in One Dimensional $\v{S}=1/2$ Heisenberg Model
  with Dimerization and Quadrumerization

abstract:
  The one dimensional $S=1/2$ Heisenberg model with dimerization ($1-j$) and
quadrumerization ($\delta$) in the magnetic field is studied by means of the
numerical exact diagonalization of finite size systems and the conformal field
theory. It is found that the magnetization plateau at half of the saturation
value exists for $\delta \neq 0$. For $\delta = 0$, this model is described by
the conformal field theory with central charge $c=1$ at this value of
magnetization. The critical exponent $\nu$ which characterizes the
$\delta$-dependence of the width of the plateau is calculated using the level
spectroscopy method. The $j$-dependence of the critical exponent $\nu$ is found
to be non-monotonic and discontinuous at $j = 0$. The effective theory of the
magnetization plateau is also presented for various limiting cases.


title:
One Dimensional Kondo Lattice Model Studied by the Density Matrix
  Renormalization Group Method

abstract:
  Recent developments of the theoretical investigations on the one-dimensional
Kondo lattice model by using the density matrix renormalization group (DMRG)
method are discussed in this review. Short summaries are given for the
zero-temperature DMRG, the finite-temperature DMRG, and also its application to
dynamic quantities. Away from half-filling, the paramagnetic metallic state is
shown to be a Tomonaga-Luttinger liquid with the large Fermi surface. For the
large Fermi surface its size is determined by the sum of the densities of the
conduction electrons and the localized spins. The correlation exponent K_rho of
this metallic phase is smaller than 1/2. At half-filling the ground state is
insulating. Excitation gaps are different depending on channels, the spin gap,
the charge gap and the quasiparticle gap. Temperature dependence of the spin
and charge susceptibilities and specific heat are discussed. Particularly
interesting is the temperature dependence of various excitation spectra, which
show unusual properties of the Kondo insulators.


title: Single hole dynamics in dimerized spin liquids

abstract:
  The dynamics of a single hole in quantum antiferromagnets is influenced by
magnetic fluctuations. In the present work we consider two situations. The
first one corresponds to a single hole in the two leg t-J spin ladder. In this
case the wave function renormalization is relatively small and the
quasiparticle residue of the S=1/2 state remains close to unity. However at
large t/J there are higher spin (S=3/2,5/2,..) bound states of the hole with
the magnetic excitations, and therefore there is a crossover from
quasiparticles with S=1/2 to quasiparticles with higher spin.
  The second situation corresponds to a single hole in two coupled
antiferromagnetic planes very close to the point of antiferromagnetic
instability. In this case the hole wave function renormalization is very strong
and the quasiparticle residue vanishes at the point of instability.


title:
Anisotropic suppression of quasiparticle weight in two-dimensional
  electron system with partially flat Fermi surface: two-loop
  renormalization-group analysis

abstract:
  Two-loop renormalization-group analysis for a two-dimensional electron system
with a partially flat Fermi surface has been carried out. We found that, {\it
irrespective of pairing mechanism}, the quasiparticle weight is anisotropically
suppressed due to logarithmically singular processes in the flat regions of the
Fermi surface. When the energy scale decreases, the quasiparticle weight is the
most strongly suppressed around the center of the flat region, which
qualitatively agrees with the anisotropic pseudogap behavior suggested through
the angle-resolved photoemission spectroscopy experiments for underdoped
Bi$_2$Sr$_2$CaCu$_2$O$_8$.


title: Tetragonal states from epitaxial strain on metal films

abstract:
  The tetragonal states produced by isotropic pseudomorphic epitaxial strain in
the (001) plane on a tetragonal phase of a crystal are calculated for V, Ti,
Rb, Li, K, Sr from first-principles electronic theory. It is shown that each
metal has two tetragonal phases corresponding to minima of the total energy
with respect to tetragonal deformations, hence are equilibrium phases, and that
the equilibrium phases are separated by a region of inherent instability. The
equilibrium phase for any strained tetragonal state can thus be uniquely
identified. Lattice constants and relative energies of the two phases and the
saddle point between them are tabulated, as well as the tetragonal elastic
constants of each phase.


title: The Uneven Distribution of Numbers in Nature

abstract:
  Suppose you look at today's stock prices and bet on the value of the first
digit. One could guess that a fair bet should correspond to the frequency of
$1/9 = 11.11%$ for each digit from 1 to 9. This is by no means the case, and
one can easily observe a strong prevalence of the small values over the large
ones. The first three integers 1,2 and 3 alone have globally a frequency of 60%
while the other six values 4, 5, 6, 7, 8 and 9 appear only in 40% of the cases.
This situation is actually much more general than the stock market and it
occurs in a variety of number catalogs related to natural phenomena. The first
observation of this property traces back to S. Newcomb in 1881 but a more
precise account was given by F. Benford in 1938. In this note we illustrate
these observations with the enlightening specific example of the stock market.
We also identify the general mechanism for the origin of this uneven
distribution in the multiplicative nature of fluctuations in economics and in
many natural phenomena. This provides a natural explanation for the ubiquitous
presence of the Benford's law in many different phenomena with the common
element that their fluctuations refer to a fraction of their values. This
brings us close to the problem of the spontaneous origin of scale invariant
properties in various phenomena which is a debated question at the frontier of
different fields.


title: Minimal Surfaces, Screw Dislocations and Twist Grain Boundaries

abstract:
  Large twist-angle grain boundaries in layered structures are often described
by Scherk's first surface whereas small twist-angle grain boundaries are
usually described in terms of an array of screw dislocations. We show that
there is no essential distinction between these two descriptions and that, in
particular, their comparative energetics depends crucially on the core
structure of their screw-dislocation topological defects.

