title:
Random Field and Random Anisotropy Effects in Defect-Free
  Three-Dimensional XY Models

abstract:
  Monte Carlo simulations have been used to study a vortex-free XY ferromagnet
with a random field or a random anisotropy on simple cubic lattices. In the
random field case, which can be related to a charge-density wave pinned by
random point defects, it is found that long-range order is destroyed even for
weak randomness. In the random anisotropy case, which can be related to a
randomly pinned spin-density wave, the long-range order is not destroyed and
the correlation length is finite. In both cases there are many local minima of
the free energy separated by high entropy barriers. Our results for the random
field case are consistent with the existence of a Bragg glass phase of the type
discussed by Emig, Bogner and Nattermann.


title: Exactly solvable two-dimensional quantum spin models

abstract:
  A method is proposed for constructing an exact ground-state wave function of
a two-dimensional model with spin 1/2. The basis of the method is to represent
the wave function by a product of fourth-rank spinors associated with the sites
of a lattice and the metric spinors corresponding to bonds between nearest
neighbor sites. The function so constructed is an exact wave function of a
14-parameter model. The special case of this model depending on one parameter
is analyzed in detail. The ground state is always a nondegenerate singlet, and
the spin correlation functions decay exponentially with distance. The method
can be generalized for models with spin 1/2 to other types of lattices.


title: Kinetics of moisture absorption in mixtures for iron foundry

abstract:
  The moisture absorption in granulated materials used in foundry technologies
is analyzed. The absorption process has a diffusive behavior mainly. A simple
experimental technique, in which the wet weight increment was recorded as the
experimental parameter and an analytic method with computing procedure to find
the parameters characterizing the process was used. The determination of these
parameters by traditional methods is a very difficult task so, very refined and
expensive trials are needed. The fitting of the model permits to determine the
diffusion coefficient and the moisture concentration in the separation surface
between the sample and the environment. The concentration profiles are
established for different times. Finally, the possibility of occurrence of
superimposed diffusive processes in some materials is analyzed and the
diffusion coefficient and the amount of moisture incorporated by each process
are calculated.


title:
Dynamical Monte Carlo Study of Equilibrium Polymers: Effects of High
  Density and Ring Formation

abstract:
  An off-lattice Monte Carlo algorithm for solutions of equilibrium polymers
(EP) is proposed. At low and moderate densities this is shown to reproduce
faithfully the (static) properties found recently for flexible linear EP using
a lattice model. The molecular weight distribution (MWD) is well described in
the dilute limit by a Schultz-Zimm distribution and becomes purely exponential
in the semi-dilute limit.
  Additionally, very concentrated molten systems are studied. The MWD remains a
pure exponential in contrast to recent claims. The mean chain mass is found to
increase faster with density than in the semi-dilute regime due to additional
entropic interactions generated by the dense packing of spheres.
  We also consider systems in which the formation of rings is allowed so that
both the linear chains and the rings compete for the monomers. In agreement
with earlier predictions the MWD of the rings reveals a strong singularity
whereas the MWD of the coexisting linear chains remains essentially unaffected.


title:
Generalized Boltzmann equation for a trapped Bose-condensed gas using
  the Kadanoff-Baym formalism

abstract:
  Using the Kadanoff-Baym non-equilibrium Green's function formalism, we derive
kinetic equations for the non-condensate atoms at finite temperatures which
include the effect of binary collisions between atoms. The effect of collisions
is included using the second-order self-energy given by the Beliaev (gapless)
approximation. We limit our discussion to finite temperatures where we can use
the single-particle Hartree-Fock spectrum for the excited atoms. In this limit,
we can neglect the off-diagonal propagators ($\tilde{g}_{12}$ and
$\tilde{g}_{21}$). As expected, this leads to the kinetic equations and
collision integrals used in recent work by Zaremba, Nikuni, and Griffin (ZNG)
[1]. We also derive a consistent equation of motion for the condensate
wavefunction, involving a finite-temperature generalization of the well-known
Gross-Pitaevskii equation which includes a dissipative term, as well as the
mean field of the non-condensate.


title:
Monte Carlo Study of the Anisotropic Heisenberg Antiferromagnet on the
  Triangular Lattice

abstract:
  We report a Monte Carlo study of the classical antiferromagnetic Heisenberg
model with easy axis anisotropy on the triangular lattice. Both the free energy
cost for long wavelength spin waves as well as for the formation of free
vortices are obtained from the spin stiffness and vorticity modulus
respectively. Evidence for two distinct Kosterlitz-Thouless types of
defect-mediated phase transitions at finite temperatures is presented.


title:
The $E\otimes e$ Jahn-Teller Polaron in Comparison with the Holstein
  Polaron

abstract:
  Based on an exact expression for the self-energy of the Jahn-Teller polaron,
we find that symmetry of pseudospin rotation makes the vertex correction much
less effective than that for the Holstein polaron. This ineffectiveness brings
about a smaller effective mass m^* and a quantitatively differenent
large-to-small polaron crossover, as examined by exact diagonalization in a
two-site system. In the strong-coupling and antiadiabatic region, a rigorous
analytic expression is found for m^*.


title: The Kondo effect in periodic narrow-band systems

abstract:
  The Kondo divergences owing to interaction of current carriers with local
moments in highly correlated electron systems are considered within the Hubbard
and s-d exchange models with infinitely strong on-site interaction, the
many-electron Hubbard representation being used. The picture of density of
states containing a peak at the Fermi level is obtained. Various forms of the
self-consistent approximation are used. The problem of the violation of
analytical properties of the Green's function is discussed. Smearing of the
"Kondo" peak owing to spin dynamics and finite temperatures is investigated.


title:
A Combinatorial Interpretation of the Free Fermion Condition of the
  Six-Vertex Model

abstract:
  The free fermion condition of the six-vertex model provides a 5 parameter
sub-manifold on which the Bethe Ansatz equations for the wavenumbers that enter
into the eigenfunctions of the transfer matrices of the model decouple, hence
allowing explicit solutions. Such conditions arose originally in early
field-theoretic S-matrix approaches. Here we provide a combinatorial
explanation for the condition in terms of a generalised Gessel-Viennot
involution. By doing so we extend the use of the Gessel-Viennot theorem,
originally devised for non-intersecting walks only, to a special weighted type
of \emph{intersecting} walk, and hence express the partition function of $N$
such walks starting and finishing at fixed endpoints in terms of the single
walk partition functions.


title:
Equations of state for solids at high pressures and temperatures from
  shock-wave data

abstract:
  This paper deals with the analytic derivation of a complete equation of state
(EOS) for solids from shock-wave data in the range of pressures and
temperatures, attained by detonation of chemical explosives. The caloric, the
thermal and the incomplete EOS are determined as well. An equation for the
shock temperature along the Hugoniot curve is derived also.


title:
Spatial evolutionary prisoner's dilemma game with three strategies and
  external constraints

abstract:
  The emergence of mutual cooperation is studied in a spatially extended
evolutionary prisoner's dilemma game in which the players are located on the
sites of cubic lattices for dimensions d=1, 2, and 3. Each player can choose
one of the three strategies: cooperation (C), defection (D) or Tit for Tat (T).
During the evolutionary process the randomly chosen players adopt one of their
neighboring strategies if the chosen neighbor has higher payoff. Morover, an
external constraint imposes that the players always cooperate with probability
p. The stationary state phase diagram are computed by both using generalized
mean-field approximations and Monte Carlo simulations. Nonequilibrium second
order phase transitions assosiated with the extinction of one of the possible
strategies are found and the corresponding critical exponents belong to the
directed percolation universality class. It is shown that forcing externally
the collaboration does not always produce the desired result.


title:
Transport properties and phase diagram of the disordered lattice
  vibration model

abstract:
  We study the transport and localization properties of scalar vibrations on a
lattice with random bond strength by means of the transfer matrix method. This
model has been recently suggested as a means to investigate the vibrations and
heat conduction properties of structural glasses. In three dimensions we find a
very rich phase diagram. The delocalization transition is split, so that
between the localized and diffusive phases which have been identified in the
Anderson problem, we observe a phase with anomalous, sub-exponential
localization. For low frequencies, we find a strongly conducting phase with
ballistic and super-diffusive transport, reflecting a diverging diffusivity.
The last phase generates an anomalous heat conductivity which grows with the
system size. These phases are the counterparts of those identified in an
earlier study of the normal modes.


title: Metal-insulator transition in system with topological disorder

abstract:
  Metal-insulator transition in anisotropic disordered Anderson model with both
topological and diagonal disorder is investigated numerically. For four sets of
the model parameters we found the critical disorder and the critical exponent
and prove that they do not depend on the transport direction. The system size
independent critical conductance distribution P_c(g) is found for each critical
point. The form of P_c(g) depends on the model parameters and on the direction
of the treansport. Arguments in favor of the one-parameter scaling are
presented.
  This paper has been withdrawn


title:
2D scaling approach for the first order transition in
  Bi_2Sr_2CaCu_2O_{8+y}

abstract:
  We analyze recent measurements by Ooi et al. of the angular dependence of the
first order vortex phase transition in Bi_2Sr_2CaCu_2O_{8+y}. The experimental
findings of these authors are interpreted in terms of a 2D versus 3D scaling
approach. Invoking a crossover from a 3D to a 2D scaling approach, it shown,
that the apparent failure of a 3D scaling approach away from the bulk
transition temperature is due to a crossover to decoupled independent
superconducting slabs.


title: The non-extensive version of the Kolmogorov-Sinai entropy at work

abstract:
  We address the problem of applying the Kolmogorov-Sinai method of entropic
analysis, expressed in a generalized non-extensive form, to the dynamics of the
logistic map at the chaotic threshold, which is known to be characterized by a
power law rather than exponential sensitivity to initial conditions. The
computer treatment is made difficult, if not impossible, by the multifractal
nature of the natural invariant distribution: Thus the statistical average is
carried out on the power index. The resulting entropy time evolution becomes a
smooth and linear function of time with the non-extensive index Q < 1
prescribed by the heuristic arguments of earlier work, thereby showing how to
make the correct entropic prediction in the spirit of the single-trajectory
approach of Kolmogorov.


title: Re-entrant spin susceptibility of a superconducting grain

abstract:
  We study the spin susceptibility chi of a small, isolated superconducting
grain. Due to the interplay between parity effects and pairing correlations,
the dependence of chi on temperature T is qualitatively different from the
standard BCS result valid in the bulk limit. If the number of electrons on the
grain is odd, chi shows a re-entrant behavior as a function of temperature.
This behavior persists even in the case of ultrasmall grains where the mean
level spacing is much larger than the BCS gap. If the number of electrons is
even, chi(T) is exponentially small at low temperatures.


