title:
Effective Interactions and Volume Energies in Charge-Stabilized
  Colloidal Suspensions

abstract:
  Charge-stabilized colloidal suspensions can be conveniently described by
formally reducing the macroion-microion mixture to an equivalent one-component
system of pseudo-particles. Within this scheme, the utility of a linear
response approximation for deriving effective interparticle interactions has
been demonstrated [M. J. Grimson and M. Silbert, Mol. Phys. 74, 397 (1991)].
Here the response approach is extended to suspensions of finite-sized macroions
and used to derive explicit expressions for (1) an effective electrostatic pair
interaction between pseudo-macroions and (2) an associated volume energy that
contributes to the total free energy. The derivation recovers precisely the
form of the DLVO screened-Coulomb effective pair interaction for spherical
macroions and makes manifest the important influence of the volume energy on
thermodynamic properties of deionized suspensions. Excluded volume corrections
are implicitly incorporated through a natural modification of the inverse
screening length. By including nonlinear response of counterions to macroions,
the theory may be generalized to systematically investigate effective many-body
interactions.


title:
Doping dependence of the Neel temperature in Mott-Hubbard
  antiferromagnets: Effect of vortices

abstract:
  The rapid destruction of long-range antiferromagnetic order upon doping of
Mott-Hubbard antiferromagnetic insulators is studied within a generalized
Berezinskii-Kosterlitz-Thouless renormalization group theory in accordance with
recent calculations suggesting that holes dress with vortices. We calculate the
doping-dependent Neel temperature in good agreement with experiments for
high-Tc cuprates. Interestingly, the critical doping where long-range order
vanishes at zero temperature is predicted to be xc ~ 0.02, independently of any
energy scales of the system.


title:
Translational Invariance in Models for Low-Temperature Properties of
  Glasses

abstract:
  We report on a refined version of our spin-glass type approach to the
low-temperature physics of structural glasses. Its key idea is based on a Born
von Karman expansion of the interaction potential about a set of reference
positions in which glassy aspects are modeled by taking the harmonic
contribution within this expansion to be random. Within the present refined
version the expansion at the harmonic level is reorganized so as to respect the
principle of global translational invariance. By implementing this principle,
we have for the first time a mechanism that fixes the distribution of the
parameters characterizing the local potential energy configurations responsible
for glassy low-temperature anomalies solely in terms of assumptions about
interactions at a microscopic level.


title: Low Energy Excitations of Yb4As3 in a Magnetic Field

abstract:
  We discuss the effects of an applied magnetic field on the low energy
excitations in the low temperature phase of Yb4As3. We show also why the
magnetic interaction of the Yb{3+} ions is nearly of an isotropic Heisenberg
spin-1/2 type. A small anisotropy due to an intrachain dipolar interaction
leads to the opening of a gap when a magnetic field is applied. The model
agrees with available experimental data. Simple experiments are suggested in
order to further test the present theory.


title: Dynamical Transverse Meissner Effect and Transition in Moving Bose Glass

abstract:
  We study moving periodic structures in presence of correlated disorder using
renormalisation group. We find that the effect of disorder persists at all
velocities resulting at zero temperature in a Moving Bose Glass phase with
transverse pinning. At non zero temperature we find two distinct moving glass
phases. We predict a sharp transition, as velocity increases between the Moving
Bose Glass, where the transverse Meissner effect persists in the direction
transverse to motion and a Correlated Moving Glass at high velocity, where it
disappears. Experimental consequences for vortex lattices and charge density
wave (CDW) systems are discussed.


title:
Designing emissive conjugated polymers with small optical gaps: a step
  towards organic polymeric infrared lasers

abstract:
  We show that chemical modification of the trans-polyacetylene structure that
involves substitution of the backbone hydrogen atoms with conjugated side
groups, leads to reduction of the backbone bond alternation as well as
screening of the effective Coulomb interaction. Consequently the optical gap of
the substituted material is smaller than the parent polyene with the same
backbone length, and the excited state ordering is conducive to efficient
photoluminescence. The design of organic polymeric infrared lasers, in the
ideal long chain limit, thereby becomes possible.


title: Susceptibility Amplitude Ratios Near a Lifshitz Point

abstract:
  The susceptibility amplitude ratio in the neighborhood of a uniaxial Lifshitz
point is calculated at one-loop level using field-theoretic and
$\epsilon_{L}$-expansion methods. We use the Schwinger parametrization of the
propagator in order to split the quadratic and quartic part of the momenta, as
well as a new special symmetry point suitable for renormalization purposes. For
a cubic lattice (d = 3), we find the result $\frac{C_{+}}{C_{-}} = 3.85$.


title: Driven Tunneling: Chaos and Decoherence

abstract:
  Chaotic tunneling in a driven double-well system is investigated in absence
as well as in the presence of dissipation. As the constitutive mechanism of
chaos-assisted tunneling, we focus on the dynamics in the vicinity of
three-level crossings in the quasienergy spectrum. The coherent quantum
dynamics near the crossing is described satisfactorily by a three-state model.
It fails, however, for the corresponding dissipative dynamics, because
incoherent transitions due to the interaction with the environment indirectly
couple the three states in the crossing to the remaining quasienergy states.
The asymptotic state of the driven dissipative quantum dynamics partially
resembles the, possibly strange, attractor of the corresponding damped driven
classical dynamics, but also exhibits characteristic quantum effects.


title:
Loss of Pi-Junction Behaviour in an Interacting Impurity Josephson
  Junction

abstract:
  Using a generalization of the non-crossing approximation which incorporates
Andreev reflection, we study the properties of an infinite-U Anderson impurity
coupled to two superconducting leads. In the regime where $\Delta$ and $T_K$
are comparable, we find that the position of the sub-gap resonance in the
impurity spectral function develops a strong anomalous phase dependence-- its
energy is a minimum when the phase difference between the superconductors is
equal to $\pi$. Calculating the Josephson current through the impurity, we find
that $\pi$-junction behaviour is lost as the position of the bound-state moves
above the Fermi energy.


title:
Central limit theorems for nonlinear hierarchical sequences of random
  variables

abstract:
  We study central limit theorems for certain nonlinear sequences of random
variables. In particular, we prove the central limit theorems for the bounded
conductivity of the random resistor networks on hierarchical lattices.


title: Orientational Melting in Carbon Nanotube Ropes

abstract:
  Using Monte Carlo simulations, we investigate the possibility of an
orientational melting transition within a "rope" of (10,10) carbon nanotubes.
When twisting nanotubes bundle up during the synthesis, orientational
dislocations or twistons arise from the competition between the anisotropic
inter-tube interactions, which tend to align neighboring tubes, and the torsion
rigidity that tends to keep individual tubes straight. We map the energetics of
a rope containing twistons onto a lattice gas model and find that the onset of
a free "diffusion" of twistons, corresponding to orientational melting, occurs
at T_OM > 160 K.


title: Electrical Noise From Phase Separation In Pr2/3Ca1/3MnO3 Single Crystal

abstract:
  Low frequency electrical noise measurements have been used to probe the
electronic state of the perovskite-type manganese oxide Pr2/3Ca1/3MnO3 versus
temperature and in the vicinity of the field-induced transition from the
insulating, charge-ordered state (I-CO) to the metallic, ferromagnetic state
(M-F). At high temperature we have observed a high level of the excess noise
with mainly a gaussian distribution of the resistance fluctuations, and the
associated power spectral density has a standard 1/f dependence. However, in
the hysteretic region, where the electrical resistance depends dramatically on
the sample history, we have observed a huge non-gaussian noise characterized by
two level fluctuator-like switching (TLS) in the time domain. We discuss the
origin of the noise in terms of percolative behavior of the conductivity. We
speculate that the dominant fluctuators are manganese clusters switching
between the M-F and the I-CO phases.


title: Tunneling between the Edges of Two Lateral Quantum Hall Systems

abstract:
  The edge of a two-dimensional electron system (2DES) in a magnetic field
consists of one-dimensional (1D) edge-channels that arise from the confining
electric field at the edge of the specimen$^{1-3}$. The crossed electric and
magnetic fields, E x B, cause electrons to drift parallel to the sample
boundary creating a chiral current that travels along the edge in only one
direction. Remarkably, in an ideal 2DES in the quantum Hall regime all current
flows along the edge$^{4-6}$. Quantization of the Hall resistance, $R_{xy}=
h/Ne^{2}$, arises from occupation of N 1D edge channels, each contributing a
conductance of $e^{2}/h^{7-11}$. To explore this unusual one-dimensional
property of an otherwise two-dimensional system, we have studied tunneling
between the edges of 2DESs in the regime of integer quantum Hall effect (QHE).
In the presence of an atomically precise, high-quality tunnel barrier, the
resultant interaction between the edge states leads to the formation of new
energy gaps and an intriguing dispersion relation for electrons traveling along
the barrier. The absence of tunneling features due to the electron spin and the
persistence of a conductance peak at zero bias are not consistent with a model
of weakly interacting edge states.


title: Conserved Mass Models and Particle Systems in One Dimension

abstract:
  In this paper we study analytically a simple one dimensional model of mass
transport. We introduce a parameter $p$ that interpolates between continuous
time dynamics ($p\to 0$ limit) and discrete parallel update dynamics ($p=1$).
For each $p$, we study the model with (i) both continuous and discrete masses
and (ii) both symmetric and asymmetric transport of masses. In the asymmetric
continuous mass model, the two limits $p=1$ and $p\to 0$ reduce respectively to
the $q$-model of force fluctuations in bead packs [S.N. Coppersmith et. al.,
Phys. Rev. E. {\bf 53}, 4673 (1996)] and the recently studied asymmetric random
average process [J. Krug and J. Garcia, cond-mat/9909034]. We calculate the
steady state mass distribution function $P(m)$ assuming product measure and
show that it has an algebraic tail for small $m$, $P(m)\sim m^{-\beta}$ where
the exponent $\beta$ depends continuously on $p$. For the asymmetric case we
find $\beta(p)=(1-p)/(2-p)$ for $0\leq p <1$ and $\beta(1)=-1$ and for the
symmetric case, $\beta(p)=(2-p)^2/(8-5p+p^2)$ for all $0\leq p\leq 1$. We
discuss the conditions under which the product measure ansatz is exact. We also
calculate exactly the steady state mass-mass correlation function and show that
while it decouples in the asymmetric model, in the symmetric case it has a
nontrivial spatial oscillation with an amplitude decaying exponentially with
distance.