categories: cond-mat.soft cond-mat.mtrl-sci cond-mat.stat-mech math.DG


title: Bunching Transitions on Vicinal Surfaces and Quantum N-mers

abstract:
  We study vicinal crystal surfaces with the terrace-step-kink model on a
discrete lattice. Including both a short-ranged attractive interaction and a
long-ranged repulsive interaction arising from elastic forces, we discover a
series of phases in which steps coalesce into bunches of n steps each. The
value of n varies with temperature and the ratio of short to long range
interaction strengths. We propose that the bunch phases have been observed in
very recent experiments on Si surfaces. Within the context of a mapping of the
model to a system of bosons on a 1D lattice, the bunch phases appear as quantum
n-mers.


title:
Phase-Ordering Dynamics with an Order-Parameter-Dependent Mobility: The
  Large-n Limit

abstract:
  The effect of an order-parameter dependent mobility (or kinetic coefficient),
on the phase-ordering dynamics of a system described by an n-component vector
order parameter is addressed at zero temperature in the large-n limit. We
consider cases in which the mobility or kinetic coefficient vanishes when the
magnitude of the order parameter takes its equilibrium value. In the large-n
limit, the system is exactly soluble for both conserved and non-conserved order
parameter. In the non-conserved case, the scaling form for the correlation
function and it's Fourier transform, the structure factor, is established, with
the characteristic length growing as a power of time. In the conserved case,
the structure factor is evaluated and found to exhibit a multi-scaling
behaviour, with two growing length scales differing by a logarithmic factor. In
both cases, the rate of growth of the length scales depends on the manner in
which the mobility or kinetic coefficient vanishes as the magnitude of the
order parameter approaches its equilibrium value.


title: A Progress Report on the SO(5) Theory of High T_c Superconductivity

abstract:
  In this talk I give a brief update on the recent progress in the SO(5) theory
of high T_c superconductivity (Science, 275: 1089,1997). Reviewed topics
include SO(5) ladders, the unification of BCS and SDW quasi-particles in the
SO(5) theory and the microscopic origin of the condensation energy.


title: Mean-Field Theory for Spin Ladders Using Angular-Momentum Coupled Bases

abstract:
  We study properties of two-leg Heisenberg spin ladders in a mean-field
approximation using a variety of angular-momentum coupled bases. The mean-field
theory proposed by Gopalan, Rice, and Sigrist, which uses a rung basis, assumes
that the mean-field ground state consists of a condensate of spin-singlets
along the rungs of the ladder. We generalize this approach to larger
angular-momentum coupled bases which incorporate---by their mere definition---a
substantial fraction of the important short-range structure of these materials.
In these bases the mean-field ground-state remains a condensate of spin
singlet---but now with each involving a larger fraction of the spins in the
ladder. As expected, the ``purity'' of the ground-state, as judged by the
condensate fraction, increases with the size of the elementary block defining
the basis. Moreover, the coupling to quasiparticle excitations becomes weaker
as the size of the elementary block increases. Thus, the weak-coupling limit of
the theory becomes an accurate representation of the underlying mean-field
dynamics. We illustrate the method by computing static and dynamic properties
of two-leg ladders in the various angular-momentum coupled bases.


title: Thickness evolution of coercivity in ultrathin magnetic films

abstract:
  The thickness evolution of in-plane magnetization reversal in ultrathin films
is studied with a theoretical model that takes account of surface roughness
typical of epitaxial growth. Guided by N\'{e}el's model, step edge sites of
monolayer-height islands are assigned a two-fold anisotropy in addition to a
four-fold anisotropy at all sites. Coercivity is found to depend essentially on
both the film thickness and the partial coverage of the topmost layer. Its
qualitative features are determined primarily by sample geometry and the size
of the step anisotropy compared to the domain wall energy. Magnetostatic
interactions change the results quantitatively, but not qualitatively. Their
effect is understood by comparing calculations with and without their
inclusion.


title:
A Strong-Coupling Approach to the Magnetization Process of Polymerized
  Quantum Spin Chains

abstract:
  Polymerized quantum spin chains (i.e. spin chains with a periodic modulation
of the coupling constants) exhibit plateaux in their magnetization curves when
subjected to homogeneous external magnetic fields. We argue that the
strong-coupling limit yields a simple but general explanation for the
appearance of plateaux as well as of the associated quantization condition on
the magnetization. We then proceed to explicitly compute series for the plateau
boundaries of trimerized and quadrumerized spin-1/2 chains. The picture is
completed by a discussion how the universality classes associated to the
transitions at the boundaries of magnetization plateaux arise in many cases
from a first order strong-coupling effective Hamiltonian.


title: X-ray Investigation of the Magneto-elastic Instability of alpha'-NaV2O5

abstract:
  We present an X-ray diffuse scattering study of the pretransitional
structural fluctuations of the magneto-elastic transition in alpha'-NaV2O5.
This transition is characterized by the appearance below Tsp~35K of satellite
reflections at the reduced wave vector (1/2,1/2,1/4). A large regime of
structural fluctuations is measured up to 90 K. These fluctuations are three
dimensional between Tsp and ~50K and quasi-one dimensional above ~60K. At 40 K
the anisotropy ratio is found to be (xib :xia :xic)= (3.8 : 1.8 : 1), which
reveals the importance of transverse interactions in the stabilization of the
low temperature phase. We discuss our results within the framework of recent
theories dealing with the simultaneous occurrence of a charge ordering, a spin
gap and a lattice distortion in this intriguing compound.


title:
Quasielastic magnetic scattering of neutrons at the systems with heavy
  fermions

abstract:
  The theory of the quasielastic magnetic scattering of neutrons at the spin
liquid with RVB (resonance valence bonds) correlations is presented.
Calculations demonstrate that the dependence of the scattering cross section on
the energy transfer reproduces experimental shape of quasielastic peak in heavy
fermion systems. It is shown that Fermi statistics of the spin liquid
elementary excitations leads to the oscillations of the quasielastic scattering
total cross section as a function of the momentum transfer.