title:
Anomaly of AC resistance in magnetic nanoparticle alloys at
  spin-glass-like transition

abstract:
  (withdrawn) A combined study of magnetic susceptibility and AC resistance was
performed on melt-spun Cu-Co granular magnetic ribbons. The AC resistance as a
function of temperature has a sharp maximum. We associate it with a diverging
correlation length at the temperature of collective freezing of magnetic
moments via increasing magnetic losses in the induced non-uniform field.
Application of this model to the experimental data allows a direct
determination of the critical exponent of correlation length on both sides of
the transition. Giant AC magnetoresistance is observed at the freezing
temperature.


title:
Magnetic circular dichroism in X-ray fluorescence of Heusler alloys at
  threshold excitation

abstract:
  The results of fluorescence measurements of magnetic circular dichroism (MCD)
in Mn L_2,L_3 X-ray emission and absorption for Heusler alloys NiMnSb and
Co2MnSb are presented. Very intense resonance Mn L_3 emission is found at the
Mn 2p_3/2 threshold and is attributed to a peculiarity of the threshold
excitation in materials with the half-metallic character of the electronic
structure. A theoretical model for the description of resonance scattering of
polarized x-rays is suggested.


title:
Self-organized hierarchical structure in a plastic network of chaotic
  units

abstract:
  Formation of a layered structure is studied in a globally coupled map of
chaotic units with a plastic coupling strength that changes depending on the
states of units globally and an external input. In the parameter region
characterized by weakly chaotic and desynchronized dynamics, units
spontaneously form a hierarchical structure due to the influence of the input,
with regards to the coupling strength. This hierarchical structure is not fixed
in time, and is successively reorganized. It is found that the distribution of
lifetimes of the structure obeys a power law. The possible relevance of the
present result to information processing in the brain is briefly discussed.

categories: cond-mat.dis-nn chao-dyn cond-mat.stat-mech nlin.CD


title:
Kinetics of stochastically-gated diffusion-limited reactions and
  geometry of random walk trajectories

abstract:
  In this paper we study the kinetics of diffusion-limited, pseudo-first-order
A + B -> B reactions in situations in which the particles' intrinsic
reactivities vary randomly in time. That is, we suppose that the particles are
bearing "gates" which interchange randomly and independently of each other
between two states - an active state, when the reaction may take place, and a
blocked state, when the reaction is completly inhibited. We consider four
different models, such that the A particle can be either mobile or immobile,
gated or ungated, as well as ungated or gated B particles can be fixed at
random positions or move randomly. All models are formulated on a
$d$-dimensional regular lattice and we suppose that the mobile species perform
independent, homogeneous, discrete-time lattice random walks. The model
involving a single, immobile, ungated target A and a concentration of mobile,
gated B particles is solved exactly. For the remaining three models we
determine exactly, in form of rigorous lower and upper bounds, the large-N
asymptotical behavior of the A particle survival probability. We also realize
that for all four models studied here such a probalibity can be interpreted as
the moment generating function of some functionals of random walk trajectories,
such as, e.g., the number of self-intersections, the number of sites visited
exactly a given number of times, "residence time" on a random array of lattice
sites and etc. Our results thus apply to the asymptotical behavior of the
corresponding generating functions which has not been known as yet.


title:
Interpretation of Inelastic Neutron Scattering Data Using the Phase
  Diagram of Hole-Doped Cuprates

abstract:
  Inelastic Neutron Scattering (INS) data in LSCO, YBCO and Bi2212 are
discussed. In the literature, the INS spectra remain far from being
comprehensively understood. We show that local (Q-integrated) susceptibility
data and the energy dependence of the spin susceptibility at antiferromagnetic
vector, Q = (p,p), can be interpreted by using the phase diagram for hole-doped
cuprates. We analyze also the origin of the resonant peak which relates to the
order parameter for long-range phase coherence in hole-doped cuprates. Thus, we
present an interpretation of neutron data in LSCO, YBCO and Bi2212 published so
far.


title: BEC and the New World of Coherent Matter Waves

abstract:
  This paper is based on three tutorial lectures given at the CRM Summer School
in Banff, Alberta, June 27-July 10, 1999 on "Theoretical Physics at the End of
the Twentieth Century". They will be published by Springer-Verlag. Lecture I
gives a quick overview of recent studies of BEC in trapped atomic gases and
reviews some properties of a trapped non-interacting Bose gas. Lecture II
mainly discusses the collective modes of a pure condensate, based on the T=0 GP
equation of motion. The Stringari quantum hydrodynamic theory is introduced.
Lecture III discusses the coupled dynamics of the condensate and non-condensate
at finite temperatures. A recent derivation of the two-fluid hydrodynamic
equations is sketched. This theory predicts a new relaxational mode associated
with the fact that the condensate and non-condensate may not be in diffusive
equilibrium.


title: Asymmetric Unimodal Maps: Some Results from q-generalized Bit Cumulants

abstract:
  In this study, using q-generalized bit cumulants (q is the nonextensivity
parameter of the recently introduced Tsallis statistics), we investigate the
asymmetric unimodal maps. The study of the q-generalized second cumulant of
these maps allows us to determine, for the first time, the dependence of the
inflexion paremeter pairs (z_1,z_2) to the nonextensivity parameter q. This
behaviour is found to be very similar to that of the logistic-like maps
(z_1=z_2=z) reported recently by Costa et al. [Phys.Rev.E 56 (1997) 245].


title:
Artifactual log-periodicity in finite size data: Relevance for
  earthquake aftershocks

abstract:
  The recently proposed discrete scale invariance and its associated
log-periodicity are an elaboration of the concept of scale invariance in which
the system is scale invariant only under powers of specific values of the
magnification factor. We report on the discovery of a novel mechanism for such
log-periodicity relying solely on the manipulation of data. This ``synthetic''
scenario for log-periodicity relies on two steps: (1) the fact that
approximately logarithmic sampling in time corresponds to uniform sampling in
the logarithm of time; and (2) a low-pass-filtering step, as occurs in
constructing cumulative functions, in maximum likelihood estimations, and in
de-trending, reddens the noise and, in a finite sample, creates a maximum in
the spectrum leading to a most probable frequency in the logarithm of time. We
explore in detail this mechanism and present extensive numerical simulations.
We use this insight to analyze the 27 best aftershock sequences studied by
Kisslinger and Jones [1991] to search for traces of genuine log-periodic
corrections to Omori's law, which states that the earthquake rate decays
approximately as the inverse of the time since the last main shock. The
observed log-periodicity is shown to almost entirely result from the
``synthetic scenario'' owing to the data analysis. From a statistical point of
view, resolving the issue of the possible existence of log-periodicity in
aftershocks will be very difficult as Omori's law describes a point process
with a uniform sampling in the logarithm of the time. By construction, strong
log-periodic fluctuations are thus created by this logarithmic sampling.


title: Antiferromagnetic Correlations and the Pseudogap in HTS Cuprates

abstract:
  Evidence is presented from NMR 1/T1T and ARPES data for the sudden
disappearance of 2D antiferromagnetic (AF) correlations in the lightly
overdoped region (p=0.19) at the T=0 metal-insulator transition where the
pseudogap energy falls to zero. AF fluctuations thus appear to be intimately
associated with the pseudogap and serve primarily to weaken superconductivity,
strongly reducing the condensation energy and superfluid density.


title:
Factors affecting the Optimal Design of High-Tc Superconductors - the
  Pseudogap and Critical Currents

abstract:
  The impact of the normal-state pseudogap, present in all optimal and
underdoped HTS cuprates, on critical currents and critical temperature is
surveyed. With the opening of the pseudogap around a doping state of p=0.19 the
condensation energy and superfluid density are rapidly suppressed due to
reduction in the normal-state spectral weight. Even by optimal doping (p=0.16)
these measures of the 'strength' of superconductivity are diminished by up to
40%. This results in a sharp reduction in critical currents and irreversibility
field, respectively. The optimal doping state where these properties are
maximised is therefore not at maximum Tc but in the lightly overdoped region
where the pseudogap energy falls to zero at p=0.19. The presence of impurities
and grain boundaries further heightens these effects.


title:
Density Matrix Renormalization Group Study of the S=1/2 Anisotropic
  Antiferromagnetic Heisenberg Chains with Quasiperiodic Exchange Modulation

abstract:
  The low energy behavior of the S=1/2 antiferromagnetic XY-like XXZ chains
with precious mean quasiperiodic exchange modulation is studied by the density
matrix renormalization group method. It is found that the energy gap of the
chain with length N scales as $\exp (-cN^{\omega})$ with nonuniversal exponent
$\omega$ if the Ising component of the exhange coupling is antiferromagnetic.
This behavior is expected to be the characteristic feature of the quantum spin
chains with relevant aperiodicity. This is in contrast to the XY chain for
which the precious mean exchange modulation is marginal and the gap scales as
$N^{-z}$. On the contrary, it is also verified that the energy gap scales as
$N^{-1}$ if the Ising component of the exhange coupling is ferromagnetic. Our
results are not only consistent with the recent bosonization analysis of Vidal,
Mouhanna and Giamarchi but also clarify the nature of the strong coupling
regime which is inaccesssible by the bosonization approach.


title: Effects of Non-local Stress on the Determination of Shear Banding Flow

abstract:
  We analyze the steady planar shear flow of the modified Johnson-Segalman
model, which has an added non-local term. We find that the new term allows for
unambiguous selection of the stress at which two ``phases'' coexist, in
contrast to the original model. For general differential constitutive models we
show the singular nature of stress selection in terms of a saddle connection
between fixed points in the equivalent dynamical system. The result means that
stress selection is unique under most conditions for space non-local models
Finally, illustrated by simple models, we show that stress selection generally
depends on the form of the non-local terms (weak universality).


title: Electronic Transport Properties of Quasicrystalline Thin Films

abstract:
  Quasicrystals are assumed to be electronically stabilized by a Hume-Rothery
type mechanism. This explains most of the peculiar properties of quasicrystals.
The stabilization is investigated by electronic transport properties, as they
depend sensitively on the stabilizing interaction between the static structure
and the conduction electrons. Thin-film techniques provide samples which are
well suited for systematic investigations as a function of composition and
structural quality for Al-Cu-Fe and Al-Pd-Re i-phases. For a narrow range of
composition, large transport anomalies occur, reaching a
metal-insulator-transition in thin films of i-Al-Pd-Re. We discuss this in the
framework of a resonant scattering of the conduction electrons with the
quasicrystalline structure, leading to a reduced electronic mobility and
density of states (DOS) at the Fermi energy $E_F$.