title:
Temperature scaling, glassiness and stationarity in the Bak-Sneppen
  model

abstract:
  We show that the emergence of criticality in the locally-defined Bak-Sneppen
model corresponds to separation over a hierarchy of timescales. Near to the
critical point the model obeys scaling relations, with exponents which we
derive numerically for a one-dimensional system. We further describe how the
model can be related to the glass model of Bouchaud [{\em J. Phys. I France
{\bf 2}, 1705 (1992)}], and we use this insight to comment on the usual
assumption of stationarity in the Bak-Sneppen model. Finally, we propose a
general definition of self-organised criticality which is in partial agreement
with other recent definitions.


title: Phase Slips and Interfaces in D-Wave Superconductors

abstract:
  We consider a model (100) interface between two d-wave superconductors. By
solving the Bogoliubov de Gennes equation on a tight binding lattice, we study
the properties of the interface as a function of the interface barrier. We
contrast the two scenarios: (i) an order parameter phase difference of
$\theta=0$ across the interface, and (ii) a phase slip of $\theta=\pi$ across
the interface. We find resonant sub-gap structure in the density of states only
when there is a phase slip present. We show that the local s-wave and ``p-wave
like'' order parameter components are strongly influenced by the barrier
profile and by the phase slip. The temperature dependence of the local p-wave
order parameter follows the underlying bulk $T_c$ for p-wave superconductivity
implying that this could be measured by a suitable tunneling experiment. We
also calculate the Josephson critical current as a function of the strength of
the insulating barrier at the interface.


title:
Magnetic field induced directional localization in a 2D rectangular
  lattice

abstract:
  We study the effect of a perpendicular uniform magnetic field on the
dissipative conductivity of a rectangular lattice with anisotropic hopping,
$t_x\neq t_y $. We show that the magnetic field may enhance dramatically the
directional anisotropy in the conductivity. The effect is a measurable physical
realization of Aubry's duality in Harper systems.


title:
Influence of parallel magnetic fields on a single-layer two-dimensional
  electron system with a hopping mechanism of conductivity

abstract:
  Large positive (P) magnetoresistance (MR) has been observed in parallel
magnetic fields in a single 2D layer in a delta-doped GaAs/AlGaAs
heterostructure with a variable-range-hopping (VRH) mechanism of conductivity.
Effect of large PMR is accompanied in strong magnetic fields by a substantial
change in the character of the temperature dependence of the conductivity. This
implies that spins play an important role in 2D VRH conductivity because the
processes of orbital origin are not relevant to the observed effect. A possible
explanation involves hopping via double occupied states in the upper Hubbard
band, where the intra-state correlation of spins is important.


title:
Dynamics of the 2D two-component plasma near the Kosterlitz-Thouless
  transition

abstract:
  We study the dynamics of a classical, two-component plasma in two dimensions,
in the vicinity of the Kosterlitz-Thouless (KT) transition where the system
passes from a dielectric low-temperature phase (consisting of bound pairs) to a
conducting phase. We use two ``complementary'' analytical approaches and
compare to simulations. The conventional, ``intuitive'' approach is built on
the KT picture of independently relaxing, bound pairs. A more formal approach,
working with Mori projected dynamic correlation functions, avoids to assume the
pair picture from the start. We discuss successes and failures of both
approaches, and suggest a way to combine the advantages of both.


title: Growth Optimal Investment and Pricing of Derivatives

abstract:
  We introduce a criterion how to price derivatives in incomplete markets,
based on the theory of growth optimal strategy in repeated multiplicative
games. We present reasons why these growth-optimal strategies should be
particularly relevant to the problem of pricing derivatives. We compare our
result with other alternative pricing procedures in the literature, and discuss
the limits of validity of the lognormal approximation. We also generalize the
pricing method to a market with correlated stocks. The expected estimation
error of the optimal investment fraction is derived in a closed form, and its
validity is checked with a small-scale empirical test.


title: Identifying the bottom line after a stock market crash

abstract:
  In this empirical paper we show that in the months following a crash there is
a distinct connection between the fall of stock prices and the increase in the
range of interest rates for a sample of bonds. This variable, which is often
referred to as the interest rate spread variable, can be considered as a
statistical measure for the disparity in lenders' opinions about the future; in
other words, it provides an operational definition of the uncertainty faced by
economic agents. The observation that there is a strong negative correlation
between stock prices and the spread variable relies on the examination of 8
major crashes in the United States between 1857 and 1987. That relationship
which has remained valid for one and a half century in spite of important
changes in the organization of financial markets can be of interest in the
perspective of Monte Carlo simulations of stock markets.


title:
Phenomenological Theory of Superconductivity and Magnetism in
  Ho$_{1-x}$Dy$_x$Ni$_2$B$_2$C

abstract:
  The coexistence of the superconductivity and magnetism in the
Ho$_{1-x}$Dy$_x$Ni$_2$B$_2$C is studied by using Ginzburg-Landau theory. This
alloy shows the coexistence and complex interplay of superconducting and
magnetic order. We propose a phenomenological model which includes two magnetic
and two superconducting order parameters accounting for the multi-band
structure of this material. We describe phenomenologically the magnetic
fluctuations and order and demonstrate that they lead to anomalous behavior of
the upper critical field. The doping dependence of $T_c$ in
Ho$_{1-x}$Dy$_x$Ni$_2$B$_2$C showing a reentrance behavior are analyzed
yielding a very good agreement with experimental data.


title:
Cauchy magnetic field component and magnitude distribution studied by
  the zero-field muon spin relaxation technique

abstract:
  Zero-field muon spin relaxation (ZF-$\mu $SR) data for dilute spin magnetic
systems have been widely interpreted with what is called a Kubo-Toyabe form
based on a Lorentzian distribution of local field components. We derive here
the proper magnetic field \textit{magnitude} distribution using independent and
uncorrelated \textit{component} distributions. Our result is then compared to
the previously accepted formula for ZF-$\mu $SR. We discuss the origins of the
magnetic field component and magnitude distributions. Further we found that
after rescaling the magnetic field, the differences that are amenable to
experimental examination are quite small, although the interpretations behind
them are quite different.


title: Self-trapped Exciton and Franck-Condon Spectra Predicted in LaMnO$_3$

abstract:
  Because the ground state has cooperative Jahn-Teller order, electronic
excitations in LaMnO$_3$ are predicted to self-trap by local rearrangement of
the lattice. The optical spectrum should show a Franck-Condon series, that is,
a Gaussian envelope of vibrational sidebands. Existing data are reinterpreted
in this way. The Raman spectrum is predicted to have strong multiphonon
features.


title:
Stable integration of isolated cell membrane patches in a nanomachined
  aperture: a step towards a novel device for membrane physiology

abstract:
  We investigate the microscopic contact of a cell/semiconductor hybrid. The
semiconductor is nanostructured with the aim of single channel recording of ion
channels in cell membranes. This approach will overcome many limitations of the
classical patch-clamp technique. The integration of silicon-based devices
'on-chip' promises novel types of experiments on single ion channels.


title:
Effect of symmetry breaking perturbations in the one-dimensional SU(4)
  spin-orbital model

abstract:
  We study the effect of symmetry breaking perturbations in the one-dimensional
SU(4) spin-orbital model. We allow the exchange in spin ($J_1$) and orbital
($J_2$) channel to be different and thus reduce the symmetry to SU(2) $\otimes$
SU(2). A magnetic field $h$ along the $S^z$ direction is also applied. Using
the formalism developped by Azaria et al we extend their analysis of the
isotropic $J_1=J_2$, h=0 case and obtain the low-energy effective theory near
the SU(4) point in the asymmetric case. An accurate analysis of the
renormalization group flow is presented with a particular emphasis on the
effect of the anisotropy. In zero magnetic field, we retrieve the same
qualitative low-energy physics than in the isotropic case. In particular, the
massless behavior found on the line $J_1=J_2>K/4$ extends in a large
anisotropic region. We discover though that the anisotropy plays its trick in
allowing non trivial scaling behaviors of the physical quantities. When a
magnetic field is present the effect of the anisotropy is striking. In addition
to the usual commensurate-incommensurate phase transition that occurs in the
spin sector of the theory, we find that the field may induce a second
transition of the KT type in the remaining degrees of freedom to which it does
not couple directly. In this sector, we find that the effective theory is that
of an SO(4) Gross-Neveu model with an h-dependent coupling that may change its
sign as h varies.


title:
Evolution of Fragments Formed at the Rupture of a Knotted Alkane
  Molecule

abstract:
  Common experience tells us that a knot significantly weakens the polymer
strand in which it is tied, which in turn leads to more facile chain rupture
under tensile loading. Using first-principles molecular dynamics calculations
we describe the dynamical evolution of the radicals that form after chain
rupture of a single knotted alkane molecule in their very early stages of life.
They are able to recombine, to form cyclic alkanes and to undergo
disproportionation phenomena with nearby chain segments. The breaking of a
single knotted polymer chain under mechanical loading is thus predicted to
reveal phenomena falling in the domain of ultrafast spectroscopy.


title:
Giant 1/f noise in perovskite manganites: evidence of the percolation
  threshold

abstract:
  We discovered an unprecedented magnitude of the 1/f noise near the Curie
temperature Tc in low-Tc manganites. The scaling behavior of the 1/f noise and
the resistance provides strong evidence of the percolation nature of the
ferromagnetic transition in the polycrystalline samples. The step-like changes
of the resistance with temperature, observed for single crystals, suggest that
the size of the ferromagnetic domains depends on the size of crystallites.


title: The Fermi surface of Bi2Sr2CaCu2O8

abstract:
  We study the Fermi surface of Bi2Sr2CaCu2O8 (Bi2212) using angle resolved
photoemission (ARPES) with a momentum resolution of ~ 0.01 of the Brillouin
zone. We show that, contrary to recent suggestions, the Fermi surface is a
large hole barrel centered at (pi,pi), independent of the incident photon
energy.


title: Three-dimensional Josephson-junction arrays in the quantum regime

abstract:
  We study the quantum phase transition properties of a three-dimensional
periodic array of Josephson junctions with charging energy that includes both
the self and mutual junction capacitances. We use the phase fluctuation algebra
between number and phase operators, given by the Euclidean group E_2, and we
effectively map the problem onto a solvable quantum generalization of the
spherical model. We obtain a phase diagram as a function of temperature,
Josephson coupling and charging energy. We also analyze the corresponding
fluctuation conductivity and its universal scaling form in the vicinity of the
zero-temperature quantum critical point.