title: Structure and stability of finite gold nanowires

abstract:
  Finite gold nanowires containing less than 1000 atoms are studied using the
molecular dynamics simulation method and embedded atom potential. Nanowires
with the face-centered cubic structure and the (111) oriented cross-section are
prepared at T=0 K. After annealing and quenching the structure and vibrational
properties of nanowires are studied at room temperature. Several of these
nanowires form multi-walled structures of lasting stability. They consist of
concentrical cylindrical sheets and resemble multi-walled carbon nanotubes.
Vibrations are investigated by diagonalization of the dynamical matrix. It was
found that several percents of vibrational modes are unstable because of
uncompleted restructuring of initial fcc nanowires.


title:
Droplet Phenomenology and Mean Field in a Frustrated and Disordered
  System

abstract:
  The low lying excited states of the three-dimensional minimum matching
problem are studied numerically. The excitations' energies grow with their size
and confirm the droplet picture. However, some low energy, infinite size
excitations create multiple valleys in the energy landscape. These states
violate the droplet scaling ansatz, and are consistent with mean field
predictions. A similar picture may apply to spin glasses whereby the droplet
picture describes the physics at small length scales, while mean field
describes that at large length scales.


title: Selection, Mutations and Codon Usage in Bacterial Model

abstract:
  We present a statistical model of bacterial evolution based on the coupling
between codon usage and tRNA abundance. Such a model interprets this aspect of
the evolutionary process as a balance between the codon homogenization effect
due to mutation process and the improvement of the translation phase due to
natural selection. We develop a thermodynamical description of the asymptotic
state of the model. The analysis of naturally occurring sequences shows that
the effect of natural selection on codon bias not only affects genes whose
products are largely required at maximal growth rate conditions but also gene
products that undergo rapid transient increases.


title:
Disorder Driven Roughening Transitions of Elastic Manifolds and Periodic
  Elastic Media

abstract:
  The simultaneous effect of both disorder and crystal-lattice pinning on the
equilibrium behavior of oriented elastic objects is studied using scaling
arguments and a functional renormalization group technique. Our analysis
applies to elastic manifolds, e.g., interfaces, as well as to periodic elastic
media, e.g., charge-density waves or flux-line lattices. The competition
between both pinning mechanisms leads to a continuous, disorder driven
roughening transition between a flat state where the mean relative displacement
saturates on large scales and a rough state with diverging relative
displacement. The transition can be approached by changing the impurity
concentration or, indirectly, by tuning the temperature since the pinning
strengths of the random and crystal potential have in general a different
temperature dependence. For D dimensional elastic manifolds interacting with
either random-field or random-bond disorder a transition exists for 2<D<4, and
the critical exponents are obtained to lowest order in \epsilon=4-D. At the
transition, the manifolds show a superuniversal logarithmic roughness. Dipolar
interactions render lattice effects relevant also in the physical case of D=2.
For periodic elastic media, a roughening transition exists only if the ratio p
of the periodicities of the medium and the crystal lattice exceeds the critical
value p_c=6/\pi\sqrt{\epsilon}. For p<p_c the medium is always flat. Critical
exponents are calculated in a double expansion in \mu=p^2/p_c^2-1 and
\epsilon=4-D and fulfill the scaling relations of random field models.


title: Modeling Grain Boundaries using a Phase Field Technique

abstract:
  We propose a two dimensional frame-invariant phase field model of grain
impingement and coarsening. One dimensional analytical solutions for a stable
grain boundary in a bicrystal are obtained, and equilibrium energies are
computed. We are able to calculate the rotation rate for a free grain between
two grains of fixed orientation. For a particular choice of functional
dependencies in the model the grain boundary energy takes the same analytic
form as the microscopic (dislocation) model of Read and Shockley.


title:
Rotating wave approximation: systematic expansion and application to
  coupled spin pairs

abstract:
  We propose a new treatment of the dynamics of a periodically time-dependent
Liouvillian by mapping it onto a time-independent problem and give a systematic
expansion for its effective Liouvillian. In the case of a two-level system, the
lowest order contribution is equivalent to the well-known rotating wave
approximation. We extend the formalism to a pair of coupled two-level systems.
For this pair, we find two Rabi frequencies and we can give parameter regimes
where the leading order of the expansion is suppressed and higher orders become
important. These results might help to investigate the interaction of tunneling
systems in mixed crystals by providing a tool for the analysis of echo
experiments.


title: Models of the Pseudogap State of Two-Dimensional Systems

abstract:
  We analyze a number of ``nearly exactly'' solvable models of electronic
spectrum of two-dimensional systems with well-developed fluctuations of short
range order of ``dielectric'' (e.g. antiferromagnetic) or ``superconducting''
type, which lead to the formation of anisotropic pseudogap state on certain
parts of the Fermi surface. We formulate a recurrence procedure to calculate
one-electron Green's function which takes into account all Feynman diagrams in
perturbation series and is based upon the approximate Ansatz for higher-order
terms in this series. Detailed results for spectral densities and density of
states are presented. We also discuss some important points concerning the
justification of our Ansatz for higher-order contributions.


title: C-axis electronic Raman scattering in Bi_2Sr_2CaCu_2O_{8+\delta}

abstract:
  We report a c-axis-polarized electronic Raman scattering study of
Bi_2Sr_2CaCu_2O_{8+\delta} single crystals. In the normal state, a resonant
electronic continuum extends to 1.5 eV and gains significant intensity as the
incoming photon energy increases. In the superconducting state, a coherence
2\Delta peak appears around 50 meV, with a suppression of the scattering
intensity at frequencies below the peak position. The peak energy, which is
higher than that seen with in-plane polarizations, signifies distinctly
different dynamics of quasiparticle excitations created with out-of-plane
polarization.


