title: Propagation of Sound in Two-Species Bose-Einstein Condensates

abstract:
  We investigate the propagation of zero sound in two-species interpenetrating
Bose-Einstein condensates. In very elongated clouds, this propagation is shown
to be essentially one-dimensional. We also present 1D numerical experiments
that exhibit the existence of two different sound pulses with velocities
consistent with the theoretical predictions.


title: Structure of the Hilbert-space of the infinite-dimensional Hubbard model

abstract:
  An iterative procedure for the explicit construction of the nontrivial
subspace of all symmetry-adapted configurations with non-zero weight in the
ground-state of the infinite-dimensional Hubbard model is developed on the
basis of a symmetrized representation of the transition operators on a sequence
of Bethe-Lattices of finite depth. The relation ship between these operators
and the well known mapping of the infinite-dimensional Hubbard model onto an
effective impurity problem coupled to a (self-consistent) bath on
non-interacting electrons is given. As an application we calculate the
properties of various Hubbard stars and give estimates for the half-filled
Hubbard model with up to 0.1% accuracy.


title: Point-Contact Conductances at the Quantum Hall Transition

abstract:
  On the basis of the Chalker-Coddington network model, a numerical and
analytical study is made of the statistics of point-contact conductances for
systems in the integer quantum Hall regime. In the Hall plateau region the
point-contact conductances reflect strong localization of the electrons, while
near the plateau transition they exhibit strong mesoscopic fluctuations. By
mapping the network model on a supersymmetric vertex model with GL(2|2)
symmetry, and postulating a two-point correlator in keeping with the rules of
conformal field theory, we derive an explicit expression for the distribution
of conductances at criticality. There is only one free parameter, the power law
exponent of the typical conductance. Its value is computed numerically to be
X_t = 0.640 +/- 0.009. The predicted conductance distribution agrees well with
the numerical data. For large distances between the two contacts, the
distribution can be described by a multifractal spectrum solely determined by
X_t. Our results demonstrate that multifractality can show up in appropriate
transport experiments.


title: Analyticity in Hubbard models

abstract:
  The Hubbard model describes a lattice system of quantum particles with local
(on-site) interactions. Its free energy is analytic when \beta t is small, or
\beta t^2/U is small; here, \beta is the inverse temperature, U the on-site
repulsion and t the hopping coefficient.
  For more general models with Hamiltonian H = V + T where V involves local
terms only, the free energy is analytic when \beta ||T|| is small,
irrespectively of V. The Gibbs state exists in the thermodynamic limit, is
exponentially clustering and thermodynamically stable. These properties are
rigorously established in this paper.


title: Negative Josephson coupling in the Kondo strong coupling limit

abstract:
  We Show that pair hopping through a Kondo singlet give rise to a negative
Josephson coupling. Thus, our calculation supports the existance of staggered
pair correlations in the strong coupling limit of a one dimensional Kondo
lattice.


title:
Temperature dependence of the diffuse scattering fine structure in
  equiatomic CuAu

abstract:
  The temperature dependence of the diffuse scattering fine structure from
disordered equiatomic CuAu was studied using {\it in situ} x-ray scattering. In
contrast to Cu$_3$Au the diffuse peak splitting in CuAu was found to be
relatively insensitive to temperature. Consequently, no evidence for a
divergence of the antiphase length-scale at the transition temperature was
found. At all temperatures studied the peak splitting is smaller than the value
corresponding to the CuAuII modulated phase. An extended Ginzburg-Landau
approach is used to explain the temperature dependence of the diffuse peak
profiles in the ordering and modulation directions. The estimated mean-field
instability point is considerably lower than is the case for Cu$_3$Au.


title: Quasiparticle transport in the vortex state of YBa_2Cu_3O_6.9

abstract:
  The effect of vortices on quasiparticle transport in cuprate superconductors
was investigated by measuring the low temperature thermal conductivity of
YBa_2Cu_3O_6.9 in magnetic fields up to 8 T. The residual linear term (as T \to
0) is found to increase with field, directly reflecting the occupation of
extended quasiparticle states. A study for different Zn impurity concentrations
reveals a good agreement with recent calculations for a d-wave superconductor,
thereby shedding light on the nature of scattering by both impurities and
vortices. It also provides a quantitative measure of the gap near the nodes.


title:
Effective Gauge Theories, The Renormalization Group, and
  High-Temperature Superconductivity

abstract:
  These lectures serve as an introduction to the renormalization group approach
to effective field theories, with emphasis on systems with a Fermi surface. For
such systems, demanding appropriate scaling with respect to the renormalization
group for the appropriate excitations leads directly to the important concept
of quasiparticles and the connexion between large-N_f treatments and
renormalization group running in theory space. In such treatments N_f denotes
the number of effective fermionic degrees of freedom above the Fermi surface;
this number is roughly proportional to the size of the Fermi surface. As an
application of these ideas, non-trivial infra red structure in three
dimensional U(1) gauge theory is discussed, along with applications to the
normal phase physics of high-T_c superconductors, in an attempt to explain the
experimentally observed deviations from Fermi liquid behaviour. Specifically,
the direct current resistivity of the theory is computed at finite
temperatures, $T$, and is found to acquire $O(1/N_f)$ corrections to the linear
$T$ behaviour. Such scaling corrections are consistent with recent experimental
observations in high T_c superconducting cuprates.


title: Phase transition curves for mesoscopic superconducting samples

abstract:
  We compute the phase transition curves for mesoscopic superconductors.
Special emphasis is given to the limiting shape of the curve when the magnetic
flux is large. We derive an asymptotic formula for the ground state of the
Schr\"odinger equation in the presence of large applied flux. The expansion is
shown to be sensitive to the smoothness of the domain. The theoretical results
are compared to recent experiments.


title: Realistic Models of Biological Motion

abstract:
  The origin of biological motion can be traced back to the function of
molecular motor proteins. Cytoplasmic dynein and kinesin transport organelles
within our cells moving along a polymeric filament, the microtubule. The motion
of the myosin molecules along the actin filaments is responsible for the
contraction of our muscles. Recent experiments have been able to reveal some
important features of the motion of individual motor proteins, and a new
statistical physical description - often referred to as ``thermal ratchets'' -
has been developed for the description of motion of these molecules. In this
approach the motors are considered as Brownian particles moving along
one-dimensional periodic structures due to the effect of nonequilibrium
fluctuations. Assuming specific types of interaction between the particles the
models can be made more realistic. We have been able to give analytic solutions
for our model of kinesin with elastically coupled Brownian heads and for the
motion of the myosin filament where the motors are connected through a rigid
backbone. Our theoretical predictions are in a very good agreement with the
various experimental results. In addition, we have considered the effects
arising as a result of interaction among a large number of molecular motors,
leading to a number of novel cooperative transport phenomena.


title: Charge dynamics and optical conductivity of the t-J model

abstract:
  The dynamic charge susceptibility and the optical conductivity are calculated
in the planar t-J model within the memory function method, working directly in
terms of Hubbard operators. The density fluctuation spectrum consists of a
damped sound-like mode for small wave vectors and a broad high energy peak
($\sim t$) for large momenta. The study of the optical conductivity shows that
electron scattering from spin fluctuations leads to the Drude-frequency
dependent relaxation rate which exhibits a crossover from $\omega^{3/2}$
behavior at low frequencies ($\omega <2|\mu|$), to a linear $\omega$-dependence
for frequencies larger than $2|\mu|$. Due to the spin-polaron nature of charge
carriers, extra absorbtions arise starting at a frequency $\omega \agt J$. The
obtained results are in a good agreement with exact diagonalization studies.


title:
Interacting electrons in magnetic fields: Tracking potentials and
  Jastrow-product wavefunctions

abstract:
  The Schrodinger equation for an interacting spinless electron gas in a
nonuniform magnetic field admits an exact solution in Jastrow product form when
the fluctuations in the magnetic field track the fluctuations in the scalar
potential. For tracking realizations in a two-dimensional electron gas, the
degeneracy of the lowest Landau level persists, and the ``tracking'' solutions
span the ground state subspace. In the context of the fractional quantum Hall
problem, the Laughlin wave function is shown to be a tracking solution.
Tracking solutions for screened Coulomb interactions are also constructed. The
resulting wavefunctions are proposed as variational wave functions with
potentially lower energy in the case of non-negligible Landau level mixing than
the Laughlin function.


title:
Localization of Phonons in Two Component Superlattice with Random
  Thicknesses of the Layers

abstract:
  Two component superlattice film of 2N layers is considered and dimensionally
quantised spectrum of phonon-s is found. The problem of localization of
phonon-s in the superlattice with random thicknesses of the layers is
investigated. The Landauer resistance of the transport of phonon-s is
calculated exactly. For short range disorder the numerical analyses shows, that
at frequency \omega=0, there is delocalized state and the correlation length
index \nu is equal to 2.


title: Orbital Magnetism in Disordered Mesoscopic Metals

abstract:
  The theory of orbital magnetism in disordered metals is reviewed, and
extended to include a broad range of temperatures and fields. Sample-to-sample
fluctuations in the orbital magnetic susceptibility are studied. In a given
sample these fluctuations manifest themselves in aperiodic sample-specific
oscillations of susceptibility and magnetization, when the strength of the
magnetic field is changed.


title:
Temperature and magnetic-field dependence of the conductivity of YBaCuO
  films in the vicinity of superconducting transition: Effect of
  Tc-inhomogeneity

abstract:
  Temperature and magnetic field dependences of the conductivity of YBaCuO
films in the transition region are analyzed taking into account spatial
inhomogeneity in transition temperature, Tc.
  (i) An expression for the superconducting contribution to conductivity,
\sigma_s(T,H,Tc), of a homogeneous superconductor for H<<Hc2(T=0) is obtained
using the solution of the Ginzburg-Landau equation in form of perturbation
expansions [S.Ullah, A.T.Dorsey, PRB 44, 262 (1991)].
  (ii) The error in \sigma_s(T,H,Tc) occurring due to the presence of
Tc-inhomogeneity is calculated and plotted on an H-T plane diagram. These
calculations use an effective medium approximation and a Gaussian distribution
of Tc.
  (iii) Measuring the temperature dependences of a voltage, induced by a
focused electron beam, we determine spatial distributions of the critical
temperature for YBaCuO microbridges with a 2 micron resolution. A typical
Tc-distribution dispersion is found to be approximately 1K. For such
dispersion, error in \sigma_s(T,H,Tc) due to Tc-inhomogeneity exceeds 30% for
magnetic fields H < 1 T and temperatures |T-Tc| < 0.5 K.
  (iv) Experimental R(T,H) dependences of resistance are well described by a
numerical solution of a set of Kirchoff equations for the resistor network
based on the measured spatial distributions of Tc and the expression for
\sigma_s(T,H,Tc).


