title: Magnetotransport in nearly antiferromagnetic metals

abstract:
  We present a theory of the magnetotransport in weakly disordered metals close
to an antiferromagnetic quantum-critical point. The anisotropic scattering from
critical spin fluctuations is strongly influenced by weak but isotropic
scattering from small amounts of disorder. This leads to a large regime where
the resistivity obeys a scaling form rho=rho_0+Delta rho = rho_0+T^{3/2}
f(T/\rho_0,(p-p_c)/rho_0,B/rho_0^{3/2}), where rho_0 is the residual
resistivity, B the magnetic field and p-p_c>0 measures the distance from the
quantum-critical point on the paramagnetic side of the phase diagram. Orbital
effects of the magnetic field are most pronounced in very clean samples for not
too low temperatures, where the resistivity for increasing magnetic field
crosses over from a linear temperature dependence Delta rho =T*sqrt{rho_0} to a
resistivity linear in B and independent of T and rho_0. At higher magnetic
fields Delta rho saturates at a value proportional to T^{1.5} or T^2/(p-p_c).
Deviations from scaling, the interplay of orbital and spin contributions of the
magnetic field and experimental test of the spin-fluctuation model are
discussed in detail.


title:
Systems with Correlations in the Variance: Generating Power-Law Tails in
  Probability Distributions

abstract:
  We study how the presence of correlations in physical variables contributes
to the form of probability distributions. We investigate a process with
correlations in the variance generated by (i) a Gaussian or (ii) a truncated
L\'{e}vy distribution. For both (i) and (ii), we find that due to the
correlations in the variance, the process ``dynamically'' generates power-law
tails in the distributions, whose exponents can be controlled through the way
the correlations in the variance are introduced. For (ii), we find that the
process can extend a truncated distribution {\it beyond the truncation cutoff},
which leads to a crossover between a L\'{e}vy stable power law and the present
``dynamically-generated'' power law. We show that the process can explain the
crossover behavior recently observed in the $S&P500$ stock index.


title: Electronic Dephasing in the Quantum Hall Regime

abstract:
  By means of degenerate four-wave mixing (FWM), we investigate the quantum
coherence of electron-hole pairs in the presence of a two-dimensional electron
gas in modulation-doped GaAs-AlGaAs quantum wells in the quantum Hall effect
regime. With increasing magnetic field, we observe a crossover from Markovian
to non-Markovian behavior, as well as large jumps in the decay time of the FWM
signal at even Landau level filling factors. The main observations can be
qualitatively reproduced by a model which takes into account scattering by the
collective excitations of the two-dimensional electron gas.


title: Composition Patterning in Systems Driven by Competing Dynamics

abstract:
  We study an alloy system where short-ranged, thermally-driven diffusion
competes with externally imposed, finite-ranged, athermal atomic exchanges, as
is the case in alloys under irradiation. Using a Cahn-Hilliard-type approach,
we show that when the range of these exchanges exceeds a critical value,
labyrinthine concentration patterns at a mesoscopic scale can be stabilized.
Furthermore, these steady-state patterns appear only for a window of the
frequency of forced exchanges. Our results suggest that ion beams may provide a
novel route to stabilize and tune the size of nanoscale structural features in
materials.


title: Band structure of the Jahn-Teller polaron from Quantum Monte Carlo

abstract:
  A path-integral representation is constructed for the Jahn-Teller polaron
(JTP). It leads to a perturbation series that can be summed exactly by the
diagrammatic Quantum Monte Carlo technique. The ground-state energy, effective
mass, spectrum and density of states of the three-dimensional JTP are
calculated with no systematic errors. The band structure of JTP interacting
with dispersionless phonons, is found to be similar to that of the Holstein
polaron. The mass of JTP increases exponentially with the coupling constant. At
small phonon frequencies, the spectrum of JTP is flat at large momenta, which
leads to a strongly distorted density of states with a massive peak at the top
of the band.


title: Supercooling across first-order phase transitions in vortex matter

abstract:
  Hysteresis in cycling through first-order phase transitions in vortex matter,
akin to the well-studied phenomenon of supercooling of water, has been
discussed in literature. Hysteresis can be seen while varying either
temperature T or magnetic field H (and thus the density of vortices). Our
recent work on phase transitions with two control variables shows that the
observable region of metastability of the supercooled phase would depend on the
path followed in H-T space, and will be larger when T is lowered at constant H
compared to the case when H is lowered at constant T. We discuss the effect of
isothermal field variations on metastable supercooled states produced by
field-cooling. This path dependence is not a priori applicable to metastability
caused by reduced diffusivity or hindered kinetics.


title:
Magnetization plateaux and jumps in a class of frustrated ladders: A
  simple route to a complex behaviour

abstract:
  We study the occurrence of plateaux and jumps in the magnetization curves of
a class of frustrated ladders for which the Hamiltonian can be written in terms
of the total spin of a rung. We argue on the basis of exact diagonalization of
finite clusters that the ground state energy as a function of magnetization can
be obtained as the minimum - with Maxwell constructions if necessary - of the
energies of a small set of spin chains with mixed spins. This allows us to
predict with very elementary methods the existence of plateaux and jumps in the
magnetization curves in a large parameter range, and to provide very accurate
estimates of these magnetization curves from exact or DMRG results for the
relevant spin chains.


title: Finite-temperature correlations in the trapped Bose gas

abstract:
  Previous functional integral methods for translationally invariant systems
have been extended to the case of a confining trap potential. Essentially all
finite-temperature properties of the repulsive Bose gas in a paraboloidal trap
can be determined this way. New analytical results reported here are for the
finite-temperature two-point correlation functions below the critical
temperatures in d=3,2 and 1 dimensions. Only for d=3 are correlations both long
range and coherent - at long range consistent with the existence of a coherent
state of the condensate in the trap. The two-dimensional condensate is
marginally stable in the sense that correlations decay by a power law.


title: Can one have preroughening of vicinal surfaces?

abstract:
  We discuss the possibility that, besides roughening, a vicinal surface could
display preroughening (PR), and consider the possible mechanisms for its
promotion. Within the framework of a terrace-step-kink model, it turns out that
a PR transition is possible, and could be induced by a short-range repulsion
between parallel kinks along the same step or on adjacent steps, or even by
some kind of extended range step-step repulsion. We discuss the possible
relevance of this phenomenon to the anomalous roughening behaviour recently
reported for Ag(115).


title: Field theory for trapped atomic gases

abstract:
  In this course we give a selfcontained introduction to the quantum field
theory for trapped atomic gases, using functional methods throughout. We
consider both equilibrium and nonequilibrium phenomena. In the equilibrium
case, we first derive the appropriate Hartree-Fock theory for the properties of
the gas in the normal phase. We then turn our attention to the properties of
the gas in the superfluid phase, and present a microscopic derivation of the
Bogoliubov and Popov theories of Bose-Einstein condensation and the
Bardeen-Cooper-Schrieffer theory of superconductivity. The former are
applicable to trapped bosonic gases such as rubidium, lithium, sodium and
hydrogen, and the latter in particular to the fermionic isotope of atomic
lithium. In the nonequilibrium case, we discuss various topics for which a
field-theoretical approach is especially suited, because they involve physics
that is not contained in the Gross-Pitaevskii equation. Examples are quantum
kinetic theory, the growth and collapse of a Bose condensate, the phase
dynamics of bosonic and fermionic superfluids, and the collisionless collective
modes of a Bose gas below the critical temperature.


title: Investigations of a two-mode atom laser model

abstract:
  Atom lasers based on rf-outcoupling from a trapped Bose-Einstein condensate
can be described by a set of generalized, coupled Gross-Pitaevskii equations
(GPE).
  If not only one but two radio frequencies are used for outcoupling, the atoms
emerging from the trap have two different energies and the total wavefunction
of the untrapped spin-state is a coherent superposition which leads to a pulsed
atomic beam.
  We present results for such a situation obtained from a 1D-GPE model for
magnetically trapped Rb-87 in the F=1 state. The wavefunction of the atomic
beam can be approximated by a sum of two Airy functions. In the limit of weak
coupling we calculate the intensity analytically.


title:
Photoemission Study of Rare-Earth Ditelluride Compounds (ReTe_2 : Re =
  La, Pr, Sm, and Gd)

abstract:
  We studied the electronic structure of rare-earth ditelluride (ReTe_2 : Re =
La, Pr, Sm, and Gd) using photoemission spectroscopy. From the x-ray
photoelectron spectroscopy (XPS) study of the 3d core levels of rare-earth
elements, we found that all the rare earth elements are trivalent. We have also
made theoretical calculations using the Gunnarsson and Schoenhammer
approximation and multiplet calculations for the rare earth elements to find
that the La and Gd~3d peaks are well explained using our calculations. There is
no considerable change in the line-shape of the Te~3d peaks depending on
different rare earth elements. On ther other hand, valence band spectra studied
with the ultraviolet photoelectron spectroscopy (UPS) show a small change in
the Te p band depending on rare-earth elements. According to the UPS data,
LaTe_2 has very low carrier density at the Fermi level while SmTe_2 and PrTe_2
show strongly metallic band structure effects near the Fermi level.


title: Coulomb Blockade Ratchet

abstract:
  We investigate the transport properties of a new class of ratchets. The
device is constructed by applying an ac voltage to the metallic single electron
tunneling transistor, and a net transport current is induced by the
time-dependent bias-voltage, although the voltage value is on the average zero.
The mechanism underlying this phenomenon is the Coulomb blockade of the single
electron tunneling. The directions and the values of the induced net currents
can be well controlled by the gate-voltage. A net transport current has also
been observed even in the absence of the external bias-voltages, which is
attributed to the noise in the circuit.


title: High Frequency Dynamics of Amorphous Silica

abstract:
  We present the results of extensive molecular dynamics computer simulations
in which the high frequency dynamics of silica, nu>0.5 THz, is investigated in
the viscous liquid state as well as in the glass state. We characterize the
properties of high frequency sound modes by analyzing J_l(q,nu) and J_t(q,nu),
the longitudinal and transverse current correlation function, respectively. For
wave-vectors q>0.4 Angstrom^{-1} the spectra are sitting on top of a flat
background which is due to multiphonon excitations. In the acoustic frequency
band, i.e. for nu<20 THz, the intensity of J_l(q,nu) and J_t(q,nu) in the
liquid and the glass approximately proportional to temperature, in agreement
with the harmonic approximation. In contrast to this, strong deviations from a
linear scaling are found for nu>20 THz. The dynamic structure factor S(q,nu)
exhibits for q>0.23 Angstrom^{-1} a boson peak which is located nearly
independent of q around 1.7 THz. We show that the low frequency part of the
boson peak is mainly due to the elastic scattering of transverse acoustic modes
with frequencies around 1 THz. The strength of this scattering depends on q and
is largest around q=1.7 Angstrom^{-1}, the location of the first sharp
diffraction peak in the static structure factor. By studying S(q,nu) for
different system sizes we show that strong finite size effects are present in
the low frequency part of the boson peak in that for small systems part of its
intensity is missing. We discuss the consequences of these finite size effects
for the structural relaxation.