title:
Negative magnetoresistance in the nearest-neighbour hopping conduction
  in granular gold film

abstract:
  The low temperature (0.5-55 K) conduction of semicontinuous gold film vacuum
deposited at T \approx 50 K is studied. The film is near the percolation
threshold (thickness 3.25 nm). Its resistance is extremely sensitive to the
applied voltage U. At low enough U the film behaves as an insulator
(two-dimensional granular metal). In this state the dependences R(T) \propto
\exp (1/T) (for T \leq 20 K) and R(U) \propto \exp (1/U)) (for T \leq 1 K and U
> 0.1 V) are observed. Magnetoresistance (MR) is negative and can be described
by \Delta R(H)/R(0) \propto -H^2/T. This negative MR which manifests itself for
nearest-neighbour hopping is rather uncommon and, up to now, has not been
clarified. The possible mechanisms of such case of negative MR are discussed.


title:
Simple model of self-organized biological evolution as completely
  integrable dissipative system

abstract:
  The Bak-Sneppen model of self-organized biological evolution of an infinite
ecosystem of randomly interacting species is represented in terms of an
infinite set of variables which can be considered as an analog to the set of
integrals of motion of completely integrable system. Each of this variables
remains to be constant but its influence on the evolution process is restricted
in time and after definite moment its value is excluded from description of the
system dynamics.


title:
Exact results for the Kardar--Parisi--Zhang equation with spatially
  correlated noise

abstract:
  We investigate the Kardar--Parisi--Zhang (KPZ) equation in $d$ spatial
dimensions with Gaussian spatially long--range correlated noise ---
characterized by its second moment $R(\vec{x}-\vec{x}') \propto
|\vec{x}-\vec{x}'|^{2\rho-d}$ --- by means of dynamic field theory and the
renormalization group. Using a stochastic Cole--Hopf transformation we derive
{\em exact} exponents and scaling functions for the roughening transition and
the smooth phase above the lower critical dimension $d_c = 2 (1+\rho)$. Below
the lower critical dimension, there is a line $\rho_*(d)$ marking the stability
boundary between the short-range and long-range noise fixed points. For $\rho
\geq \rho_*(d)$, the general structure of the renormalization-group equations
fixes the values of the dynamic and roughness exponents exactly, whereas above
$\rho_*(d)$, one has to rely on some perturbational techniques. We discuss the
location of this stability boundary $\rho_* (d)$ in light of the exact results
derived in this paper, and from results known in the literature. In particular,
we conjecture that there might be two qualitatively different strong-coupling
phases above and below the lower critical dimension, respectively.


title:
Ferromagnetic resonance force microscopy on microscopic cobalt single
  layer films

abstract:
  We report mechanical detection of ferromagnetic resonance signals from
microscopic Co single layer thin films using a magnetic resonance force
microscope (MRFM). Variations in the magnetic anisotropy field and the
inhomogeneity of were clearly observed in the FMR spectra of microscopic Co
thin films 500 and 1000 angstrom thick and 40 X 200 micron^2 in lateral extent.
This demonstrates the important potential that MRFM detection of FMR holds for
microscopic characterization of spatial distribution of magnetic properties in
magnetic layered materials and devices.


title:
Magnetic Field Dependence of the Low-Temperature Specific Heat in
  PrInAg_2: Support for a Non-Magnetic Heavy-Fermion Ground State

abstract:
  In order to elucidate the nature of the ground state in the heavy-electron
system PrInAg_2, we measured its specific heat for temperatures between 60 mK
and 2 K in magnetic fields up to 9 T. The peak maximum of the low temperature
anomaly in the specific heat (interpreted as Kondo anomaly) shifts from 0.42 K
at zero field to 0.45 K at 6 T, and to 0.5 K at 9 T. The data at 3 T (after
subtracting the low temperature tail due to a Schottky anomaly) is practically
indistinguishable from the zero field data. A low temperature nuclear Schottky
anomaly from a hyperfine enhancement of the Pr nuclei was observed in field.
Both enhanced hyperfine interaction and the insensitivity of the specific heat
anomaly to fields support the hypothesis that the Kondo effect in this system
has a non-magnetic origin.


title:
Study of Harper's Equation for the 2-D Systems of Antiferromagnetically
  Correlated Electrons in an External Magnetic Field

abstract:
  Considering interacting (antiferromagnetically correlated) electrons, we
derive a generalized Harper's equation for the square lattice of infinite size.
We obtain an analytic expression for the density of states from the newly
derived Harper's equation. We present a predicted phase diagram of staggered
magnetization in the plane of temperature vs doping rate and discover a
possibility of reentrant behavior of the staggered magnetization even in the
presence of applied magnetic field. It is shown that below a critical electron
correlation strength (Coulomb repulsion) the staggered magnetization in the
presence of magnetic field vanishes at an even denominator $q$ value but not at
odd $q$ of a given magnetic flux quantum per plaquette, $p/q$.


title:
Effect of the cluster size in modeling the H_2 desorption and
  dissociative adsorption on Si(001)

abstract:
  Three different clusters, Si_9H_12, Si_15H_16, and Si_21H_20, are used in
density-functional theory calculations in conjunction with ab initio
pseudopotentials to study how the energetics of H_2 dissociativ e adsorption on
and associative desorption from Si(001) depends on the cluster size. The
results are compared to five-layer slab calculations using the same
pseudopotentials and high qu ality plane-wave basis set. Several
exchange-correlation functionals are employed. Our analysis suggests that the
smaller clusters generally overestimate the activation barriers and reaction
energy. The Si_21H_20 cluster, however, is found to predict reaction
energetics, with E_{a}^{des}=56 +- 3 kcal/mol (2.4 +- 0.1 eV), reasonably close
(though still different) to that obtained from the slab calculations.
Differences in the calculated activation energies are discussed in relation to
the efficiency of clusters to describe the properties of the clean Si(001)-2x1
surface.