title: Interactions Between Charged Rods Near Salty Surfaces

abstract:
  Using both theoretical modeling and computer simulations we study a model
system for DNA interactions in the vicinity of charged membranes. We focus on
the polarization of the mobile charges in the membranes due to the nearby
charged rods (DNA) and the resulting screening of their fields and inter-rod
interactions. We find, both within a Debye-Huckel model and in Brownian
dynamics simulations, that the confinement of the mobile charges to the surface
leads to a qualitative reduction in their ability to screen the charged rods to
the degree that the fields and resulting interactions are not finite-ranged as
in systems including a bulk salt concentration, but rather decay algebraically
and the screening effect is more like an effective increase in the multipole
moment of the charged rod.


title:
Z_2 Gauge Theory of Electron Fractionalization in Strongly Correlated
  Systems

abstract:
  We develop a new theoretical framework for describing and analyzing exotic
phases of strongly correlated electrons which support excitations with
fractional quantum numbers. Starting with a class of microscopic models
believed to capture much of the essential physics of the cuprate
superconductors, we derive a new gauge theory - based upon a {\it discrete}
Ising or Z_2 symmetry - which interpolates naturally between an
antiferromagnetic Mott insulator and a conventional d-wave superconductor. We
explore the intervening regime, and demonstrate the possible existence of an
exotic fractionalized insulator - the nodal liquid - as well as various more
conventional insulating phases exhibiting broken lattice symmetries. A crucial
role is played by vortex configurations in the Z_2 gauge field.
Fractionalization is obtained if they are uncondensed. Within the insulating
phases, the dynamics of these Z_2 vortices in two dimensions (2d) is described,
after a duality transformation, by an Ising model in a transverse field - the
Ising spins representing the Z_2 vortices. The presence of an unusual Berry's
phase term in the gauge theory, leads to a doping-dependent "frustration" in
the dual Ising model, being fully frustrated at half-filling. The Z_2 gauge
theory is readily generalized to a variety of different situations - in
particular, it can also describe 3d insulators with fractional quantum numbers.
We point out that the mechanism of fractionalization for d>1 is distinct from
the well-known 1d spin-charge separation. Other interesting results include a
description of an exotic fractionalized superconductor in two or higher
dimensions.


title:
Bandwidths Statistics from the Eigenvalue Moments for Harper-Hofstadter
  Problem

abstract:
  I propose a method for studying the product of bandwidths for the
Harper-Hofstader model. This method requires knowledge of the moments of the
midband energies. I conjectured a general formula for these moments. I computed
the asymptotic representation for the product of bandwidths in the limit of a
weak magnetic flux using Szego's theorem for Hankel matrices. I then give a
first approximation for the edge of the butterfly spectrum and discuss its
connection with P. Levy's formula for Brownian motion .


title: Pseudogaps in the 2D half-filled Hubbard model

abstract:
  We study the pseudogaps in the spectra of the half-filled 2D Hubbard model
using both finite-size and dynamical cluster approximation (DCA) quantum Monte
Carlo calculations. A charge pseudogap, accompanied by non-Fermi liquid
behavior in the self energy, is shown to persist in the thermodynamic limit.
The DCA (finite-size) method systematically underestimates (overestimates) the
width of the pseudogap. A spin pseudogap is not seen at half-filling.


title: Indirect RKKY Interaction in the Nearly Nested Fermi Liquid

abstract:
  The indirect RKKY interaction in a layered metal with nearly nested (almost
squared) Fermi surface is evaluated analytically. The final expressions are
obtained in closed form as a combination of Bessel functions. We show that the
expected ``2k_F'' oscillations occur as the far asymptote of our expressions,
where a value of the effective Fermi momentum k_F depends on the direction in
r-space. We demonstrate the existence of the intermediate asymptote of the
interaction which is of the sign-reversal antiferromagnetic type. This part of
RKKY interaction is the only term surviving in the limit of exact nesting. A
good accordance of our analytical formulas with numerical findings is found
until the interatomic distances.


title:
Coexistence of Weak Localization and a Metallic Phase in Si/SiGe Quantum
  Wells

abstract:
  Magnetoresistivity measurements on p-type Si/SiGe quantum wells reveal the
coexistence of a metallic behavior and weak localization. Deep in the metallic
regime, pronounced weak localization reduces the metallic behavior around zero
magnetic field without destroying it. In the insulating phase, a positive
magnetoresistivity emerges close to B=0, possibly related to spin-orbit
interactions.


title:
Theoretical model for the superconducting and magnetically ordered
  borocarbides

abstract:
  We present a theory of superconductivity in presence of a general magnetic
structure in a form suitable for the description of complex magnetic phases
encountered in borocarbides. The theory, complemented with some details of the
band structure and with the magnetic phase diagram, may explain the nearly
reentrant behaviour and the anisotropy of the upper critical field of HoNi2B2C.
The onset of the helical magnetic order depresses superconductivity via the
reduction of the interaction between phonons and electrons caused by the
formation of magnetic Bloch states. At mean field level, no additional
suppression of superconductivity is introduced by the incommensurability of the
helical phase.


title: Phase Diagram of Heavy-Ion Irradiated Bi_2Sr_2CaCu_2O_8

abstract:
  Using dynamic and thermodynamic magnetization measurements, we analyze the
phase diagram of Bi_2Sr_2CaCu_2O_8delta single crystals containing amorphous
columnar defects created by heavy-ion irradiation. Reversible magnetization
experiments yield the respective magnitudes of the pinning energy and entropy
contributions to the free energy of the vortex lattice. It appears that the
entropy contribution in the London regime is relatively minor in both
unirradiated and irradiated crystals, except in the case of high density of
columns and inductions B that are smaller than the interaction field H_int ~
B_phi / 6. The dependence of the entropy contribution on vortex- and defect
density correlates well with measurements of the irreversibility line H_irr,
which shows a sharp increase at H_int.


title:
Translational symmetry breaking in two-dimensional antiferromagnets and
  superconductors

abstract:
  It was argued many years ago that translational symmetry breaking due to the
appearance of spin-Peierls ordering (or bond-charge stripe order) is a
fundamental property of the quantum paramagnetic states of a large class of
square lattice antiferromagnets. Recently, such states were shown to be a
convenient point of departure for studying translational symmetry breaking in
doped antiferromagnets: these results are briefly reviewed here with an
emphasis on experimental implications. In the presence of stronger frustration,
it was also argued that the insulating antiferromagnet can undergo a transition
to a deconfined state with no lattice symmetry breaking. This transition is
described by a fully-frustrated Ising model in a transverse field: details of
this earlier derivation of the Ising model are provided here--this is motivated
by the reappearance of the same Ising model in a recent study of the
competition between antiferromagnetism and d-wave superconductivity by Senthil
and Fisher (cond-mat/9910224).


title: Off-Equilibrium Dynamics at Very Low Temperatures in 3d Spin Glasses

abstract:
  We present a high statistic systematic study of the overlap correlation
function well below the critical temperature in the three dimensional Gaussian
spin glass. The off-equilibrium correlation function has been studied
confirming the power law behavior for the dynamical correlation length. In
particular we have computed the dynamical critical exponent $z$ in a wide range
of temperatures, $0.35 \le T \le 0.9$, obtaining a dependence $z(T)=6.2/T$ in a
very good agreement with recent experiments. Moreover, we report a study of the
violation of the fluctuation-dissipation theorem for very low temperatures
$T=0.5$ and $T=0.35$. All our numerical results avoid a droplet model
interpretation even when $T$ is as low as $T=0.35$.


title: Hydrodynamic Approach to Vortex Lifetime in Trapped Bose Condensates

abstract:
  We study a vortex in a two-dimensional, harmonically trapped Bose-Einstein
condensate at zero temperature. Through a variational calculation using a trial
condensate wave function and a nonlinear Schroedinger Lagrangian, we obtain the
effective potential experienced by a vortex at an arbitrary position in the
condensate, and find that an off-center vortex will move in a circular
trajectory around the trap center. We find the frequency of this precession to
be smaller than the elementary excitation frequencies in the cloud.
  We also study the radiation of sound from a moving vortex in an infinite,
uniform system, and discuss the validity of this as an approximation for the
trapped case. Furthermore, we estimate the lifetime of a vortex due to
imperfections in the trapping potential.


title:
Cherenkov radiation from fluxon in a stack of coupled long Josephson
  junctions

abstract:
  We present a systematic study of the Cherenkov radiation of Josephson plasma
waves by fast moving fluxon in a stack of coupled long Josephson junctions for
different fluxon modes. It is found that at some values of parameters
current-voltage characteristic may exhibit a region of the back-bending on the
fluxon step. In the opposite limit the emission of the Cherenkov radiation
takes place. In the annular junctions of moderate length the interaction of the
emitted waves with fluxon results in the novel resonances which emerge on the
top of the fluxon step. We present more exact formulas which describe the
position of such resonances taking into account difference between junction and
non-linear corrections. The possibility of direct detection of the Cherenkov
radiation in junctions of linear geometry is discussed.


title: Different Scenarios for Critical Glassy Dynamics

abstract:
  We study the role of different terms in the $N$-body potential of glass
forming systems on the critical dynamics near the glass transition. Using a
simplified spin model with quenched disorder, where the different terms of the
real $N$-body potential are mapped into multi-spin interactions, we identified
three possible scenarios. For each scenario we introduce a ``minimal'' model
representative of the critical glassy dynamics near, both above and below, the
critical transition lin e. For each ``minimal'' model we discuss the low
temperature equilibrium dynamics.


title: Dependence of Conductance on Percolation Backbone Mass

abstract:
  On two-dimensional percolation clusters at the percolation threshold, we
study $<\sigma(M_B,r)>$, the average conductance of the backbone, defined by
two points separated by Euclidean distance $r$, of mass $M_B$. We find that
with increasing $M_B$ and for fixed r, $<\sigma(M_B,r)>$ asymptotically {\it
decreases} to a constant, in contrast with the behavior of homogeneous sytems
and non-random fractals (such as the Sierpinski gasket) in which conductance
increases with increasing $M_B$. We explain this behavior by studying the
distribution of shortest paths between the two points on clusters with a given
$M_B$. We also study the dependence of conductance on $M_B$ slightly above the
percolation threshold.