title:
Exact Zero Temperature Correlation Functions for Two-Leg Hubbard Ladders
  and Carbon Nanotubes

abstract:
  Motivated by recent work of Lin, Balents, and Fisher (cond-mat/9801285), we
compute correlation functions at zero temperature for weakly coupled two-leg
Hubbard ladders and (N,N) armchair carbon nanotubes. Lin et al. argued that
such systems renormalize towards the SO(8) Gross-Neveu model, an integrable
theory. We exploit this integrability to perform the computation at the SO(8)
invariant point. Terms breaking this symmetry can be treated systematically in
perturbation theory, leading to a model with the same qualitative features.
Using said correlators, we determine the optical conductivity, the single
particle spectral function, and the I-V curve for tunneling into the system
from an external lead. The frequency, \omega, dependent optical conductivity is
determined exactly for \omega < 3m (m being the fermion particle mass in the
SO(8) Gross-Neveu model). It is characterized by a sharp ``exciton'' peak
followed by the onset of the particle-hole continuum. Similarly, we obtain the
exact single particle spectral function for energies less than 3m. The latter
possesses a delta function peak arising from single particle excitations,
together with a two particle continuum. The final quantity we compute is the
tunneling I-V curve to lowest non-vanishing order in the tunneling matrix
elements. For this quantity, we present exact results for voltages, V <
(1+\sqrt(3))m. The resulting differential conductance is marked by a finite
jump at the energy of the onset of tunneling into the continuum of two particle
states. All calculations are done through form-factor expansions of correlation
functions. Finite truncations of such expansions give exact closed form
expressions below given energy thresholds. To facilitate these calculations, we
compute a number of previously unknown one and two particle form factors.


title: Vibronic Mechanism of the Isotope and Pressure Effects in Cuprates

abstract:
  The doped cuprates are considered to be a system of local spinless bosons
moving in a lattice of the hole pseudo-Jahn-Teller centers CuO_{4}^{5-}. A
detailed qualitative and quantitative description of the vibronic mechanism
determining both the isotope substitution and the external pressure effect on
T_c is given. This mechanism can properly interpret the principal peculiarities
of isotopic and baric effects in cuprates except the region of the well
developed percolation phenomena


title:
Nonperturbative Renormalization Group Function for Quantum Hall Plateau
  Transitions Imposed by Global Symmetries

abstract:
  As a unified theory of integer and fractional quantum Hall plateau
transitions, a nonperturbative theory of the two-parameter scaling
renormalization group function is presented. By imposing global symmetries
known as ``the law of corresponding states'', we seek a possible form of
renormalization group flows. Asking for consistency with the result from
weak-localization perturbation theory, such restriction is so intense that we
can analytically determine its concrete form. Accordingly, the critical
exponent $\nu$ and the irrelevant scaling index $y$ are obtained analytically
and turn out to be irrational. Their values ($\nu\approx 2.1$, $y\approx 0.3$)
agree favorably well with experiments and numerics.


title: Stability of the vortex lattice in D-wave superconductors

abstract:
  Use is made of Onsager's hydrodynamic equation to derive the vibration
spectrum of the vortex lattice in d-wave superconductor. In particular the
rhombic lattice (i.e. the $45^\circ$ tilted square lattice) is found to be
stable for $B>H_{cr}(t)$. Here $H_{cr}(t)$ denotes the critical field at which
the vortex lattice transition takes place.


title:
How universal is the one-particle Green's function of a Luttinger
  liquid?

abstract:
  The one-particle Green's function of the Tomonaga-Luttinger model for
one-dimensional interacting Fermions is discussed. Far away from the origin of
the plane of space-time coordinates the function falls off like a power law.
The exponent depends on the direction within the plane. For a certain form of
the interaction potential or within an approximated cut-off procedure the
different exponents only depend on the strength of the interaction at zero
momentum and can be expressed in terms of the Luttinger liquid parameters
$K_{\rho}$ and $K_{\sigma}$ of the model at hand. For a more general
interaction and directions which are determined by the charge velocity
$v_{\rho}$ and spin velocity $v_{\sigma}$ the exponents also depend on the
smoothness of the interaction at zero momentum and the asymptotic behavior of
the Green's function is not given by the Luttinger liquid parameters alone.
This shows that the physics of large space-time distances in Luttinger liquids
is less universal than is widely believed.


title:
Dissipative Interaction and Anomalous Surface Absorption of Bulk Phonons
  at a Two-Dimensional Defect in a Solid

abstract:
  We predict an extreme sensitivity to the dissipative losses of the resonant
interaction of bulk phonons with a 2D defect in a solid. We show that the total
resonant reflection of the transverse phonon at the 2D defect, described
earlier without an account for dissipation, occurs only in the limit of
extremely weak dissipation and is changed into almost total transmission by
relatively weak bulk absorption. Anomalous surface absorption of the transverse
phonon, when one half of the incident acoustic energy is absorbed at the 2D
defect, is predicted for the case of "intermediate" bulk dissipation.


title: Josephson effect between Bose condensates

abstract:
  I present an overview of the physics of the Josephson effect between Bose
condensed systems, with emphasis on the recently achieved BEC's in trapped
alkali gases. I focus mostly on those physical phenomena that are likely to be
observed only (or more easily) in these novel systems. Thus I omit the
discussion of problems which may be viewed as straightforward applications of
well known Josephson physics. In particular, I review the external and the
internal Josephson effects, and discuss how in the latter case it may be
possible to explore the crossover between collective Josephson behavior and
independent boson Rabi dynamics. I also describe novel macroscopic quantum
phenomena such as self-trapping and interference between separate Bose
condensates.


title: Long-range coherence and mesoscopic transport in N-S metallic structures

abstract:
  We review the mesoscopic transport in a diffusive proximity superconductor
made of a normal metal (N) in metallic contact with a superconductor (S). The
Andreev reflection of electrons on the N-S interface is responsible for the
diffusion of electron pairs in N. Superconducting-like properties are induced
in the normal metal. In particular, the conductivity of the N metal is locally
enhanced by the proximity effect. A re-entrance of the metallic conductance
occurs when all the energies involved (e.g. temperature and voltage) are small.
The relevant characteristic energy is the Thouless energy which is $\hbar$
divided by the diffusion time for an electron travelling throughout the sample.
In loop-shaped devices, a 1/T temperature-dependent oscillation of the
magnetoresistance arises with a large amplitude from the long-range coherence
of low-energy pairs.


title: Dynamic Properties of Random Heteropolymer with Correlations in Sequence

abstract:
  The some dynamic properties of a random heteropolymer in the condensed state
are studied in the mode coupling approximation. In agreement with recent report
a dynamic friction increasing is predicted for the random heteropolymer with
power-law correlations in comparison with exponential correlations. In the case
of strong power-law correlations the dynamic friction function diverge in the
thermodynamic limit. The qualitative explanation is given for the monomer's
dynamics slowing down and diverged energetic barrier between the frozen and
random coil states. The possible relations with protein's function and
evolution are discussed.


title: Disorder effects and electronic conductance in metallic carbon nanotubes

abstract:
  Disorder effects on the density of states and electronic conduction in
metallic carbon nanotubes are analyzed by a tight binding model with Gaussian
bond disorder. Metallic armchair and zigzag nanotubes are considered. We obtain
a conductance which becomes smaller by the factor 1/2 ~ 1/3 from that of the
clean nanotube. This decrease mainly comes from lattice fluctuations of the
width which is comparable to thermal fluctuations. We also find that
suppression of electronic conductance around the Fermi energy due to disorder
is smaller than that of the inner valence (and conduction) band states. This is
a consequence of the extended nature of electronic states around the Fermi
energy between the valence and conduction bands, and is a property typical of
the electronic structures of metallic carbon nanotubes.


title: Quantum mechanics with coordinate-dependent mass

abstract:
  We study a motion of quantum particles, whose properties depend on one
coordinate so that they can move freely in the perpendicular direction. A
rotationally-symmetric Hamiltonian is derived and applied to study a general
interface formed between two semiconductors. We predict a new type of electron
states, localized at the interface. They appear whenever the two bulk
dispersions intersect. These shallow states lie near the point of intersection
and are restricted to a finite range of perpendicular momentum. The scattering
of carriers by the interface is discussed.


title:
Proximity Effect, Andreev Reflections, and Charge Transport in
  Mesoscopic Superconducting-Semiconducting Heterostructures

abstract:
  In the quasi-twodimensional (Q2D) electron gas of an InAs channel between an
AlSb substrate and superconducting Niobium layers the proximity effect induces
a pair potential so that a Q2D mesoscopic
superconducting-normal-superconducting (SNS) junction forms in the channel. The
pair potential is calculated with quasiclassical Green's functions in the clean
limit. For such a junction alternating Josephson currents and current-voltage
characteristics (CVCs) are computed, using the non-equilibrium quasiparticle
wavefunctions which solve the time-dependent Bogoliubov-de Gennes Equations.
The CVCs exhibit features found experimentally by the Kroemer group: A steep
rise of the current at small voltages ("foot") changes at a "corner current" to
a much slower increase of current with higher voltages, and the zero-bias
differential resistance increases with temperature. Phase-coherent multiple
Andreev reflections and the associated Cooper pair transfers are the physical
mechanisms responsible for the oscillating Josephson currents and the CVCs.
Additional experimental findings not reproduced by the theory require model
improvements, especially a consideration of the external current leads which
should give rise to hybrid quasiparticle/collective mode excitations.


title:
Critical temperature of an anisotropic superconductor containing both
  nonmagnetic and magnetic impurities

abstract:
  The combined effect of both nonmagnetic and magnetic impurities on the
superconducting transition temperature is studied theoretically within the BCS
model. An expression for the critical temperature as a function of potential
and spin-flip scattering rates is derived for a two-dimensional superconductor
with arbitrary in-plane anisotropy of the superconducting order parameter,
ranging from isotropic s-wave to d-wave (or any pairing state with nonzero
angular momentum) and including anisotropic s-wave and mixed (d+s)-wave as
particular cases. This expression generalizes the well-known Abrikosov-Gor'kov
formula for the critical temperature of impure superconductors. The effect of
defects and impurities in high temperature superconductors is discussed.