title: Correlated Electrons in Carbon Nanotubes

abstract:
  Single-wall carbon nanotubes are almost ideal systems for the investigation
of exotic many-body effects due to non-Fermi liquid behavior of interacting
electrons in one dimension. Recent theoretical and experimental results are
reviewed with a focus on electron correlations. Starting from a microscopic
lattice model we derive an effective phase Hamiltonian for conducting
single-wall nanotubes with arbitrary chirality. The parameters of the
Hamiltonian show very weak dependence on the chiral angle, which makes the
low-energy physics of conducting nanotubes universal. The temperature-dependent
resistivity and frequency-dependent optical conductivity of nanotubes with
impurities are evaluated within the Luttinger-like model. Localization effects
are studied. In particular, we found that intra-valley and inter-valley
electron scattering can not coexist at low energies. Low-energy properties of
clean nanotubes are studied beyond the Luttinger liquid approximation. The
strongest Mott-like electron instability occurs at half filling. In the Mott
insulating phase electrons at different atomic sublattices form characteristic
bound states. The energy gaps of $0.01-0.1$ eV occur in all modes of elementary
excitations. We finally discuss observability of the Mott insulating phase in
transport experiments. The accent is made on the charge transfer from external
electrodes which results in a deviation of the electron density from
half-filling.


title: The Values Distribution in a Competing Shares Financial Market Model

abstract:
  We present our competing shares financial market model and describe its
behaviour by numerical simulation. We show that in the critical region the
distribution avalanches of the market value as defined in this model has a
power-law distribution with exponent around 2.3. In this region the price
returns distribution is truncated Levy stable with exponent near the observed
value.


title: Kadanoff-Baym Equations with Initial Correlations

abstract:
  The Kadanoff-Baym equations (KBE) are usually derived under the assumption of
the weakening of initial correlations (Bogolyubov's condition) and, therefore,
fail to correctly describe the short time behavior. We demonstrate that this
assumption is not necessary. Using functional derivatives techniques, we
present a straightforward generalization of the KBE which allows to include
arbitrary initial correlations and which is more general than previous
derivations. As a result, an additional collision integral is obtained which is
being damped out after a few collisions. Our results are complemented with
numerical investigations showing the effect of initial correlations.


title:
Influence of surface irregularities on barriers for vortex entry in
  type-II superconductors

abstract:
  A theoretical description of the influence of surface irregularities (such as
wedge-like cracks) on the Bean-Livingston energy barrier is presented. A
careful quantitative estimate of the field of the first vortex entry H* into a
homogeneous bulk superconductor with crack-like defects is obtained. This
estimate is based on an exact analytical solution for the Meissner current
distribution in a bulk superconductor with a single two-dimensional infinitely
thin crack. This thin crack is characterized by the lowest H* value, and, thus,
appears to be the best gate for vortex entry.


title: Critical Effects at 3D Wedge-Wetting

abstract:
  We show that continuous filling or wedge-wetting transitions are possible in
3D wedge-geometries made from (angled) substrates exhibiting first-order
wetting transitions and develop a comprehensive fluctuation theory yielding a
complete classification of the critical behaviour. Our fluctuation theory is
based on the derivation of a Ginzburg criterion for filling and also an exact
transfer-matrix analysis of a novel effective Hamiltonian which we propose as a
model for wedge fluctuation effects. The influence of interfacial fluctuations
is shown to be very strong and, in particular, leads to a remarkable universal
divergence of the interfacial roughness $\xi_{\perp}\sim (T_F-T)^{-1/4}$ on
approaching the filling temperature $T_F$, valid for all possible types of
intermolecular forces.


title:
Persistent currents and magnetic flux trapping in fragments of carbon
  deposits containing multiwalled nanotubes

abstract:
  It is found that the magnetization curves of samples of fragments of cathode
carbon deposits with a high content of multiwalled nanotubes exhibit a
pronounced irreversible character, attesting to the induction of persistent
currents in the samples and to magnetic flux trapping, as happens in a multiply
connected superconducting structure. A decrease of the trapped flux in time
could not be observed at low (helium) temperatures with a measurement time of
about 20 h. For intermediate (~30K) and room temperatures the trapped magnetic
flux decays slowly with characteristic relaxation times of the order of 150 and
15 h, respectively.


title:
Non-ohmic hopping transport in a-YSi: from isotropic to directed
  percolation

abstract:
  Variable range hopping transport has been investigated in amorphous YSi from
30 mK to 2 K as a function of the temperature T and electric field F. For F=0,
where conduction is along sinuous paths (isotropic percolation), the
conductance G depends on T according to an Efros-Shklovskii law. The nonlinear
I-V's were studied up to very high fields for which G no longer depends on T
(directed percolation : current paths along F). We show that heating effects
are negligible. Then, we show that for low F values,
ln(G(F,T))=ln(G(0,T))-eFL/kT. The order of magnitude (5-10 nm) and the T
dependence of L agree with theoretical predictions. From L, we extract the
dielectric constant. The very high electric field data do not agree with the
theoretical predictions, which could be due to tunneling across the mobility
edge. For intermediate electric fields our data evidence the influence of the
straightening of the paths (transition from isotropic to directed percolation).
The statistical properties of the segments of the paths where the current flows
against the electrical force are extracted for the first time.


title:
Orbital Ordering in the Manganites :- Resonant X-ray Scattering
  Predictions at the Manganese L_II and L_III Edges

abstract:
  It is proposed that the observation of orbital ordering in manganite
materials should be possible at the L_II and L_III edges of manganese using
X-ray resonant scattering. If performed, dipole selection rules would make the
measurements much more direct than the disputed observations at the manganese K
edge. They would yield specific information about the type and mechanism of the
ordering not available at the K edge, as well as permitting the effects of
orbital ordering and Jahn-Teller ordering to be detected and distinguished from
one another. Predictions are presented based on atomic multiplet calculations,
indicating distinctive dependence on energy, as well as on polarization and on
the azimuthal angle around the scattering vector.


title: Analyzing Fragmentation of Simple Fluids with Percolation Theory

abstract:
  We show that the size distributions of fragments created by high energy
nuclear collisions are remarkably well reproduced within the framework of a
parameter free percolation model. We discuss two possible scenarios to explain
this agreement and suggest that percolation could be an universal mechanism to
explain the fragmentation of simple fluids.


title:
Comment II on J.M. Salazar and L. Brenig's article: ``Computer
  Simulations and Kinetic Theory of an Inelastic Granular Gas''

abstract:
  In two recent articles Salazar and Brenig question the validity of kinetic
theory for granular gases and fluids, on the based of a supposedly exact
hierarchy of coupled equations for the velocity moments, which the authors
derive from the BBGKY-hierarchy. Their derivation contains several errors,
which are exposed. Moreover, they support their findings with results from
direct monte carlo simulation (DSMC) of the Boltzmann equation, which
supposedly show that Haff's homogeneous cooling law only holds for times
shorter than one mean free time. However, their DSMC results have no physical
significance as they are carried out at a density, twice close packing.


title:
Annular Long Josephson Junctions in a Magnetic Field: Engineering and
  Probing the Fluxon Interaction Potential

abstract:
  The interaction of a Josephson fluxon with an external magnetic field-induced
potential in a long Josephson junction is investigated experimentally. The
thermal activation of the fluxon from a potential well is observed and
experiments probing its predicted quantum properties are discussed. A method
for engineering a magnetic double-well potential for a fluxon is proposed and
the use of the coupled fluxon states for quantum computation is suggested.


title: Exact singlet bond ground states for electronic models

abstract:
  We have proposed several 1D and 2D electronic models with the exact ground
state. The ground state wave function of these models is represented in terms
of "singlet bond" functions consisting of homopolar and ionic configurations.
The Hamiltonians of the models include the correlated hopping of electrons,
pair hopping terms and the spin interactions. One of the 1D models demonstrates
spontaneous breaking of the translational symmetry. We studied also the model
describing the transition point between the phases with and without an
off-diagonal long-range order.


title:
Evaporation of the pancake-vortex lattice in weakly-coupled layered
  superconductors

abstract:
  We calculate the melting line of the pancake-vortex system in a layered
superconductor, interpolating between two-dimensional (2D) melting at high
fields and the zero-field limit of single-stack evaporation. Long-range
interactions between pancake vortices in different layers permit a mean-field
approach, the ``substrate model'', where each 2D crystal fluctuates in a
substrate potential due to the vortices in other layers. We find the thermal
stability limit of the 3D solid, and compare the free energy to a 2D liquid to
determine the first-order melting transition and its jump in entropy.


title:
Acoustic Cyclotron Resonance and Giant High Frequency Magnetoacoustic
  Oscillations in Metals with Locally Flattened Fermi Surface

abstract:
  We consider the effect of local flattening on the Fermi surface (FS) of a
metal upon geometric oscillations of the velocity and attenuation of ultrasonic
waves in the neighborhood of the acoustic cyclotron resonance. It is shown that
such peculiarities of the local geometry of the FS can lead to a significant
enhancement of both cyclotron resonance and geometric oscillations.
Characteristic features of the coupling of ultrasound to shortwave cyclotron
waves arising due to the local flattening of the FS are analyzed.
  PACS numbers 71.18.+y; 72.15.Gd; 72.15.-v


title: A Kondo impurity in a disordered metal: Anderson's theorem revisited

abstract:
  We consider a local moment which is coupled by a non-random Kondo $J$ to a
band of conduction electrons in a random potential. We prove an analog of
Anderson's theorem in a large-N limit of this model. The theorem states that
when the disorder is weak, the disorder-averaged low-temperature thermodynamics
is independent of the strength of the disorder; remarkably, it further states
that fluctuation effects in the long-time limit are {\it independent even of
the realization of the disorder}. We discuss the relationship of this theorem
to theoretical and experimental studies of similar problems.