title:
Single Electron Tunneling at Large Conductance: The Semiclassical
  Approach

abstract:
  We study the linear conductance of single electron devices showing Coulomb
blockade phenomena. Our approach is based on a formally exact path integral
representation describing electron tunneling nonperturbatively. The
electromagnetic environment of the device is treated in terms of the
Caldeira-Leggett model. We obtain the linear conductance from the Kubo formula
leading to a formally exact expression which is evaluated in the semiclassical
limit. Specifically we consider three models. First, the influence of an
electromagnetic environment of arbitrary impedance on a single tunnel junction
is studied focusing on the limits of large tunneling conductance and high to
moderately low temperatures. The predictions are compared with recent
experimental data. Second, the conductance of an array of N tunnel junctions is
determined in dependence on the length N of the array and the environmental
impedance. Finally, we consider a single electron transistor and compare our
results for large tunneling conductance with experimental findings.


title:
Effect of the electromagnetic environment on arrays of small normal
  metal tunnel junctions: Numerical and experimental investigation

abstract:
  We present results of a set of experiments to investigate the effect of
dissipative external electromagnetic environment on tunneling in linear arrays
of junctions in the weak tunneling regime. The influence of this resistance
decreases as the number of junctions in the chain increases and ultimately
becomes negligible. Further, there is a value of external impedance, typically
\~5 k$\Omega$, at which the half-width of the zero-voltage dip in the
conductance curve shows a maximum. Some new analytical formulae, based on the
phase-correlation theory, along with numerical results will be presented.


title:
The adsorption-desorption model and its application to vibrated granular
  materials

abstract:
  We investigate both analytically and by numerical simulation the kinetics of
a microscopic model of hard rods adsorbing on a linear substrate, a model which
is relevant for compaction of granular materials. The computer simulations use
an event-driven algorithm which is particularly efficient at very long times.
For a small, but finite desorption rate, the system reaches an equilibrium
state very slowly, and the long-time kinetics display three successive regimes:
an algebraic one where the density varies as $1/t$, a logarithmic one where the
density varies as $1/\ln(t)$, followed by a terminal exponential approach. The
characteristic relaxation time of the final regime, though incorrectly
predicted by a mean field arguments, can be obtained with a systematic
gap-distribution approach. The density fluctuations at equilibrium are also
investigated, and the associated time-dependent correlation function exhibits a
power law regime followed by a final exponential decay. Finally, we show that
denser particle packings can be obtained by varying the desorption rate during
the process.


title: Energy of Magnetic Vortices in Rotating Superconductor

abstract:
  We carry out a systematic analytic investigation of stationary and
cylindrically symmetric vortex configurations for simple models representing an
incompressible non-relativistic superconductor in a rigidly rotating
background. It is shown that although the magnetic and kinetic contributions to
the energy per unit length of such a vortex are separately modified by the
background angular velocity, its effect on the total energy per unit length
cancels out. For a type II superconductor threaded by a parallel array of such
vortices, this result implies that the relevant macroscopic magnetic field
strength H will not be equal to the large scale average <B> of the local
magnetic induction B (as has previously been suggested) but instead that H will
simply be equal to the external London field that characterizes the value of B
outside the vortices.


title:
Electronic Transport in a Three-dimensional Network of 1-D Bismuth
  Quantum Wires

abstract:
  The resistance R of a high density network of 6 nm diameter Bi wires in
porous Vycor glass is studied in order to observe its expected semiconductor
behavior. R increases from 300 K down to 0.3 K. Below 4 K, where R varies
approximately as ln(1/T), the order-of-magnitude of the resistance rise, as
well as the behavior of the magnetoresistance are consistent with localization
and electron-electron interaction theories of a one-dimensional disordered
conductor in the presence of strong spin-orbit scattering. We show that this
behaviour and the surface-enhanced carrier density may mask the proposed
semimetal-to-semiconductor transition for quantum Bi wires.


title:
Exact solution of a partially asymmetric exclusion model using a
  deformed oscillator algebra

abstract:
  We study the partially asymmetric exclusion process with open boundaries. We
generalise the matrix approach previously used to solve the special case of
total asymmetry and derive exact expressions for the partition sum and currents
valid for all values of the asymmetry parameter q. Due to the relationship
between the matrix algebra and the q-deformed quantum harmonic oscillator
algebra we find that q-Hermite polynomials, along with their orthogonality
properties and generating functions, are of great utility. We employ two
distinct sets of q-Hermite polynomials, one for q<1 and the other for q>1. It
turns out that these correspond to two distinct regimes: the previously studied
case of forward bias (q<1) and the regime of reverse bias (q>1) where the
boundaries support a current opposite in direction to the bulk bias. For the
forward bias case we confirm the previously proposed phase diagram whereas the
case of reverse bias produces a new phase in which the current decreases
exponentially with system size.


title: Phase boundary dynamics in a one-dimensional non-equilibrium lattice gas

abstract:
  We study dynamics of a phase boundary in a one-dimensional lattice gas, which
is initially put into a non-equilibrium configuration and then is let to evolve
in time by particles performing nearest-neighbor random walks constrained by
hard-core interactions. Initial non-equilibrium configuration is characterized
by an $S$-shape density profile, such that particles density from one side of
the origin (sites $X \leq 0$) is larger (high density phase, HDP) than that
from the other side (low-density phase, LDP). We suppose that all the lattice
gas particles, except for the rightmost particle of the HDP, have symmetric
hopping probabilities. The rightmost particle of the HDP, which determines the
position of the phase separating boundary, is subject to a constant force $F$,
oriented towards the HDP; in our model this force mimics an effective tension
of the phase separating boundary. We find that, in the general case, the mean
displacement $\bar{X(t)}$ of the phase boundary grows with time as $\bar{X(t)}
= \alpha(F) t^{1/2}$, where the prefactor $\alpha(F)$ depends on $F$ and on the
initial densities in the HDP and LDP. We show that $\alpha(F)$ can be positive
or negative, which means that depending on the physical conditions the HDP may
expand or get compressed. In the particular case when $\alpha(F) = 0$, i.e.
when the HDP and LDP coexist with each other, the second moment of the phase
boundary displacement is shown to grow with time sublinearly, $\bar{X^2(t)} =
\gamma t^{1/2}$, where the prefactor $\gamma$ is also calculated explicitly.
Our analytical predictions are shown to be in a very good agreement with the
results of Monte Carlo simulations.


title: Fragile vs strong liquids: a saddles ruled scenario

abstract:
  In the context of the energy landscape description of supercooled liquids, we
propose an explanation for the different behaviour of fragile and strong
liquids. Above the Goldstein crossover temperature Tx, diffusion is interpreted
as a motion in the phase space among unstable stationary points of the
potential energy, that is among saddles. In this way two mechanisms of
diffusion arise: mechanism A takes place when the system crosses potential
energy barriers along stable uphill directions, while mechanism B consists in
finding unstable downhill directions out of a saddle. Depending on the mutual
value of the efficiency temperatures of A and B, we obtain two very different
behaviours of the viscosity, reproducing the usual classification of liquids in
fragile and strong. Moreover, this scenario very naturally predicts the
possibility of a fragile-to-strong crossover when lowering the temperature.


title:
Two-Species Reaction-Diffusion System with Equal Diffusion Constants:
  Anomalous Density Decay at Large Times

abstract:
  We study a two-species reaction-diffusion model where A+A->0, A+B->0 and
B+B->0, with annihilation rates lambda0, delta0 > lambda0 and lambda0,
respectively. The initial particle configuration is taken to be randomly mixed
with mean densities nA(0) > nB(0), and with the two species A and B diffusing
with the same diffusion constant. A field-theoretic renormalization group
analysis suggests that, contrary to expectation, the large-time density of the
minority species decays at the same rate as the majority when d<=2. Monte Carlo
data supports the field theory prediction in d=1, while in d=2 the
logarithmically slow convergence to the large-time asymptotics makes a
numerical test difficult.


title:
Spectral Representation for the Effective Macroscopic Response of a
  Polycrystal: Application to Third-Order Nonlinear Susceptibility

abstract:
  Erratum:
  In our paper, we show that the spectral representation for isotropic
two-component composites also applies to uniaxial polycrystals. We have learned
that this result was, in fact, first conjectured by G.W. Milton. While our
derivation is more detailed, our result for the spectral function is the same
as Milton's. We very much regret not having been aware of this work at the time
of writing our paper.
  Original abstract:
  We extend the spectral theory used for the calculation of the effective
linear response functions of composites to the case of a polycrystalline
material with uniaxially anisotropic microscopic symmetry. As an application,
we combine these results with a nonlinear decoupling approximation as modified
by Ma et al., to calculate the third-order nonlinear optical susceptibility of
a uniaxial polycrystal, assuming that the effective dielectric function of the
polycrystal can be calculated within the effective-medium approximation.


title: Nonlinear sigma model study of a frustrated spin ladder

abstract:
  A model of two-leg spin-S ladder with two additional frustrating diagonal
exchange couplings J_{D}, J_{D}' is studied within the framework of the
nonlinear sigma model approach. The phase diagram has a rich structure and
contains 2S gapless phase boundaries which split off the boundary to the fully
saturated ferromagnetic phase when J_{D} and J_{D}' become different. For the
S=1/2 case, the phase boundaries are identified as separating two topologically
distinct Haldane-type phases discussed recently by Kim et al.
(cond-mat/9910023).


title: Emergence of stable and fast folding protein structures

abstract:
  The number of protein structures is far less than the number of sequences. By
imposing simple generic features of proteins (low energy and compaction) on all
possible sequences we show that the structure space is sparse compared to the
sequence space. Even though the sequence space grows exponentially with N (the
number of amino acids) we conjecture that the number of low energy compact
structures only scales as ln N. This implies that many sequences must map onto
countable number of basins in the structure space. The number of sequences for
which a given fold emerges as a native structure is further reduced by the dual
requirements of stability and kinetic accessibility. The factor that determines
the dual requirement is related to the sequence dependent temperatures,
T_\theta (collapse transition temperature) and T_F (folding transition
temperature). Sequences, for which \sigma =(T_\theta-T_F)/T_\theta is small,
typically fold fast by generically collapsing to the native-like structures and
then rapidly assembling to the native state. Such sequences satisfy the dual
requirements over a wide temperature range. We also suggest that the functional
requirement may further reduce the number of sequences that are biologically
competent. The scheme developed here for thinning of the sequence space that
leads to foldable structures arises naturally using simple physical
characteristics of proteins. The reduction in sequence space leading to the
emergence of foldable structures is demonstrated using lattice models of
proteins.


title:
Electronic structure and magnetism of Fe$_{3-x}$V$_{x}$X (X = Si, Ga and
  Al) alloys by the KKR-CPA method

abstract:
  We present first principles charge- and spin-selfconsistent electronic
structure computations on the Heusler-type disordered alloys Fe$_{3-x}$V$_{x}$X
for three different metalloids X=(Si, Ga and Al). In these calculations we use
the methodology based on the Korringa-Kohn- Rostoker formalism and the
coherent-potential approximation (KKR-CPA), generalized to treat disorder in
multi-component complex alloys.