title:
Quantum dynamics in canonical and micro-canonical ensembles. Part I.
  Anderson localization of electrons

abstract:
  The new numerical approach for consideration of quantum dynamics and
calculations of the average values of quantum operators and time correlation
functions in the Wigner representation of quantum statistical mechanics has
been developed. The time correlation functions have been presented in the form
of the integral of the Weyl's symbol of considered operators and the Fourier
transform of the product of matrix elements of the dynamic propagators. For the
last function the integral Wigner- Liouville's type equation has been derived.
The numerical procedure for solving this equation combining both molecular
dynamics and Monte Carlo methods has been developed. For electrons in
disordered systems of scatterers the numerical results have been obtained for
series of the average values of the quantum operators including position and
momentum dispersions, average energy, energy distribution function as well as
for the frequency dependencies of tensor of electron conductivity and
permittivity according to quantum Kubo formula. Zero or very small value of
static conductivity have been considered as the manifestation of Anderson
localization of electrons in 1D case. Independent evidence of Anderson
localization comes from the behaviour of the calculated time dependence of
position dispersion.


title:
Temperature Effect on the Operation of Elementary Quantum-Dot Spin Gates
  by the Example of the NOT-AND Gate

abstract:
  The effect of temperature on the operation of the elementary quantum-dot spin
gates for single-electron computing is studied theoretically within the
framework of the Hubbard model by the example of the NOT-AND gate. The
calculated values of the uniform external magnetic field necessary to realize
the whole truth table proved to be unreasonably high for the implementation of
the NOT-AND logical function even at the liquid helium temperature. This result
appears to be common to all spin gates. Thus, finite temperatures seem to be a
serious obstacle for the practical realization of ground state calculations in
quantum dot spin gates.


title:
An exact representation of the fermion dynamics in terms of Poisson
  processes and its connection with Monte Carlo algorithms

abstract:
  We present a simple derivation of a Feynman-Kac type formula to study
fermionic systems. In this approach the real time or the imaginary time
dynamics is expressed in terms of the evolution of a collection of Poisson
processes. A computer implementation of this formula leads to a family of
algorithms parametrized by the values of the jump rates of the Poisson
processes. From these an optimal algorithm can be chosen which coincides with
the Green Function Monte Carlo method in the limit when the latter becomes
exact.

categories: cond-mat cs.DS hep-lat math-ph math.MP quant-ph


title:
Quantum-Mechanical Position Operator and Localization in Extended
  Systems

abstract:
  We introduce a fundamental complex quantity, $z_{L}$, which allows us to
discriminate between a conducting and non-conducting thermodynamic phase in
extended quantum systems. Its phase can be related to the expectation value of
the position operator, while its modulus provides an appropriate definition of
a localization length. The expressions are valid for {\it any} fractional
particle filling. As an illustration we use $z_{L}$ to characterize insulator
to ``superconducting'' and Mott transitions in one-dimensional lattice models
with infinite on-site Coulomb repulsion at quarter filling.


title:
A few electrons per ion scenario for the B=0 metal-insulator transition
  in two dimensions

abstract:
  We argue on the basis of experimental numbers that the B=0 metal-insulator
transition in two dimensions, observed in Si-MOSFETs and in other
two-dimensional systems, is likely to be due to a few strongly interacting
electrons, which also interact strongly with the random positively ionized
impurities. At the insulating side the electrons are all bound in pairs to the
ions. On the metallic side free electrons exist which are scattered by ions
dressed with electron-pairs and therefore alter the bare scattering potential
of the ions. The physics at the metallic side of the transition is argued to be
controlled by the classical to quantum transport cross-over leading to the
observed non-monotonous dependence of the resistivity on temperature. This few
electrons per ion scenario appears to be an experimentally realistic and
testable scenario, which can also serve as a starting point for further
theoretical analysis of the two-dimensional metal-insulator transition.


title: Short Time Quantum AC response of a system of Nanomagnets

abstract:
  We calculate the magnetisation relaxation in the short-time regime for an
ensemble of nanomagnets in the presence of a low-frequency external AC biasing
field, at temperatures lower than the magnetic anisotropy energy of the
individual nanomagnets. It is then found that the relaxation is strongly
affected by AC fields with amplitude larger than that of the $T_2$ fluctuations
in the nuclear field. This will allow experimental probing of the nuclear spin
relaxation mechanism.


title:
High frequency magneto-impedance of double perovskite
  $La_{1.2}Sr_{1.8}Mn_{2}O_{7}$: secondary transitions at high temperatures

abstract:
  Radio frequency magneto-impedance measurements clearly reveal a pronounced
anomaly at 260K besides the main MI transition at 125K in the double perovskite
material $La_{1.2}Sr_{1.8}Mn_{2}O_{7}$. This feature is not seen clearly in
static resistivity and magnetization measurements. We suggest that this anomaly
represents short range magnetic correlations enhanced at radio frequencies,
with the easy axis along the c-axis .


title: Non-linear microwave impedance of short and long Josephson Junctions

abstract:
  The non-linear dependence on applied $ac$ field ($b_{\omega}$) or current ($%
i_{\omega}$) of the microwave (ac) impedance $R_{\omega}+iX_{\omega}$ of both
short and long Josephson junctions is calculated under a variety of excitation
conditions. The dependence on the junction width is studied, for both field
symmetric (current anti-symmetric) and field anti-symmetric (current symmetric)
excitation configurations.The resistance shows step-like features every time a
fluxon (soliton) enters the junction, with a corresponding phase slip seen in
the reactance. For finite widths the interference of fluxons leads to some
interesting effects which are described. Many of these calculated results are
observed in microwave impedance measurements on intrinsic and fabricated
Josephson junctions in the high temperature superconductors, and new effects
are suggested. When a $% dc$ field ($b_{dc}$) or current ($i_{dc}$) is applied,
interesting phase locking effects are observed in the ac impedance
$Z_{\omega}$. In particular an almost periodic dependence on the dc bias is
seen similar to that observed in microwave experiments at very low dc field
bias. These results are generic to all systems with a $\cos (\phi)$ potential
in the overdamped limit and subjected to an ac drive.


title:
Quantum Environments: Spin Baths, Oscillator Baths, and applications to
  Quantum Magnetism

abstract:
  The low-energy physics of systems coupled to their surroundings is understood
by truncating to effective Hamiltonians; these tend to reduce to a few
canonical forms, involving coupling to "baths" of oscillators or spins. The
method for doing this is demonstrated using examples from magnetism,
superconductivity, and measurement theory, as is the way one then solves for
the low-energy dynamics. Finally, detailed application is given to the exciting
recent Quantum relaxation and tunneling work in naomagnets.


title: Quantum Relaxation of Ensembles of Nanomagnets

abstract:
  Recent theory and experiment in crystals of molecular magnets suggest that
fundamental tests of the decoherence mechanisms of macroscopic quantum
phenomena may be feasible in these systems (which are almost ideal quantum spin
glasses). We review the results, and suggest new experiments.


title: Superconductivity in correlated disordered two-dimensional electron gas

abstract:
  We calculate the dynamic effective electron-electron interaction potential
for a low density disordered two-dimensional electron gas. The disordered
response function is used to calculate the effective potential where the
scattering rate is taken from typical mobilities from recent experiments. We
investigate the development of an effective attractive pair potential for both
disordered and disorder free systems with correlations determined from existing
numerical simulation data. The effect of disorder and correlations on the
superconducting critical temperature Tc is discussed.


title:
Conductivity of Metallic Si:B near the Metal-Insulator Transition:
  Comparison between Unstressed and Uniaxially Stressed Samples

abstract:
  The low-temperature dc conductivities of barely metallic samples of p-type
Si:B are compared for a series of samples with different dopant concentrations,
n, in the absence of stress (cubic symmetry), and for a single sample driven
from the metallic into the insulating phase by uniaxial compression, S. For all
values of temperature and stress, the conductivity of the stressed sample
collapses onto a single universal scaling curve. The scaling fit indicates that
the conductivity of si:B is proportional to the square-root of T in the
critical range. Our data yield a critical conductivity exponent of 1.6,
considerably larger than the value reported in earlier experiments where the
transition was crossed by varying the dopant concentration. The larger exponent
is based on data in a narrow range of stress near the critical value within
which scaling holds. We show explicitly that the temperature dependences of the
conductivity of stressed and unstressed Si:B are different, suggesting that a
direct comparison of the critical behavior and critical exponents for stress-
tuned and concentration-tuned transitions may not be warranted.


title:
Metal-insulator transition in disordered 2DEG including temperature
  effects

abstract:
  We calculate self-consistently the mutual dependence of electron correlations
and electron-defect scattering for a two dimensional electron gas at finite
temperature. We employ an STLS approach to calculate the electron correlations
while the electron scattering rate off Coulombic impurities and surface
roughness is calculated using self-consistent current-relaxation theory. The
methods are combined and self-consistently solved. We discuss a metal-insulator
transition for a range of disorder levels and electron densities. Our results
are in good agreement with recent experimental observations.


title:
Finite-size scaling of helix-coil transitions in poly-alanine studied by
  multicanonical simulations

abstract:
  We report results from multicanonical simulations of poly-alanine.
Homopolymers of up to 30 amino acids were considered and various thermodynamic
quantities as a function of temperature calculated. We study the nature of the
observed helix-coil transition and present estimates for critical exponents.


title: Sign-time distributions for interface growth

abstract:
  We apply the recently introduced distribution of sign-times (DST) to
non-equilibrium interface growth dynamics. We are able to treat within a
unified picture the persistence properties of a large class of relaxational and
noisy linear growth processes, and prove the existence of a non-trivial scaling
relation. A new critical dimension is found, relating to the persistence
properties of these systems. We also illustrate, by means of numerical
simulations, the different types of DST to be expected in both linear and
non-linear growth mechanisms.


title:
Asymmetric double barrier resonant tunneling structures with improved
  characteristics

abstract:
  We present a self-consistent calculation, based on the global coherent
tunnelling model, and show that structural asymmetry of double barrier resonant
tunnelling structures significantly modifies the current-voltage
characteristics compared to the symmetric structures. In particular, a suitably
designed asymmetric structure can produce much larger peak current and absolute
value of the negative differential conductivity than its commonly used
symmetric counterpart.


title:
Relationship between resistivity and specific heat in a canonical
  non-magnetic heavy fermion alloy system: UPt_5-xAu_x

abstract:
  UPt_(5-x)Au_x alloys form in a single crystal structure, cubic AuBe_5-type,
over a wide range of concentrations from x = 0 to at least x = 2.5. All
investigated alloys, with an exception for x = 2.5, were non-magnetic. Their
electronic specific heat coefficient $\gamma$ varies from about 60 (x = 2) to
about 700 mJ/mol K^2 (x = 1). The electrical resistivity for all alloys has a
Fermi-liquid-like temperature variation, \rho = \rho_o + AT^2, in the limit of
T -> 0 K. The coefficient A is strongly enhanced in the heavy-fermion regime in
comparison with normal and transition metals. It changes from about 0.01 (x =
0) to over 2 micro-ohm cm/K^2 (x = 1). A/\gamma^2, which has been postulated to
have a universal value for heavy-fermions, varies from about 10^-6 (x = 0, 0.5)
to 10^-5 micro-ohm cm (mol K/mJ)^2 (x > 1.1), thus from a value typical of
transition metals to that found for some other heavy-fermion metals. This ratio
is unaffected, or only weakly affected, by chemical or crystallographic
disorder. It correlates with the paramagnetic Curie-Weiss temperature of the
high temperature magnetic susceptibility.