title:
Dynamical Mean-Field Solution for a Model of Metal-Insulator Transitions
  in Moderately Doped Manganites

abstract:
  We propose that a specific spatial configuration of lattice sites that
energetically favor {\it 3+} or {\it 4+} Mn ions in moderately doped manganites
constitutes approximately a spatially random two-energy-level system. Such an
effect results in a mechanism of metal-insulator transitions that appears to be
different from both the Anderson transition and the Mott-Hubbard transition.
Correspondingly, a disordered Kondo lattice model is put forward, whose
dynamical mean-field solution agrees reasonably with experiments.


title:
Symmetry Origin of the Phase Transitions and Phase Separation in
  Manganites at Low Doping

abstract:
  We analyze the symmetry changes of the paramagnetic to the A-type
antiferromagnetic and to the ferromagnetic phase transitions in undoped and
moderately doped LaMnO$_3$, respectively. We show that in the
orthorhombic-distorted perovskite manganites the phase separation at low doping
is associated with the noncollinear nature of the magnetic orders permitted by
symmetry. A simple model for the competition between the two phase transitions
is put forward within the framework of Landau theory of phase transitions.


title:
Landau Theory of the Phase Transitions in Half Doped Manganites:
  Interplay of Magnetic, Charge and Structural Orders

abstract:
  The order parameters of the magnetic, charge and structural orders at
half-doped manganites are identified. A corresponding Landau theory of the
phase transitions is formulated. Many structural and thermodynamical behaviors
are accounted for and clarified within the framework. In particular, the theory
provides a unified picture for the scenario of the phase transitions and their
nature with respect to the variation of the tolerance factor of the manganites.
It also accounts for the origin of the incommensurate nature of the orbital
order and its subsequently accompanying antiferromagnetic order.


title: Phenomenological model of high-Tc superconductivity: a MCS model

abstract:
  Since the introduction of a MCS (Magnetic Coupling between Stripes)
phenomenological model [cond-mat/9902355, to be published in J.
Superconductivity] for hole-doped cuprates many new experimental data have been
presented in the literature as evidence in support of the MCS model. We
consider here recent data and the MCS model which is based upon experimental
facts, namely, the presence of (i) stripes; (ii) spin fluctuations, and (iii)
two order parameters (for pairing and for long-phase coherence) in hole-doped
cuprates. We discuss also the superconductivity in the s-wave NdCeCuO cuprate.


title:
Electron localization in sound absorption oscillations in the quantum
  Hall effect regime

abstract:
  The absorption coefficient for surface acoustic waves in a piezoelectric
insulator in contact with a GaAs/AlGaAs heterostructure (with two-dimensional
electron mobility $\mu= 1.3\times 10^5 cm^2/V\cdot s)$ at T=4.2K) via a small
gap has been investigated experimentally as a function of the frequency of the
wave, the width of the vacuum gap, the magnetic field, and the temperature. The
magnetic field and frequency dependencies of the high-frequency conductivity
(in the region 30-210 MHz) are calculated and analyzed. The experimental
results can be explained if it assumed that there exists a fluctuation
potential in which current carrier localization occurs. The absorption of the
surface acoustic waves in an interaction with two-dimensional electrons
localized in the energy "tails" of Landau levels is discussed.


title: Triangle Distribution and Equation of State for Classical Rigid Disks

abstract:
  The triangle distribution function f^(3) for three mutual nearest neighbors
in the plane describes basic aspects of short-range order and statistical
thermodynamics in two-dimensional many-particle systems. This paper examines
prospects for constructing a self-consistent calculation for the rigid-disk
system f^(3). We present several identities obeyed by f^(3). A rudimentary
closure suggested by scaled-particle theory is introduced. In conjunction with
three of the basic identities, this closure leads to a unique f^(3) over the
entire density range. The pressure equation of state exhibits qualitatively
correct behavior in both the low density and the close-packed limits, but no
intervening phase transition appears. We discuss extensions to improved disk
closures, and to the three-dimensional rigid sphere system.


title: On Matrix Product States for Periodic Boundary Conditions

abstract:
  The possibility of a matrix product representation for eigenstates with
energy and momentum zero of a general m-state quantum spin Hamiltonian with
nearest neighbour interaction and periodic boundary condition is considered.
The quadratic algebra used for this representation is generated by 2m operators
which fulfil m^2 quadratic relations and is endowed with a trace. It is shown
that {\em not} every eigenstate with energy and momentum zero can be written as
matrix product state. An explicit counter-example is given. This is in contrast
to the case of open boundary conditions where every zero energy eigenstate can
be written as a matrix product state using a Fock-like representation of the
same quadratic algebra.


title: Meanders: Exact Asymptotics

abstract:
  We conjecture that meanders are governed by the gravitational version of a
c=-4 two-dimensional conformal field theory, allowing for exact predictions for
the meander configuration exponent \alpha=\sqrt{29}(\sqrt{29}+\sqrt{5})/12, and
the semi-meander exponent {\bar\alpha}=1+\sqrt{11}(\sqrt{29}+\sqrt{5})/24. This
result follows from an interpretation of meanders as pairs of fully packed
loops on a random surface, described by two c=-2 free fields. The above values
agree with recent numerical estimates. We generalize these results to a score
of meandric numbers with various geometries and arbitrary loop fugacities.

categories: cond-mat.stat-mech hep-th math-ph math.CO math.MP


title: Paths to Self-Organized Criticality

abstract:
  We present a pedagogical introduction to self-organized criticality (SOC),
unraveling its connections with nonequilibrium phase transitions. There are
several paths from a conventional critical point to SOC. They begin with an
absorbing-state phase transition (directed percolation is a familiar example),
and impose supervision or driving on the system; two commonly used methods are
extremal dynamics, and driving at a rate approaching zero. We illustrate this
in sandpiles, where SOC is a consequence of slow driving in a system exhibiting
an absorbing-state phase transition with a conserved density. Other paths to
SOC, in driven interfaces, the Bak-Sneppen model, and self-organized directed
percolation, are also examined. We review the status of experimental
realizations of SOC in light of these observations.


title:
Long-range spin-pairing order and spin defects in quantum spin-1/2
  ladders

abstract:
  For w-legged antiferromagnetic spin-1/2 Heisenberg ladders, a long-range
spin-pairing order can be identified which enables the separation of the space
spanned by finite-range (covalent) valence-bond configurations into w+1
subspaces. Since every subspace has an equivalent counter subspace connected by
translational symmetry, twofold degeneracy, breaking traslational symmetry is
found except for the subspace where the ground state of w=even belongs to. In
terms of energy ordering, (non)degeneracy and the discontinuities introduced in
the long-range spin-pairing order by topological spin defects, the differences
between even and odd ladders are explained in a general and systematic way.


title:
Stable and Metastable Structures of Cobalt on Cu(001): An ab initio
  Study

abstract:
  We report results of density-functional theory calculations on the
structural, magnetic, and electronic properties of (1x1)-structures of Co on
Cu(001) for coverages up to two monolayers. In particular we discuss the
tendency towards phase separation in Co islands and the possibility of
segregation of Cu on top of the Co-film. A sandwich structure consisting of a
bilayer Co-film covered by 1ML of Cu is found to be the lowest-energy
configuration. We also discuss a bilayer c(2x2)-alloy which may form due to
kinetic reasons, or be stabilized at strained surface regions. Furthermore, we
study the influence of magnetism on the various structures and, e.g., find that
Co adlayers induce a weak spin-density wave in the copper substrate.


title:
Topological effects in ring polymers (II): Influence of persistence
  length

abstract:
  The interplay of topological constraints and persistence length of ring
polymers in their own melt is investigated by means of dynamical Monte Carlo
simulations of a three dimensional lattice model. We ask if the results are
consistent with an asymptotically regime where the rings behave like (compact)
{\em lattice animals} in a self-consistent network of topological constraints
imposed by neighbouring rings. Tuning the persistence length provides an
efficient route to increase the ring overlap required for this mean-field
picture to hold: The {\em effective} Flory exponent for the ring size decreases
down to $\nu \stackrel{<}{\sim} 1/3$ with increasing persistence length.
Evidence is provided for the emergence of one additional characteristic length
scale $\dtop \propto N^0$, only weakly dependent on the persistence length and
much larger than the excluded volume screening length $\xi$. At distances
larger than \dtop the conformational properties of the rings are governed by
the topological interactions, at smaller distances rings and their linear chain
counterparts become similar. (At distances smaller than $\xi$ both
architectures are identical.) However, the crossover between both limits is
intricate and broad as a detailed discussion of the local fractal dimension
(e.g., obtained from the static structure factor) reveals.


title: Phenomenological Renormalization Group Methods

abstract:
  Some renormalization group approaches have been proposed during the last few
years which are close in spirit to the Nightingale phenomenological procedure.
In essence, by exploiting the finite size scaling hypothesis, the approximate
critical behavior of the model on infinite lattice is obtained through the
exact computation of some thermal quantities of the model on finite clusters.
In this work some of these methods are reviewed, namely the mean field
renormalization group, the effective field renormalization group and the finite
size scaling renormalization group procedures. Although special emphasis is
given to the mean field renormalization group (since it has been, up to now,
much more applied an extended to study a wide variety of different systems) a
discussion of their potentialities and interrelations to other methods is also
addressed.


title: A Novel Longitudinal Mode in the Coupled Quantum Chain Compound KCuF3

abstract:
  Inelastic neutron scattering measurements are reported that show a new
longitudinal mode in the antiferromagnetically ordered phase of the spin-1/2
quasi-one-dimensional antiferromagnet KCuF3. This mode signals the cross-over
from one-dimensional to three-dimensional behavior and indicates a reduction in
the ordered spin moment of a spin-1/2 antiferromagnet. The measurements are
compared with recent quantum field theory results and are found to be in
excellent agreement. A feature of the data not predicted by theory is a damping
of the mode by decay processes to the transverse spin-wave branches.