title:
The strongly enhanced magnetic excitations near the quantum critical
  point of Cr$_{1-x}$V$_x$ and why strong exchange enhancement need not imply
  heavy fermion behavior

abstract:
  Inelastic neutron scattering reveals strong spin fluctuations with energies
as high as 0.4eV in the nearly antiferromagnetic metal Cr$_{0.95}$V$_{0.05}$.
The magnetic response is well described by a modified Millis-Monien-Pines
function. From the low-energy response, we deduce a large exchange enhancement,
more than an order of magnitude larger than the corresponding enhancement of
the low-temperature electronic heat capacity $\gamma T$. A scaling relationship
between $\gamma$ and the inverse of the wavevector- averaged spin relaxation
rate $\Gamma_{\text{ave}}$ is demonstrated for a number of magnetically
correlated metals


title:
Non-Self Averaging in Autocorrelations for Potts Models on Quenched
  Random Gravity Graphs

abstract:
  We investigate the non-self-averaging properties of the dynamics of Ising,
4-state Potts and 10-state Potts models in single-cluster Monte Carlo
simulations on quenched ensembles of planar, trivalent Phi3 random graphs,
which we use as an example of relevant quenched connectivity disorder.
  We employ a novel application of scaling techniques to the cumulative
probability distribution of the autocorrelation times for both the energy and
magnetisation in order to discern non-self-averaging. Although the specific
results discussed here are for quenched random graphs, the method has quite
general applicability.


title:
New Evidence of Earthquake Precursory Phenomena in the 17 Jan. 1995 Kobe
  Earthquake, Japan

abstract:
  Significant advances, both in the theoretical understanding of rupture
processes in heterogeneous media and in the methodology for characterizing
critical behavior, allows us to reanalyze the evidence for criticality and
especially log-periodicity in the previously reported chemical anomalies that
preceded the Kobe earthquake. The ion ($Cl^-$, $K^+$, $Mg^{++}$, $NO_3^{-}$ and
$SO_4^{--}$) concentrations of ground-water issued from deep wells located near
the epicenter of the 1995 Kobe earthquake are taken as proxies for the
cumulative damage preceding the earthquake. Using both a parametric and
non-parametric analysis, the five data sets are compared extensively to
synthetic time series. The null-hypothesis that the patterns documented on
these times series result from noise decorating a simple power law is rejected
with a very high confidence level.


title: Coherent vs incoherent pairing in 2D systems near magnetic instability

abstract:
  We study the superconductivity in 2D fermionic systems near antiferromagnetic
instability, assuming that the pairing is mediated by spin fluctuations. This
pairing involves fully incoherent fermions and diffusive spin excitations. We
show that the competition between fermionic incoherence and strong pairing
interaction yields the pairing instability temperature $T_{ins}$ which
increases and saturates as the magnetic correlation length $\xi \to \infty$. We
argue that in this quantum-critical regime the pairing problem is qualitatively
different from the BCS one.


title:
Photoemission Spectral Weight Transfer and Mass Renormalization in the
  Fermi-Liquid System La$_{1-x}$Sr$_x$TiO$_{3+y/2}$

abstract:
  We have performed a photoemission study of La$_{1-x}$Sr$_x$TiO$_{3+y/2}$ near
the filling-control metal-insulator transition (MIT) as a function of hole
doping. Mass renormalization deduced from the spectral weight and the width of
the quasi-particle band around the chemical potential $\mu$ is compared with
that deduced from the electronic specific heat. The result implies that, near
the MIT, band narrowing occurs strongly in the vicinity of $\mu$. Spectral
weight transfer occurs from the coherent to the incoherent parts upon
antiferromagnetic ordering, which we associate with the partial gap opening at
$\mu$.


title:
Spin correlation functions and susceptibilities in the easy-plane XXZ
  chain

abstract:
  We present a Green's-function theory of magnetic short-range order in the
$S=1/2$ easy-plane XXZ chain based on the projection method for the dynamic
spin susceptibility and a decoupling of three-spin operator products
introducing vertex parameters. The longitudinal and transverse static
susceptibilities and two-point correlation functions of arbitrary range are
calculated self-consistently for all wavenumbers, temperatures, and anisotropy
parameters $ -1\leq \Delta \leq 1$. In the easy-plane ferromagnetic region
$(\Delta < 0)$, the longitudinal correlators of spins at distance $n$ change
sign at a finite temperature $T_0(n,\Delta)$, in reasonable agreement with
recent data obtained by finite-chain diagonalizations. The temperature
dependence of the uniform static susceptibilities exhibits a maximum which is
explained as an effect of magnetic short-range order which decreases with
increasing temperature.


title:
Explanation of Giant Cluster Coexistence in Doped Manganites and Other
  Compounds

abstract:
  Computational studies show the generation of large coexisting metallic and
insulating clusters with equal electronic density in models for manganites. The
clusters are induced by disorder on exchange and hopping amplitudes near
first-order transitions of the non-disordered strongly coupled system. The
random-field Ising model is used to explain the qualitative aspects of our
results. Percolative characteristics are natural in this context. Our results
explain the recently experimentally discovered micrometer size inhomogeneities
in manganites. The conclusions are general and apply to a variety of compounds.


title: Monte Carlo Simulation of the Three-dimensional Ising Spin Glass

abstract:
  We study the 3D Edwards-Anderson model with binary interactions by Monte
Carlo simulations. Direct evidence of finite-size scaling is provided, and the
universal finite-size scaling functions are determined. Using an iterative
extrapolation procedure, Monte Carlo data are extrapolated to infinite volume
up to correlation length \xi = 140. The infinite volume data are consistent
with both a continuous phase transition at finite temperature and an essential
singularity at finite temperature. An essential singularity at zero temperature
is excluded.


title:
A possible role of D^- band in hopping conductivity and metal-insulator
  transition in 2D structures

abstract:
  A simple two-band model is suggested explaining recently reported unusual
features for hopping magnetoresistance and the metal-insulator transition in 2D
structures. The model implies that the conductivity is dominated by the upper
Hubbard band (D^- band). Experimental studies of hopping magnetoresistance for
Si delta doped GaAs/AlGaAs heterostructure give additional evidences for the
model.


title: Low temperature ellipsometry of NaV2O5

abstract:
  The dielectric function of alpha'NaV2O5 was measured with electric field
along the a and b axes in the photon energy range 0.8-4.5 eV for temperatures
down to 4K. We observe a pronounced decrease of the intensity of the 1 eV peak
upon increasing temperature with an activation energy of about 25meV,
indicating that a finite fraction of the rungs becomes occupied with two
electrons while others are emptied as temperature increases. No appreciable
shifts of peaks were found s in the valence state of individual V atoms at the
phase transition is very small. A remarkable inflection of this temperature
dependence at the phase transition at 34 K indicates that charge ordering is
associated with the low temperature phase.


title:
The effective potential of N-vector models: a field-theoretic study to
  O(\epsilon^3)

abstract:
  We study the effective potential of three-dimensional O($N$) models. In
statistical physics the effective potential represents the free-energy density
as a function of the order parameter (Helmholtz free energy), and, therefore,
it is related to the equation of state. In particular, we consider its
small-field expansion in the symmetric (high-temperature) phase, whose
coefficients are related to the zero-momentum $2j$-point renormalized coupling
constants $g_{2j}$. For generic values of $N$, we calculate $g_{2j}$ to three
loops in the field-theoretic approach based on the $\epsilon$-expansion. The
estimates of $g_{2j}$, or equivalently of $r_{2j}\equiv g_{2j}/g_4^{j-1}$, are
obtained by a constrained analysis of the series that takes into account the
exact results in one and zero dimensions.


title: BEC in Nonextensive Statistical Mechanics

abstract:
  We discuss the Bose-Einstein condensation (BEC) for an ideal gas of bosons in
the framework of Tsallis's nonextensive statistical mechanics. We study the
corrections to the standard BEC formulas due to a weak nonextensivity of the
system. In particular, we consider three cases in the D-dimensional space: the
homogeneous gas, the gas in a harmonic trap and the relativistic homogenous
gas. The results show that small deviations from the extensive Bose statistics
produce remarkably large changes in the BEC transition temperature.


title: Depinning of kinks in a Josephson-junction ratchet array

abstract:
  We have measured the depinning of trapped kinks in a ratchet potential using
a fabricated circular array of Josephson junctions. Our ratchet system consists
of a parallel array of junctions with alternating cell inductances and
junctions areas. We have compared this ratchet array with other circular
arrays. We find experimentally and numerically that the depinning current
depends on the direction of the applied current in our ratchet ring. We also
find other properties of the depinning current versus applied field, such as a
long period and a lack of reflection symmetry, which we can explain
analytically.


title: Note on the Kaplan-Yorke dimension and linear transport coefficients

abstract:
  A number of relations between the Kaplan-Yorke dimension, phase space
contraction, transport coefficients and the maximal Lyapunov exponents are
given for dissipative thermostatted systems, subject to a small external field
in a nonequilibrium stationary state. A condition for the extensivity of phase
space dimension reduction is given. A new expression for the transport
coefficients in terms of the Kaplan-Yorke dimension is derived. Alternatively,
the Kaplan-Yorke dimension for a dissipative macroscopic system can be
expressed in terms of the transport coefficients of the system. The agreement
with computer simulations for an atomic fluid at small shear rates is very
good.


title:
S-wave Scattering Length from Effective Positronium-Positronium
  Interaction for Bose-Einstein Condensates

abstract:
  The s-wave scattering length for the Positronium-Positronium interaction is
estimated semi-phenomenologically with the long-range van der Waals force and
the short-range repulsive potential that represents the hard core between two
positronium. The obtained value of the scattering length is $a \sim 0.44{\rm
nm}$, and its stability is also checked for different parametrizations. Using
this value, the Gross-Pitaevskii equation can be fixed for the Positronium
Bose-Einstein condensates (Ps BEC). The static properties of Ps BEC are studied
from the solutions of that equation. The phase-transition temperature shift of
the Ps BEC due to the Ps-Ps interaction is also evaluated with the mean-field
approximation.


title:
Linking numbers for self-avoiding walks and percolation: application to
  the spin quantum Hall transition

abstract:
  Non-local twist operators are introduced for the O(n) and Q-state Potts
models in two dimensions which, in the limits n -> 0 (resp. Q -> 1) count the
numbers of self-avoiding loops (resp. percolation clusters) surrounding a given
point. This yields many results, for example the distribution of the number of
percolation clusters which must be crossed to connect a given point to an
infinitely distant boundary. These twist operators correspond to (1,2) in the
Kac classification of conformal field theory, so that their higher-point
correlations, which describe linking numbers around multiple points, may be
computed exactly. As an application we compute the exact value \sqrt 3/2 for
the dimensionless conductivity at the spin Hall transition, as well as the
shape dependence of the mean conductance in an arbitrary simply connected
geometry with two extended edge contacts.