title: Singularities and Pseudogaps in the Density of States of Peierls Chains

abstract:
  We develop a non-perturbative method to calculate the density of states (DOS)
of the fluctuating gap model describing the low-energy physics of electrons on
a disordered Peierls chain. For real order parameter field we calculate the DOS
at the Fermi energy exactly as a functional of the disorder for a chain of
finite length L. Averaging rho (0) with respect to a Gaussian probability
distribution of the Peierls order parameter, we show that in the thermodynamic
limit the average DOS at the Fermi energy diverges for any finite value of the
correlation length above the Peierls transition. Pseudogap behavior emerges
only if the Peierls order parameter is finite and sufficiently large.


title:
Novel Transport Mechanism for Interacting Electrons in Disordered
  Systems: Variable-Range Resonant Tunneling

abstract:
  To interpret recent experimental observations of the phononless hopping
conduction, we suggest a novel transport mechanism according to which the
current-carrying single electrons move via quantum resonant tunneling between
localized states brought into resonance by fast electron hops in their
environment.


title: Zero Temperature Dynamics of the Weakly Disordered Ising Model

abstract:
  The Glauber dynamics of the pure and weakly disordered random-bond 2d Ising
model is studied at zero-temperature. A single characteristic length scale,
$L(t)$, is extracted from the equal time correlation function. In the pure
case, the persistence probability decreases algebraically with the coarsening
length scale. In the disordered case, three distinct regimes are identified: a
short time regime where the behaviour is pure-like; an intermediate regime
where the persistence probability decays non-algebraically with time; and a
long time regime where the domains freeze and there is a cessation of growth.
In the intermediate regime, we find that $P(t)\sim L(t)^{-\theta'}$, where
$\theta' = 0.420\pm 0.009$. The value of $\theta'$ is consistent with that
found for the pure 2d Ising model at zero-temperature. Our results in the
intermediate regime are consistent with a logarithmic decay of the persistence
probability with time, $P(t)\sim (\ln t)^{-\theta_d}$, where $\theta_d =
0.63\pm 0.01$.


title: Normal state properties of an interacting large polaron gas

abstract:
  A simple approach to the many-polaron problem for both weak and intermediate
electron-phonon coupling and valid for densities much smaller than those
typical of metals is presented. Within the model the total energy, the
collective modes and the single-particle properties are studied and compared
with the available theories. It is shown the occurrence of a charge density
wave instability in the intermediate coupling regime.


title:
Fermionic Ising Glasses with BCS Pairing Interaction. Tricritical
  Behaviour

abstract:
  We have examined the role of the BCS pairing mechanism in the formation of
the magnetic moment and henceforth a spin glass (SG) phase by studying a
fermionic Sherrington-Kirkpatrick model with a local BCS coupling between the
fermions. This model is obtained by using perturbation theory to trace out the
conduction electrons degrees of freedom in conventional superconducting alloys.
The model is formulated in the path integral formalism where the spin operators
are represented by bilinear combinations of Grassmann fields and it reduces to
a single site problem that can be solved within the static approximation with a
replica symmetric Ansatz. We argue that this is a valid procedure for values of
temperature above the de Almeida-Thouless instability line. The phase diagram
in the T-g plane, where g is the strength of the pairing interaction, for fixed
variance J^2/N of the random couplings J_{ij}, exhibits three regions: a normal
paramagnetic (NP) phase, a spin glass (SG) phase and a pairing (PAIR) phase
where there is formation of local pairs.The NP and PAIR phases are separated by
a second order transition line g=g_{c}(T) that ends at a tricritical point
T_{3}=0.9807J, g_{3}=5,8843J, from where it becomes a first order transition
line that meets the line of second order transitions at T_{c}=0.9570J that
separates the NP and the SG phases. For T<T_{c} the SG phase is separated from
the PAIR phase by a line of first order transitions.
  These results agree qualitatively with experimental data in
Gd_{x}Th_{1-x}RU_{2}.


title:
Reply to "Comment on `Roughening Transition of Interfaces in Disordered
  Systems'"

abstract:
  We reply to the recent comment cond-mat/9810097 on our original Letter
`Roughening Transition of Interfaces in Disordered Systems', Phys. Rev. Lett.
81, 1469 (1998).


title:
Comment on ``On loss of quantum coherence in an interacting Fermi gas''
  (cond-mat/9810297)

abstract:
  In a recent paper \cite{EH} Eriksen and Hedegard (EH) claimed that they
``clarified'' our path integral calculation \cite{GZ1} and found an error in
our analysis. Here we will demonstrate that this claim is based on
mathematically incorrect and physically meaningless procedure invented by the
authors \cite{EH}. The main problem in the EH analysis is due to their
erroneous calculation of Gaussian path integrals and has nothing to do with the
momentum recoil of a particle interacting with an external bath.


title: Complex periodic potentials with real band spectra

abstract:
  This paper demonstrates that complex PT-symmetric periodic potentials possess
real band spectra. However, there are significant qualitative differences in
the band structure for these potentials when compared with conventional real
periodic potentials. For example, while the potentials V(x)=i\sin^{2N+1}(x),
(N=0, 1, 2, ...), have infinitely many gaps, at the band edges there are
periodic wave functions but no antiperiodic wave functions. Numerical analysis
and higher-order WKB techniques are used to establish these results.


title: From BCS to BEC Superconductivity: Spectroscopic Consequences

abstract:
  The evolution from BCS to BEC superconductivity in the s-wave and d-wave
channels is analyzed at zero temperature for a two-dimensional superconductor.
Spectroscopic quantities for s-wave and d-wave systems present fundamental
differences when particle density and attraction strength are varied. A
detailed analysis of single quasiparticle properties (excitation spectrum,
momentum distribution, spectral function and density of states) indicates that
the evolution of these spectroscopic quantities in the d-wave case is not
smooth, unlike the situation encountered for the s-wave system.


title:
Incremental expansions for the ground state energy of the
  two-dimensional Hubbard model

abstract:
  A generalization of Faddeev's approach of the 3-body problem to the many-body
problem leads to the method of increments. This method was recently applied to
account for the ground state properties of Hubbard-Peierls chains (JETP Letters
67 (1998) 1052). Here we generalize this approach to two-dimensional square
lattices and explicitely treat the incremental expansion up to third order.
Comparing our numerical results with various other approaches (Monte Carlo,
cumulant approaches) we show that incremental expansions are very efficient
because good accuracy with those approaches is achieved treating lattice
segments composed of 8 sites only.


title:
Anomalous pinning behavior in an incommensurate two-chain model of
  friction

abstract:
  Pinning phenomena in an incommensurate two-chain model of friction are
studied numerically. The pinning effect due to the breaking of analyticity
exists in the present model. The pinning behavior is, however, quite different
from that for the breaking of analyticity state of the Frenkel-Kontorova model.
When the elasticity of chains or the strength of interchain interaction is
changed, pinning force and maximum static frictional force show anomalously
complicated behavior accompanied by a successive phase transition and they
vanish completely under certain conditions.


title:
Heat Capacity Evidence for the Suppression of Skyrmions at Large Zeeman
  Energy

abstract:
  Measurements on a multilayer two-dimensional electron system (2DES) near
Landau level filling $\nu$=1 reveal the disappearance of the nuclear spin
contribution to the heat capacity as the ratio $\tilde{g}$ between the Zeeman
and Coulomb energies exceeds a critical value $\tilde{g}_c \approx$0.04. This
disappearance suggests the vanishing of the Skyrmion-mediated coupling between
the lattice and the nuclear spins as the spin excitations of the 2DES make a
transition from Skyrmions to single spin-flips above $\tilde{g}_c$. Our
experimental $\tilde{g}_c$ is smaller than the calculated $\tilde{g}_c$=0.054
for an ideal 2DES; we discuss possible origins of this discrepancy.


title: Plasmons in coupled bilayer structures

abstract:
  We calculate the collective charge density excitation dispersion and spectral
weight in bilayer semiconductor structures {\it including effects of interlayer
tunneling}. The out-of-phase plasmon mode (the ``acoustic'' plasmon) develops a
long wavelength gap in the presence of tunneling with the gap being
proportional to the square root (linear power) of the tunneling amplitude in
the weak (strong) tunneling limit. The in-phase plasmon mode is qualitatively
unaffected by tunneling. The predicted plasmon gap should be a useful tool for
studying many-body effects.


title:
Spin relaxation of conduction electrons in polyvalent metals: A
  realistic calculation

abstract:
  Relaxation of electronic spins in metals is significantly enhanced whenever a
Fermi surface crosses Brillouin zone boundaries, special symmetry points, or
lines of accidental degeneracy. A realistic calculation shows that if aluminum
had one valence electron, its spin relaxation would be slower by nearly two
orders of magnitude. This not only solves a longstanding experimental puzzle,
but also provides a way of tailoring spin dynamics of electrons in a conduction
band.


title: Phonon-assisted tunneling in asymmetric resonant tunneling structures

abstract:
  Based on the dielectric continuum model, we calculated the phonon assisted
tunneling (PAT) current of general double barrier resonant tunneling structures
(DBRTSs) including both symmetric and antisymmetric ones. The results indicate
that the four higher frequency interface phonon modes (especially the one which
peaks at either interface of the emitter barrier) dominate the PAT processes,
which increase the valley current and decrease the PVR of the DBRTSs. We show
that an asymmetric structure can lead to improved performance.


title: Hall magnetocapacitance in two-dimensional electron systems

abstract:
  The magnetocapacitance of a two-dimensional electron system (2DES) is
investigated experimentally, both under and away from Quantum Hall (QH)
conditions, at frequencies between 1 kHz and 100 MHz. The nature of the
capacitive signal in a bounded 2DES is determined by a resistive cut-off
frequency 1/$\tau \propto \sigma_{xx}$, the longitudinal magnetoconductivity. A
new response mechanism is reported for angular frequencies $\omega > 1/\tau$,
which is controlled by the $\em transverse$ or Hall conductivity $\sigma_{xy}$
and the boundaries of the sample, even at frequencies far below those of the
Edge Magnetoplasma resonances and away from the QH-conditions.


title: Semi-Markov Random Walks and Universality in Ising-like Chains

abstract:
  We exhibit a one to one correspondence between some universal probabilistic
properties of the ordering coordinate of one-dimensional Ising-like models and
a class of continuous time random walks. This correspondence provides an new
qualitative picture of the properties of the ordering coordinate of the Ising
model.