title:
Polymers and manifolds in static random flows: a renormalization group
  study

abstract:
  We study the dynamics of a polymer or a D-dimensional elastic manifold
diffusing and convected in a non-potential static random flow (the ``randomly
driven polymer model''). We find that short-range (SR) disorder is relevant for
d < 4 for directed polymers (each monomer sees a different flow) and for d < 6
for isotropic polymers (each monomer sees the same flow) and more generally for
d<d_c(D) in the case of a manifold. This leads to new large scale behavior,
which we analyze using field theoretical methods. We show that all divergences
can be absorbed in multilocal counter-terms which we compute to one loop order.
We obtain the non trivial roughness zeta, dynamical z and transport exponents
phi in a dimensional expansion. For directed polymers we find zeta about 0.63
(d=3), zeta about 0.8 (d=2) and for isotropic polymers zeta about 0.8 (d=3). In
all cases z>2 and the velocity versus applied force characteristics is
sublinear, i.e. at small forces v(f) f^phi with phi > 1. It indicates that this
new state is glassy, with dynamically generated barriers leading to trapping,
even by a divergenceless (transversal) flow. For random flows with long-range
(LR) correlations, we find continuously varying exponents with the ratio gL/gT
of potential to transversal disorder, and interesting crossover phenomena
between LR and SR behavior. For isotropic polymers new effects (e.g. a sign
change of zeta - zeta_0) result from the competition between localization and
stretching by the flow. In contrast to purely potential disorder, where the
dynamics gets frozen, here the dynamical exponent z is not much larger than 2,
making it easily accessible by simulations. The phenomenon of pinning by
transversal disorder is further demonstrated using a two monomer ``dumbbell''
toy model.


title:
Current-voltage scaling of chiral and gauge-glass models of
  two-dimensional superconductors

abstract:
  The scaling behavior of the current-voltage characteristics of chiral and
gauge glass models of disordered superconductors, are studied numerically, in
two dimensions. For both models, the linear resistance is nonzero at finite
temperatures and the scaling analysis of the nonlinear resistivity is
consistent with a phase transition at T=0 temperature characterized by a
diverging correlation length $\xi \propto T^{-\nu_{T}}$ and thermal critical
exponent $\nu_{T}$. The values of $\nu_{T}$, however, are found to be different
for the chiral and gauge glass models, suggesting different universality
classes, in contrast to the result obtained recently in three dimensions.


title:
Manifestation of classical bifurcation in the spectrum of the integrable
  quantum dimer

abstract:
  We analyze the classical and quantum properties of the integrable dimer
problem. The classical version exhibits exactly one bifurcation in phase space,
which gives birth to permutational symmetry broken trajectories and a
separatrix. The quantum analysis yields all tunneling rates (splittings) in
leading order of perturbation. In the semiclassical regime the eigenvalue
spectrum obtained by numerically exact diagonalization allows to conclude about
the presence of a separatrix and a bifurcation in the corresponding classical
model.


title:
Crossover between the Dense Electron-Hole Phase and the BCS Excitonic
  Phase in Quantum Dots

abstract:
  Second order perturbation theory and a Lipkin-Nogami scheme combined with an
exact Monte Carlo projection after variation are applied to compute the
ground-state energy of $6\le N\le 210$ electron-hole pairs confined in a
parabolic two-dimensional quantum dot. The energy shows nice scaling properties
as N or the confinement strength is varied. A crossover from the high-density
electron-hole phase to the BCS excitonic phase is found at a density which is
roughly four times the close-packing density of excitons.


title:
Finite Temperature Effects in One-dimensional Mott-Hubbard Insulator:
  Angle-Resolved Photoemission Study of Na_{0.96}V_{2}O_{5}

abstract:
  We have made an angle-resolved photoemission study of a one-dimensional (1D)
Mott-Hubbard insulator Na_{0.96}V_{2}O_{5} and found that the spectra of the V
3d lower Hubbard band are strongly dependent on the temperature. We have
calculated the one-particle spectral function of the one-dimensional t-J model
at finite temperatures by exact diagonalization and compared them with the
experimental results. Good overall agreement is obtained between experiment and
theory. The strong finite temperature effects are discussed in terms of the
existence of the ``Fermi surface'' of the spinon band.


title: Surface Critical Behavior in Systems with Absorbing States

abstract:
  We present a general scaling theory for the surface critical behavior of
non-equilibrium systems with phase transitions into absorbing states. The
theory allows for two independent surface exponents which satisfy generalized
hyperscaling relations. As an application we study a generalized version of
directed percolation with two absorbing states. We find two distinct surface
universality classes associated with inactive and reflective walls. Our results
indicate that the exponents associated with these two surface universality
classes are closely connected.


title: Decoherence of Bose-Einstein condensates in traps at finite temperature

abstract:
  The phase diffusion of the order parameter of trapped Bose-Einstein
condensates at temperatures large compared to the mean trap frequency is
determined, which gives the fundamental limit of the line-width of an atom
laser. In addition a prediction of the correlation time of the number
fluctuations in the condensate is made and related to the phase diffusion via
the fluctuation-dissipation relation.


title:
New quantum Monte Carlo study of quantum critical phenomena with
  Trotter-number-dependent finite-size scaling and non-equilibrium relaxation

abstract:
  We propose a new efficient scheme for the quantum Monte Carlo study of
quantum critical phenomena in quantum spin systems. Rieger and Young's
Trotter-number-dependent finite-size scaling in quantum spin systems and Ito
{\it et al.}'s evaluation of the transition point with the non-equilibrium
relaxation in classical spin systems are combined and generalized. That is,
only one Trotter number and one inverse temperature proportional to system
sizes are taken for each system size, and the transition point of the
transformed classical spin model is estimated as the point at which the order
parameter shows power-law decay. The present scheme is confirmed by the
determination of the critical phenomenon of the one-dimensional $S=1/2$
asymmetric XY model in the ground state. The estimates of the transition point
and the critical exponents $\beta$, $\gamma$ and $\nu$ are in good agreement
with the exact solutions. The dynamical critical exponent is also estimated as
${\mit \Delta}=2.14(I4(Jpm 0.06$, which is consistent with that of the
two-dimensional Ising model.