title:
Slow fracton modes in antiferromagnetic correlations of high-Tc
  superconductors

abstract:
  In cuprate superconductors superconductivity develops as a unique cross over
between the two extremal phases characterizing these compounds. The extremal
phases are known to be an antiferromagnetic (AF) Mott insulator one, on one
side, and a non-superconducting itinerant metal on the other. Numerous
inelastic neutron scattering studies on underdoped samples shows most directly
that the static AF order of the parent phase does not disappear by doping but
transforms in such a way that at least fluctuating and local AF order persists,
coexisting with superconductivity. Structurally, the model of stripe-type
correlations accounts for the coexistence while its genuine dynamics might hide
the most interesting physics of cuprates, including the pairing mechanism.
Indeed, there is an impressive accumulation of knowledge in favour of AF
correlations underlying the pairing. Some crucial properties of these
correlations have been recently pointed out: they should involve small velocity
collective modes of the wave vector identical to that of static AF order while
their energy scale (<50 meV) should be rather low compared with any other
involved. In this work we show on phenomenological ground that the very concept
of a disordered AF phase interpenetrated with non-magnetic boundaries
comprises, irrespective of structural details of stripe order, the dynamical
ingredients (fractons) which may be important for pairing. The model elaborated
below introduces a new energy scale (10-50 meV) associated with slow (or almost
localized) and well-defined AF excitations, interpreting simultaneously most of
the neutron scattering results in a consistent and a natural way.


title: Scaling at the chaos threshold in an interacting quantum dot

abstract:
  The chaotic mixing by random two-body interactions of many-electron Fock
states in a confined geometry is investigated numerically and compared with
analytical predictions. Two distinct regimes are found in the dependence of the
inverse participation ratio in Fock space I on the dimensionless conductance of
the quantum dot g and the excitation energy E. In both regimes I>>1, but only
the small-g regime is described by the golden rule. The crossover region is
characterized by a maximum in a scaling function that becomes more pronounced
with increasing excitation energy. The scaling parameter that governs the
transition is (E/g)ln g.


title:
Enhancement of structural rearrangement in glassy systems under shear
  flow

abstract:
  We extend the analysis of the mean field schematic model recently introduced
for the description of glass forming liquids to the case of a supercooled fluid
subjected to a shear flow of rate $\gamma$. After quenching the system to a low
temperature $T$, a slow glassy regime is observed before stationarity is
achieved at the characteristic time $\tau_g$. $\tau_g$ is of the order of the
usual equilibration time without shear $\tau_g^o$ for weak shear, $\gamma
\tau_g ^o<1$. For larger shear, $\gamma \tau_g ^o>1$, local rearrangement of
dense regions is instead enhanced by the flow, and $\tau_g \simeq 1/(T\gamma)$.


title: Cooper problem in the vicinity of Anderson transition

abstract:
  We study numerically the ground state properties of the Cooper problem in the
three-dimensional Anderson model. It is shown that attractive interaction
creates localized pairs in the metallic noninteracting phase. This localization
is destroyed at sufficiently weak disorder. The phase diagram for the
delocalization transition in the presence of disorder and interaction is
determined.


title: The dynamical structure factor in disordered systems

abstract:
  We study the spectral width as a function of the external momentum for the
dynamical structure factor of a disordered harmonic solid, considered as a toy
model for supercooled liquids and glasses. Both in the context of single-link
coherent potential approximation and of a single-defect approximation, two
different regimes are clearly identified: if the density of states at zero
energy is zero, the Rayleigh $p^4$ law is recovered for small momentum. On the
contrary, if the disorder induces a non vanishing density of states at zero
energy, a linear behaviour is obtained. The dynamical structure factor is
numerically calculated in lattices as large as $96^3$, and satisfactorily
agrees with the analytical computations.


title: Thermal behavior of Quantum Cellular Automaton wires

abstract:
  We investigate the effect of a finite temperature on the behavior of logic
circuits based on the principle of Quantum Cellular Automata (QCA) and of
ground state computation. In particular, we focus on the error probability for
a wire of QCA cells that propagates a logic state. A numerical model and an
analytical, more approximate, model are presented for the evaluation of the
partition function of such a system and, consequently, of the desired
probabilities. We compare the results of the two models, assessing the limits
of validity of the analytical approach, and provide estimates for the maximum
operating temperature.


title:
Electric field induced suppression of universal conductance fluctuations
  and dephasing in disordered systems

abstract:
  We report a novel phenomenon that the universal conductance fluctuations
(UCF) can be suppressed by a small electric field E. The experiment has been
carried out on single crystals of Si doped heavily with P and B beyond the
critical composition of insulator to metal transition. The phenomenon is
identified as a consequence of electric field induced dephasing of the electron
wavefunction. Over the range of measurements, the observed dephasing rate
(\tau_\phi^{-1}) varied as \tau_\phi^{-1} = aT + bE^q with q \approx 1.3 and
for E >> E^*, a cross-over field, \tau_\phi^{-1} \sim E^q, independent of T.
This experiment also establishes that the UCF can be utilized as a sensitive
electron "interferometer" to measure dephasing rate.


title: Formation and Equilibrium Properties of Living Polymer Brushes

abstract:
  Polydisperse brushes obtained by reversible radical chain polymerization
reaction onto a solid substrate with surface-attached initiators, are studied
by means of an off-lattice Monte Carlo algorithm of living polymers (LP).
Various properties of such brushes, like the average chain length and the
conformational orientation of the polymers, or the force exerted by the brush
on the opposite container wall, reveal power-law dependence on the relevant
parameters. The observed molecular weight distribution (MWD) of the grafted LP
decays much more slowly than the corresponding LP bulk system due to the
gradient of the monomer density within the dense pseudo-brush which favors
longer chains. Both MWD and the density profiles of grafted polymers and chain
ends are well fitted by effective power laws whereby the different exponents
turn out to be mutually self-consistent for a pseudo-brush in the
strong-stretching regime.


title: Persistence exponents of non-Gaussian processes in statistical mechanics

abstract:
  Motivated by certain problems of statistical physics we consider a stationary
stochastic process in which deterministic evolution is interrupted at random
times by upward jumps of a fixed size. If the evolution consists of linear
decay, the sample functions are of the "random sawtooth" type and the level
dependent persistence exponent \theta can be calculated exactly. We then
develop an expansion method valid for small curvature of the deterministic
curve. The curvature parameter g plays the role of the coupling constant of an
interacting particle system. The leading order curvature correction to \theta
is proportional to g^{2/3}. The expansion applies in particular to exponential
decay in the limit of large level, where the curvature correction considerably
improves the linear approximation. The Langevin equation, with Gaussian white
noise, is recovered as a singular limiting case.


title: Correlated random band matrices: localization-delocalization transitions

abstract:
  We study the statistics of eigenvectors in correlated random band matrix
models. These models are characterized by two parameters, the band width B(N)
of a Hermitian N times N matrix and the correlation parameter C(N) describing
correlations of matrix elements along diagonal lines. The correlated band
matrices show a much richer phenomenology than models without correlation as
soon as the correlation parameter scales sufficiently fast with matrix size. In
particular, for B(N) and C(N) increasing like the square root of N the model
shows a localization-delocalization transition of the quantum Hall type.


title: Super lattice formation of an array of volatile wetting droplets

abstract:
  For an ordered array of critical volatile wetting droplets the formation of a
super lattice by an Ostwald-ripening like competition process is considered.
The underlying diffusion problem is treated within a quasistatic approximation
and to first order in the inverse droplets distance. The approach is rather
general but a square lattice and a triangular lattice are studied explicitly.
Dispersion relations for the super-lattice growth of these arrays are
calculated.


title: Efficiency and current reversals in spatially inhomogeneous ratchets

abstract:
  Efficiency of generation of net unidirectional current in an adiabatically
driven symmetric periodic potential system is studied.
  The efficiency shows a maximum, in the case of an inhomogeneous system with
spatially varying periodic friction coefficient, as a function of temperature.
The ratchet is not most efficient when it gives maximum current. The direction
of current may also be reversed as a function of noise strength when, instead,
an asymmetric periodic potential is considered.


title: Broken symmetry of row switching in 2D Josephson junction arrays

abstract:
  We present an experimental and theoretical study of row switching in
two-dimensional Josephson junction arrays. We have observed novel dynamic
states with peculiar percolative patterns of the voltage drop inside the
arrays. These states were directly visualized using laser scanning microscopy
and manifested by fine branching in the current-voltage characteristics of the
arrays. Numerical simulations show that such percolative patterns have an
intrinsic origin and occur independently of positional disorder. We argue that
the appearance of these dynamic states is due to the presence of various
metastable superconducting states in arrays.


title: Spin Waves in Canted Phases: An Application to Doped Manganites

abstract:
  We present the effective lagrangian for low energy and momentum spin waves in
canted phases at next to leading order in the derivative expansion. The
symmetry breaking pattern SU(2) --> 1 of the internal spin group and that of
the crystallographic space group imply that there is one ferromagnetic and one
antiferromagnetic spin wave. The interaction of the spin waves with the charge
carriers is also discussed for canted, ferromagnetic and antiferromagnetic
phases. All this together allows us to write the doping dependence of the
dispersion relation parameters for doped manganites. We point out that the spin
waves posses distinctive characteristics which may allow us to experimentally
differentiate canted phases from phase separation regions in doped manganites.


title:
Spike-train responses of a pair of Hodgkin-Huxley neurons coupled by
  excitatory and inhibitory synapses and axons with time delay

abstract:
  Numerical calculations have been made on the spike-train response of a pair
of Hodgkin-Huxley (HH) neurons coupled by synapses and axons with time delay.
The recurrent excitatory-excitatory, inhibitory-inhibitory,
excitatory-inhibitory, and inhibitory-excitatory couplings are adopted. The
coupled, excitable HH neurons are assumed to receive the two kinds of
spike-train inputs: the transient input consisting of M impulses for the finite
duration (M: integer) and the sequential input with the constant interspike
interval (ISI). The HH neurons in all the kinds of couplings are found to play
a role of memory storage with on-off switching. When the coupling strength and
the time delay are changed, the distribution of the output ISI $T_{\rm o}$
shows bifurcation (multifurcation), metastability and chaotic behavior. The
calculation of the time correlation shows that the synchronization between the
two HH neurons is well preserved even when the distribution of their $T_{\rm
o}$ is chaotic. The correlation dimension of the cycles of $T_{\rm o}$ is shown
to depend not only on the model parameters but also on the type of input ISIs.