title:
Phase Diagram of the 2-Leg Heisenberg Ladder with Alternating
  Dimerization

abstract:
  Using the Lanczos method we determine the phase diagram of the 2-leg
AF-Heisenberg ladder with alternating dimerization. It consists of a resonating
valence bond phase and a dimer phase separated by a critical line. Our results
are in good agreement with previous conjectures obtained with the non linear
sigma model.


title:
Disorder, Order, and Domain Wall Roughening in the 2d Random Field Ising
  Model

abstract:
  Ground states and domain walls are investigated with exact combinatorial
optimization in two-dimensional random field Ising magnets. The ground states
break into domains above a length scale that depends exponentially on the
random field strength squared. For weak disorder, this paramagnetic structure
has remnant long-range order of the percolation type. The domain walls are
super-rough in ordered systems with a roughness exponent $\zeta$ close to 6/5.
The interfaces exhibit rare fluctuations and multiscaling reminiscent of some
models of kinetic roughening and hydrodynamic turbulence.


title: Hausdorff dimension, anyonic distribution functions, and duality

abstract:
  We obtain the distribution functions for anyonic excitations classified into
equivalence classes labeled by Hausdorff dimension $h$ and as an example of
such anyonic systems, we consider the collective excitations of the Fractional
Quantum Hall Effect (FQHE). We also introduce the concept of duality between
such classes, defined by $\tilde{h}=3-h$. In this way, we confirm that the
filling factors for which the FQHE were observed just appears into these
classes and the internal duality for a given class $h$ or $\tilde{h}$ is
between quasihole and quasiparticle excitations for these FQHE systems.
Exchanges of dual pairs $(\nu,\tilde{\nu})$, suggests conformal invariance. A
connection between equivalence classes $h$ and the modular group for the
quantum phase transitions of the FQHE is also obtained. A $\beta-$function is
also defined for the complex conductivity which embodies the $h$ classes.


title: An ordered stack of spin valves in a layered magnetoresistive perovskite

abstract:
  The layered compound La2-2xSr1+2xMn2O7 (x=0.3) consists of bilayers of
metallic MnO2 sheets separated by insulating material. The compound exhibits
markedly anisotropic magnetoresistance at temperatures well below the
three-dimensional magnetic ordering temperature TC=90 K in addition to colossal
magnetoresistance around TC. We present neutron diffraction data which show
that the magnetic structure of this material switches from antiferromagnetic
stacking of the (ferromagnetically ordered) sheets in zero field to
ferromagnetic stacking in a field of 1.5 Tesla. The data are the first to be
collected on any manganite as a function of applied field, exactly as the
magnetoresistance data themselves are collected. They provide a natural
explanation of the low-field magnetoresistance in the ordered phase in terms of
spin-polarised tunnelling between the magnetic layers and suggest that the
material is a bulk stack of spin-valve devices.


title: Interplay of Chemical Bonding and Magnetism in Fe_4N, Fe_3N, Fe_2N

abstract:
  Using spin density functional theory we have carried out a comparative study
of chemical bonding and magnetism in Fe_4N, Fe_3N and Fe_2N. All of these
compounds form close packed Fe lattices, while N occupies octahedral
interstitial positions. High spin fcc Fe and hypothetical FeN with rock salt
structure have been included in our study as reference systems. We find strong,
covalent Fe-N bonds as a result of a substantial \sigma-type p-d hybridisation,
with some charge transfer to N. Those Fe d orbitals which contribute to the p-d
bonds, do no longer participate in the exchange splitting of the Fe d bands.
Because of the large exchange fields, the majority spin d bands are always
fully occupied, while the minority spin d bands are close to half-filling, thus
optimizing the Fe d-d covalent bonding. As a consequence, in good approximation
the individual Fe moments decrease in steps of 0.5 \mu_B from fcc iron (2.7
\mu_B) via Fe_4N (2.7 and 1.97 mu_B}), \chem{Fe_3N} (1.99 \mu_B) to \zeta -
Fe_2N (1.43 \mu_B).


title:
Evidence for saturation of channel transmission from conductance
  fluctuations in atomic-size point contacts

abstract:
  The conductance of atomic size contacts has a small, random, voltage
dependent component analogous to conductance fluctuations observed in diffusive
wires (UCF). A new effect is observed in gold contacts, consisting of a marked
suppression of these fluctuations when the conductance of the contact is close
to integer multiples of the conductance quantum. Using a model based on the
Landauer-Buettiker formalism we interpret this effect as evidence that the
conductance tends to be built up from fully transmitted (i.e., saturated)
channels plus a single, which is partially transmitted.


title: Theory of Doping and Defects in III-V Nitrides

abstract:
  Doping problems in GaN and in AlGaN alloys are addressed on the basis of
state-of-the-art first-principles calculations. For n-type doping we find that
nitrogen vacancies are too high in energy to be incorporated during growth, but
silicon and oxygen readily form donors. The properties of oxygen, including
DX-center formation, support it as the main cause of unintentional n-type
conductivity. For p-type doping we find that the solubility of Mg is the main
factor limiting the hole concentration in GaN. We discuss the beneficial
effects of hydrogen during acceptor doping. Compensation of acceptors by
nitrogen vacancies may occur, becoming increasingly severe as x increases in
Al_x Ga_(1-x)N alloys.


title: Parallel dynamics of extremely diluted symmetric Q-Ising neural networks

abstract:
  The parallel dynamics of extremely diluted symmetric Q-Ising neural networks
is studied for arbitrary Q using a probabilistic approach. In spite of the
extremely diluted architecture the feedback correlations arising from the
symmetry prevent a closed-form solution in contrast with the extremely diluted
asymmetric model. A recursive scheme is found determining the complete time
evolution of the order parameters taking into account all feedback. It is based
upon the evolution of the distribution of the local field, as in the fully
connected model. As an illustrative example an explicit analysis is carried out
for the Q=2 and Q=3 model. These results agree with and extend the partial
results existing for Q=2. For Q>2 the analysis is entirely new. Finally,
equilibrium fixed-point equations are derived and a capacity-gain function
diagram is obtained.


title: Spin dynamics of hole doped Y2BaNiO5

abstract:
  Starting from a multiband Hamiltonian containing the relevant Ni and O
orbitals, we derive an effective Hamiltonian $H_{eff}$ for the low energy
physics of doped Y$_{2}$BaNiO$_{5}.$ For hole doping, $H_{eff}$ describes O
fermions interacting with S=1 Ni spins in a chain, and cannot be further
reduced to a simple one-band model. Using numerical techniques, we obtain a
dynamical spin structure factor with weight inside the Haldane gap. The nature
of these low-energy excitations is identified and the emerging physical picture
is consistent with most of the experimental information in Y%
$_{2-x}$Ca$_{x}$BaNiO$_{5}$


title: Multidimensional Bosonization

abstract:
  Bosonization of degenerate fermions yields insight both into Landau Fermi
liquids, and into non-Fermi liquids. We begin our review with a pedagogical
introduction to bosonization, emphasizing its applicability in spatial
dimensions greater than one. After a brief historical overview, we present the
essentials of the method. Well known results of Landau theory are recovered,
demonstrating that this new tool of many-body theory is robust. Limits of
multidimensional bosonization are tested by considering several examples of
non-Fermi liquids, in particular the composite fermion theory of the
half-filled Landau level. Nested Fermi surfaces present a different challenge,
and these may be relevant in the cuprate superconductors. We conclude by
discussing the future of multidimensional bosonization.

categories: cond-mat.str-el cond-mat.mes-hall cond-mat.supr-con hep-th


title: Continuum Double Exchange Model

abstract:
  We present a continuum model for doped manganites which consist of two
species of quantum spin 1/2 fermions interacting with classical spin fields.
The phase structure at zero temperature turns out to be considerably rich:
antiferromagnetic insulator, antiferromagnetic two band conducting, canted two
band conducting, canted one band conducting and ferromagnetic one band
conducting phases are identified, all of them being stable against phase
separation. There are also regions in the phase diagram where phase separation
occurs.


title:
Quantum atomic delocalization vs. structural disorder in amorphous
  silicon

abstract:
  Quantum effects on the atom delocalization in amorphous silicon have been
studied by path-integral Monte Carlo simulations from 30 to 800 K. The quantum
delocalization is appreciable vs. topological disorder, as seen from structural
observables such as the radial distribution function (RDF). At low
temperatures, the width of the first peak in the RDF increases by a factor of
1.5 due to quantum effects. The overall anharmonicity of the solid vibrations
at finite temperatures in amorphous silicon is clearly larger than in the
crystalline material. Low-energy vibrational modes are mainly located on
coordination defects in the amorphous material.


title:
Nearly universal crossing point of the specific heat curves of Hubbard
  models

abstract:
  A nearly universal feature of the specific heat curves C(T,U) vs. T for
different U of a general class of Hubbard models is observed. That is, the
value C_+ of the specific heat curves at their high-temperature crossing point
T_+ is almost independent of lattice structure and spatial dimension d, with
C_+/k_B \approx 0.34. This surprising feature is explained within second order
perturbation theory in U by identifying two small parameters controlling the
value of C_+: the integral over the deviation of the density of states
N(\epsilon) from a constant value, characterized by \delta N=\int d\epsilon
|N(\epsilon)-1/2|, and the inverse dimension, 1/d.


title: Force distribution in a scalar model for non-cohesive granular material

abstract:
  We study a scalar lattice model for inter-grain forces in static,
non-cohesive, granular materials, obtaining two primary results. (i) The
applied stress as a function of overall strain shows a power law dependence
with a nontrivial exponent, which moreover varies with system geometry. (ii)
Probability distributions for forces on individual grains appear Gaussian at
all stages of compression, showing no evidence of exponential tails. With
regard to both results, we identify correlations responsible for deviations
from previously suggested theories.


title: Anisotropic pseudogap in the half-filling 2-d Hubbard model at finite T

abstract:
  We have studied the pseudogap formation in the single-particle spectra of the
half-filling two-dimensional Hubbard model. Using a Green's function with the
one-loop self-energy correction of the spin and charge fluctuations, we have
numerically calculated the self-energy, the spectral function, and the density
of states in the weak-coupling regime at finite temperature. Pseudogap
formations have been observed in both the density of states and the spectral
function at the Fermi level. The pseudogap in the spectral function is
explained by the non-Fermi-liquid-like nature of the self-energy. The anomalous
behavior in the self-energy is caused by both the strong antiferromagnetic spin
fluctuation and the nesting condition on the non-interacting Fermi surface. In
the present approximation, we find a logarithmic singularity in the integrand
of the imaginary part of the self-energy. Concerning the energy dependence of
the spectral function and the self-energy, two theorems are proved. They give a
necessary condition in the self-energy to produce the pseudogap at the Fermi
level. The pseudogap in the spectral function is highly momentum dependent on
the Fermi surface. It opens initially in the $(\pm \pi,0)$, $(0,\pm \pi)$
regions as the normal state pseudogap observed in the high-$T_c$
superconductors and if the interaction is increased, it spreads to other Fermi
surface sectors. The anisotropy of the pseudogap is produced by the low-energy
enhancement of the spin excitation around ${\bf Q}=(\pi,\pi)$ and the flatness
of the band dispersion around the saddle point.