title:
Girvin-MacDonald-Platzman Collective Mode at General Filling Factors:
  Magneto-Roton Minimum at Half-Filled Landau Level

abstract:
  The single mode approximation has proved useful for the excitation spectrum
at $\nu=1/3$. We apply it to general fractions and find that it predicts $n$
magneto-roton minima in the dispersion of the Girvin-MacDonald-Platzman
collective mode for the fractional quantum Hall states at $\nu=n/(2n+1)$, and
one magneto-roton minimum for both the composite Fermi sea and the paired
composite fermion state. Experimental relevance of the results will be
considered.


title:
A Convergent Method for Calculating the Properties of Many Interacting
  Electrons

abstract:
  A method is presented for calculating binding energies and other properties
of extended interacting systems using the projected density of transitions
(PDoT) which is the probability distribution for transitions of different
energies induced by a given localized operator, the operator on which the
transitions are projected. It is shown that the transition contributing to the
PDoT at each energy is the one which disturbs the system least, and so, by
projecting on appropriate operators, the binding energies of equilibrium
electronic states and the energies of their elementary excitations can be
calculated. The PDoT may be expanded as a continued fraction by the recursion
method, and as in other cases the continued fraction converges exponentially
with the number of arithmetic operations, independent of the size of the
system, in contrast to other numerical methods for which the number of
operations increases with system size to maintain a given accuracy. These
properties are illustrated with a calculation of the binding energies and
zone-boundary spin- wave energies for an infinite spin-1/2 Heisenberg chain,
which is compared with analytic results for this system and extrapolations from
finite rings of spins.


title: Time evolution of wave-packets in quasi-1D disordered media

abstract:
  We have investigated numerically the quantum evolution of a wave-packet in a
quenched disordered medium described by a tight-binding Hamiltonian with
long-range hopping (band random matrix approach). We have obtained clean data
for the scaling properties in time and in the bandwidth b of the packet width
and its fluctuations with respect to disorder realizations. We confirm that the
fluctuations of the packet width in the steady-state show an anomalous scaling
and we give a new estimate of the anomalous scaling exponent. This anomalous
behaviour is related to the presence of non-Gaussian tails in the distribution
of the packet width. Finally, we have analysed the steady state probability
profile and we have found finite band corrections of order 1/b with respect to
the theoretical formula derived by Zhirov in the limit of infinite bandwidth.
In a neighbourhood of the origin, however, the corrections are $O(1/\sqrt{b})$.


title: The Conductivity Tensor for the Hubbard Model

abstract:
  A new theoretical analysis of the current-charge and charge-charge
propagators is presented for the Hubbard model, using the static approximation
for the Composite Operator Method. This approximation manifestly preserves a
sum rule which governs the single-site dynamics. We compare our results with
those obtained by numerical analysis.


title: Correlations in optical phonon spectra of complex solids

abstract:
  Spectral correlations in the optical phonon spectrum of a solid with a
complex unit cell are analysed using the Wigner-Dyson statistical approach.
Despite the fact that all force constants are real, we find that the statistics
are predominantly of the GUE type depending on the location within the
Brillouin zone of a crystal and the unit cell symmetry. Analytic and numerical
results for the crossover from GOE to GUE statistics are presented.


title: The XY model on the one-dimensional superlattice: static properties

abstract:
  The XY model (s=1/2) on the one-dimensional alternating superlattice (closed
chain) is solved exactly by using a generalized Jordan-Wigner transformation
and the Green function method. Closed expressions are obtained for the
excitation spectrum, the internal energy, the specific heat, the average
magnetization per site, the static transverse susceptibility and the two-spin
correlation function in the field direction at arbitrary temperature. At T=0 it
is shown that the system presents multiple second order phase transitions
induced by the transverse field, which are associated to the zero energy mode
with wave number equal to 0 or $\pi$. It is also shown that the average
magnetization as a function of the field presents, alternately, regions of
plateaux (disordered phases) and regions of variable magnetization (ordered
phases). The static correlation function presents an oscillating behaviour in
the ordered phase and its period goes to infinity at the critical point.


title:
Effect of the spin-orbit interaction on the band structure and
  conductance of quasi-one-dimensional systems

abstract:
  We discuss the effect of the spin-orbit interaction on the band structure,
wave functions and low temperature conductance of long quasi-one-dimensional
electron systems patterned in two-dimensional electron gases (2DEG). Our model
for these systems consists of a linear (Rashba) potential confinement in the
direction perpendicular to the 2DEG and a parabolic confinement transverse to
the 2DEG. We find that these two terms can significantly affect the band
structure introducing a wave vector dependence to subband energies, producing
additional subband minima and inducing anticrossings between subbands. We
discuss the origin of these effects in the symmetries of the subband wave
functions.


title: Order parameter for two-dimensional critical systems with boundaries

abstract:
  Conformal transformations can be used to obtain the order parameter for
two-dimensional systems at criticality in finite geometries with fixed boundary
conditions on a connected boundary. To the known examples of this class (such
as the disk and the infinite strip) we contribute the case of a rectangle. We
show that the order parameter profile for simply connected boundaries can be
represented as a universal function (independent of the criticality model)
raised to the power eta/2. The universal function can be determined from the
Gaussian model or equivalently a problem in two-dimensional electrostatics. We
show that fitting the order parameter profile to the theoretical form gives an
accurate route to the determination of eta. We perform numerical simulations
for the Ising model and percolation for comparison with these analytic
predictions, and apply this approach to the study of the planar rotor model.


title: Band-Gap Engineering in two-dimensional periodic photonic crystals

abstract:
  A theoretical investigation is made of the dispersion characteristics of
plasmons in a two-dimensional periodic system of semiconductor (dielectric)
cylinders embedded in a dielectric (semiconductor) background. We consider both
square and hexagonal arrangements and calculate extensive band structures for
plasmons using a plane-wave method within the framework of a local theory. It
is found that such a system of semiconductor-dielectric composite can give rise
to huge full band gaps (with a gap to midgap ratio $\approx 2$) within which
plasmon propagation is forbidden. The most interesting aspect of this
investigation is the huge lowest gap occurring below a threshold frequency and
extending up to zero. The maximum magnitude of this gap is defined by the
plasmon frequency of the inclusions or the background as the case may be. In
general we find that greater the dielectric (and plasmon frequency) mismatch,
the larger this lowest band-gap. Whether or not some higher energy gaps appear,
the lowest gap is always seen to exist over the whole range of filling fraction
in both geometries. Just like photonic and phononic band-gap crystals,
semiconducting band-gap crystals should have important consequences for
designing useful semiconductor devices in solid state plasmas.


title: Buckling instability in type-II superconductors with strong pinning

abstract:
  We predict a novel buckling instability in the critical state of thin type-II
superconductors with strong pinning. This elastic instability appears in high
perpendicular magnetic fields and may cause an almost periodic series of flux
jumps visible in the magnetization curve. As an illustration we apply the
obtained criteria to a long rectangular strip.


title: Generalised Fluctuation Formula

abstract:
  We develop a General Fluctuation Formula for phase variables that are odd
under time reversal. Simulations are used to verify the new formula.


title:
Quantum phase transitions and collapse of the Mott gap in the
  $d=1+\epsilon$ dimensional half-filled Hubbard model

abstract:
  We study the low-energy asymptotics of the half-filled Hubbard model with a
circular Fermi surface in $d=1+\epsilon$ continuous dimensions, based on the
one-loop renormalization-group (RG) method. Peculiarity of the $d=1+\epsilon$
dimensions is incorporated through the mathematica structure of the elementary
particle-partcile (PP) and particle-hole (PH) loops: infrared logarithmic
singularity of the PH loop is smeared for $\epsilon>0$. The RG flows indicate
that a quantum phase transition (QPT) from a metallic phase to the Mott
insulator phase occurs at a finite on-site Coulomb repulsion $U$ for
$\epsilon>0$. We also discuss effects of randomness.


title: The triangular Ising antiferromagnet in a staggered field

abstract:
  We study the equilibrium properties of the nearest-neighbor Ising
antiferromagnet on a triangular lattice in the presence of a staggered field
conjugate to one of the degenerate ground states. Using a mapping of the ground
states of the model without the staggered field to dimer coverings on the dual
lattice, we classify the ground states into sectors specified by the number of
``strings''. We show that the effect of the staggered field is to generate
long-range interactions between strings. In the limiting case of the
antiferromagnetic coupling constant J becoming infinitely large, we prove the
existence of a phase transition in this system and obtain a finite lower bound
for the transition temperature. For finite J, we study the equilibrium
properties of the system using Monte Carlo simulations with three different
dynamics. We find that in all the three cases, equilibration times for low
field values increase rapidly with system size at low temperatures. Due to this
difficulty in equilibrating sufficiently large systems at low temperatures, our
finite-size scaling analysis of the numerical results does not permit a
definite conclusion about the existence of a phase transition for finite values
of J. A surprising feature in the system is the fact that unlike usual glassy
systems, a zero-temperature quench almost always leads to the ground state,
while a slow cooling does not.


title: Nucleation of Superconductivity in a Mesoscopic Loop of Finite Width

abstract:
  The normal/superconducting phase boundary Tc has been calculated for
mesoscopic loops, as a function of an applied perpendicular magnetic field H.
While for thin-wire loops and filled disks the Tc(H) curves are well known, the
intermediate case, namely mesoscopic loops of finite wire width, have been
studied much less. The linearized first Ginzburg-Landau equation is solved with
the proper normal/vacuum boundary conditions both at the internal and at the
external loop radius. For thin-wire loops the Tc(H) oscillations are perfectly
periodic, and the Tc(H) background is parabolic (this is the usual Little-Parks
effect). For loops of thicker wire width, there is a crossover magnetic field
above which Tc(H) becomes quasi-linear, with the period identical to the Tc(H)
of a filled disk (i.e. pseudoperiodic oscillations). This dimensional
transition is similar to the 2D-3D transition for thin films in a parallel
field, where vortices start penetrating the material as soon as the film
thickness exceeds the temperature dependent coherence length by a factor 1.8.
For the presently studied loops, the crossover point is controlled by a similar
condition. In the high field '3D' regime, a giant vortex state establishes,
where only a surface superconducting sheath near the sample's outer radius is
present.


title: Magnetic Properties of Weakly Doped Antiferromagnets

abstract:
  We study the spin excitations and the transverse susceptibility of a
two-dimensional antiferromagnet doped with a small concentration of holes in
the t-J model. The motion of holes generates a renormalization of the magnetic
properties. The Green's functions are calculated in the self-consistent Born
approximation. It is shown that the long-wavelength spin waves are
significantly softened and the shorter-wavelength spin waves become strongly
damped as the doping increases. The spin wave velocity is reduced by the
coherent motion of holes, and not increased as has been claimed elsewhere. The
transverse susceptibility is found to increase considerably with doping, also
as a result of coherent hole motion. Our results are in agreement with
experimental data for the doped copper oxide superconductors.