title: Coulomb drag in intermediate magnetic fields

abstract:
  We investigated theoretically the Coulomb drag effect in coupled 2D electron
gases in a wide interval of magnetic field and temperature $ 1/\tau \ll
\omega_c \ll E_F/\hbar$, $T \ll E_F$,
  $\tau$ being intralayer scattering time, $\omega_c$ being the cyclotron
frequency. We show that the quantization of the electron spectrum leads to rich
parametric dependences of drag transresistance on temperature and magnetic
field. This is in contrast to usual resistance. New small energy scales are
found to cut typical excitation energies to values lower than temperature. This
may lead to a linear temperature dependence of transresistance even in a
relatively weak magnetic field and can explain some recent experimental data.
  We present a novel mechanism of Coulomb drag when the current in the active
layer causes a magnetoplasmon wind and the magnetoplasmons are absorbed by the
electrons of the passive layer providing a momentum transfer. We derived
general relations that describe the drag as a result of resonant tunneling of
magnetoplasmons.


title: Skyrmions in disordered heterostructures}

abstract:
  We have investigated the effect of weak disorder on the ground state of a two
dimensional electron gas in the quantum Hall regime at filling factors slightly
deviating from unity. The skyrmions and antiskyrmions are found to be present
even at filling factor $\nu=1$. They may be strongly squeezed by the disorder.
We have studied temperature effect on skyrmion density and size.


title: Coulomb Blockade without Tunnel Junctions

abstract:
  We find that tunnel junctions are not needed to provide single electron
effects in a metallic island. Eventually the tunnel junction may be replaced by
an arbitrary scatterer. It is important that even a {\it diffusive} scatterer
provides a sufficient isolation for single electron effects to persist. To
formulate this in exact terms, we derive and analyze the effective action that
describes an arbitrary scatterer. We also consider the fluctuations of the
effective charging energy.


title:
Influence of disorder on the perpendicular magnetoresistance of magnetic
  multilayers

abstract:
  The effect of disorder on the perpendicular magnetoresistance of magnetic
multilayers is investigated theoretically. Various kinds of disorder are
considered: (i) interface substitutional disorder and (ii) bulk disorder in the
various layers and in the leads. The calculations are based upon the
non-equilibrium Green's function formalism, together with the recursion method
for calculating the real-space Green's function.


title: Optimum ground states for spin-3/2 ladders with two legs

abstract:
  We construct the exact ground state for an antiferromagnetic spin-3/2 model
on the two-leg ladder as an optimum ground state. The ground state contains a
discrete parameter "sigma"=+/-1 and a continuous parameter "a" which controls
z-axis anisotropy. For most values of "a" the global ground state is unique. It
has vanishing sublattice magnetization and exponentially decaying correlation
functions. By using the transfer matrix technique, we calculate exactly the
fluctuations of the magnetization, the nearest-neighbour correlation, and the
longitudinal correlation length as functions of the parameters.


title: Universal 1/f Noise from Dissipative SOC Models

abstract:
  We introduce a model able to reproduce the main features of 1/f noise:
hyper-universality (the power-law exponents are independent on the dimension of
the system; we show here results in d=1,2) and apparent lack of a low-frequency
cutoff in the power spectrum. Essential ingredients of this model are an
activation-deactivation process and dissipation.


title: Low-temperature structural model of hcp solid C$_{70}$

abstract:
  We report intermolecular potential-energy calculations for solid C_${70}$ and
determine the optimum static orientations of the molecules at low temperature;
we find them to be consistent with the monoclinic structural model proposed by
us in an earlier report [Solid State Commun. {\bf 105), 247 (1998)]. This model
indicates that the C_5 axis of the molecule is tilted by an angle $\approx$18^o
from the monoclinic b axis in contrast with the molecular orientation proposed
by Verheijen {\it et al.} [J. Chem. Phys. {\bf 166}, 287 (1992)] where the C_5
axis is parallel to the monoclinic b axis. In this calculation we have
incorporated the effective bond charge Coulomb potential together with the
Lennard-Jones potential between the molecule at the origin of the monoclinic
unit cell and its six nearest neighbours, three above and three below. The
minimum energy configuration for the molecular orientations turns out to be at
$\theta$=18^o, $\phi$=8^o, and $\psi$=5^o, where $\theta$, $\phi$, and $\psi$
define the molecular orientations.


title:
Magnon Wave-function and Impurity Effects in S=1 Antiferromagnetic
  Chains: A Large-n Approach

abstract:
  A large-n approximation to the S=1 antiferromagnetic chain, using the
symmetric tensor representation and its conjugate, is developed to order 1/n in
order to calculate the magnon wave-function and to study the effect of
modifying the exchange coupling from J to J' on a single link. It is shown that
a magnon boundstate exists below the Haldane gap for arbitrarily small negative
J'-J but only above a certain critical value of J'-J for positive values. In
the former case the binding energy vanishes as the square of (J-J').


title: Critical Behaviour of a Homogeneous Bose Gas at Finite Temperature

abstract:
  A homogeneous non-relativistic Bose gas is investigated at finite temperature
using renormalization group methods. The phase transition is shown to be second
order and the effective chemical potential and the effective quartic coupling
vanish at the critical temperature. We obtain the critical exponent $\nu =0.73$
at leading order in the derivative expansion.


title: Scattering approach to parametric pumping

abstract:
  A d.c. current can be pumped through a quantum dot by periodically varying
two independent parameters $X_1$ and $X_2$, like a gate voltage or magnetic
field. We present a formula that relates the pumped current to the parametric
derivatives of the scattering matrix $S(X_1,X_2)$ of the system. As an
application we compute the statistical distribution of the pumped current in
the case of a chaotic quantum dot.