title:
Dissipation-Driven Breakdown of Universality in Two-Dimensional
  Superconductors

abstract:
  The influence of gapless dissipative degrees of freedom on the
superconductor-insulator transition in two dimensions is investigated. We
develop a series expansion for the free energy of a (2+1)-dimensional XY model
coupled to a bosonic heat bath that can be approximately summed to all orders.
The calculation explicitly conserves topological excitations. We derive the
zero temperature phase diagram and the free energy critical exponent, and find
a transition from universal to non-universal scaling behavior as the coupling
to the dissipative environment is increased, implying the existence of a new
universality class.


title:
Field Theoretical Analysis of On-line Learning of Probability
  Distributions

abstract:
  On-line learning of probability distributions is analyzed from the field
theoretical point of view. We can obtain an optimal on-line learning algorithm,
since renormalization group enables us to control the number of degrees of
freedom of a system according to the number of examples. We do not learn
parameters of a model, but probability distributions themselves. Therefore, the
algorithm requires no a priori knowledge of a model.

categories: cond-mat.dis-nn adap-org hep-th nlin.AO physics.data-an


title: Anomalous dielectric after-effect in ferroelectric KH2PO4

abstract:
  Dielectric permittivity of KH2PO4 single crystals and pressed powders was
measured in the presence of dc electric bias fields. Usually if the bias was
switched at time t = 0, real and imaginary permittivity jumped to new values
then decreased as logt . This well-known 'glassy' effect is attributed to
switchable microdomains that are present accidentally in single crystals but
are systematically more numerous in powders. A very different after-effect was
observed in a narrow T range around 7-8 K. Real permittivity jumped to a lower
value then increased with t as a stretched exponential with a T-dependent time
constant. This lay near the Arrhenius law of a known, but unassigned
dispersion. Such a dispersion is a typical effect of a point defect at low
concentration in a crystal, and is here attributed to the elementary movement
of a domain wall, consisting of a single H-bond reversal. The anomalous
after-effect results from the interaction between these point defects and the
microdomain system. Several other H-bond ferroelectrics exhibited similar
effects. They offer an opportunity to study 'glassy' and 'crystalline' slow
dynamic effects that coexist and interact.


title: Derivation of the Semi-circle Law from the Law of Corresponding States

abstract:
  We show that, for the transition between any two quantum Hall states, the
semi-circle law and the existence of a duality symmetry follow solely from the
consistency of the law of corresponding states with the two-dimensional scaling
flow. This puts these two effects on a sound theoretical footing, implying that
both should hold exactly at zero temperature, independently of the details of
the microscopic electron dynamics. This derivation also shows how the
experimental evidence favours taking the two-dimensional flow seriously for the
whole transition, and not just near the critical points.


title: Exact solution of the random bipartite matching model

abstract:
  In this paper we present the exact solution for the average minimum energy of
the random bipartite matching model with an arbitrary finite number of elements
where random paired interactions are described by independent exponential
distribution. This solution confirms the Parisi conjecture proposed for this
model earlier, as well as the result of the replica solution of this model in
the thermodynamic limit.


title: From antiferromagnetism to d-wave superconductivity in the 2D t-J model

abstract:
  We have found that the two dimensional t-J model, for the physical parameter
range J/t = 0.4 reproduces the main experimental qualitative features of
High-Tc copper oxide superconductors: d-wave superconducting correlations are
strongly enhanced upon small doping and clear evidence of off diagonal long
range order is found at the optimal doping \delta ~ 0.15. On the other hand
antiferromagnetic long range order, clearly present at zero hole doping, is
suppressed at small hole density with clear absence of antiferromagnetism at
\delta >~ 0.1.


title:
How does a quadratic term in the energy dispersion modify the
  single-particle Green's function of the Tomonaga-Luttinger model?

abstract:
  We calculate the effect of a quadratic term in the energy dispersion on the
low-energy behavior of the Green's function of the spinless Tomonaga-Luttinger
model (TLM). Assuming that for small wave-vectors q = k - k_F the fermionic
excitation energy relative to the Fermi energy is v_F q + q^2 / (2m), we
explicitly calculate the single-particle Green's function for finite but small
values of lambda = q_c /(2k_F). Here k_F is the Fermi wave-vector, q_c is the
maximal momentum transfered by the interaction, and v_F = k_F / m is the Fermi
velocity. Assuming equal forward scattering couplings g_2 = g_4, we find that
the dominant effect of the quadratic term in the energy dispersion is a
renormalization of the anomalous dimension. In particular, at weak coupling the
anomalous dimension is tilde{gamma} = gamma (1 - 2 lambda^2 gamma), where gamma
is the anomalous dimension of the TLM. We also show how to treat the change of
the chemical potential due to the interactions within the functional
bosonization approach in arbitrary dimensions.


title:
Random antiferromagnetic quantum spin chains: Exact results from scaling
  of rare regions

abstract:
  We study XY and dimerized XX spin-1/2 chains with random exchange couplings
by analytical and numerical methods and scaling considerations. We extend
previous investigations to dynamical properties, to surface quantities and
operator profiles, and give a detailed analysis of the Griffiths phase. We
present a phenomenological scaling theory of average quantities based on the
scaling properties of rare regions, in which the distribution of the couplings
follows a surviving random walk character. Using this theory we have obtained
the complete set of critical decay exponents of the random XY and XX models,
both in the volume and at the surface. The scaling results are confronted with
numerical calculations based on a mapping to free fermions, which then lead to
an exact correspondence with directed walks. The numerically calculated
critical operator profiles on large finite systems (L<=512) are found to follow
conformal predictions with the decay exponents of the phenomenological scaling
theory. Dynamical correlations in the critical state are in average
logarithmically slow and their distribution show multi-scaling character. In
the Griffiths phase, which is an extended part of the off-critical region
average autocorrelations have a power-law form with a non-universal decay
exponent, which is analytically calculated. We note on extensions of our work
to the random antiferromagnetic XXZ chain and to higher dimensions.


title: Chlorine adsorption on the Cu(111) surface

abstract:
  We investigate the adsorption of chlorine on the Cu(111) surface with full
potential all-electron density functional calculations. Chlorine adsorption at
the fcc hollow sites is slightly preferred over that at the hcp hollow. The
adsorption geometry is in excellent agreement with electron diffraction and ion
scattering data. Adsorption energies and surface diffusion barriers are close
to those deduced from experiment.


title:
Adsorption of Large Ions from an Electrolyte Solution: A Modified
  Poisson-Boltzmann Equation

abstract:
  The behavior of electrolyte solutions close to a charged surface is studied
theoretically. A modified Poisson-Boltzmann equation which takes into account
the volume excluded by the ions in addition to the electrostatic interactions
is presented. In a formal lattice gas formalism the modified Poisson-Boltzmann
equation can be obtained from a mean-field approximation of the partition
function. In an alternative phenomenological approach, the same equation can be
derived by including the entropy of the solvent molecules in the free energy.
In order to visualize the effect of the steric repulsion, a simple case of a
single, highly charged, flat surface is discussed. This situation resembles
recent adsorption experiments of large ions onto a charged monolayer. A simple
criterion for the importance of the steric effects is expressed in terms of the
surface charge density and the size of the ions. It is shown that when these
effects are important a saturated layer is formed near the surface. A modified
Grahame equation relating the ion concentration at the surface to the surface
charge density is obtained.


title: Semiclassical approach to the thermodynamics of spin chains

abstract:
  Using the PQSCHA semiclassical method, we evaluate thermodynamic quantities
of one-dimensional Heisenberg ferro- and antiferromagnets. Since the PQSCHA
reduces their evaluation to classical-like calculations, we take advantage of
Fisher's exact solution to get all results in an almost fully analytical way.
Explicitly considered here are the specific heat, the correlations length and
susceptibility. Good agreement with Monte Carlo simulations is found for S>1
antiferromagnets, showing that the relevance of the topological terms and of
the Haldane gap is significant only for the lowest spin values and
temperatures.


title:
Enhancement of Pairing Correlation and Spin Gap through Suppression of
  Single-Particle Dispersion in One-Dimensional Models

abstract:
  We investigate the effects of suppression of single-particle dispersion near
the Fermi level on the spin gap and the singlet-pairing correlation by using
the exact diagonalization method for finite-size systems. We consider strongly
correlated one-dimensional models, which have flat band dispersions near wave
number k=\pi/2, if the interactions are switched off. Our results for strongly
correlated models show that the spin gap region expands as the single-particle
dispersion becomes flatter. The region where the singlet-pairing correlation is
the most dominant also expands in models with flatter band dispersions. Based
on our numerical results, we propose a pairing mechanism induced by the
flat-band dispersion.


title: Wannier-functions characterization of floating bonds in a-Si

abstract:
  We investigate the electronic structure of over-coordinated defects in
amorphous silicon via density-functional total-energy calculations, with the
aim of understanding the relationship between topological and electronic
properties on a microscopic scale. Maximally-localized Wannier functions are
computed in order to characterize the bonding and the electronic properties of
these defects. The five-fold coordination defects give rise to delocalized
states extending over several nearest neighbors, and therefore to very
polarizable bonds and anomalously high Born effective charges for the defective
atoms.


title: Monopoles and fractional vortices in chiral superconductors

abstract:
  We discuss two exotic objects, which must be experimentally identified in
chiral superfluids and superconductors. These are (i) the vortex with
fractional quantum number (N=1/2 in chiral superfluids, and N=1/2 and N=1/4 in
chiral superconductors), which plays the part ofthe Alice string in
relativistic theories; and (ii) the hedgehog in the l-field, which is the
counterpart of the Dirac magnetic monopole. These objects of different
dimensions are topologically connected. They form the combined object which is
called nexus in relativistic theories. In chiral superconductors the nexus has
magnetic charge emanating radially from the hedgehog, while the half-quantum
vortices play the part of the Dirac string. Each of them supplies the
fractional magnetic flux to the hedgehog, representing 1/4 of the
"conventional" Dirac string. We discuss the topological interaction of the
superconductor's nexus with the `t Hooft-Polyakov magnetic monopole, which can
exist in GUT. The `t Hooft-Polyakov magnetic monopole and the hedgehog with the
same magnetic charge are topologically confined by a piece of the Abrikosov
vortex. Other properties of half-quantum vortices and monopoles are discussed
as well, including fermion zero modes.


title: Anomalous resonance phenomena of solitary waves with internal modes

abstract:
  We investigate the non-parametric, pure ac driven dynamics of nonlinear
Klein-Gordon solitary waves having an internal mode of frequency $\Omega_i$. We
show that the strongest resonance arises when the driving frequency
$\delta=\Omega_i/2$, whereas when $\delta=\Omega_i$ the resonance is weaker,
disappearing for nonzero damping. At resonance, the dynamics of the kink center
of mass becomes chaotic. As we identify the resonance mechanism as an {\em
indirect} coupling to the internal mode due to its symmetry, we expect similar
results for other systems.