title: Stress in frictionless granular material: Adaptive Network Simulations

abstract:
  We present a minimalistic approach to simulations of force transmission
through granular systems. We start from a configuration containing cohesive
(tensile) contact forces and use an adaptive procedure to find the stable
configuration with no tensile contact forces. The procedure works by
sequentially removing and adding individual contacts between adjacent beads,
while the bead positions are not modified. In a series of two-dimensional
realizations, the resulting force networks are shown to satisfy a linear
  constraint among the three components of average stress, as anticipated by
recent theories. The coefficients in the linear constraint remain nearly
  constant for a range of shear loadings up to about .6 of the normal loading.
The spatial distribution of contact forces shows strong concentration along
``force chains". The probability of contact forces of magnitude f shows an
exponential falloff with f. The response to a local perturbing force is
concentrated along two characteristic rays directed downward and laterally.


title:
Dynamic Vortex Phases and Pinning in Superconductors with Twin
  Boundaries

abstract:
  We investigate the pinning and driven dynamics of vortices interacting with
twin boundaries using large scale molecular dynamics simulations on samples
with near one million pinning sites. For low applied driving forces, the vortex
lattice orients itself parallel to the twin boundary and we observe the
creation of a flux gradient and vortex free region near the edges of the twin
boundary. For increasing drive, we find evidence for several distinct dynamical
flow phases which we characterize by the density of defects in the vortex
lattice, the microscopic vortex flow patterns, and orientation of the vortex
lattice. We show that these different dynamical phases can be directly related
to microscopically measurable voltage - current V(I) curves and voltage noise.
By conducting a series of simulations for various twin boundary parameters we
derive several vortex dynamic phase diagrams.


title: A two-species continuum model for aeolian sand ripples

abstract:
  We formulate a continuum model for aeolian sand ripples consisting of two
species of grains: a lower layer of relatively immobile clusters, with an upper
layer of highly mobile grains moving on top. We predict analytically the ripple
wavelength, initial ripple growth rate and threshold saltation flux for ripple
formation. Numerical simulations show the evolution of realistic ripple
profiles from initial surface roughness via ripple growth and merger.


title: Charge and orbital order in half-doped manganites

abstract:
  An explanation is given for the charge order, orbital order and insulating
state observed in half-doped manganese oxides, such as
Nd$_{1/2}$Sr$_{1/2}$MnO$_{3}$. The competition between the kinetic energy of
the electrons and the magnetic exchange energy drives the formation of
effectively one-dimensional ferromagnetic zig-zag chains. Due to a topological
phase factor in the hopping, the chains are intrinsically insulating and
orbital-ordered. Most surprisingly, the strong Coulomb interaction between
electrons on the same Mn-ion leads to the experimentally observed charge
ordering. For doping less than 1/2 the system is unstable towards phase
separation into a ferromagnetic metallic and charge-ordered insulating phase.


title: A deterministic sandpile automaton revisited

abstract:
  The Bak-Tang-Wiesenfeld (BTW) sandpile model is a cellular automaton which
has been intensively studied during the last years as a paradigm for
self-organized criticality. In this paper, we reconsider a deterministic
version of the BTW model introduced by Wiesenfeld, Theiler and McNamara, where
sand grains are added always to one fixed site on the square lattice. Using the
Abelian sandpile formalism we discuss the static properties of the system. We
present numerical evidence that the deterministic model is only in the BTW
universality class if the initial conditions and the geometric form of the
boundaries do not respect the full symmetry of the square lattice.


title: Molecular shape and flexoelectricity

abstract:
  We performed Monte Carlo simulations of systems of wedge-shaped objects
formed from Gay-Berne ellipsoids joined to Lennard-Jones spheres. We studied
two different wedge shapes, one more asymmetric than the other. The bend and
splay flexoelectric coefficients were measured in the isotropic and smectic
phases using linear response theory, and found to be negligibly small in the
isotropic phase. We found a close connection between the properties of the
intermolecular potential and the flexoelectric coefficients measured in the
smectic phase. In particular, we found negligible bend coefficients for both
shapes and a larger magnitude of the splay coefficient for the more prominent
wedge, in accord with Meyer's original mechanism for flexoelectricity. The less
prominent wedge produced a splay flexoelectric coefficient with the opposite
sign due to the attractive tail of the intermolecular potential and the
relative narrowness of the molecular head.


title: Inelastic scattering and shot noise in diffusive mesoscopic conductors

abstract:
  A short summary of the drift-diffusion-Langevin formalism for calculating
finite-frequency shot noise in diffusive conductors is presented. Two new
results are included in this presentation. First, we arrive at a simple (but
accurate) phenomenological expression for the semiclassical distribution
function of electrons in the presence of electron-electron scattering. Second,
it is shown that in thin samples, low-frequency shot noise may be large even if
the sample length is much larger than the electron-phonon relaxation length.


title:
Quasiparticle States near the Surface and the Domain Wall in a p_x \pm i
  p_y-Wave Superconductor

abstract:
  The electronic states near a surface or a domain wall in the p_x \pm i p_y
-wave superconductor are studied. This state has been recently suggested as the
superconducting state of Sr_2 Ru O_4. The p_x \pm i p_y-wave paring state
breaks the time reversal symmetry and induces a magnetic field. The obtained
temperature dependence of the magnetic field is consistent with the observed
\mu SR data.


title:
Shrinking limits of silicon MOSFET's: Numerical study of 10-nm-scale
  devices

abstract:
  We have performed numerical modeling of dual-gate ballistic n-MOSFET's with
channel length of the order of 10 nm, including the effects of quantum
tunneling along the channel and through the gate oxide. Our analysis includes a
self-consistent solution of the full (two-dimensional) electrostatic problem,
with account of electric field penetration into the heavily-doped electrodes.
The results show that transistors with channel length as small as 8 nm can
exhibit either a transconductance up to 4,000 mS/mm or gate modulation of
current by more than 8 orders of magnitude, depending on the gate oxide
thickness. These characteristics make the devices satisfactory for logic and
memory applications, respectively, though their gate threshold voltage is
rather sensitive to nanometer-scale variations in the channel length.


title:
Three-Dimensional Ordering in Weakly Coupled Antiferromagnetic Ladders
  and Chains

abstract:
  A theoretical description is presented for low-temperature magnetic-field
induced three-dimensional (3D) ordering transitions in strongly anisotropic
quantum antiferromagnets, consisting of weakly coupled antiferromagnetic
spin-1/2 chains and ladders. First, effective continuum field theories are
derived for the one-dimensional subsystems. Then the Luttinger parameters,
which determine the low-temperature susceptibilities of the chains and ladders,
are calculated from the Bethe ansatz solution for these effective models. The
3D ordering transition line is obtained using a random phase approximation for
the weak inter-chain (inter-ladder) coupling. Finally, considering a Ginzburg
criterion, the fluctuation corrections to this approach are shown to be small.
The nature of the 3D ordered phase resembles a Bose condensate of integer-spin
magnons. It is proposed that for systems with higher spin degrees of freedom,
e.g. N-leg spin-1/2 ladders, multi-component condensates can occur at high
magnetic fields.


title: Broken Symmetry and Nonequilibrium Superfluid 3He

abstract:
  These lecture notes provide an introduction to non-equilibrium superfluid
3He, with applications to high-frequency excitations, including aspects of
symmetry breaking in 3He and its effects on collective mode dynamics. In 1957
Landau predicted that liquid 3He would support propagating shear waves at low
temperatures, i.e. a transverse sound mode. Such waves have recently been
observed at low temperatures in the superfluid B-phase of liquid 3He. These
observations provide a beautiful example of the effect of spontaneous symmetry
breaking in condensed matter. I discuss the theory of transverse wave
propagation in 3He and the recent detection of these waves by magneto-acoustic
rotation of the 3He polarization in a magnetic field.


title: Superconductivity in a Toy Model of the Pseudogap State

abstract:
  We analyze superconducting state (both s and d - wave) in a simple exactly
solvable model of pseudogap state, induced by short - range order fluctuations
(e.g. antiferromagnetic), which is based upon model Fermi - surface with "hot
patches". It is shown that superconducting energy gap averaged over these
fluctuations is non zero even for the temperatures larger than mean - field T_c
of superconducting transition in a sample as a whole. For temperatures T>T_c
superconductivity apparently exists within separate regions ("drops"). We study
the spectral density and the density of states and demonstrate that
superconductivity signals itself in these already for T>T_c, while at T_c
itself nothing special happens from this point of view. These anomalies are in
qualitative agreement with a number experiments on underdoped cuprates.


title: Magneto-acoustic rotation of transverse waves in 3He-B

abstract:
  In superfluid 3He-B the off-resonant coupling of the J=2-, M=+/- 1 order
parameter collective modes to transverse current excitations stabilizes
propagating transverse waves with low damping for frequencies above that of the
J=2- modes. Right- (RCP) and left circularly polarized (LCP) transverse modes
are degenerate in zero field; however, a magnetic field with H || q lifts this
degeneracy giving rise to the acoustic analog of circular birefringence and an
acoustic Faraday effect for linearly polarized transverse sound waves. We
present theoretical results for the temperature, pressure and field dependence
of the Faraday rotation angle, and compare the theory with recent measurements.
The analysis provides a direct measurement of the Lande' g-factor for the J=2-
modes, and new information on the magnitude of f-wave pairing correlations in
3He-B.


title: Bogomol'nyi Limit For Magnetic Vortices In Rotating Superconductor

abstract:
  This work is the sequel of a previous investigation of stationary and
cylindrically symmetric vortex configurations for simple models representing an
incompressible non-relativistic superconductor in a rigidly rotating
background. In the present paper, we carry out our analysis with a generalized
Ginzburg-Landau description of the superconductor, which provides a
prescription for the radial profile of the normal density within the vortex.
Within this framework, it is shown that the Bogomol'nyi limit condition marking
the boundary between type I and type II behavior is unaffected by the rotation
of the background.