title:
Exact weak-coupling radius of the Holstein polaron in one, two, and
  three dimensions

abstract:
  We apply weak-coupling perturbation theory to the Holstein molecular crystal
model in order to compute an electron-phonon correlation function
characterizing the shape and size of the polaron lattice distortion in one,
two, and three dimensions. This correlation function is computed exactly to
leading order in the electron-phonon coupling constant, permitting a complete
description of correlations in any dimension for both isotropic and arbitrarily
anisotropic cases. Using this exact result, the width of the polaron is
characterized along arbitrary directions. The width of the polaron thus
determined disagrees in every dimension with some well-known characterizations
of polarons, signalling in particular the breakdown of the adiabatic
approximation and the characterizations of self-trapping associated with it.


title: Avoided Critical Behavior in O(n) Systems

abstract:
  Long-range frustrating interactions, even if their strength is infinitesimal,
can give rise to a dramatic proliferations of ground or near-ground states. As
a consequence, the ordering temperature can exhibit a discontinuous drop as a
function of the frustration. A simple model of the doped Mott insulator, where
the short-range tendency of the holes to phase separate competes with
long-range Coulomb effects, exhibits this "avoided critical" behavior. This
model may serve as a paradigm for many other systems.


title:
Competition between spin glass order and strong coupling
  superconductivity in a single-species fermion model

abstract:
  The phase diagram of a single species fermion model allowing for local
pairing superconductivity (SC) and spin glass order (SG) is derived as a
function of chemical potential \mu and ratio r=v/J between attractive coupling
v and frustrated magnetic interaction J. For ratios larger than a
characteristic r_c(\mu), superconductivity does not allow for SG order, while
for smaller values a very detailed phase diagram arises with entangled spin
glass and superconducting transitions. Our results for the Green's functions
show that superconductivity occurring in the magnetic interaction band is of
gapless type with a crossover from strongly gapless, within a certain range
below T_c, to very weakly gapless in a wide low temperature regime, and
hardgapped at T=0.


title:
Collective excitations in Bose-Einstein condensates in triaxially
  anisotropic parabolic traps

abstract:
  The wave equation of low-frequency density waves in Bose-Einstein condensates
at vanishing temperature in arbitrarily anisotropic harmonic traps is separable
in elliptic coordinates, provided the condensate can be treated in the
Thomas-Fermi approximation. We present a complete solution of the mode
functions, which are polynomials of finite order, and their eigenfrequencies
which are characterized by three integer quantum numbers.


title: Response Functions of Two-Coupled Chains of Tomonaga-Luttinger Liquids

abstract:
  Properties of fluctuations in two chains of Tomonaga-Luttinger liquids
coupled by the interchain hopping have been studied by calculating retarded
response functions $\chi^R_{\rho} (q_x,q_y;\omega)$ for charge and $\chi^R_{\p}
(q_x,q_y;\omega)$ for spin where $q_x$ and $q_y (=0$ or $\pi$) denote the
longitudinal and transverse wave vector, respectively, and $\omega$ is the
frequency. We have found the notable fact that the repulsive intrachain
interaction results in the clear enhancement of ${\rm{Im}}\chi^R_\p
(q_x,\pi;\omega)$ and the suppression of ${\rm{Im}}\chi^R_\rho
(q_x,\pi;\omega)$ at low energies. This result indicates the importance of the
dynamical effect by the spin fluctuation with $q_y = \pi$ and small $\omega$,
which has a possibility to give rise to the attractive interaction for the
electron pairing.


title:
Structure and Strength of Dislocation Junctions: An Atomic Level
  Analysis

abstract:
  The quasicontinuum method is used to simulate three-dimensional
Lomer-Cottrell junctions both in the absence and in the presence of an applied
stress. The simulations show that this type of junction is destroyed by an
unzipping mechanism in which the dislocations that form the junction are
gradually pulled apart along the junction segment. The calculated critical
stress needed for breaking the junction is comparable to that predicted by line
tension models. The simulations also demonstrate a strong influence of the
initial dislocation line directions on the breaking mechanism, an effect that
is neglected in the macroscopic treatment of the hardening effect of junctions.


title: A model for anomalous directed percolation

abstract:
  We introduce a model for the spreading of epidemics by long-range infections
and investigate the critical behaviour at the spreading transition. The model
generalizes directed bond percolation and is characterized by a probability
distribution for long-range infections which decays in $d$ spatial dimensions
as $1/r^{d+\sigma}$. Extensive numerical simulations are performed in order to
determine the density exponent $\beta$ and the correlation length exponents
$\nu_{||}$ and $\nu_\perp$ for various values of $\sigma$. We observe that
these exponents vary continuously with $\sigma$, in agreement with recent
field-theoretic predictions. We also study a model for pairwise annihilation of
particles with algebraically distributed long-range interactions.


title: Adatom Diffusion at GaN (0001) and (000bar1) Surfaces

abstract:
  The diffusion of Ga and N adatoms has been studied for the technologically
relevant wurtzite (000bar1) and (0001) surfaces employing density-functional
theory. Our calculations reveal a very different diffusivity for Ga and N
adatoms on the equilibrium surfaces: While Ga is very mobile at typical growth
temperatures, the diffusion of N is by orders of magnitudes slower. These
results give a very detailed insight of how and under which growth conditions N
adatoms can be stabilized and efficiently incorporated at the surface. We
further find that the presence of excess N strongly increases the Ga diffusion
barrier and discuss the consequences for the growth of GaN.


title: Self-Similar Interpolation in Quantum Mechanics

abstract:
  An approach is developed for constructing simple analytical formulae
accurately approximating solutions to eigenvalue problems of quantum mechanics.
This approach is based on self-similar approximation theory. In order to derive
interpolation formulae valid in the whole range of parameters of considered
physical quantities, the self-similar renormalization procedure is complimented
here by boundary conditions which define control functions guaranteeing correct
asymptotic behaviour in the vicinity of boundary points. To emphasize the
generality of the approach, it is illustrated by different problems that are
typical for quantum mechanics, such as anharmonic oscillators, double-well
potentials, and quasiresonance models with quasistationary states. In addition,
the nonlinear Schr\"odinger equation is considered, for which both eigenvalues
and wave functions are constructed.