title: Dynamic Nuclear Overhauser Shifts in Larmor beats from a quantum well

abstract:
  The significance of nuclear spin polarisation in time-resolved optical
studies of III-V semiconductors is addressed. Electron Larmor beats in
pump-probe reflectivity from a GaAs/AlGaAs quantum well show Overhauser shift
of 0.7 T due to accumulated nuclear polarisation <I>/I=0.065. This leads to
precision values of electron g-factor, elucidates nuclear spin pumping and
diffusion mechanisms in quantum wells and informs discussion of implications
for spin-electronics and transport.


title: Aging in a simple glassformer

abstract:
  Using molecular dynamics computer simulations we investigate the
out-of-equilibrium dynamics of a Lennard-Jones system after a quench from a
high temperature to one below the glass transition temperature. By studying the
radial distribution function we give evidence that during the aging the system
is very close to the critical surface of mode-coupling theory. Furthermore we
show that two-time correlation functions show a strong dependence on the
waiting time since the quench and that their shape is very different from the
one in equilibrium. By investigating the temperature and time dependence of the
frequency distribution of the normal modes we show that the energy of the
inherent structures can be used to define an effective (time dependent)
temperature of the aging system.


title:
Optimized energy calculation in lattice systems with long-range
  interactions

abstract:
  We discuss an efficient approach to the calculation of the internal energy in
numerical simulations of spin systems with long-range interactions. Although,
since the introduction of the Luijten-Bl\"ote algorithm, Monte Carlo
simulations of these systems no longer pose a fundamental problem, the energy
calculation is still an O(N^2) problem for systems of size N. We show how this
can be reduced to an O(N logN) problem, with a break-even point that is already
reached for very small systems. This allows the study of a variety of, until
now hardly accessible, physical aspects of these systems. In particular, we
combine the optimized energy calculation with histogram interpolation methods
to investigate the specific heat of the Ising model and the first-order regime
of the three-state Potts model with long-range interactions.


title:
Conditions on the Occurrence of Magnetic Moments in Quasicrystals and
  Related Phases

abstract:
  We analyze the criterion for the appearance of magnetic moments in
quasicrystals approximants and liquids Al-Mn. In a Hartree-Fock scheme, it is
shown that the Stoner criterion (Un_d(E_F) > 1) does not apply to these systems
due to the presence of the pseudogap in the density of states. We give an
generalized criterion that can take into account the particular density of
state. The main result is that the appearance of a magnetic moment on a Mn atom
depends strongly on its position in the crystalline structure. Indeed some
particular positions of the Mn atoms lead to magnetic moments whereas other
positions are more stable when the Mn is non-magnetic. This criterion is used
to analyzed the electronic structure of two approximants (1/1-Al-Pd-Mn-Si and
T-Al-Pd-Mn) calculated by the self-consistent LMTO-ASA method.


title: Magnetization in short-period mesoscopic electron systems

abstract:
  We calculate the magnetization of the two-dimensional electron gas in a
short-period lateral superlattice, with the Coulomb interaction included in
Hartree and Hartree-Fock approximations. We compare the results for a finite,
mesoscopic system modulated by a periodic potential, with the results for the
infinite periodic system. In addition to the expected strong exchange effects,
the size of the system, the type and the strength of the lateral modulation
leave their fingerprints on the magnetization.


title:
Disordered 2d quasiparticles in class D: Dirac fermions with random
  mass, and dirty superconductors

abstract:
  Disordered noninteracting quasiparticles that are governed by a Majorana-type
Hamiltonian -- prominent examples are dirty superconductors with broken
time-reversal and spin-rotation symmetry, or the fermionic representation of
the 2d Ising model with fluctuating bond strengths -- are called class D. In
two dimensions, weakly disordered systems of this kind may possess a metallic
phase beyond the insulating phases expected for strong disorder. We show that
the 2d metal phase emanates from the free Majorana fermion point, in the
direction of the RG trajectory of a perturbed WZW model. To establish this
result, we develop a supersymmetric extension of the method of nonabelian
bosonization. On the metallic side of the metal-insulator transition, the
density of states becomes nonvanishing at zero energy, by a mechanism akin to
dynamical mass generation. This feature is explored in a model of N species of
disordered Dirac fermions, via the mapping on a nonlinear sigma model, which
encapsulates a Z_2 spin degree of freedom. We compute the density of states in
a finite system, and obtain agreement with the random-matrix prediction for
class D, in the ergodic limit. Vortex disorder, which is a relevant
perturbation at the free-fermion point, changes the density of states at low
energy and suppresses the local Z_2 degree of freedom, thereby leading to a
different symmetry class, BD.


title:
Determination of the magnetization scaling exponent for single crystal
  La$_{0.8}$Sr$_{0.2}$MnO$_3$ by broadband microwave surface impedance
  measurements

abstract:
  Employing a broadband microwave reflection configuration, we have measured
the complex surface impedance, $Z_S(\omega,T)$, of single crystal
La$_{0.8}$Sr$_{0.2}$MnO$_3$, as a function of frequency (0.045-45 GHz) and
temperature (250-325 K). Through the dependence of the microwave surface
impedance on the magnetic permeability, $\hat\mu(\omega,T)$, we have studied
the local magnetic behavior of this material, and have extracted the
spontaneous magnetization, $M_0(T)$, in {\em zero applied field}. The broadband
nature of these measurements and the fact that no external field is applied to
the material provide a unique opportunity to analyze the critical behavior of
the spontaneous magnetization at temperatures very close to the ferromagnetic
phase transition. We find a Curie temperature $T_C=305.5\pm 0.5$ K and scaling
exponent $\beta=0.45\pm 0.05$, in agreement with the prediction of mean-field
theory. We also discuss other recent determinations of the magnetization
critical exponent in this and similar materials and show why our results are
more definitive.


title:
Progressive suppression of spin relaxation in 2D channels of finite
  width

abstract:
  We have investigated spatio-temporal kinetics of electron spin polarization
in semiconductor narrow 2D strip and explored the ability to manipulate spin
relaxation. Information about spin of the conduction electrons and mechanisms
of spin rotation is incorporated into transport Monte Carlo simulation program.
A model problem, involving linear-in-k splitting of the conduction band,
responsible for the D'yakonov-Perel' mechanism of spin relaxation in the
zinc-blende semiconductors and heterostructures, is solved numerically to yield
the decay of spin polarization of an ensemble of electrons in the 2D channel of
finite width. For very wide channels, a conventional 2D value of spin
relaxation is obtained. With decreasing channel width the relaxation time
soares rapidly by orders of magnitude. Surprisingly, the cross-over point
between 2D and quasi-1D behavior is found to be at tens of electron mean-free
paths. Thus, classically wide channels can effectively suppress electron spin
relaxation.


title:
One-Dimensional Partially Asymmetric Simple Exclusion Process on a Ring
  with a Defect Particle

abstract:
  The effect of a moving defect particle for the one-dimensional partially
asymmetric simple exclusion process on a ring is considered. The current of the
ordinary particles, the speed of the defect particle and the density profile of
the ordinary particles are calculated exactly. The phase diagram for the
correlation length is identified. As a byproduct, the average and the variance
of the particle density of the one-dimensional partially asymmetric simple
exclusion process with open boundaries are also computed.


title: Superconducting Fluxon Pumps and Lenses

abstract:
  We study stochastic transport of fluxons in superconductors by alternating
current (AC) rectification. Our simulated system provides a fluxon pump,
"lens", or fluxon "rectifier" because the applied electrical AC is transformed
into a net DC motion of fluxons. Thermal fluctuations and the asymmetry of the
ratchet channel walls induce this "diode" effect, which can have important
applications in devices, like SQUID magnetometers, and for fluxon optics,
including convex and concave fluxon lenses. Certain features are unique to this
novel two-dimensional (2D) geometric pump, and different from the previously
studied 1D ratchets.


title: Observation of a new excitation in the mixed-valent state of YbInCu_4

abstract:
  Infrared measurements are used to obtain conductivity as a function of
temperature and frequency in YbInCu_4, which exhibits an isostructural
transition to a mixed-valent state at T_v \simeq 42 K. In addition to a gradual
loss of spectral weight with decreasing temperature extending up to 1.5 eV,
sharp resonances appear in the mixed-valent state at 0 and 0.25 eV . These
features may be key to understanding both YbInCu_4 and the nature of the
mixed-valent Kondo state.


title:
Electronic structure of the (111) and (-1-1-1) surfaces of cubic BN: A
  local-density-functional ab initio study

abstract:
  We present ab initio local-density-functional electronic structure
calculations for the (111) and (-1-1-1) surfaces of cubic BN. The energetically
stable reconstructions, namely the N adatom, N3 triangle models on the (111),
the (2x1), boron and nitrogen triangle patterns on the (-1-1-1) surface are
investigated. Band structure and properties of the surface states are discussed
in detail.


title: Dark Solitons in Bose-Einstein Condensates

abstract:
  Dark solitons in cigar shaped Bose-Einstein condensates of Rubidium-87 are
created by a phase imprinting method. Coherent and dissipative dynamics of the
solitons has been observed.


title: Macroscopic Dielectric Constant for Microstructures of Sedimentary Rocks

abstract:
  An approximate method to calculate dielectric response and relaxation
functions for water saturated sedimentary rocks is tested for realistic
threedimensional pore space images. The test is performed by comparing the
prediction from the approximate method against the exact solution. The
approximate method is based on image analysis and local porosity theory. An
empirical rule for the specification of the length scale in local porosity
theory is advanced. The results from the exact solution are compared to those
obtained using local porosity theory and various other approximate mixing laws.
The calculation based on local porosity theory is found to yield improved
quantitative agreement with the exact result.


title: Neighbor-junction state effect on the fluxon motion in a Josephson stack

abstract:
  We study experimentally and theoretically the influence of phase-whirling
(resistive) state in one junction of a two-fold Josephson stack on the fluxon
motion in the other junction. In experiment, we measure the fluxon velocity
versus current in one junction as a function of the state (Meissner or
resistive) of the neighboring junction. The analysis, made for the limit of
high fluxon density, shows that the interaction with the resistive state
results in an increase of the effective damping for the moving fluxon and,
therefore, in reduction of its velocity. Numerical simulations confirm this
result for various fluxon densities. The experimental data are in good
agreement with the theoretical predictions. In addition, the fluxon step
measured experimentally has a rather peculiar structure with back and forth
bending regions which is understood as a manifestation of the photon absorption
in the neighboring junction.