title:
Density-matrix functional theory of the Hubbard model: An exact
  numerical study

abstract:
  A density functional theory for many-body lattice models is considered in
which the single-particle density matrix is the basic variable. Eigenvalue
equations are derived for solving Levy's constrained search of the interaction
energy functional W, which is expressed as the sum of Hartree-Fock energy and
the correlation energy E_C. Exact results are obtained for E_C of the Hubbard
model on various periodic lattices. The functional dependence of E_C is
analyzed by varying the number of sites, band filling and lattice structure.
The infinite one-dimensional chain and one-, two-, or three-dimensional finite
clusters with periodic boundary conditions are considered. The properties of
E_C are discussed in the limits of weak and strong electronic correlations, as
well as in the crossover region. Using an appropriate scaling we observe a
pseudo-universal behavior which suggests that the correlation energy of
extended systems could be obtained quite accurately from finite cluster
calculations. Finally, the behavior of E_C for repulsive (U>0) and attractive
(U<0) interactions are contrasted.


title:
Waveguide diffusion modes and slowdown of D'yakonov-Perel' spin
  relaxation in narrow 2-D semiconductor channels

abstract:
  We have shown that in narrow 2D semiconductor channels the D'yakonov-Perel'
spin relaxation rate is strongly reduced. This relaxation slowdown appears in
special waveguide diffusion modes which determine the propagation of spin
density in long channels. Experiments are suggested to detect the theoretically
predicted effects. A possible application is a field effect transistor operated
with injected spin current.


title: Raman spectroscopy of InN films grown on Si

abstract:
  We have used Raman spectroscopy to study indium nitride thin films grown by
molecular beam epitaxy on (111) silicon substrates at temperatures between 450
and 550 C. The Raman spectra show well defined peaks at 443, 475, 491, and 591
cm{-1}, which correspond to the A_1(TO), E_1(TO), E_2^{high}, and A_1(LO)
phonons of the wurtzite structure, respectively. In backscattering normal to
the surface the A_1(TO) and E_1(TO) peaks are very weak, indicating that the
films grow along the hexagonal c axis. The dependence of the peak width on
growth temperature reveals that the optimum temperature is 500 C, for which the
fullwidth of the E_2^{high} peak has the minimum value of 7 cm{-1}. This small
value, comparable to previous results for InN films grown on sapphire, is
evidence of the good crystallinity of the films.


title: Diffusion-controlled reactions in presence of polymers

abstract:
  We study the properties of diffusion controlled reactions with traps or
reaction sites attached to polymer chains or to a star polymer. Using a field
theoretical renormalization group approach we obtain the scaling of the moments
of concentration of diffusing particles near the core of a star polymer and
calculate numerical values of the exponents governing the scaling of the
reaction rate.


title: Order parameter model for unstable multilane traffic flow

abstract:
  We discuss a phenomenological approach to the description of unstable vehicle
motion on multilane highways that explains in a simple way the observed
sequence of the phase transitions "free flow -> synchronized motion -> jam" as
well as the hysteresis in the transition "free flow <-> synchronized motion".
We introduce a new variable called order parameter that accounts for possible
correlations in the vehicle motion at different lanes. So, it is principally
due to the "many-body" effects in the car interaction, which enables us to
regard it as an additional independent state variable of traffic flow. Basing
on the latest experimental data (cond-mat/9905216) we assume that these
correlations are due to a small group of "fast" drivers. Taking into account
the general properties of the driver behavior we write the governing equation
for the order parameter. In this context we analyze the instability of
homogeneous traffic flow manifesting itself in both of the mentioned above
phase transitions where, in addition, the transition "synchronized motion ->
jam" also exhibits a similar hysteresis. Besides, the jam is characterized by
the vehicle flows at different lanes being independent of one another. We
specify a certain simplified model in order to study the general features of
the car cluster self-formation under the phase transition "free flow <->
synchronized motion". In particular, we show that the main local parameters of
the developed cluster are determined by the state characteristics of vehicle
motion only.


title: Colloids with polymer stars: The interaction

abstract:
  We derive the short distance interaction of star polymers in a colloidal
solution. We calculate the corresponding force between two stars with arbitrary
numbers of legs f_1 and f_2. We show that a simple scaling theory originally
derived for high f_1, f_2 nicely matches with the results of elaborated
renormalization group analysis for f_1 + f_2 =< 6 generalizing and confirming a
previous conjecture based only on scaling results for f_1=f_2=1,2.


title:
Density Profile of the One-Dimensional Partially Asymmetric Simple
  Exclusion Process with Open Boundaries

abstract:
  The one-dimensional partially asymmetric simple exclusion process with open
boundaries is considered. The stationary state, which is known to be
constructed in a matrix product form, is studied by applying the theory of
q-orthogonal polynomials. Using a formula of the q-Hermite polynomials, the
average density profile is computed in the thermodynamic limit. The phase
diagram for the correlation length, which was conjectured in the previous
work[J. Phys. A {\bf 32} (1999) 7109], is confirmed.


title: Signatures of an intermediate 2d Coulomb phase at low temperatures

abstract:
  The study of the ground state of spinless fermions in 2d disordered clusters
(Phys. Rev. Lett. {\bf 83}, 1826 (1999)) has suggested the existence of a new
quantum phase for intermediate Coulomb energy to kinetic energy ratios $r_s$.
Exact diagonalization of the same small clusters show that its low energy
excitations (quantum ergodicity above a few ``hexatic'' excitations
characterized by oriented currents) significantly differ from those occuring in
the Fermi glass (weak $r_s$) and in the pinned Wigner crystal (large $r_s$).
The ``hexatic'' excitations vanish for temperatures of order of the Fermi
temperature.


title: Electron-hole liquid in the hexaborides

abstract:
  We investigate the energetics of the electron-hole liquid in stoichiometric
divalent metal hexaborides. The ground state energy of an electron-hole plasma
is calculated using RPA and Hubbard schemes and compared to the binding energy
of a single exciton. Intervalley scattering processes play an important role in
increasing this binding energy and stabilizing a dilute Bose gas of excitons.


title: Statistical Self-Similarity of One-Dimensional Growth Processes

abstract:
  For one-dimensional growth processes we consider the distribution of the
height above a given point of the substrate and study its scale invariance in
the limit of large times. We argue that for self-similar growth from a single
seed the universal distribution is the Tracy-Widom distribution from the theory
of random matrices and that for growth from a flat substrate it is some other,
only numerically determined distribution. In particular, for the polynuclear
growth model in the droplet geometry the height maps onto the longest
increasing subsequence of a random permutation, from which the height
distribution is identified as the Tracy-Widom distribution.


title: The Dielectric Breakdown Model at Small $\eta$: Pole Dynamics

abstract:
  We consider the dielectric breakdown model in the limit $\eta\to 0^+$. A
differential equation describing the surface growth is derived; this equation
is KPZ plus a term causing linear instability, and includes a short-distance
regularization similar to a viscosity. The equation exhibits an interesting
dynamics in terms of poles, permitting us to derive a large family of solutions
to the equation of motion. In terms of poles, we find all stationary
configurations of the surface, and analytically calculate their stability. For
each value of the viscosity, only one stable configuration is found, but this
configuration is nonlinearly unstable in the presence of an exponentially small
amount of noise. The present approach may be useful in understanding the
dynamics for finite $\eta$, in particular the DLA model, in terms of
perturbations to the infinitesimal $\eta$ problem.


title:
Universal aspects of vacancy-mediated disordering dynamics: the effect
  of external fields

abstract:
  We investigate the disordering of an initially phase-segregated binary alloy,
due to a highly mobile defect which couples to an electric or gravitational
field. Using both mean-field and Monte Carlo methods, we show that the late
stages of this process exhibit dynamic scaling, characterized by a set of
exponents and scaling functions. A new scaling variable emerges, associated
with the field. While the scaling functions carry information about the field
and the boundary conditions, the exponents are universal. They can be computed
analytically, in excellent agreement with simulation results.


title:
Proximity effect, quasiparticle transport, and local magnetic moment in
  ferromagnet-d-wave superconductor junctions

abstract:
  The proximity effect, quasiparticle transport, and local magnetic moment in
ferromagnet--d-wave superconductor junctions with {110}-oriented interface are
studied by solving self-consistently the Bogoliubov-de Gennes equations within
an extended Hubbard model. It is found that the proximity induced order
parameter oscillates in the ferromagnetic region. The modulation period is
shortened with the increased exchange field while the oscillation amplitude is
depressed by the interfacial scattering. With the determined superconducting
energy gap, a transfer matrix method is proposed to compute the subgap
conductance within a scattering approach. Many novel features including the
zero-bias conductance dip and splitting are exhibited with appropriate values
of the exchange field and interfacial scattering strength. The conductance
spectrum can be influenced seriously by the spin-flip interfacial scattering.
In addition, a sizable local magnetic moment near the {110}-oriented surface of
the d-wave superconductor is discussed.


title:
Activation gaps for the fractional quantum Hall effect: realistic
  treatment of transverse thickness

abstract:
  The activation gaps for fractional quantum Hall states at filling fractions
$\nu=n/(2n+1)$ are computed for heterojunction, square quantum well, as well as
parabolic quantum well geometries, using an interaction potential calculated
from a self-consistent electronic structure calculation in the local density
approximation. The finite thickness is estimated to make $\sim$30% correction
to the gap in the heterojunction geometry for typical parameters, which
accounts for roughly half of the discrepancy between the experiment and
theoretical gaps computed for a pure two dimensional system. Certain model
interactions are also considered. It is found that the activation energies
behave qualitatively differently depending on whether the interaction is of
longer or shorter range than the Coulomb interaction; there are indications
that fractional Hall states close to the Fermi sea are destabilized for the
latter.


title: Ground State Structure of Spin Glasses

abstract:
  We investigate the ground state structure of Ising spin glasses in zero
magnetic field by determining how the ground state changes in a fixed finite
block far from the boundaries when the boundary conditions are changed. We
find, both in two and three dimensions, that the probability of a change in the
block ground state configuration tends to zero as the system size tends to
infinity. This indicates a trivial ground state structure, as predicted by the
droplet theory.


title: Universal Selftrapping in Nonlinear Tight-binding Lattices

abstract:
  We show that nonlinear tight-binding lattices of different geometries and
dimensionalities, display an universal selftrapping behavior. First, we
consider the single nonlinear impurity problem in various tight-binding
lattices, and use the Green's function formalism for an exact calculation of
the minimum nonlinearity strength to form a stationary bound state. For all
lattices, we find that this critical nonlinearity parameter (scaled by the
energy of the bound state), in terms of the nonlinearity exponent, falls inside
a narrow band, which converges to e^(1/2) at large exponent values. Then, we
use the Discrete Nonlinear Schroedinger (DNLS) equation to examine the
selftrapping dynamics of a single excitation, initially localized on the single
nonlinear site, and compute the critical nonlinearity parameter for abrupt
dynamical selftrapping. For a given nonlinearity exponent, this critical
nonlinearity, properly scaled, is found to be nearly the same for all lattices.
Same results are obtained when generalizing to completely nonlinear lattices,
suggesting an underlying selftrapping universality behavior for all nonlinear
(even disordered) tight-binding lattices described by DNLS.