title: Double-exchange via degenerate orbitals

abstract:
  We consider the double-exchange for systems in which doped electrons occupy
degenerate orbitals, treating the realistic situation with double degenerate
$e_g$ orbitals. We show that the orbital degeneracy leads in general to
formation of anisotropic magnetic structures and that in particular, depending
on the doping concentration, the layered magnetic structures of the A-type and
chain-like structures of the C-type are stabilized. The phase-diagram that we
obtain provides an explanation for the experimentally observed magnetic
structures of some over-doped (electron-doped) manganites of the type
Nd$_{1-x}$Sr$_x$MnO$_3$, Pr$_{1-x}$Sr$_x$MnO$_3$ and Sm$_{1-x}$Ca$_x$MnO$_3$
with $x > 0.5$.


title:
Quantum dissipation due to the interaction with chaotic
  degrees-of-freedom and the correspondence principle

abstract:
  Both in atomic physics and in mesoscopic physics it is sometimes interesting
to consider the energy time-dependence of a parametrically-driven chaotic
system. We assume an Hamiltonian ${\cal H}(Q,P;x(t))$ where $x(t)=Vt$. The
velocity $V$ is slow in the classical sense but not necessarily in the
quantum-mechanical sense. The crossover (in time) from ballistic to diffusive
energy-spreading is studied. The associated irreversible growth of the average
energy has the meaning of dissipation. It is found that a dimensionless
velocity $v_{PR}$ determines the nature of the dynamics, and controls the route
towards quantal-classical correspondence (QCC). A perturbative regime and a
non-perturbative semiclassical regime are distinguished.


title:
Symmetry Breaking with a Slant: Topological Defects after an
  Inhomogeneous Quench

abstract:
  We show that, in second-order phase transformations induced by an
inhomogeneous quench, the density of topological defects is drastically
suppressed as the velocity with which the quench propagates becomes smaller
than the speed at which the front of the broken symmetry phase spreads. The
velocity of the broken symmetry phase front is approximately given by the ratio
of the healing length to relaxation time at freeze-out, that is at the instant
when the critical slowing down results in a transition from the adiabatic to
the impulse behavior in the order parameter. Experimental implications are
briefly discussed.


title: The Folding Funnel Landscape of the Peptide Met-enkephalin

abstract:
  We study the free energy landscape of the small peptide Met-enkephalin. Our
data were obtained from a generalized-ensemble Monte Carlo simulation taking
the interactions among all atoms into account. We show that the free energy
landscape resembles that of a funnel, indicating that this peptide is a good
folder. Our work demonstrates that the energy landscape picture and folding
concept, developed in the context of simplified protein models, can also be
used to describe the folding in more realistic models.


title:
Random-phase Approximation Treatment Of Edge Magnetoplasmons: Edge-state
  Screening And Nonlocality

abstract:
  A random-phase approximation (RPA) treatment of edge magnetoplasmons (EMP) is
presented for strong magnetic fields, low temperatures, and integer filling
factors \nu. It is valid for negligible dissipation and lateral confining
potentials smooth on the scale of the magnetic length \ell_{0} but sufficiently
steep that the Landau-level (LL) flattening can be neglected. LL coupling,
screening by edge states, and nonlocal contributions to the current density are
taken into account. In addition to the fundamental mode with typical dispersion
relation \omega\sim q_x \ln(q_{x}), fundamental modes with {\it acoustic}
dispersion relation \omega\sim q_x are obtained for \nu>2. For \nu=1,2 a {\bf
dipole} mode exists, with dispersion relation \omega\sim q_x^3, that is
directly related to nonlocal responses.


title:
Universal relaxational dynamics near two-dimensional quantum-critical
  points

abstract:
  We describe the nonzero temperature (T), low frequency (\omega) dynamics of
the order parameter near quantum critical points in two spatial dimensions (d),
with a special focus on the regime \hbar\omega << k_B T. For the case of a
`relativistic', O(n)-symmetric, bosonic quantum field theory we show that, for
small \epsilon=3-d, the dynamics is described by an effective classical model
of waves with a quartic interaction. We provide analytical and numerical
analyses of the classical wave model directly in d=2. We describe the crossover
from the finite frequency, "amplitude fluctuation", gapped quasiparticle mode
in the quantum paramagnet (or Mott insulator), to the zero frequency "phase" (n
>= 2) or "domain wall" (n=1) relaxation mode near the ordered state. For static
properties, we show how a surprising, duality-like transformation allows an
exact treatment of the strong-coupling limit for all n. For n=2, we compute the
universal T dependence of the superfluid density below the Kosterlitz-Thouless
temperature, and discuss implications for the high temperature superconductors.
For n=3, our computations of the dynamic structure factor relate to neutron
scattering experiments on La_{1.85}Sr_{0.15}CuO_4, and to light scattering
experiments on double layer quantum Hall systems. We expect that closely
related effective classical wave models will apply also to other quantum
critical points in d=2.


title: Universal Level dynamics of Complex Systems

abstract:
  . We study the evolution of the distribution of eigenvalues of a $N\times N$
matrix subject to a random perturbation drawn from (i) a generalized Gaussian
ensemble (ii) a non-Gaussian ensemble with a measure variable under the change
of basis. It turns out that, in the case (i), a redefinition of the parameter
governing the evolution leads to a Fokker-Planck equation similar to the one
obtained when the perturbation is taken from a standard Gaussian ensemble (with
invariant measure). This equivalence can therefore help us to obtain the
correlations for various physically-significant cases modeled by generalized
Gaussian ensembles by using the already known correlations for standard
Gaussian ensembles.
  For large $N$-values, our results for both cases (i) and (ii) are similar to
those obtained for Wigner-Dyson gas as well as for the perturbation taken from
a standard Gaussian ensemble. This seems to suggest the independence of
evolution, in thermodynamic limit, from the nature of perturbation involved as
well as the initial conditions and therefore universality of dynamics of the
eigenvalues of complex systems.


title:
Current-carrying states in superconductor/insulator and
  superconductor/semiconductor superlattices in the mesoscopic regime

abstract:
  We discuss some of the basic theoretical aspects of current-carrying states
in superconducting superlattices with tunnel barriers in the mesoscopic regime,
when the superconducting layer thickness is small compared to the BCS coherence
length but large compared to the atomic scale. We establish the necessary
conditions for the observation of the classical Josephson effect (with
sinusoidal current-phase dependence) and derive self-consistent analytical
expressions for the critical Josephson current. These expressions are
proportional to an additional small factor and have unusual temperature
dependence as compared with the single-junction case. For certain parameter
values, the superconducting gap exhibits an exponential decrease due to
pair-breaking effect of the supercurrent. The supercurrent can completely
destroy the superconductivity of the system above a certain characteristic
temperature (lower than the transition temperature of individual layers). In
this paper, we also study the effect of intrabarrier exchange interactions. We
show that this effect is strongly enhanced compared with the single- junction
case and can manifest itself in an exponential decrease of the critical
temperature.


title:
Phase diagram and magnons in quasi-one-dimensional dipolar
  antiferromagnets

abstract:
  We investigate antiferromagnetic spin chains, which are coupled by a weak
antiferromagnetic exchange interaction on a hexagonal lattice. We particulary
study the role of the dipole-dipole interaction within the framework of a
Heisenberg model with nearest-neighbor exchange and additional dipolar
interaction. We find several commensurate and incommensurate phases depending
on the ratio of dipolar energy to interchain-exchange energy due to their
competing qualtity. The ground-state analysis is supplemented by a stability
analysis by means of a linear spin-wave theory. In comparison with experiments
(CsMnBr_3, RbMnBr_3) we obtain good agreement for the energy gaps. From this we
conclude, that the dipolar interaction is the most important source of
anisotropy in these Mn-compounds.


title: Dynamic Density Functional Theory of Fluids

abstract:
  We present a new time-dependent Density Functional approach to study the
relaxational dynamics of an assembly of interacting particles subject to
thermal noise. Starting from the Langevin stochastic equations of motion for
the velocities of the particles we are able by means of an approximated closure
to derive a self-consistent deterministic equation for the temporal evolution
of the average particle density.
  The closure is equivalent to assuming that the equal-time two-point
correlation function out of equilibrium has the same properties as its
equilibrium version.
  The changes in time of the density depend on the functional derivatives of
the grand canonical free energy functional $F[\rho]$ of the system. In
particular the static solutions of the equation for the density correspond to
the exact equilibrium profiles provided one is able to determine the exact form
of $F[\rho]$. In order to assess the validity of our approach we performed a
comparison between the Langevin dynamics and the dynamic density functional
method for a one-dimensional hard-rod system in three relevant cases and found
remarkable agreement, with some interesting exceptions, which are discussed and
explained. In addition, we consider the case where one is forced to use an
approximate form of $F[\rho]$.
  Finally we compare the present method with the stochastic equation for the
density proposed by other authors [Kawasaki,Kirkpatrick etc.] and discuss the
role of the thermal fluctuations.


title:
Interface pinning and slow ordering kinetics on infinitely ramified
  fractal structures

abstract:
  We investigate the time dependent Ginzburg-Landau (TDGL) equation for a non
conserved order parameter on an infinitely ramified (deterministic) fractal
lattice employing two alternative methods: the auxiliary field approach and a
numerical method of integration of the equations of evolution. In the first
case the domain size evolves with time as $L(t)\sim t^{1/d_w}$, where $d_w$ is
the anomalous random walk exponent associated with the fractal and differs from
the normal value 2, which characterizes all Euclidean lattices. Such a power
law growth is identical to the one observed in the study of the spherical model
on the same lattice, but fails to describe the asymptotic behavior of the
numerical solutions of the TDGL equation for a scalar order parameter. In fact,
the simulations performed on a two dimensional Sierpinski Carpet indicate that,
after an initial stage dominated by a curvature reduction mechanism \`a la
Allen-Cahn, the system enters in a regime where the domain walls between
competing phases are pinned by lattice defects.
  The lack of translational invariance determines a rough free energy
landscape, the existence of many metastable minima and the suppression of the
marginally stable modes, which in translationally invariant systems lead to
power law growth and self similar patterns. On fractal structures as the
temperature vanishes the evolution is frozen, since only thermally activated
processes can sustain the growth of pinned domains.