categories: cond-mat.stat-mech cond-mat.mes-hall math-ph math.MP nlin.PS patt-sol


title:
Molecular dynamics of folding of secondary structures in Go-type models
  of proteins

abstract:
  We consider six different secondary structures of proteins and construct two
types of Go-type off-lattice models: with the steric constraints and without.
The basic aminoacid-aminoacid potential is Lennard Jones for the native
contacts and a soft repulsion for the non-native contacts. The interactions are
chosen to make the target secondary structure be the native state of the
system. We provide a thorough equilibrium and kinetic characterization of the
sequences through the molecular dynamics simulations with the Langevin noise.
Models with the steric constraints are found to be better folders and to be
more stable, especially in the case of the $\beta$-structures. Phononic spectra
for vibrations around the native states have low frequency gaps that correlate
with the thermodynamic stability. Folding of the secondary structures proceeds
through a well defined sequence of events. For instance, $\alpha$-helices fold
from the ends first. The closer to the native state, the faster establishment
of the contacts. Increasing the system size deteriorates the folding
characteristics. We study the folding times as a function of viscous friction
and find a regime of moderate friction with the linear dependence. We also
consider folding when one end of a structure is pinned which imitates
instantaneous conditions when a protein is being synthesized. We find that,
under such circumstances, folding of helices is faster and of the
$\beta$-sequences slower.


title:
Coexistence of superconductivity and ferromagnetism in ferromagnetic
  metals

abstract:
  In this paper we address the question of coexistence of superconductivity and
ferromagnetism in the high temperature superconductor
RuSr$_2$GdCu$_2$O$_{8-\delta}$. Using a field theoretical approach we study a
one-fermion effective model of a ferromagnetic superconductor in which the
quasiparticles responsible for the ferromagnetism form the Cooper pairs as
well. We discuss the physical features which are different in this model and
the standard BCS model and consider their experimental consequences.


title:
Landauer Theory, Inelastic Scattering and Electron Transport in
  Molecular Wires

abstract:
  In this paper we address the topic of inelastic electron scattering in
mesoscopic quantum transport. For systems where only elastic scattering is
present, Landauer theory provides an adequate description of transport that
relates the electronic current to single-particle transmission and reflection
probabilities. A formalism proposed recently by Bonca and Trugman facilitates
the calculation of the one-electron transmission and reflection probabilities
for inelastic processes in mesoscopic conductors connected to one-dimensional
ideal leads. Building on their work, we have developed a self-consistent
procedure for the evaluation of the non-equilibrium electron distributions in
ideal leads connecting such mesoscopic conductors to electron reservoirs at
finite temperatures and voltages. We evaluate the net electronic current
flowing through the mesoscopic device by utilizing these non-equilibrium
distributions. Our approach is a generalization of Landauer theory that takes
account of the Pauli exclusion principle for the various competing elastic and
inelastic processes while satisfying the requirement of particle conservation.
As an application we examine the influence of elastic and inelastic scattering
on conduction through a two site molecular wire with longitudinal phonons using
the Su-Schrieffer-Heeger model of electron-phonon coupling.


title: Two-dimensional frustrated Heisenberg model: Variational study

abstract:
  The stability of the ferromagnetic phase of the 2D quantum spin-1/2 model
with nearest-neighbor ferro- and next-nearest neighbor antiferromagnetic
interactions is studied. It turns out that values of exchange integrals at
which the ferromagnetic state becomes unstable with respect to a creation of
one and two magnon are different. This difference shows that the classical
approximation is inapplicable to the study of the transition from the
ferromagnetic to the singlet state in contrast with 1D case. This problem is
investigated using a variational function of new type. It is based on the boson
representation of spin operators which is different from the Holstein-Primakoff
approximation. This allows us to obtain the accurate estimate of the transition
point and to study the character of the phase transition.


title: Epidemics and percolation in small-world networks

abstract:
  We study some simple models of disease transmission on small-world networks,
in which either the probability of infection by a disease or the probability of
its transmission is varied, or both. The resulting models display epidemic
behavior when the infection or transmission probability rises above the
threshold for site or bond percolation on the network, and we give exact
solutions for the position of this threshold in a variety of cases. We confirm
our analytic results by numerical simulation.


title:
Magnetic penetration depth in electron-doped cuprates - evidence for gap
  nodes

abstract:
  The in-plane penetration depth \lambda(T) is measured in electron-doped
single crystals Nd1.85Ce0.15CuO4-x (NCCO) and Pr1.85Ce0.15CuO4-x (PCCO) using a
11 MHz LC resonator. In NCCO, \lambda(T) exhibits a minimum at 3.8 K and a
pronounced upturn down to 0.4 K due to the paramagnetic contribution of Nd3+
ions. The London penetration depth contribution is linear in T. The
paramagnetic contribution is absent in PCCO, where \lambda(T)~T^2 at low
temperatures. Our results indicate the presence of nodes in the superconducting
gap, i.e., non s-wave symmetry of the order parameter in electron-doped
cuprates.


title: Cross-linking of micelles by gemini surfactants

abstract:
  We investigate the effects of gemini surfactants, telechelic chain and lipids
on the nature of micelles formed by conventional single-tail surfactants in
water by carrying out Monte Carlo simulations. In a mixture of gemini and
single-tail surfactants in water we find direct evidence of micelles of
predominantly single-tail surfactants some of which are dynamically
cross-linked by gemini surfactants when the concentrations of the geminis is
only a few mole percent and their spacers are {\it hydrophilic}. In contrast,
mixtures of lipids and single-tail surfactants in water form only isolated
micelles, each consisting of a mixture of both species, without cross-links.


title:
Fractional-flux Little-Parks resistance oscillations in disordered
  superconducting Au$_{0.7}$In$_{0.3}$ cylinders

abstract:
  Resistance of disordered superconducting Au$_{0.7}$In$_{0.3}$ cylindrical
films was measured as a function of applied magnetic field. In the
high-temperature part of the superconducting transition regime, the resistance
oscillated with a period of $h/2e$ in the unit of the enclosed magnetic flux.
However, at lower temperatures, the resistance peaks split. We argue that this
splitting is due to the emergence of an oscillation with a period of $h/4e$,
half of the flux quantum for paired electrons. The possible physical origin of
the $h/4e$ resistance oscillation is discussed in the context of new minima in
the free energy of a disordered superconducting cylinder.


title:
Two-dimensional weak localization effects in high temperature
  superconductor Nd{2-x}Ce{x}CuO{4-d}

abstract:
  A systematic study of the resistivity and Hall effect in single crystal
Nd{2-x}Ce{x}CuO{4-d} films (0.12 < x < 0.20) is presented, with special
emphasis on the low-temperature dependence of the normal state conductance.
Two-dimensional weak localization effects are found both in a normally
conducting underdoped sample (x = 0.12) and in situ superconducting optimally
doped (x = 0.15) or overdoped (x = 0.18) samples in a high magnetic field B >
B{c2}. The phase coherence time and the effective thickness of a CuO{2}
conducting layer {d} (~ 1.5 A) have been estimated by fitting 2D weak
localization theory expressions to the magnetoresistivity data for magnetic
fields perpendicular to the {ab} plane and in plane. Estimates of the parameter
{d} ensure the condition of strong carrier confinement and justify a model
consisting of almost decoupled 2D metallic sheets for the Nd{2-x}Ce{x}CuO{4-d}
single crystal.


title: The Cluster Processor: New Results

abstract:
  We present a progress report on the Cluster Processor, a special-purpose
computer system for the Wolff simulation of the three-dimensional Ising model,
including an analysis of simulation results obtained thus far. These results
allow, within narrow error margins, a determination of the parameters
describing the phase transition of the simple-cubic Ising model and its
universality class. For an improved determination of the correction-to-scaling
exponent, we include Monte Carlo data for systems with nearest-neighbor and
third-neighbor interactions in the analysis.


title: Dynamics of the Number of Trades of Financial Securities

abstract:
  We perform a parallel analysis of the spectral density of (i) the logarithm
of price and (ii) the daily number of trades of a set of stocks traded in the
New York Stock Exchange. The stocks are selected to be representative of a wide
range of stock capitalization. The observed spectral densities show a different
power-law behavior. We confirm the $1/f^2$ behavior for the spectral density of
the logarithm of stock price whereas we detect a $1/f$-like behavior for the
spectral density of the daily number of trades.


title:
Auxiliary particle theory of threshold singularities in photoemission
  and X-ray absorption spectra: Test of a conserving T-matrix approximation

abstract:
  We calculate the exponents of the threshold singularities in the
photoemission spectrum of a deep core hole and its X-ray absorption spectrum in
the framework of a systematic many-body theory of slave bosons and
pseudofermions (for the empty and occupied core level). In this representation,
photoemission and X-ray absorption can be understood on the same footing; no
distinction between orthogonality catastrophe and excitonic effects is
necessary. We apply the conserving slave particle T-matrix approximation
(CTMA), recently developed to describe both Fermi and non-Fermi liquid behavior
systems with strong local correlations, to the X-ray problem as a test case.
The numerical results for both photoemission and X-ray absorption are found to
be in agreement with the exact infrared powerlaw behavior in the weak as well
as in the strong coupling regions. We point out a close relation of the CTMA
with the parquet equation approach of Nozi{\`e}res et al.


title:
Quantum effects on the BKT phase transition of two-dimensional Josephson
  arrays

abstract:
  The phase diagram of two dimensional Josephson arrays is studied by means of
the mapping to the quantum XY model. The quantum effects onto the
thermodynamics of the system can be evaluated with quantitative accuracy by a
semiclassical method, the {\em pure-quantum self-consistent harmonic
approximation}, and those of dissipation can be included in the same framework
by the Caldeira-Leggett model. Within this scheme, the critical temperature of
the superconductor-to-insulator transition, which is a
Berezinskii-Kosterlitz-Thouless one, can be calculated in an extremely easy way
as a function of the quantum coupling and of the dissipation mechanism.
Previous quantum Monte Carlo results for the same model appear to be rather
inaccurate, while the comparison with experimental data leads to conclude that
the commonly assumed model is not suitable to describe in detail the real
system.


title: Excitonic lasing in semiconductor quantum wires

abstract:
  Direct experimental evidences for excitonic lasing is obtained in optically
pumped V-groove quantum wire structures. We demonstrate that laser emission at
a temperature of 10 K arises from a population inversion of localized excitons
within the inhomogenously-broadened luminescence line. At the lasing threshold,
we estimate a maximum exciton density of about 1.8 105cm-1.