title:
Field-induced magnetic reorientation and effective anisotropy of a
  ferromagnetic monolayer within spin wave theory

abstract:
  The reorientation of the magnetization of a ferromagnetic monolayer is
calculated with the help of many-body Green's function theory. This allows, in
contrast to other spin wave theories, a satisfactory calculation of magnetic
properties over the entire temperature range of interest since interactions
between spin waves are taken into account. A Heisenberg Hamiltonian plus a
second-order uniaxial single-ion anisotropy and an external magnetic field is
treated by the Tyablikov (Random Phase Approximation: RPA) decoupling of the
exchange interaction term and the Anderson-Callen decoupling of the anisotropy
term. The orientation of the magnetization is determined by the spin components
$\la S^\alpha\ra$ ($\alpha=x,y,z$), which are calculated with the help of the
spectral theorem. The knowledge of the orientation angle $\Theta_0$ allows a
non-perturbative determination of the temperature dependence of the effective
second-order anisotropy coefficient. Results for the Green's function theory
are compared with those obtained with mean-field theory (MFT). We find
significant differences between these approaches.


title: Scale Invariance and Lack of Self-Averaging in Fragmentation

abstract:
  We derive exact statistical properties of a class of recursive fragmentation
processes. We show that introducing a fragmentation probability 0<p<1 leads to
a purely algebraic size distribution in one dimension, P(x) ~ x^{-2p}. In d
dimensions, the volume distribution diverges algebraically in the small
fragment limit, P(V)\sim V^{-\gamma} with \gamma=2p^{1/d}. Hence, the entire
range of exponents allowed by mass conservation is realized. We demonstrate
that this fragmentation process is non-self-averaging. Specifically, the
moments Y_\alpha=\sum_i x_i^{\alpha} exhibit significant fluctuations even in
the thermodynamic limit.


title: A Monte-Carlo approach to Poisson-Boltzmann like free energy functionals

abstract:
  A simple technique is proposed for numerically determining equilibrium ion
distribution functions belonging to free energies of the Poisson-Boltzmann
type. The central idea is to perform a conventional Monte-Carlo simulation
using the free energy as the "Hamiltonian" entering the Metropolis criterion
and the spatially discretized density as degrees of freedom. This approach is
complementary to the possibility of numerically solving the differential
equations corresponding to the variational problem, but it is much easier to
implement and to generalize. Its utility is demonstrated in two examples:
valence mixtures and hard core interactions of ions surrounding a charged rod.


title: A Generalized Ginzburg-Landau Approach to Second Harmonic Generation

abstract:
  We develop a generalized Ginzburg-Landau theory for second harmonic
generation (SHG) in magnets by expanding the free energy in terms of the order
parameter in the magnetic phase and the susceptibility tensor in the
corresponding high-temperature phase. The non-zero components of the SHG
susceptibility in the ordered phase are derived from the symmetries of the
susceptibility tensor in the high-temperature phase and the symmetry of the
order parameter. In this derivation, the dependence of the SHG susceptibility
on the order parameter follows naturally, and therefore its nonreciprocal
optical properties.
  We examine this phenomenology for the magnetoelectric compound Cr$_2$O$_3$ as
well as for the ferroelectromagnet YMnO$_3$.


title: On possible experimental realizations of directed percolation

abstract:
  Directed percolation is one of the most prominent universality classes of
nonequilibrium phase transitions and can be found in a large variety of models.
Despite its theoretical success, no experiment is known which clearly
reproduces the critical exponents of directed percolation. The present work
compares suggested experiments and discusses possible reasons why the
observation of the critical exponents of directed percolation is obscured or
even impossible.


title: Spontaneous Magnetization of Composite Fermions

abstract:
  It is argued that the composite fermion liquid is a promising candidate for
an observation of the elusive, interaction driven magnetization first proposed
by Bloch seven decades ago. In analogy to what is theoretically believed to be
the case for the idealized electron gas in zero magnetic field, this
spontaneously broken symmetry phase is predicted to occur prior to a transition
into the Wigner crystal.


title:
On some upper bounds for spin velocity and instability of gapless state
  of 1-d Heisenberg chains

abstract:
  We derive upper bounds for spin velocity in half-integer-spin Heisenberg
antiferromagnetic chains. We relate these upper bounds to the instability of
the gapless state, which is observed in frustrated systems.


title: A scaling theory of 3D spinodal turbulence

abstract:
  A new scaling theory for spinodal decomposition in the inertial hydrodynamic
regime is presented. The scaling involves three relevant length scales, the
domain size, the Taylor microscale and the Kolmogorov dissipation scale. This
allows for the presence of an inertial "energy cascade", familiar from theories
of turbulence, and improves on earlier scaling treatments based on a single
length: these, it is shown, cannot be reconciled with energy conservation. The
new theory reconciles the t^{2/3} scaling of the domain size, predicted by
simple scaling, with the physical expectation of a saturating Reynolds number
at late times.


title:
Nucleation of vortex arrays in rotating anisotropic Bose-Einstein
  condensates

abstract:
  The nucleation of vortices and the resulting structures of vortex arrays in
dilute, trapped, zero-temperature Bose-Einstein condensates are investigated
numerically. Vortices are generated by rotating a three-dimensional,
anisotropic harmonic atom trap. The condensate ground state is obtained by
propagating the Gross-Pitaevskii equation in imaginary time. Vortices first
appear at a rotation frequency significantly larger than the critical frequency
for vortex stabilization. This is consistent with a critical velocity mechanism
for vortex nucleation. At higher frequencies, the structures of the vortex
arrays are strongly influenced by trap geometry.


title:
A new approach for the analytic computation of the Instantaneous Normal
  Modes spectrum

abstract:
  In the context of the Instantaneous Normal Mode approach, the spectrum of the
Hessian of Hamiltonian is a key quantity to describe liquids behaviour. The
determination of the spectrum represents a major task for theoretical studies,
and has been addressed recently in various works. In this paper a new approach
for the analytic computation of the Hessian spectrum is presented. The one
dimensional case for a system of particles interacting via a purely repulsive
potential at low density is analyzed in details and the spectrum is computed
exactly also in the localized sector. Finally, the possible extensions of the
method are discussed, together with a comparison with different approaches to
the problem.


title:
Zener transitions between dissipative Bloch bands. II: Current Response
  at Finite Temperature

abstract:
  We extend, to include the effects of finite temperature, our earlier study of
the interband dynamics of electrons with Markoffian dephasing under the
influence of uniform static electric fields. We use a simple two-band
tight-binding model and study the electric current response as a function of
field strength and the model parameters. In addition to the Esaki-Tsu peak,
near where the Bloch frequency equals the damping rate, we find current peaks
near the Zener resonances, at equally spaced values of the inverse electric
field. These become more prominenent and numerous with increasing bandwidth (in
units of the temperature, with other parameters fixed). As expected, they
broaden with increasing damping (dephasing).


title:
Reconciling the correlation length for high-spin Heisenberg
  antiferromagnets

abstract:
  We present numerical results for the antiferromagnetic Heisenberg model
(AFHM) that definitively confirm that chiral perturbation theory, corrected for
cutoff effects in the AFHM, leads to a correct field-theoretical description of
the low-temperature behavior of the spin correlation length for spins $S \geq
1/2$. With two independent quantum Monte Carlo algorithms and a
finite-size-scaling technique, we explore correlation lengths up to $\xi
\approx 10^5$ lattice spacings a for spins S=1 and 5/2. We show how the recent
prediction of cutoff effects by P. Hasenfratz is approached for moderate
$\xi/a={\cal O}(100)$, and smoothly connects with other approaches to modeling
the AFHM at smaller correlation lengths.


title: Intersecting Loop Models on Z^D: Rigorous Results

abstract:
  We consider a general class of (intersecting) loop models in D dimensions,
including those related to high-temperature expansions of well-known spin
models. We find that the loop models exhibit some interesting features - often
in the ``unphysical'' region of parameter space where all connection with the
original spin Hamiltonian is apparently lost. For a particular n=2, D=2 model,
we establish the existence of a phase transition, possibly associated with
divergent loops. However, for n >> 1 and arbitrary D there is no phase
transition marked by the appearance of large loops. Furthermore, at least for
D=2 (and n large) we find a phase transition characterised by broken
translational symmetry.


title: The Exact Solution of the SU(3) Hubbard Model

abstract:
  The Bethe ansatz equations of the 1-D SU(3) Hubbard model are systematically
derived by diagonalizing the inhomogeneous transfer matrix of the XXX model. We
first derive the scattering matrix of the SU(3) Hubbard model through the
coordinate Bethe ansatz method. Then, with the help quantum inverse scattering
method we solve the nested transfer matrix and give the eigenvalues, the
eigenvectors and the Bethe ansatz equations. Finally, we obtain the exactly
analytic solution for the ground state.


title:
Cluster variation - Pade` approximants method for the simple cubic Ising
  model

abstract:
  The cluster variation - Pade` approximant method is a recently proposed tool,
based on the extrapolation of low/high temperature results obtained with the
cluster variation method, for the determination of critical parameters in
Ising-like models. Here the method is applied to the three-dimensional simple
cubic Ising model, and new results, obtained with an 18-site basic cluster, are
reported. Other techniques for extracting non-classical critical exponents are
also applied and their results compared with those by the cluster variation -
Pade` approximant method.


title: Renormalized Theory of Infinitely Driven Lattice Gases

abstract:
  We use field theoretic renormalization group methods to study the critical
behavior of a recently proposed Langevin equation for driven lattice gases
under infinitely fast drive.
  We perform an expansion around the upper critical dimension, $d_c=4$, and
obtain the critical exponents to one loop order. The main features of the two
loop calculation are also outlined. The renormalized theory is shown to exhibit
a behavior different from the standard field theory for the DLG with finite
driving,
  {\em i.e.} it is not mean field like.


title:
Wetting layer thickness and early evolution of epitaxially strained thin
  films

abstract:
  We propose a physical model which explains the existence of finite thickness
wetting layers in epitaxially strained films. The finite wetting layer is shown
to be stable due to the variation of the non-linear elastic free energy with
film thickness. We show that anisotropic surface tension gives rise to a
metastable enlarged wetting layer. The perturbation amplitude needed to
destabilize this wetting layer decreases with increasing lattice mismatch. We
observe the development of faceted islands in unstable films.