title:
The Renormalization-Group peculiarities of Griffiths and Pearce: What
  have we learned?

abstract:
  We review what we have learned about the "Renormalization-Group
peculiarities" which were discovered about twenty years ago by Griffiths and
Pearce, and which questions they asked are still widely open. We also mention
some related developments.


title: Topological origin of the phase transition in a mean-field model

abstract:
  We argue that the phase transition in the mean-field XY model is related to a
particular change in the topology of its configuration space. The nature of
this topological transition can be discussed on the basis of elementary Morse
theory using the potential energy per particle V as a Morse function. The value
of V where such a topological transition occurs equals the thermodynamic value
of V at the phase transition and the number of (Morse) critical points grows
very fast with the number of particles N. Furthermore, as in statistical
mechanics, also in topology the way the thermodynamic limit is taken is
crucial.


title: Quantized Roentgen Effect in Bose-Einstein Condensates

abstract:
  A classical dielectric moving in a charged capacitor can create a magnetic
field (Roentgen effect). A quantum dielectric, however, will not produce a
magnetization, except at vortices. The magnetic field outside the quantum
dielectric appears as the field of quantized monopoles.


title: Self-organized criticality and directed percolation

abstract:
  A sandpile model with stochastic toppling rule is studied. The control
parameters and the phase diagram are determined through a MF approach, the
subcritical and critical regions are analyzed. The model is found to have some
similarities with directed percolation, but the existence of different boundary
conditions and conservation law leads to a different universality class, where
the critical state is extended to a line segment due to self-organization.
These results are supported with numerical simulations in one dimension. The
present model constitute a simple model which capture the essential difference
between ordinary nonequilibrium critical phenomena, like DP, and self-organized
criticality.


title:
Correlation energy of an electron gas in strong magnetic fields at high
  densities

abstract:
  The high-density electron gas in a strong magnetic field B and at zero
temperature is investigated. The quantum strong-field limit is considered in
which only the lowest Landau level is occupied. It is shown that the
perturbation series of the ground-state energy can be represented in analogy to
the Gell-Mann Brueckner expression of the ground-state energy of the field-free
electron gas. The role of the expansion parameter is taken by r_B= (2/3 \pi^2)
(B/m^2) (\hbar r_s /e)^3 instead of the field-free Gell-Mann Brueckner
parameter r_s. The perturbation series is given exactly up to o(r_B) for the
case of a small filling factor for the lowest Landau level.


title:
Hall Effect of La2/3(Ca,Pb)1/3MnO3 Single Crystals near the Critical
  Temperature

abstract:
  The Hall resistivity rho_{xy} of a La_{2/3}(Ca,Pb)_{1/3}MnO_3 single crystal
has been measured as a function of temperature and field. The overall behavior
is similar to that observed previously in thin-films. At 5 K, rho_{xy} is
positive and linear in field, indicating that the anomalous contribution $R_S$
is negligible. However, the effective carrier density in a free electron model
is n_{eff}=2.4 holes/Mn, even larger than the 0.85-1.9 holes/Mn reported for
thin-films and far larger than the 0.33 holes/Mn expected from the doping
level. As temperature increases, a strong, negative contribution to rho_{xy}
appears, that we ascribe to R_S. Using detailed magnetization data, we separate
the ordinary (\propto B) and anomalous (\propto M) contributions. Below T_C,
R_S \propto rho_{xx}, indicating that magnetic skew scattering is the dominant
mechanism in the metallic ferromagnetic regime. At and above the
resistivity-peak temperature, we find that rho_{xy}/rho_{xx}M is a constant,
independent of temperature and field. This implies that the anomalous Hall
coefficient is proportional to the magnetoresistance. A different explanation
based on two fluid model is also presented.


title: Temperature Dependence of Facet Ridges in Crystal Surfaces

abstract:
  The equilibrium crystal shape of a body-centered solid-on-solid (BCSOS) model
on a honeycomb lattice is studied numerically. We focus on the facet ridge
endpoints (FRE). These points are equivalent to one dimensional KPZ-type growth
in the exactly soluble square lattice BCSOS model. In our more general context
the transfer matrix is not stochastic at the FRE points, and a more complex
structure develops. We observe ridge lines sticking into the rough phase where
thesurface orientation jumps inside the rounded part of the crystal. Moreover,
the rough-to-faceted edges become first-order with a jump in surface
orientation, between the FRE point and Pokrovsky-Talapov (PT) type critical
endpoints. The latter display anisotropic scaling with exponent $z=3$ instead
of familiar PT value $z=2$.


title:
Fourth-Order Cumulants to Characterize the Phase Transitions of a Spin-1
  Ising Model

abstract:
  Fourth-order cumulants of physical quantities have been used to characterize
the nature of a phase transition. In this paper we report some Monte Carlo
simulations to illustrate the behavior of fourth-order cumulants of
magnetization and energy across second and first-order transitions in the phase
diagram of a well known spin-1 Ising model. Simple ideas from the theory of
thermodynamic fluctuations are used to account for the behavior of these
cumulants.


title: The Markov approximation for the atomic output coupler

abstract:
  The regions of validity of the Markov approximation for the coupling of atoms
out of an atomic trap are determined. We consider radio-frequency output
coupling in the presence of gravity and collisional repulsion, and Raman output
coupling. The Markov approximation is crucial in most theoretical descriptions
of an atom laser that assume a continuous process of output coupling from a
trapped Bose-Einstein condensate. In this regime many techniques proved to be
useful for modeling the optical laser, such as master equations, can be used to
describe the dynamics of the damping of the condensate mode undergoing output
coupling.


title: Two-Dimensional Quantum Spin Systems with Ladder and Plaquette Structure

abstract:
  We investigate low-energy properties of two-dimensional quantum spin systems
with the ladder and plaquette structures, which are described by a generalized
antiferromagnetic Heisenberg model with both of the bond and spin alternations.
By exploiting a non-linear $\sigma$ model technique and a modified spin wave
approach, we evaluate the spin gap and the spontaneous magnetization to discuss
the quantum phase transition between the ordered and disordered states. We
argue how the spin-gapped phase is driven to the antiferromagnetic phase in the
phase diagram.


title: An SO(5) model of p-wave superconductivity and ferromagnetism

abstract:
  We study an SO(5) model unifying p-wave superconductivity and ferromagnetism.
If only a single p-wave pairing wavefunction is involved, the p-wave
superconducting (pSC) and ferromagnetic (F) states can be unified to form
10-dimensional multiplet of SO(5). The collective modes in pSC and F states are
studied from this viewpoint in terms of a non-linear sigma model.


title: Dynamics of Highly Supercooled Liquids

abstract:
  The diffusivity of tagged particles is demonstrated to be heterogeneous on
time scales comparable to or less than the structural relaxation time %taking
place at the interparticle distance in a highly supercooled liquid via 3D
molecular dynamics simulation. The particle motions in the relatively active
regions dominantly contribute to the mean square displacement, giving rise to a
diffusion constant systematically larger than the Stokes-Einstein value. The
van Hove self-correlation function $G_s(r,t)$ is shown to have a large $r$ tail
which can be scaled in terms of $r/t^{1/2}$ for $t \ls 3 \tau_\alpha$, where
$\tau_\alpha \cong$ the stress relaxation time. Its presence indicates
heterogeneous diffusion in the active regions. However, the diffusion process
eventually becomes homogeneous on time scales longer than the life time of the
heterogeneity structure ($\sim 3 \tau_{\alpha}$).


title:
The Kosterlitz-Thouless and magnetic transition temperatures in layered
  magnets with a weak easy-plane anisotropy

abstract:
  The two-dimensional (2D) Heisenberg magnet with a weak easy-plane anisotropy
is considered. A renormalization group (RG) analysis in this model is performed
for both quantum and classical cases. A crossover from the Heisenberg to 2D XY
model is discussed. The magnetic transition owing to the interlayer coupling is
considered. Analytical results for the Kosterlitz-Thouless and Curie (Neel)
temperatures are derived with account of two-loop corrections. The results are
compared with experimental data, e.g. on K2CuF4, and turn out to provide a
quantitative description, unlike the standard one-loop results.


title: Charge screening and magnetic anisotropy in RCo5 compounds

abstract:
  An analysis of magnetic anisotropy in RCo5 compounds is performed with
account of screening of ion point charges by conduction electrons. A crucial
role of non-uniform distribution of screening electrons (the terms containing
derivatives of charge density) is demonstrated. Influence of anisotropy of
screening, that is connected with the anisotropy of the Fermi surface, on the
magnetic anisotropy sign is discussed.


title: Landau-Ginzburg Theories for Non-Abelian Quantum Hall States

abstract:
  We construct Landau-Ginzburg effective field theories for fractional quantum
Hall states -- such as the Pfaffian state -- which exhibit non-Abelian
statistics. These theories rely on a Meissner construction which increases the
level of a non-Abelian Chern-Simons theory while simultaneously projecting out
the unwanted degrees of freedom of a concomitant enveloping Abelian theory. We
describe this construction in the context of a system of bosons at Landau level
filling factor $\nu=1$, where the non-Abelian symmetry is a
dynamically-generated SU(2) continuous extension of the discrete particle-hole
symmetry of the lowest Landau level. We show how the physics of quasiparticles
and their non-Abelian statistics arises in this Landau-Ginzburg theory. We
describe its relation to edge theories -- where a coset construction plays the
role of the Meissner projection -- and discuss extensions to other states.


title: Co-operative Kondo Effect in the two-channel Kondo Lattice

abstract:
  We discuss the possibility of a co-operative Kondo effect driven by channel
interference in a Kondo lattice where local moments are coupled to a single
Fermi sea via two orthogonal scattering channels. In this situation, the
channel quantum number is not conserved. We argue that the absence of channel
conservation causes the Kondo effect in the two channels to constructively
interfere, giving rise to a superconducting condensate of composite pairs,
formed between the local moments and the conduction electrons. Our arguments
are based on the observation that a heavy Fermi surface gives rise to zero
modes for Kondo singlets to fluctuate between screening channels of different
symmetry, producing a divergent composite pair susceptibility. Secondary
screening channels couple to these divergent fluctuations, promoting an
instability into a state with long-range composite order. We present detailed a
detailed mean-field theory for this superconducting phase, and discuss the
possible implications for heavy fermion physics.


title: Thermodynamics for Fractal Statistics

abstract:
  We consider for an anyon gas its termodynamics properties taking into account
the fractal statistics obtained by us recently. This approach describes the
anyonic excitations in terms of equivalence classes labeled by fractal
parameter or Hausdorff dimension $h$. An exact equation of state is obtained in
the high-temperature and low-temperature limits, for gases with a constant
density of states.