title: How localized is an extended quantum system ?

abstract:
  We elaborate on a geometric characterization of the electromagnetic
properties of matter. A fundamental complex quantity, z_{L}, is introduced to
study the localization properties of extended quantum systems. z_L, which
allows us to discriminate between conducting and non-conducting thermodynamic
phases, has an illuminating physical (and geometric) interpretation. Its phase
can be related to the expectation value of the position operator (and a Berry
phase), while its modulus is associated with quantum electric polarization
fluctuations (and a quantum metric). We also study the scaling behavior of z_L
in the one-dimensional repulsive Hubbard model.


title:
Magneto-roton excitation of fractional quantum Hall effect: Comparison
  between theory and experiment

abstract:
  A major obstacle toward a {\em quantitative} verification, by comparison to
experiment, of the theory of the excitations of the fractional quantum Hall
effect has been the lack of a proper understanding of disorder. We circumvent
this problem by studying the {\em neutral} magneto-roton excitations, whose
energy is expected to be largely insensitive to disorder. The calculated
energies of the roton at 1/3, 2/5 and 3/7 fillings of the lowest Landau level
are in good agreement with those measured experimentally.


title:
Absence of a Magnetic Phase Transition in Heisenberg, Hubbard, and
  Kondo-lattice (s-f) Films

abstract:
  The possibility of a magnetic phase transition in Heisenberg, Hubbard, and
s-f (Kondo-lattice) films is investigated. It is shown that, for any finite
temperature and any finite number of layers, the magnetization within every
layer must vanish. Thus, the Mermin-Wagner theorem is extended to a variety of
system geometries. We also comment on the microscopic interpretation of the
transition from two to three dimensions in the limit of an infinite number of
layers.

categories: cond-mat.str-el cond-mat.stat-mech math-ph math.MP


title:
Field-theoretic methods for systems of particles with exotic exclusion
  statistics

abstract:
  We calculate the partition function of a gas of particles obeying Haldane
exclusion statistics, using a definition of a Hilbert space having a
`fractional dimension' and constructing appropriate coherent states. The
fractional dimension is expressed though the form of the identity operator in
the Hilbert space. We find that there many possible generalisations of the
Pauli exclusion principle, with particular choices of the scalar product
leading to consistency either with Haldane's original definition of the
effective dimensionality of the Hilbert space or with the combinatorial
procedure invoked by Haldane and Wu. We explicitly demonstrate that at low
particle densities these definitions are equivalent.


title:
Inequivalence of Dynamical Ensembles in a Generalised Driven Diffusive
  Lattice Gas

abstract:
  We generalise the Driven Diffusive Lattice Gas (DDLG) model by using a
combination of Kawasaki and Glauber dynamics. We find via Monte Carlo
simulations and perturbation studies that the simplest possible generalisation
of the equivalence of the canonical and grand-canonical ensembles, which holds
in equilibrium, does not apply for this class of nonequilibrium systems.


title: Multilevel blocking Monte Carlo simulations for quantum dots

abstract:
  This article provides an introduction to the ideas behind the multilevel
blocking (MLB) approach to the fermion sign problem in path-integral Monte
Carlo simulations, and also gives a detailed discussion of MLB results for
quantum dots. MLB can turn the exponential severity of the sign problem into an
algebraic one, thereby enabling numerically exact studies of otherwise
inaccessible systems. Low-temperature simulation results for up to eight
strongly correlated electrons in a parabolic 2D quantum dot are presented.


title: Importance of matrix elements in the ARPES spectra of BISCO

abstract:
  We have carried out extensive first-principles angle-resolved photointensity
(ARPES) simulations in Bi2212 wherein the photoemission process is modelled
realistically by taking into account the full crystal wavefunctions of the
initial and final states in the presence of the surface. The spectral weight of
the ARPES feature associated with the CuO_2 plane bands is found to undergo
large and systematic variations with $k_\parallel$ as well as the energy and
polarization of the incident photons. These theoretical predictions are in good
accord with the corresponding measurements, indicating that the remarkable
observed changes in the spectral weights in Bi2212 are essentially a matrix
element effect and that the importance of matrix elements should be kept in
mind in analyzing the ARPES spectra in the high-Tc's.


title: Molecular Simulations of Dewetting

abstract:
  We have studied the breakup and subsequent fluid flow in very thin films of
partially wetting liquid on solid substrates, using molecular dynamics
simulations. The liquid is made of short chain molecules interacting with
Lennard-Jones interactions, and the solid is modeled as a clean crystal lattice
whose atoms have thermal oscillations. Films below a critical thickness are
found to exhibit a spontaneous spinodal-like instability leading to dry
patches, as predicted theoretically and observed in some experiments. Liquid
withdrawing from a dry patch collects in a moving rim whose fluid dynamics is
only partially in agreement with earlier predictions.


title:
The effect of interstitial clusters and vacancies on the STM image of
  graphite

abstract:
  Making use of the tight-binding Green's function technique, we have
calculated the STM images of graphite with surface and sub-surface defects,
while taking into account the relaxation of the lattice due to defects. We have
demonstrated that two different physical mechanisms may result in the formation
of hillocks in the STM images: buckling of the graphite surface due to
interstitials between the uppermost graphite layers and the enhancement of the
electron density of states close to the Fermi energy on the carbon atoms in the
vicinity of vacancies. Our results indicate that small hillocks may originate
both from the interstitial clusters and from the vacancies. By contrast,
however, large hillocks in excess of 10 \AA~ in diameter can be caused only by
interstitial clusters.


title: Two-dimensional periodic frustrated Ising models in a transverse field

abstract:
  We investigate the interplay of classical degeneracy and quantum dynamics in
a range of periodic frustrated transverse field Ising systems at zero
temperature. We find that such dynamics can lead to unusual ordered phases and
phase transitions, or to a quantum spin liquid (cooperative paramagnetic) phase
as in the triangular and kagome lattice antiferromagnets, respectively. For the
latter, we further predict passage to a bond-ordered phase followed by a
critical phase as the field is tilted. These systems also provide exact
realizations of quantum dimer models introduced in studies of high temperature
superconductivity.


title: Generalization in the Hopfield Model

abstract:
  In the Hopfield model the ability of the network to generalization is studied
in the case of the network trained by one input image ({\it the standard}).


title: Two-phase behavior in strained thin films of hole-doped manganites

abstract:
  We present a study of the effect of biaxial strain on the electrical and
magnetic properties of thin films of manganites. We observe that manganite
films grown under biaxial compressive strain exhibit island growth morphology
which leads to a non-uniform distribution of the strain. Transport and magnetic
properties of these films suggest the coexistence of two different phases, a
metallic ferromagnet and an insulating antiferromagnet. We suggest that the
high strain regions are insulating while the low strain regions are metallic.
In such non-uniformly strained samples, we observe a large magnetoresistance
and a field-induced insulator to metal transition.


title: Simple model of a limit order-driven market

abstract:
  We introduce and study a simple model of a limit order-driven market. Traders
in this model can either trade at the market price or place a limit order, i.e.
an instruction to buy (sell) a certain amount of the stock if its price falls
below (raises above) a predefined level. The choice between these two options
is purely random (there are no strategies involved), and the execution price of
a limit order is determined simply by offsetting the most recent market price
by a random amount. Numerical simulations of this model revealed that despite
such minimalistic rules the price pattern generated by the model has such
realistic features as ``fat'' tails of the price fluctuations distribution,
characterized by a crossover between two power law exponents, long range
correlations of the volatility, and a non-trivial Hurst exponent of the price
signal.


title: Chromatic Polynomials, Potts Models and All That

abstract:
  The q-state Potts model can be defined on an arbitrary finite graph, and its
partition function encodes much important information about that graph,
including its chromatic polynomial, flow polynomial and reliability polynomial.
The complex zeros of the Potts partition function are of interest both to
statistical mechanicians and to combinatorists. I give a pedagogical
introduction to all these problems, and then sketch two recent results: (a)
Construction of a countable family of planar graphs whose chromatic zeros are
dense in the whole complex q-plane except possibly for the disc |q-1| < 1. (b)
Proof of a universal upper bound on the q-plane zeros of the chromatic
polynomial (or antiferromagnetic Potts-model partition function) in terms of
the graph's maximum degree.

categories: cond-mat.stat-mech hep-lat math-ph math.CO math.MP


title:
Monte Carlo simulations and numerical solutions of short-time critical
  dynamics

abstract:
  Recent progress in numerical study of the short-time critical dynamics is
briefly reviewed.


title:
Operator-valued Riemann-Hilbert problem for correlation functions of the
  XXZ spin chain

abstract:
  The generating functional of correlation functions for the XXZ spin chain is
considered in the thermodynamic limit. We derive a system of integro-difference
equations that prescribe this functional. On the basis of this system we
establish the operator-valued Riemann-Hilbert problem for correlation functions
of the XXZ spin chain.


title: Andreev Spectroscopy for Superconducting Phase Qubits

abstract:
  We propose a new method to measure the coherence time of superconducting
phase qubits based on the analysis of the magnetic-field dependent dc nonlinear
Andreev current across a high-resistance tunnel contact between the qubit and a
dirty metal wire and derive a quantitative relation between the subgap I-V
characteristic and the internal correlation function of the qubit.


title:
Structural and dynamical properties of sodium silicate melts: An
  investigation by molecular dynamics computer simulation

abstract:
  We present the results of large scale computer simulations in which we
investigate the static and dynamic properties of sodium disilicate and sodium
trisilicate melts. We study in detail the static properties of these systems,
namely the coordination numbers, the temperature dependence of the Q^(n)
species and the static structure factor, and compare them with experiments. We
show that the structure is described by a partially destroyed tetrahedral SiO_4
network and the homogeneously distributed sodium atoms which are surrounded on
average by 16 silicon and other sodium atoms as nearest neighbors. We compare
the diffusion of the ions in the sodium silicate systems with that in pure
silica and show that it is much slower in the latter. The sodium diffusion is
characterized by an activated hopping through the Si-O matrix which is frozen
with respect to the movement of the sodium atoms. We identify the elementary
diffusion steps for the sodium and the oxygen diffusion and find that in the
case of sodium they are related to the breaking of a Na-Na bond and in the case
of oxygen to that of a Si-O bond. From the self part of the van Hove
correlation function we recognize that at least two successive diffusion steps
of a sodium atom are spatially highly correlated with each other. With the same
quantity we show that at low temperatures also the oxygen diffusion is
characterized by activated hopping events.