title: Defects, order, and hysteresis in driven charge-density waves

abstract:
  We model driven two-dimensional charge-density waves in random media via a
modified Swift-Hohenberg equation, which includes both amplitude and phase
fluctuations of the condensate. As the driving force is increased, we find that
the defect density first increases and then decreases. Furthermore, we find
switching phenomena, due to the formation of channels of dislocations. These
results are in qualitative accord with recent dynamical x-ray scattering
experiments by Ringlandet al. and transport experiments by Lemay et al.


title:
Effect of quantum noise on Coulomb blockade in normal tunnel junctions
  at high voltages

abstract:
  We have investigated asymptotic behavior of normal tunnel junctions at
voltages where even the best ohmic environments start to look like RC
transmission lines. In the experiments, this is manifested by an exceedingly
slow approach to the linear behavior above the Coulomb gap. As expected on the
basis of the quantum theory taking into account interaction with the
environmental modes, better fits are obtained using 1/sqrt{V}- than 1/V-
dependence for the asymptote. These results agree with the horizon picture if
the frequency-dependent phase velocity is employed instead of the speed of
light in order to determine the extent of the surroundings seen by the
junction.


title: Phonon spectral function for an interacting electron-phonon system

abstract:
  Using exact diagonalzation techniques, we study a model of interacting
electrons and phonons. The spectral width of the phonons is found to be reduced
as the Coulomb interaction U is increased. For a system with two modes per
site, we find a transfer of coupling strength from the upper to the lower mode.
This transfer is reduced as U is increased. These results give a qualitative
explanation of differences between Raman and photoemission estimates of the
electron-phonon coupling constants for A3C60 (A= K, Rb).


title:
Magnetic-field dependence of dynamical vortex response in
  two-dimensional Josephson junction arrays and superconducting films

abstract:
  The dynamical vortex response of a two-dimensional array of the resistively
shunted Josephson junctions in a perpendicular magnetic field is inferred from
simulations. It is found that, as the magnetic field is increased at a fixed
temperature, the response crosses over from normal to anomalous, and that this
crossover can be characterized by a single dimensionless parameter. It is
described how this crossover should be reflected in measurements of the complex
impedance for Josephson junction arrays and superconducting films.


title: Magnetic strip waveguides

abstract:
  We analyze the spectrum of the "local" Iwatsuka model, i.e. a two-dimensional
charged particle interacting with a magnetic field which is homogeneous outside
a finite strip and translationally invariant along it. We derive two new
sufficient conditions for absolute continuity of the spectrum. We also show
that in most cases the number of open spectral gaps of the model is finite. To
illustrate these results we investigate numerically the situation when the
field is zero in the strip being screened, e.g. by a superconducting mask.


title: Chaos in one-dimensional lattices under intense laser fields

abstract:
  A model is investigated where a monochromatic, spatially homogeneous laser
field interacts with an electron in a one-dimensional periodic lattice. The
classical Hamiltonian is presented and the technique of stroboscopic maps is
used to study the dynamical behavior of the model. The electron motion is found
to be completely regular only for small field amplitudes, developing a larger
chaotic region as the amplitude increases. The quantum counterpart of the
classical Hamiltonian is derived. Exact numerical diagonalizations show the
existence of universal, random-matrix fluctuations in the electronic energy
bands dressed by the laser field. A detailed analysis of the classical phase
space is compatible with the statistical spectral analysis of the quantum
model. The application of this model to describe transport and optical
absorption in semiconductor superlattices submitted to intense infrared laser
radiation is proposed.


title:
Magnetic resonance in the antiferromagnetic and normal state of
  NH_3K_3C_60

abstract:
  We report on the magnetic resonance of NH_3K_3C_60 powders in the frequency
range of 9 to 225 GHz. The observation of an antiferromagnetic resonance below
the phase transition at 40 K is evidence for an antiferromagnetically ordered
ground state. In the normal state, above 40 K, the temperature dependence of
the spin-susceptibilty measured by ESR agrees with previous static measurements
and is too weak to be explained by interacting localized spins in an insulator.
The magnetic resonance line width has an unusual magnetic-field dependence
which is large and temperature independent in the magnetically ordered state
and decreases rapidly above the transition. These observations agree with the
suggestion that NH_3K_3C_60 is a metal in the normal state and undergoes a
Mott-Hubbard metal to insulator transition at 40 K.


title: The Debye-Waller Factor in solid 3He and 4He

abstract:
  The Debye-Waller factor and the mean-squared displacement from lattice sites
for solid 3He and 4He were calculated with Path Integral Monte Carlo at
temperatures between 5 K and 35 K, and densities between 38 nm^(-3) and 67
nm^(-3). It was found that the mean-squared displacement exhibits finite-size
scaling consistent with a crossover between the quantum and classical limits of
N^(-2/3) and N^(-1/3), respectively. The temperature dependence appears to be
T^3, different than expected from harmonic theory. An anisotropic k^4 term was
also observed in the Debye-Waller factor, indicating the presence of
non-Gaussian corrections to the density distribution around lattice sites. Our
results, extrapolated to the thermodynamic limit, agree well with recent values
from scattering experiments.


title: Two-time scales, two-temperature scenario for nonlinear rheology

abstract:
  We investigate a general scenario for ``glassy'' or ``jammed'' systems driven
by an external, non-conservative force, analogous to a shear force in a fluid.
In this scenario, the drive results in the suppression of the usual aging
process, and the correlation and response functions become time translation
invariant. The relaxation time and the response functions are then dependent on
the intensity of the drive and on temperature. We investigate this dependence
within the framework of a dynamical closure approximation that becomes exact
for disordered, fully-connected models. The relaxation time is shown to be a
decreasing function of the drive (``shear thinning'' effect). The correlation
functions below the glass transition temperature (Tc) display a two time scales
relaxation pattern, similar to that observed at equilibrium slightly above Tc.
We also study the violation of the fluctuation dissipation relationship in the
driven system. This violation is very reminiscent of the one that takes place
in a system aging below Tc at zero drive. It involves in particular the
appearance of a two-temperatures regime, in the sense of an effective
fluctuation dissipation temperature. Although our results are in principle
limited to the closure relations that hold for mean-field models, we argue that
a number of the salient features are not inherent to the approximation scheme,
and may be tested in experiments and simulations.


title: Wilson and Kadowaki-Woods Ratios in Heavy Fermions

abstract:
  Recently we have shown that a one-parameter scaling, $T_{coh}$, describes the
physical behavior of several heavy fermions in a region of their phase diagram.
In this paper we fully characterize this region, obtaining the uniform
susceptibility, the resistivity and the specific heat in terms of the coherence
temperature $T_{coh}$. This allows for an explicit evaluation of the Wilson and
the Kadowaki-Woods ratios in this regime. These quantities turn out to be
independent of the distance $|\delta|$ to the quantum critical point (QCP). The
theory of the one-parameter scaling corresponds to a local interacting model.
Although spatial correlations are irrelevant in this case, time fluctuations
are critically correlated as a consequence of the quantum character of the
transition.


title:
Investigation of Single Boron Acceptors at the Cleaved Si:B (111)
  Surface

abstract:
  The cleaved and (2 x 1) reconstructed (111) surface of p-type Si is
investigated by scanning tunneling microscopy (STM). Single B acceptors are
identified due to their characteristic voltage-dependent contrast which is
explained by a local energetic shift of the electronic density of states caused
by the Coulomb potential of the negatively charged acceptor. In addition,
detailed analysis of the STM images shows that apparently one orbital is
missing at the B site at sample voltages of 0.4 - 0.6 V, corresponding to the
absence of a localized dangling-bond state. Scanning tunneling spectroscopy
confirms a strongly altered density of states at the B atom due to the
different electronic structure of B compared to Si.


title:
Universal Behaviour of the Superfluid Fraction and Tc of He-3 in 99.5%
  Open Aerogel

abstract:
  We have investigated the superfluid transition of He-3 in a 99.5% porosity
silica aerogel. This very dilute sample shows behaviour intermediary between
bulk He-3 and He-3 confined to the denser aerogels previously studied. We
present data on both the superfluid transition temperature and the superfluid
density and compare our results with previous measurements. Finally, we show
that the suppression of the superfluid transition temperature and suppression
of the superfluid density of He-3 in aerogel follow a universal relation for a
range of aerogel samples.


title: Distribution of lipids in non-lamellar phases of their mixtures

abstract:
  We consider a model of lipids in which a head group, characterized by its
volume, is attached to two flexible tails of equal length. The phase diagram of
the anhydrous lipid is obtained within self-consistent field theory, and
displays, as a function of lipid architecture, a progression of phases:
body-centered cubic, hexagonal, gyroid, and lamellar. We then examine mixtures
of an inverted hexagonal forming lipid and a lamellar forming lipid. As the
volume fractions of the two lipids vary, we find that inverted hexagonal,
gyroid, or lamellar phases are formed. We demonstrate that the non-lamellar
forming lipid is found preferentially at locations which are difficult for the
lipid tails to reach. Variations in the volume fraction of each type of lipid
tail are on the order of one to ten per cent within regions dominated by the
tails. We also show that the variation in volume fraction is correlated
qualitatively with the variation in mean curvature of the head-tail interface.

categories: cond-mat.soft cond-mat.stat-mech physics.bio-ph physics.chem-ph


title:
Proposal for an Experiment to Test a Theory of High Temperature
  Superconductors

abstract:
  A theory for the phenomena observed in Copper-Oxide based high temperature
superconducting materials derives an elusive time-reversal and rotational
symmetry breaking order parameter for the observed pseudogap phase ending at a
quantum-critical point near the composition for the highest $T_c$. An
experiment is proposed to observe such a symmetry breaking. It is shown that
Angle-resolved Photoemission yields a current density which is different for
left and right circularly polarized photons. The magnitude of the effect and
its momentum dependence is estimated. Barring the presence of domains of the
predicted phase an asymmetry of about 0.1 is predicted at low temperatures in
moderately underdoped samples.


title:
The delocalization problem of two interacting electrons in a
  two-dimensional random potential

abstract:
  The problem of two electrons in a two-dimensional random potential is
addressed numerically. Specifically, the role of the Coulomb interaction
between electrons on localization is investigated by writing the Hamiltonian on
a localized basis and diagonalizing it exactly. The result of that procedure is
discussed in terms of level statistics, and the expectation value of the
electron-electron separation.