title:
Exact ground states for a class of one-dimensional frustrated quantum
  spin models

abstract:
  We have found the exact ground state for two frustrated quantum spin-1/2
models on a linear chain. The first model describes ferromagnet-
antiferromagnet transition point. The singlet state at this point has
double-spiral ordering. The second model is equivalent to special case of the
spin-1/2 ladder. It has non-degenerate singlet ground state with exponentially
decaying spin correlations and there is an energy gap. The exact ground state
wave function of these models is presented in a special recurrent form and
recurrence technics of expectation value calculations is developed.


title:
Photoluminescence study of V-groove quantum wires: The influence of
  disorder on the optical spectra and the carrier thermalization

abstract:
  We report on time-resolved and steady-state photoluminescence studies of
GaAs/AlGaAs V-groove quantum wire structures. Steady-state photoluminescence
experiments are performed in the temperature range from 8K to 200K. We evaluate
the relation between photoluminescence excitation and absorption and determine
experimentally an optical density in order to analyze the temperature
dependence of the photoluminescence spectra. We find that, at a temperature
above 60K, the photoexcited electron-hole pairs reach a thermal equilibrium at
the lattice temperature while, at a temperature below about 60K, they do not
reach a quasi-equilibrium in the steady-state. Time-resolved photoluminescence
studies performed at a carrier density of about 2. 104 cm-1 indicate that, at
60K, a quasi-equilibrium is reached on a time scale of 10 ps. Furthermore, the
hot carriers cool in about 100 ps to the lattice temperature. At 8K, however,
evidence of a non-thermal carrier distribution is found at the earliest times,
which suggests that carriers in extended states are not in thermal equilibrium
with carriers in localized states.


title:
Phase transition properties of a finite ferroelectric superlattice from
  the transverse Ising model

abstract:
  We consider a finite ferroelectric superlattice in which the elementary unit
cell is made up of l atomic layers of type A and n atomic layers of type B.
Based on the transverse Ising model we examine the phase transition properties
of the ferroelectric superlattice. Using the transfer matrix method we derive
the equation for the Curie temperature of the superlattice. Numerical results
are given for the dependence of the Curie temperature on the thickness and
exchange constants of the superlattice.


title:
Analytical results for a continuum model of crystalline tensionless
  surfaces. I. Variational mean field study

abstract:
  We study analytically the equilibrium and near-equilibrium properties of a
model of surfaces relaxing via linear surface diffusion and subject to a
lattice potential. We employ the variational mean field formalism introduced by
Saito for the study of the sine Gordon model. In equilibrium, our variational
theory predicts a first order roughening transition between a flat low
temperature phase and a rough high temperature phase with the properties of the
linear molecular beam epitaxy equation. The study of a Gaussian approximation
to the Langevin dynamics of the system indicates that the surface shows
hysteresis when we continuously tune temperature. Out of equilibrium, this
Langevin dynamics approach shows that the surface mobility can have different
behaviours as a function of a driving flux. Some considerations are made
regarding different dimensionalities and underlying lattices, and connections
are drawn to related models or different approaches to the same model we study.


title:
Impurity relaxation mechanism for dynamic magnetization reversal in a
  single domain grain

abstract:
  The interaction of coherent magnetization rotation with a system of two-level
impurities is studied. Two different, but not contradictory mechanisms, the
`slow-relaxing ion' and the `fast-relaxing ion' are utilized to derive a system
of integro-differential equations for the magnetization. In the case that the
impurity relaxation rate is much greater than the magnetization precession
frequency, these equations can be written in the form of the Landau-Lifshitz
equation with damping. Thus the damping parameter can be directly calculated
from these microscopic impurity relaxation processes.


title: Vortex formation in a stirred Bose-Einstein condensate

abstract:
  Using a focused laser beam we stir a Bose-Einstein condensate of 87Rb
confined in a magnetic trap and observe the formation of a vortex for a
stirring frequency exceeding a critical value. At larger rotation frequencies
we produce states of the condensate for which up to four vortices are
simultaneously present. We have also measured the lifetime of the single vortex
state after turning off the stirring laser beam.


title: Test of a simple and flexible S8 model molecule in alpha-s8 crystals

abstract:
  Alpha S8 is the most stable crystalline form, at ambient pressure and
temperature (STP), of elemental sulfur. In this paper we analyze the zero
pressure low temperature part of the phase diagram of this crystal, in order to
test a simple and flexible model molecule. The calculations consist in a series
of molecular dynamics (MD) simulations, performed in the constant pressure-
constant temperature ensemble. Our calculations show that this model, that
gives good results for three crystalline phases at STP and T>~300 K, fails at
low temperatures, predicting a structural phase transition at 200 K where there
should be none.


title:
Charge Ordering and Phase Competition in the Layered Perovskite
  Lasr2mn2o7

abstract:
  Charge-lattice fluctuations are observed in the layered perovskite manganite
LaSr2Mn2O7 by Raman spectroscopy as high as 340 K and with decreasing
temperature they become static and form a charge ordered (CO) phase below
TCO=210 K. In the static regime, superlattice reflections are observed through
neutron and x-ray diffraction with a propagation vector (h+1/4,k-1/4,l).
Crystallographic analysis of the CO state demonstrates that the degree of
charge and orbital ordering in this manganite is weaker than the charge
ordering in three dimensional perovskite manganites. A TN=170K a type-A
antiferromagnetism (AF) develops and competes with the charge ordering, that
eventually melts below T*=100K. High resolution diffraction measurements
suggest that that CO- and AF-states do not coincide within the same region in
the material but rather co-exist as separate phases. The transition to type-A
antiferromagnetism at lower temperatures is characterized by the competition
between these two phases.


title: Magnetic ordering in GaAlAs:Mn double well structure

abstract:
  The magnetic order in the diluted magnetic semiconductor barrier of double
AlAs/GaAs: Mn quantum well structures is investigated by Monte Carlo
simulations. A confinement adapted RKKY mechanism is implemented for indirect
exchange between Mn ions mediated by holes. It is shown that, depending on the
barrier width and the hole concentration a ferromagnetic or a spin-glass order
can be established.


title:
Transport in Coupled Quantum Dots: Kondo Effect Versus
  Anti-Ferromagnetic Correlation

abstract:
  The interplay between the Kondo effect and the inter-dot magnetic interaction
in a coupled-dot system is studied. An exact result for the transport
properties at zero temperature is obtained by diagonalizing a cluster, composed
by the double-dot and its vicinity, which is connected to leads. It is shown
that the system goes continuously from the Kondo regime to an
anti-ferromagnetic state as the inter-dot interaction is increased. The
conductance, the charge at the dots and the spin-spin correlation are obtained
as a function of the gate potential.


title:
Quantum impurity dynamics in two-dimensional antiferromagnets and
  superconductors

abstract:
  We present the universal theory of arbitrary, localized impurities in a
confining paramagnetic state of two-dimensional antiferromagnets with global
SU(2) spin symmetry. The energy gap of the host antiferromagnet to spin-1
excitations, \Delta, is assumed to be significantly smaller than a typical
nearest neighbor exchange. In the absence of impurities, it was argued in
earlier work (Chubukov et al. cond-mat/9304046) that the low-temperature
quantum dynamics is universally and completely determined by the values of
\Delta and a spin-wave velocity c. Here we establish the remarkable fact that
no additional parameters are necessary for an antiferromagnet with a dilute
concentration of impurities, n_{imp} - each impurity is completely
characterized by a integer/half-odd-integer valued spin, S, which measures the
net uncompensated Berry phase due to spin precession in its vicinity. We
compute the impurity-induced damping of the spin-1 collective mode of the
antiferromagnet: the damping occurs on an energy scale \Gamma= n_{imp} (\hbar
c)^2/\Delta, and we predict a universal, asymmetric lineshape for the
collective mode peak. We argue that, under suitable conditions, our results
apply unchanged (or in some cases, with minor modifications) to d-wave
superconductors, and compare them to recent neutron scattering experiments on
YBCO by Fong et al. (cond-mat/9812047). We also describe the universal
evolution of numerous measurable correlations as the host antiferromagnet
undergoes a quantum phase transition to a Neel ordered state.


title: Charge and orbital ordering in underdoped La1-xSrxMnO3

abstract:
  We have explored spin, charge and orbitally ordered states in La1-xSrxMnO3 (0
< x < 1/2) using model Hartree-Fock calculations on d-p-type lattice models. At
x=1/8, several charge and orbitally modulated states are found to be stable and
almost degenerate in energy with a homogeneous ferromagnetic state. The present
calculation indicates that a ferromagnetic state with a charge modulation along
the c-axis which is consistent with the experiment by Yamada et al. might be
responsible for the anomalous behavior around x = 1/8.


title:
Dynamical Properties of Chiral-Glass Order in Ceramic High-Tc
  Superconductors

abstract:
  There recently accumulated growing numerical and experimental evidence that a
novel glassy zero-field phase characterized by the spontaneously broken
time-reversal symmetry, a chiral-glass phase, is realized in certain ceramic
high-$T_c$ superconductors. Existence of frustration in zero external field,
arising from the d-wave pairing symmetry of high-$T_c$ superconductors, is
essential to realize this phase. In this paper, we study the dynamical critical
properties of the chiral-glass transition by means of Monte Carlo simulations,
based on a lattice {\it XY} model with finite screening. We perform a dynamical
scaling analysis, and the results are discussed in conjunction with recent ac
magnetic susceptibility and transport measurements on high-$T_c$ ceramics.


title:
Smectic-Nematic Phase Transition as Wrinkling Transition in a Stack of
  Membranes

abstract:
  We point out that the smectic-nematic phase transition may considered as a
transition of a stack of membranes in $2+\epsilon$ dimensions, in which the
layers become so wrinkled that they interpenetrate each other are no longer
distinguishable.


title: Andreev Reflection and Proximity effect

abstract:
  The Andreev Reflection is the key mechanism for the superconducting proximity
effect. It provides phase correlations in a system of non-interacting electrons
at mesoscopic scales, i.e. over distances much larger than the microscopic
lengths : Fermi wavelength and elastic electron mean free path. This field of
research has attracted an increasing interest in the recent years in part
because of the tremendous development of nanofabrication technologies, and also
because of the richness of the involved quantum effects. In this paper we
review some recently achieved advances. We also discuss new open questions, in
particular non-equilibrium effects and proximity effect in systems with
ferromagnetic elements.