title:
Finite size scaling investigations in the quantum $\phi^4$-model with
  long-range interaction

abstract:
  In this paper, we study in details the critical behavior of the ${\cal O}(n)$
quantum $\phi^4$ model with long-range interaction decaying with the distances
r by a power law as $r^{-d-\sigma}$ in the large n-limit. The zero-temperature
critical behavior is discussed. Its alteration by the finite temperature and/or
finite sizes in the space is studied. The scaling behaviours are studied in
different regimes depending upon whether the finite temperature or the finite
sizes of the system is leading. A number of results for the correlation length,
critical amplitudes and the finite size shift, for different dimensionalities
between the lower $d_<=\sigma/2$ and the upper $d_>=3\sigma/2$ critical
dimensions, are calculated.


title:
Effect of Pressure on Tiny Antiferromagnetic Moment in the
  Heavy-Electron Compound URu_2Si_2

abstract:
  We have performed elastic neutron-scattering experiments on the
heavy-electron compound URu_2Si_2 for pressure P up to 2.8 GPa. We have found
that the antiferrmagnetic (100) Bragg reflection below T_m ~ 17.5 K is strongly
enhanced by applying pressure. For P < 1.1 GPa, the staggered moment mu_o at
1.4 K increases linearly from ~ 0.017(3) mu_B to ~ 0.25(2) mu_B, while T_m
increases slightly at a rate ~ 1 K/GPa, roughly following the transition
temperature T_o determined from macroscopic anomalies. We have also observed a
sharp phase transition at P_c ~ 1.5 GPa, above which a 3D-Ising type of
antiferromagnetic phase (mu_o ~ 0.4 mu_B) appears with a slightly reduced
lattice constant.


title: Unequal Intra-layer Coupling in a Bilayer Driven Lattice Gas

abstract:
  The system under study is a twin-layered square lattice gas at half-filling,
being driven to non-equilibrium steady states by a large, finite `electric'
field. By making intra-layer couplings unequal we were able to extend the phase
diagram obtained by Hill, Zia and Schmittmann (1996) and found that the
tri-critical point, which separates the phase regions of the stripped (S) phase
(stable at positive interlayer interactions J_3), the filled-empty (FE) phase
(stable at negative J_3) and disorder (D), is shifted even further into the
negative J_3 region as the coupling traverse to the driving field increases.
Many transient phases to the S phase at the S-FE boundary were found to be
long-lived. We also attempted to test whether the universality class of D-FE
transitions under a drive is still Ising. Simulation results suggest a value of
1.75 for the exponent gamma but a value close to 2.0 for the ratio gamma/nu. We
speculate that the D-FE second order transition is different from Ising near
criticality, where observed first-order-like transitions between FE and its
"local minimum" cousin occur during each simulation run.


title: A nonperturbative Real-Space Renormalization Group scheme

abstract:
  Based on the original idea of the density matrix renormalization group
(DMRG), i.e. to include the missing boundary conditions between adjacent blocks
of the blocked quantum system, we present a rigorous and nonperturbative
mathematical formulation for the real-space renormalization group (RG) idea
invented by L.P. Kadanoff and further developed by K.G. Wilson. This is
achieved by using additional Hilbert spaces called auxiliary spaces in the
construction of each single isolated block, which is then named a superblock
according to the original nomenclature. On this superblock we define two maps
called embedding and truncation for successively integrating out the small
scale structure. Our method overcomes the known difficulties of the numerical
DMRG, i.e. limitation to zero temperature and one space dimension.


title:
A non-perturbative real-space renormalization group scheme for the
  spin-1/2 XXX Heisenberg model

abstract:
  In this article we apply a recently invented analytical real-space
renormalization group formulation which is based on numerical concepts of the
density matrix renormalization group. Within a rigorous mathematical framework
we construct non-perturbative renormalization group transformations for the
spin-1/2 XXX Heisenberg model in the finite temperature regime. The developed
renormalization group scheme allows for calculating the renormalization group
flow behaviour in the temperature dependent coupling constant. The constructed
renormalization group transformations are applied within the ferromagnetic and
the anti-ferromagnetic regime of the Heisenberg chain. The ferromagnetic fixed
point is computed and compared to results derived by other techniques.


title:
Comment on ``Friedel phases and phases of transmission amplitudes in
  quantum scattering systems" by T. Taniguchi and M. Buettiker

abstract:
  We take a modified boundary condition at the dead end of a stub to simulate
transmission zeroes being replaced by minima and then the discontinuous phase
slip (or decrease) at the transmission zeroes are replaced by a continuous but
rapid phase slip. The modified boundary condition can be continuously tuned to
give the results of the stub with hard wall boundary condition at the dead end
of stub. Even when the phase slip is continuous one can obtain information
about the density of states in the stub region from the scattering phases.


title: Quantum phase transitions in electronic systems

abstract:
  Quantum phase transitions occur at zero temperature when some non-thermal
control-parameter like pressure or chemical composition is changed. They are
driven by quantum rather than thermal fluctuations. In this review we first
give a pedagogical introduction to quantum phase transitions and quantum
critical behavior emphasizing similarities with and differences to classical
thermal phase transitions. We then illustrate the general concepts by
discussing a few examples of quantum phase transitions occurring in electronic
systems. The ferromagnetic transition of itinerant electrons shows a very rich
behavior since the magnetization couples to additional electronic soft modes
which generates an effective long-range interaction between the spin
fluctuations. We then consider the influence of rare regions on quantum phase
transitions in systems with quenched disorder, taking the antiferromagnetic
transitions of itinerant electrons as a primary example. Finally we discuss
some aspects of the metal-insulator transition in the presence of quenched
disorder and interactions.


title:
A New Vortex Solution for Two-Component Nonlinear Schroedinger Equation
  in Anisotropic Optical Media

abstract:
  A theoretical study is given of a new type of optical vortex in nonlinear
anisotropic media. This is realized as a special solution of the two-component
non-linear Schroedinger equation. The vortex is inherent in the spin texture
that is caused by an anisotropy of dielectric tensor, for which a role of spin
is played by the Stokes vector (or pseudo-spin). By using the effective
Lagrangian for the pseudo-spin field, we give an explicit form for the vortex
solution for the case of two types of optical anisotropy; that is, nonlinear
counterpart of birefringence giving rise to the Faraday and Cotton-Mouton
effects. We also examine the evolution equation of the new vortex with respect
to the propagation direction.


title: Band structure and transport studies on PdAl2Cl8-intercalated graphite

abstract:
  The 2D Fermi surface of 1st stage PdAl2Cl8 acceptor-type graphite
intercalation compounds (GICs) has been investigated using the Shubnikov-de
Haas (SdH) effect. One fundamental frequency is observed, the angular variation
of which confirms its strongly 2D nature, as previously found through
electrical conductivity measurements. The energy spectrum can be described by
the 2D band structure model proposed by Blinowski et al. We obtain the
following parameter values: intraplane C-C interaction energy gamma_0 = 2.7 eV,
Fermi energy E_F = -1.1 eV and carrier density n_SdH= 1.1x10^27 m^-3. Some
fewer details are presented on stage 2 and 3 materials.


title:
Search for two-scale localization in disordered wires in a magnetic
  field

abstract:
  The supersymmetry technique [A. V. Kolesnikov and K. B. Efetov, Phys. Rev.
Lett. 83, 3689 (1999)] predicts a two-scale behavior of wavefunction decay in
disordered wires in the crossover regime from preserved to broken time-reversal
symmetry. We have tested this prediction by a transmission approach, relying on
the Borland conjecture that relates the decay length of the transmittance to
the decay length of the wavefunctions. Our numerical simulations show no
indication of two-scale behavior.


title: Small excitonic complexes in a disk-shaped quantum dot

abstract:
  Confined excitonic complexes in two dimensions, consisting of N_e electrons
and N_h holes, are studied by means of Bethe-Goldstone equations. Systems with
up to twelve pairs, and asymmetric configurations with N_e \ne N_h are
considered. The weak confinement regime gives indication of weak binding or
even unbinding in the triexciton and the four-exciton system, and binding in
the higher complexes.


title:
Vortex Fluctuations in High-Tc Films: Flux Noise Spectrum and Complex
  Impedance

abstract:
  The flux noise spectrum and complex impedance for a 500 {\AA} thick YBCO film
are measured and compared with predictions for two dimensional vortex
fluctuations. It is verified that the complex impedance and the flux noise
spectra are proportional to each other, that the logarithm of the flux noise
spectra for different temperatures has a common tangent with slope $\approx
-1$, and that the amplitude of the noise decreases as $d^{-3}$, where $d$ is
the height above the film at which the magnetic flux is measured. A crossover
from normal to anomalous vortex diffusion is indicated by the measurements and
is discussed in terms of a two-dimensional decoupling.


title: On the structure pf genealogical trees in the presence of selection

abstract:
  We investigate through numerical simulations the effect of selection on two
summary statistics for nucleotide variation in a sample of two genes from a
population of N asexually reproducing haploid individuals. One is the mean time
since two individuals had their most recent common ancestor ($\bar{T_s}$), and
the other is the mean number of nucleotide differences between two genes in the
sample ($\bar{d_s}$). In the case of diminishing epistasis, in which the
deleterious effect of a new mutation is attenuated, we find that the scale of
$\bar{d_s}$ with the population size depends on the mutation rate, leading then
to the onset of a sharp threshold phenomenon as N becomes large.


title:
Critical behavior in NaV2O5: the case of dielectric function and the
  aspect of universality

abstract:
  Temperature dependence in the interval 4.2-300 K and the critical behavior of
the dielectric function along c-axis in NaV2O5 have been studied in the
frequency range 1 MHz-1 GHz. These data were analyzed together with literature
data concerning specific heat, magnetic losses and ultrasonic velocity. We
found that all results support the idea about universality of the critical laws
at the transition in NaV2O5 where different physical quantities demonstrate
identical shape of the anomaly. Considerable deviations from the standard
theory of the second order transition including asymmetry of the critical
behavior above and below transition and appearance of the anomalous background
are detected.


title:
Competing effects of mass anisotropy and spin Zeeman coupling on the
  upper critical field of a mixed $d$- and s-wave superconductor

abstract:
  Based on the linearized Eilenberger equations, the upper critical field
$(H_{c2})$ of mixed d- and s-wave superconductors has been microscopically
studied with an emphasis on the competing effects of mass anisotropy and spin
Zeeman coupling. We find the mass anisotropy always enhance $H_{c2}$ while the
Zeeman interaction suppresses $H_{c2}$. As required by the thermodynamics, we
find $H_{c2}$ is saturated at zero temperature. We compare the theoretical
calculations with recent experimental data of
YBa$_{2}$Cu$_{3}$O$_{7-+AFw-delta}$.