title: Exactly solvable Kondo problem in the open $t-J$ chains

abstract:
  We study the problem of a boundary magnetic impurity coupled with the
solvable $t-J$ chains. Our model provides a good start point to understand the
Kondo problem in a Luttinger liquid as well as in a strongly correlated host.
The Kondo coupling constant $J$ can take arbitrary values, which allows us to
study the ferromagnetic and antiferromagnetic Kondo problems simultaneously. It
is shown that both the Kondo coupling and the scalar potential effectively
reconcile the spin dynamics at low temperatures. Generally, the impurity spin
is split into two ghost spins via coupling effect. The residual entropy, which
can be exactly derived for the present model, is strongly
interaction-dependent. This hints the local spin configuration is very
complicated and very different from that in the conventional Kondo problem. An
unscreening phenomenon in the antiferromagnetic regime is found, which reveals
the impurity potential plays an important role for the Kondo problem in a
strongly correlated host. The low temperature specific heat is calculated in
the framework of local Landau-Luttinger liquid theory.


title:
Spin Dynamics in the Magnetic Chains Arrays of Sr14Cu24O41: a Neutron
  Inelastic scattering Investigation

abstract:
  Below about 150 K, the spin arrangement in the chain arrays of Sr14Cu24O41 is
shown to develop in two dimensions (2D). Both the correlations and the
dispersion of the observed elementary excitations agree well with a model of
interacting dimers. Along the chains, the intra- and inter-dimer distances are
equal to 2 and about 3 times the distance (c) between neighboring Cu ions.
While the intra-dimer coupling is J about 10 meV, the inter-dimer couplings
along and between the chains are of comparable strenght, J// about -1.1 meV and
Jperp about 1.7 meV, respectively. This remarkable 2D arrangement satisfies the
formal Cu valence of the undoped compound. Our data suggest also that it is
associated with a relative sliding of one chain with respect to the next one,
which, as T decreases, develops in the chain direction. A qualitative analysis
shows that nearest inter-dimer spin correlations are ferromagnetic, which, in
such a 2D structure, could well result from frustration effects.


title:
Individual scatterers as microscopic origin of equilibration between
  spin- polarized edge channels in the quantum Hall regime

abstract:
  The equilibration length between spin-polarized edge states in the Quantum
Hall regime is measured as a function of a gate voltage applied to an electrode
on top of the edge channels. Reproducible fluctuations in the coupling are
observed and interpreted as a mesoscopic fingerprint of single spin-flip
scatterers which are turned on and off. A model to analyze macroscopic edge
state coupling in terms of individual scatterers is developed, and
characteristic values for these scatterers in our samples are extracted. For
all samples investigated, the distance between spin-flip scatterers lies
between the Drude and the quantum scattering length.


title:
Nonequilibrium phase transition in the kinetic Ising model: Existence of
  tricritical point and stochastic resonance

abstract:
  The dynamic phase transition has been studied in the two dimensional kinetic
Ising model in presence of a time varying (sinusoidal) magnetic field by Monte
Carlo simulation. The nature (continuous or discontinuous) of the transition is
characterized by studying the distribution of the order parameter and the
temperature variation of the fourth order cumulant. For the higher values of
the field amplitude, the transition observed is discontinuous and it is
continuous for lower values of the field amplitude, indicating the existence of
a tricritical point (separating the nature of the transition) on the phase
boundary. The transition is observed to be a manifestation of stochastic
resonance.


title:
Information about the Integer Quantum Hall Transition Extracted from the
  Autocorrelation Function of Spectral Determinants

abstract:
  The Autocorrelation function of spectral determinants (ASD) is used to probe
the sensitivity of a two-dimensional disordered electron gas to the system's
size L.
  For weak magnetic fields ASD is shown to depend only trivially on L, which is
a strong indication that all states are localized.
  From nontrivial dependence of ASD on L for infinite L at a Hall conductance
of 1/2 e^2/h we deduce the existence of critical wave functions at this point,
as long as the disorder strength does not exceed a critical value.


title: Spin gap in low-dimensional Mott insulators with orbital degeneracy

abstract:
  We consider the exchange Hamiltonian H_ST = -J Sum_{<rr'>} (2 S_r S_r' - 1/2)
(2 T_r T_r' - 1/2) describing two isotropic spin-1/2 Heisenberg
antiferromagnets coupled by a quartic term on equivalent bonds. The model is
relevant for systems with orbital degeneracy and strong electron-vibron
coupling in the large Hubbard repulsion limit. To investigate the ground state
properties we use a Green's Function Monte Carlo, calculating energy gaps and
correlation functions, the latter through the forward walking technique. In one
dimension we find that the ground state is a ``crystal'' of valence bond
dimers. In two dimensions, the spin gap appears to remain finite in the
thermodynamic limit, and, consistently, the staggered magnetization -- signal
of Neel long range order -- seems to vanish. From the analysis of dimer-dimer
correlation functions, however, we find no sign of a valence bond crystal. A
spin liquid appears as a plausible scenario compatible with our findings.


title: Classical behavior of a macroscopic Schroedinger cat

abstract:
  We study the dynamics of classical and quantum systems linearly interacting
with a classical environment represented by an infinite set of harmonic
oscillators. The environment induces a dynamical localization of the quantum
state into a generalized coherent state for which the $\hbar \to 0$ limit
always exists and reproduces the classical motion. We describe the consequences
of this localization on the behavior of a macroscopic system by considering the
example of a Schroedinger cat.


title:
Kink-antikink interactions in the double sine-Gordon equation and the
  problem of resonance frequencies

abstract:
  We studied the kink-antikink collision process for the "double sine-Gordon"
(DSG) equation in 1+1 dimensions at different values of the potential parameter
$R>0$. For small values of $R$ we discuss the problem of resonance frequencies.
We give qualitative explanation of the frequency shift in comparison with the
frequency of the discrete level in the potential well of isolated kink. We show
that in this region of the parameter $R$ the effective long-range interaction
between kink and antikink takes place.