title: Fractional Quantization and Fractional Quantum Hall Effect

abstract:
  A fractional quantization in a two dimensional space is proposed.
  The angular momenta of the two dimensional electrons are quantized in
fractional numbers by the boundary conditions on a multi-layered Riemann
surface. Extended wave functions for the incompressible quantum fluid states
are presented and the cohesive and the excitation energies are given.


title:
Mean Magnetic Field and Noise Cross-Correlation in Magnetohydrodynamic
  Turbulence: Results from a One-Dimensional Model

abstract:
  We show that a recently proposed [J. Fleischer and P.H. Diamond, {\em Phys.
Rev. E}{\bf 58}, R2709 (1998)] one-dimensional Burgers-like model for
magnetohydrodynamics (MHD) is in effect identical to existing models for
drifting lines and sedimenting lattices. We use the model to demonstrate,
contrary to claims in the literature, that the energy spectrum of MHD
turbulence should be independent of mean magnetic field and that
cross-correlations between the noise sources for the velocity and magnetic
fields cannot change the structure of the equations under renormalisation. We
comment on the scaling and the multiscaling properties of the stochastically
forced version of the model.


title:
Decay of Superflow Confined in Thin Torus: A Realization of Tunneling
  Quantum Fields

abstract:
  The quantum nucleation of phase slips in neutral superfluids confined in a
thin torus is investigated by means of the collective coordinate method. We
have devised, with numerical justification, a certain collective coordinate to
describe the quantum nucleation process of a phase slip. Considering the
quantum fluctuation around the local minimum of the action, we calculate the
effective mass of the phase slip. Due to the coherence of the condensate
throughout the torus, the effective mass is proportional to the circumference L
of the torus, and the decay rate has a strong exponential L-dependence.


title: Transport in Double-Crossed Luttinger Liquids

abstract:
  We study transport through two Luttinger liquids (one-dimensional electrons
interacting through a Coulomb repulsion in a metal) coupled together at {\it
two} points. External voltage biases are incorporated through boundary
conditions. We include density-density couplings as well as single-particle
hops at the contacts. For weak repulsive interactions, transport through the
wires remains undisturbed by the inter-wire couplings, which renormalise to
zero. For strong repulsive interactions, the inter-wire couplings become
strong. For symmetric barriers and no external voltage bias, a single gate
voltage is sufficient to tune for resonance transmission in both wires.
However, for asymmetric couplings or for finite external biases, the system is
insulating.


title: Finite temperature excitations of Bose gases in anisotropic traps

abstract:
  The mode frequencies of a weakly interacting Bose gas in a magnetic trap are
studied as a function of the anisotropy of the trap. As in earlier works the
generalized Hartree-Fock-Bogoliubov equations within the Popov approximation
(HFB-Popov) are used for our calculations. The new feature of our work is the
combined use of a mode expansion in a finite basis and a semiclassical
approximation of the highly excited states. The results are applied to check
the accuracy of the recently suggested equivalent zero-temperature condensate
(EZC) approximation which involves a much simpler model.


title:
Finite temperature Drude weight of the one dimensional spin 1/2
  Heisenberg model}

abstract:
  Using the Bethe ansatz method, the zero frequency contribution (Drude weight)
to the spin current correlations is analyzed for the easy plane
antiferromagnetic Heisenberg model. The Drude weight is a monotonically
decreasing function of temperature for all 0<Delta< 1, it approaches the zero
temperature value with a power law and it appears to vanish for all finite
temperatures at the isotropic Delta=1 point.


title:
Proximity Induced Josephson-Quasiparticle Process in a Single Electron
  Transistor

abstract:
  We have performed the first experiments in a superconductor - normal metal -
superconductor single electron transistor in which there is an extra
superconducting strip partially overlapping the normal metal island in good
metal-to-metal contact. Superconducting proximity effect gives rise to current
peaks at voltages below the quasiparticle threshold. We interpret these peaks
in terms of the Josephson-quasiparticle process and discuss their connection
with the proximity induced energy gap in the normal metal island.


title: A parity conserving dimer model with infinitely many absorbing states

abstract:
  We propose and study a model where, for the first time, two aspects are
present: parity conservation and infinitely many absorbing states. Whereas
steady-state simulations show that the static critical behaviour is not
affected by the presence of multiple absorbing configurations, the influence of
the initial state associated with the presence of slowly decaying memory
effects is clearly displayed in time dependent simulations. We report results
of a detailed investigation of the dependence of critical spreading exponents
on the initial particle density.


title: Fast coarsening in unstable epitaxy with desorption

abstract:
  Homoepitaxial growth is unstable towards the formation of pyramidal mounds
when interlayer transport is reduced due to activation barriers to hopping at
step edges. Simulations of a lattice model and a continuum equation show that a
small amount of desorption dramatically speeds up the coarsening of the mound
array, leading to coarsening exponents between 1/3 and 1/2. The underlying
mechanism is the faster growth of larger mounds due to their lower evaporation
rate.


title:
Anomalous Periodicity of the Current-Phase Relationship of
  Grain-Boundary Josephson Junctions in High-Tc Superconductors

abstract:
  The current-phase relation (CPR) for asymmetric 45 degree Josephson junctions
between two d-wave superconductors has been predicted to exhibit an anomalous
periodicity. We have used the single-junction interferometer to investigate the
CPR for this kind of junctions in YBCO thin films. Half-fluxon periodicity has
been experimentally found, providing a novel source of evidence for the d-wave
symmetry of the pairing state of the cuprates.


title: Universality of quantum Brownian motion

abstract:
  Are Markovian master equations for quantum Brownian motion independent of
model assumptions used in the derivation and, thus, universal? With the aim of
answering this question, we use a random band-matrix model for the system-bath
interaction to derive Markovian master equations for the time evolution of
one-dimensional quantum systems weakly coupled to a heat bath. We study in
detail two simple systems, the harmonic oscillator and the two-level system.
Our results are in complete agreement with those of earlier models, like the
Caldeira-Legget model and, in the large-band limit, with the Agarwal equations
(both with and without rotating-wave approximation). This proves the
universality of these master equations.


title:
Dynamics and thermodynamics of the pseudospin-electron model in the case
  of absence of the electron Hubbard correlation. I. The analytical
  consideration

abstract:
  Dynamics and thermodynamics of the model with local anharmonism in the case
of absence of the electron (Hubbard) correlation is investigated in the present
work. The correlation functions, mean values of pseudospin and particle number
as well as the thermodynamical potential are calculated. The calculation is
performed by diagrammatic method in the mean field approximation.
Single--particle Green functions are taken in the Hubbard--I approximation.


title: Interfacial Velocity Corrections due to Multiplicative Noise

abstract:
  The problem of velocity selection for reaction fronts has been intensively
investigated, leading to the successful marginal stability approach for
propagation into an unstable state. Because the front velocity is controlled by
the leading edge which perforce has low density, it is interesting to study the
role that finite particle number fluctuations have on this picture. Here, we
use the well-known mapping of discrete Markov processes to stochastic
differential equations and focus on the front velocity in the simple $A+A
\stackrel{\leftarrow}{\to} A$ system. Our results are consistent with a recent
(heuristic) proposal that $v_{MS} - v \sim {1\over \ln^2 {N}}$.


title:
Dynamics of transverse correlations in the spin-1/2 isotropic XY chain
  with a correlated Lorentzian disorder

abstract:
  Using a numerical approach we examine the dynamics of $zz$ correlations for
the spin-$1\over2$ isotropic XY chain with random Lorentzian intersite coupling
and transverse field at sites that depends linearly on the neighbouring
couplings. We study in detail the wave vector- and frequency-dependent $zz$
structure factor at different values of Hamiltonian parameters and temperature.
We discuss the changes in the frequency profiles of the $zz$ dynamic structure
factor caused by the introduced correlated disorder.


title:
State Orthogonalization by Building a Hilbert Space: A New Approach to
  Electronic Quantum Transport in Molecular Wires

abstract:
  Quantum descriptions of many complex systems are formulated most naturally in
bases of states that are not mutually orthogonal. We introduce a general and
powerful yet simple approach that facilitates solving such models exactly by
embedding the non-orthogonal states in a new Hilbert space in which they are by
definition mutually orthogonal. This novel approach is applied to electronic
transport in molecular quantum wires and is used to predict conductance
antiresonances of a new type that arise solely out of the non-orthogonality of
the local orbitals on different sites of the wire.


title: Polar Smectic Films

abstract:
  We report on a new experimental procedure for forming and studying polar
smectic liquid crystal films. A free standing smectic film is put in contact
with a liquid drop, so that the film has one liquid crystal/liquid interface
and one liquid crystal/air interface. This polar environment results in changes
in the textures observed in the film, including a boojum texture and a
previously unobserved spiral texture in which the winding direction of the
spiral reverses at a finite radius from its center. Some aspects of these
textures are explained by the presence of a Ksb term in the bulk elastic free
energy density that favors a combination of splay and bend deformations.


title: Ground state instability in systems of strongly interacting fermions

abstract:
  We analyze stability of a fermion system with model repulsive pair
interaction potential. The possibility for different types of restructuring of
the Fermi ground state (at sufficiently great coupling constant) is related to
the analytic properties of such potential. In particular, for the screened
Coulomb law it is shown that the restructuring cannot be of the Fermi
condensation type, known earlier for some exactly solvable models, and instead
it belongs to the class of topological transitions (TT). For this model, a
phase diagram has been built in the variables "screening parameter - coupling
constant" which displays two kinds of TT: a 5/2-kind similar to the known
Lifshitz transitions in metals, and a 2-kind characteristic for a uniform
strongly interacting system.


title: Monte Carlo Quasi-Heatbath by approximate inversion

abstract:
  When sampling the distribution P(phi) ~ exp(-|A phi|^2), a global heatbath
normally proceeds by solving the linear system A phi = eta, where eta is a
normal Gaussian vector, exactly. This paper shows how to preserve the
distribution P(phi) while solving the linear system with arbitrarily low
accuracy. Generalizations are presented.


title:
Josephson current carried by Andreev levels in superconducting quantum
  point contacts

abstract:
  The dc Josephson effect in a superconducting quantum point contact, where
supercurrent flows through a small number of channels, is reviewd. The central
role of Andreev levels is emphasized which carry the whole supercurrent in
short symmetric Josephson junctions including tunnel junctions. A simple
intuitive view of the dc Josephson effect in a quantum point contact is given
in terms of multiple Andreev reflections. The quantization of the critical
current in superconducting quantum point contacts is briefly discussed.