title: Giant Peak Effect Observed in Ultrapure YBCO Crystal

abstract:
  A giant peak in the temperature dependence of the screening current is
observed in the ac magnetic response of an ultra-pure YBa$_2$Cu$_3$O$_{6.993}$
crystal in a magnetic field. At H = 2.0 T ({\bf $H$}$||${\bf c}), the screening
current density $J_c(T)$ exhibits a 35-fold rise with 0.5 K increase in
temperature, indicating an abrupt $10^3$-fold collapse in the characteristic
volume of ordered regions in the vortex array. The peak-effect anomaly is most
pronounced for $H<$ 4.0 T, but detectable up to 7.0 T. The temperature
dependence of the equilibrium magnetization exhibits a small discontinuous jump
(for high fields) inside the peak-effect regime, suggesting that the underlying
phase transition is a weak first-order vortex-lattice melting transition.


title:
First Principles Study of Structural, Electronic and Magnetic Interplay
  in Ferroelectromagnetic Yttrium Manganite

abstract:
  We present results of local spin density approximation pseudopotential
calculations for the ferroelectromagnet, yttrium manganite (YMnO3). The origin
of the differences between ferroelectric and non-ferroelectric perovskite
manganites is determined by comparing the calculated properties of yttrium
manganite in its ferroelectric hexagonal and non-ferroelectric orthorhombic
phases. In addition, orthorhombic YMnO3 is compared with the prototypical
non-ferroelectric manganite, lanthanum manganite. We show that, while the
octahedral crystal field splitting of the cubic perovskite structure causes a
centro-symmetric Jahn-Teller distortion around the Mn3+ ion, the markedly
different splitting in hexagonal perovskites creates an electronic
configuration consistent with ferroelectric distortion. We explain the nature
of the distortion, and show that a local magnetic moment on the Mn3+ ion is a
requirement for it to occur.


title: Instance Space of the Number Partitioning Problem

abstract:
  Within the replica framework we study analytically the instance space of the
number partitioning problem. This classic integer programming problem consists
of partitioning a sequence of N positive real numbers $\{a_1, a_2,..., a_N}$
(the instance) into two sets such that the absolute value of the difference of
the sums of $a_j$ over the two sets is minimized. We show that there is an
upper bound $\alpha_c N$ to the number of perfect partitions (i.e. partitions
for which that difference is zero) and characterize the statistical properties
of the instances for which those partitions exist. In particular, in the case
that the two sets have the same cardinality (balanced partitions) we find
$\alpha_c=1/2$. Moreover, we show that the disordered model resulting from hte
instance space approach can be viewed as a model of replicators where the
random interactions are given by the Hebb rule.


title:
The role of domain walls on the vortex creep dynamics in unconventional
  superconducors

abstract:
  We investigate the influence of domain walls on the vortex dynamics in
superconductors with multi-component order parameters. We show that, due to
their complex structure domain walls can carry vortices with fractional flux
quanta. The decay of conventional vortices into fractional ones on domain walls
is examined. This decay presents an extraordinarily strong pinning mechanism
for vortices and turns domain walls occupied with pinned fractional vortices
into efficient barriers for the vortex motion. Therefore, domain walls can act
as fences for the flux flow, preventing the decay of the remnant magnetic flux
enclosed by them. Furthermore, the consequences of this property of domain
walls on the vortex dynamics are discussed in connection with observed noise in
the hysteresis cycle, using the Bean model of the critical vortex state. Based
on this picture experimental data in the unconventional superconductors
UPt$_3$, U$_{1-x}$Th$_x$Be$_{13}$ and Sr$_2$RuO$_4$ are interpreted.


title: Persistent currents in mesoscopic cavities with surface roughness

abstract:
  We consider a bidimensional discrete annular cavity with surface roughness
(SR) threaded by a magnetic flux. In the ballistic regime and at half filling,
localized border-states show up. These border states contribute coherentely to
the persistent current and the magnitude of the typical current $I_{typ}$ is
enhaced with respect to their value in the absence of confinement. This result
is robust with respect to changes in the relevant lengths of the sample.


title:
Statistical properties of phases and delay times of the one-dimensional
  Anderson model with one open channel

abstract:
  We study the distribution of phases and of Wigner delay times for a
one-dimensional Anderson model with one open channel. Our approach, based on
classical Hamiltonian maps, allows us an analytical treatment. We find that the
distribution of phases depends drastically on the parameter $\sigma_A =
\sigma/sin k$ where $\sigma^2$ is the variance of the disorder distribution and
$k$ the wavevector. It undergoes a transition from uniformity to singular
behaviour as $\sigma_A$ increases. The distribution of delay times shows
universal power law tails $~ 1/\tau^2$, while the short time behaviour is
$\sigma_A$- dependent.


title:
Test of a simple and flexible molecule model for alpha-, beta- and
  gamma-S8 crystals

abstract:
  S8 is the most stable compound of elemental sulfur in solid and liquid
phases, at ambient pressure and below 400K. Three crystalline phases of S8 have
been clearly identified in this range of thermodynamic parameters, although no
calculation of its phase diagram has been performed yet. alpha- and gamma-S8
are orientationally ordered crystals while beta-S8 is measured as
orientationally disordered. In this paper we analyze the phase diagram of S8
crystals, as given by a simple and flexible molecule model, via a series of
molecular dynamics (MD) simulations.
  The calculations are performed in the constant pressure- constant temperature
ensemble, using an algorithm that is able to reproduce structural phase
transitions.


title: Convective Term and Transversely Driven Charge-Density Waves

abstract:
  We derive the convective terms in the damping which determine the structure
of the moving charge-density wave (CDW), and study the effect of a current
flowing transverse to conducting chains on the CDW dynamics along the chains.
In contrast to a recent prediction we find that the effect is orders of
magnitude smaller, and that contributions from transverse currents of electron-
and hole-like quasiparticles to the force exerted on the CDW along the chains
act in the opposite directions. We discuss recent experimental verification of
the effect and demonstrate experimentally that geometry effects might mimic the
transverse current effect.


title:
The Angular Intensity Correlation Functions C^{(1)} and C^{(10)} for the
  Scattering of Light from Randomly Rough Dielectric and Metal Surfaces

abstract:
  We study the statistical properties of the scattering matrix S(q|k) for the
problem of the scattering of light from a randomly rough one-dimensional
surface, defined by the equation $x_3 = \zx$, where the surface profile
function $\zx$ constitutes a zero-mean, stationary, Gaussian random process,
through the effects of $S(q|k)$ on the angular intensity correlation function
C(q,k|q',k'). The existence of both the C^{(1)} and C^{(10)} correlation
functions is consistent with the amplitude of the scattered field obeying
complex Gaussian statistics in the limit of a long surface. We show that the
deviation of the statistics of the scattering matrix from circular Gaussian
statistics and the C^{(10)} correlation function are determined by exactly the
same statistical moment. As the random surface becomes rougher, the amplitude
of the scattered field no longer obeys complex Gaussian statistics but obeys
complex circular Gaussian statistics instead. In this case, the C^{(10)}
correlation function should vanish. This result is confirmed by numerical
simulation calculations.


title:
Exact duality and dual Monte-Carlo simulation for the Bosonic Hubbard
  model

abstract:
  We derive the exact dual to the Bosonic Hubbard model. The dual variables
take the form of conserved current loops (local and global). Previously this
has been done only for the very soft core model at very high density. No such
approximations are made here. In particular, the dual of the hard core model is
shown to have a very simple form which is then used to construct an efficient
Monte Carlo algorithm which is quite similar to the World Line algorithm but
with some important differences. For example, with this algorithm we can
measure easily the correlation function of the order parameter (Green
function), a quantity which is extremely difficult to measure with the standard
World Line algorithm. We demonstrate the algorithm for the one and two
dimensional hardcore Bosonic Hubbard models. We present new results especially
for the Green function and zero mode filling fraction in the two dimensional
hardcore model.


title: Statics and dynamics of a one-dimensional quantum many-body system

abstract:
  The macroscopic zero-temperature behavior of weakly- incommensurate systems
in one dimension is described in terms of solitons. The soliton density n obeys
equations displaying several types of singular interface-like solutions: (i)
equilibrium or moving boundary between the n = 0 and finite n regions, and (ii)
stationary or moving annihilation front separating solitons from antisolitons.


title:
Conformation changes and folding of proteins mediated by Davidov's
  soliton

abstract:
  We suggest that Davidov's solitons, propagating through the backbone of a
protein, can mediate conformational transition and folding of a protein to its
native state. A simple toy model is presented in which a Non Linear Schrodinger
(NLS) field interacts with another field phi, corresponding to the conformation
angles of the protein. The interaction provides the conformation field with the
energy needed in order to overcome energy barriers for folding, thus avoiding
the need for stochastic thermal activation. Such a transition is therefore a
deterministic controled process.The soliton is compensated for its energy loss
by absorption of the energy gained in the folding process. This scenario does
not change significantly even in the presence of imposed disorder, provided
that the initial momentum of the soliton is large enough.


title:
Test of renormalization predictions for universal finite-size scaling
  functions

abstract:
  We calculate universal finite-size scaling functions for systems with an
n-component order parameter and algebraically decaying interactions. Just as
previously has been found for short-range interactions, this leads to a
singular epsilon-expansion, where epsilon is the distance to the upper critical
dimension. Subsequently, we check the results by numerical simulations of spin
models in the same universality class. Our systems offer the essential
advantage that epsilon can be varied continuously, allowing an accurate
examination of the region where epsilon is small. The numerical calculations
turn out to be in striking disagreement with the predicted singularity.


title: Instantons in the working memory: implications for schizophrenia

abstract:
  The influence of the synaptic channel properties on the stability of delayed
activity maintained by recurrent neural network is studied. The duration of
excitatory post-synaptic current (EPSC) is shown to be essential for the global
stability of the delayed response. NMDA receptor channel is a much more
reliable mediator of the reverberating activity than AMPA receptor, due to a
longer EPSC. This allows to interpret the deterioration of working memory
observed in the NMDA channel blockade experiments. The key mechanism leading to
the decay of the delayed activity originates in the unreliability of the
synaptic transmission. The optimum fluctuation of the synaptic conductances
leading to the decay is identified. The decay time is calculated analytically
and the result is confirmed computationally.


title:
Polarization and localization in insulators: generating function
  approach

abstract:
  We develop the theory and practical expressions for the full
quantum-mechanical distribution of the intrinsic macroscopic polarization of an
insulator in terms of the ground state wavefunction. The central quantity is a
cumulant generating function which yields, upon successive differentiation, all
the cumulants and moments of the probability distribution of the center of mass
X/N of the electrons, defined appropriately to remain valid for extended
systems obeying twisted boundary conditions. The first moment is the average
polarization, where we recover the well-known Berry phase expression. The
second cumulant gives the mean-square fluctuation of the polarization, which
defines an electronic localization length xi_i along each direction i. It
follows from the fluctuation-dissipation theorem that in the thermodynamic
limit xi_i diverges for metals and is a finite, measurable quantity for
insulators. It is possible to define for insulators maximally-localized
``many-body Wannier functions'', which for large N become localized in
disconnected regions of the high-dimensional configuration space, establishing
a direct connection with Kohn's theory of the insulating state. Interestingly,
the expression for xi_i^2, which involves the second derivative of the
wavefunction with respect to the boundary conditions, is directly analogous to
Kohn's formula for the ``Drude weight'' as the second derivative of the energy.


title: Theoretical study of collective modes in DNA at ambient temperature

abstract:
  The instantaneous normal modes corresponding to base pair vibrations (radial
modes) and twist angle fluctuations (angular modes) of a DNA molecule model at
ambient temperature are theoretically investigated. Due to thermal disorder,
normal modes are not plane waves with a single wave number q but have a finite
and frequency dependent damping width. The density of modes rho(nu), the
average dispersion relation nu(q) as well as the coherence length xi(nu) are
analytically calculated. The Gibbs averaged resolvent is computed using a
replicated transfer matrix formalism and variational wave functions for the
ground and first excited state. Our results for the density of modes are
compared to Raman spectroscopy measurements of the collective modes for DNA in
solution and show a good agreement with experimental data in the low frequency
regime nu < 150 cm^{-1}. Radial modes extend over frequencies ranging from 50
cm^{-1} to 110 cm^{-1}. Angular modes, related to helical axis vibrations are
limited to nu < 25 cm^{-1}. Normal modes are highly disordered and coherent
over a few base pairs only (xi < 2 nm) in good agreement with neutron
scattering experiments.


title: Electron Transport in High-Tc Superconducting Grain Boundary Junctions

abstract:
  The results of dc, microwave and magnetic measurements of YBCO bicrystal
junctions on r-cut sapphire are presented. The junctions with high resistance
10-20 W and IcRN= 1-2 mV and tolerance of RNS around 30% on chip allow to
create microwave circuits with low integration (up to 10 junctions on chip).
The microwave dynamics of the junction with superconducting current-phase
relation fits with sin j relation better than 5%, that clear indicates on
tunnel conductivity between two YBCO electrodes. It was found that critical
current density depends as square root on the interface transparency in
accordance with prediction of superconducting current transport via Andreev's
bound surface states. The specific properties of current transport in high-Tc
grain boundary junctions with taking into account d-wave type of gap order in
high-Tc superconductor are discussed.


title:
Renormalization Group Approach to the Problem of Flow Through Irregular
  Packed Beds

abstract:
  We have calculated the asymptotics of Green's function of the differential
equation of nonlinear diffusion in the microscopic range with strong porosity
fluctuations in the problem of flow through irregular packed beds for arbitrary
space dimension and arbitrary covariance of porosity fluctuations.


title: Weak localization in the 2D metallic regime of Si-MOS

abstract:
  The negative magnetoresistance due to weak localization is investigated in
the two-dimensional metallic state of Si-MOS structures for high conductance
values between 35 and 120 e^2/h. The extracted phase coherence time is equal to
the momentum relaxation time at 10 K but nearly 100 times longer at the lowest
temperature. Nevertheless, only weak logarithmic corrections to the
conductivity are present in the investigated temperature and concentration
range thus proving the absence of strong quantum effects due to
electron-electron interaction. From saturation effects of the phase coherence
time a lower boundary for spin-orbit scattering of about 200 ps is estimated.


title:
Non-dynamic origin of the acoustic attenuation at high frequency in
  glasses

abstract:
  The sound attenuation in the THz region is studied down to T=16 K in glassy
glycerol by inelastic x-ray scattering. At striking variance with the decrease
found below 100 K in the GHz data, the attenuation in the THz range does not
show any T dependence. This result i) indicates the presence of two different
attenuation mechanisms, active respectively in the high and low frequency
limits; ii) demonstrates the non-dynamic origin of the attenuation of THz sound
waves, and confirms a similar conclusion obtained in SiO2 glass by molecular
dynamics; and iii) supports the low frequency attenuation mechanism proposed by
Fabian and Allen (Phys.Rev.Lett. 82, 1478 (1999)).


title:
Large scale molecular dynamics simulation of self-assembly processes in
  short and long chain cationic surfactants

abstract:
  We report on an investigation of the structural and dynamical properties of
n-nonyltrimethylammonium chloride (C9TAC) and erucyl bis [2-hydroxyethyl]
methylammonium chloride (EMAC) micelles in aqueous solution. A fully atomistic
description was used, and the time evolution was computed using molecular
dynamics.

categories: cond-mat.soft comp-gas nlin.CG physics.chem-ph


title:
Basic obstacle for electrical spin-injection from a ferromagnetic metal
  into a diffusive semiconductor

abstract:
  We have calculated the spin-polarization effects of a current in a two
dimensional electron gas which is contacted by two ferromagnetic metals. In the
purely diffusive regime, the current may indeed be spin-polarized. However, for
a typical device geometry the degree of spin-polarization of the current is
limited to less than 0.1%, only. The change in device resistance for parallel
and antiparallel magnetization of the contacts is up to quadratically smaller,
and will thus be difficult to detect.


title:
Effect of transport coefficients on the time-dependence of density
  matrix

abstract:
  For Lindblad's master equation of open quantum systems with a general
quadratic form of the Hamiltonian, the propagator of the density matrix is
analytically calculated by using path integral techniques. The time-dependent
density matrix is applied to nuclear barrier penetration in heavy ion
collisions with inverted oscillator and double-well potentials. The quantum
mechanical decoherence of pairs of phase space histories in the propagator is
studied and shown that the decoherence depends crucially on the transport
coefficients.


title:
Band structure effects on the interaction of charged particles with
  solids

abstract:
  A survey is presented of current investigations of the impact of band
structure effects on various aspects of the interaction of charged particles
with real solids. The role that interband transitions play in the decay
mechanism of bulk plasmons is addressed, and results for plasmon linewidths in
Al and Si are discussed. {\it Ab initio} calculations of the electronic energy
loss of ions moving in Al and Si are also presented, within linear response
theory, from a realistic description of the one-electron band structure and a
full treatment of the dynamic electronic response of valence electrons. Both
random and position-dependent stopping powers of valence electrons are
computed.


title: Evaporative Cooling of a Two-Component Degenerate Fermi Gas

abstract:
  We derive a quantum theory of evaporative cooling for a degenerate Fermi gas
with two constituents and show that the optimum cooling trajectory is
influenced significantly by the quantum statistics of the particles. The
cooling efficiency is reduced at low temperatures due to Pauli blocking of
available final states in each binary collision event. We compare the
theoretical optimum trajectory with experimental data on cooling a quantum
degenerate cloud of potassium-40, and show that temperatures as low as 0.3
times the Fermi temperature can now be achieved.


title:
Shifts and widths of collective excitations in trapped Bose gases by the
  dielectric formalism

abstract:
  We present predictions for the temperature dependent shifts and damping
rates. They are obtained by applying the dielectric formalism to a simple model
of a trapped Bose gas. Within the framework of the model we use lowest order
perturbation theory to determine the first order correction to the results of
Hartree-Fock-Bogoliubov-Popov theory for the complex collective excitation
frequencies, and present numerical results for the temperature dependence of
the damping rates and the frequency shifts. Good agreement with the
experimental values measured at JILA are found for the m=2 mode, while we find
disagreements in the shifts for m=0. The latter point to the necessity of a
non-perturbative treatment for an explanation of the temperature-dependence of
the m=0 shifts.


title: Mesoscopic Fluctuations of Adiabatic Charge Pumping in Quantum Dots

abstract:
  We consider the adiabatic charge transport through zero-dimensional
mesoscopic sample (quantum dot) caused by two periodically changing external
perturbations. Both the magnitude and the sign of the transmitted charge are
extremely sensitive to the configuration of the dot and to the magnetic field.
We find the correlation function characterizing the random value of this pumped
charge for arbitrary strength of the perturbation. In contrast to previous
theoretical and experimental claims, the charge is found not to have any
symmetry with respect to the inversion of magnetic field. At strong pumping
perturbation, the variance of the charge $<Q^2>$ is found to be proportional to
the length of the contour in parametric space.


title: Thermodynamics of a Tiling Model

abstract:
  A particular, two-dimensional, tiling model, composed by the so called Wang
tiles has been studied at finite temperature by Monte Carlo numerical
simulations. In absence of any thermal bath the Wang tiles give the opportunity
of building a very large number of non-periodic tilings. We can construct a
local Hamiltonian such that only perfectly matched tilings are ground states
with zero energy. This Hamiltonian has a very large degeneracy. The
thermodynamic behaviour of such a system seems to show a continuous phase
transition at non zero temperature. An order parameter with non-trivial
features is proposed. Under the critical temperature the model exhibits aging
properties. The fluctuation-dissipation theorem is violated.


title: Phonon-like and single particle dynamics in liquid lithium

abstract:
  The dynamic structure factor, S(Q,E), of liquid lithium (T=475 K) has been
determined by inelastic x-ray scattering (IXS) in the momentum transfer region
(Q = 1.4-110 nm-1). These data allow to observe how, in a simple liquid, a
phonon-like collective mode evolves towards the single particle dynamics. As a
function of Q, one finds: i) at low Q's, a sound mode with a positive
dispersion of the sound velocity, ii) at intermediate Q's, excitations whose
energy oscillates similarly to phonons in the crystal Brillouin zones, and iii)
at high Q's, the S(Q,E) approaches a Gaussian shape, indicating that the single
particle dynamics has been reached.


title: Acoustic Nature of the Boson Peak in Vitreous Silica

abstract:
  New temperature dependent inelastic x-ray (IXS) and Raman (RS) scattering
data are compared to each other and with existing inelastic neutron scattering
data in vitreous silica (v-SiO_2), in the 300 - 1775 K region. The IXS data
show collective propagating excitations up to Q=3.5 nm^-1. The temperature
behaviour of the excitations at Q=1.6 nm^-1 matches that of the boson peak
found in INS and RS. This supports the acoustic origin of the excess of
vibrational states giving rise to the boson peak in this glass.