466 research outputs found
Discrete symmetries and 1/3-quantum vortices in condensates of F=2 cold atoms
In this Letter we study discrete symmetries of mean field manifolds of
condensates of F=2 cold atoms, and various unconventional quantum vortices.
Discrete quaternion symmetries result in two species of spin defects that can
only appear in integer vortices while {\em cyclic} symmetries are found to
result in a phase shift of (or ) and therefore 1/3- (or 2/3-)
quantum vortices in condensates. We also briefly discuss 1/3-quantum vortices
in condensates of trimers.Comment: 4 pages, 2 figures included; published versio
Spin Glass Phase Transition on Scale-Free Networks
We study the Ising spin glass model on scale-free networks generated by the
static model using the replica method. Based on the replica-symmetric solution,
we derive the phase diagram consisting of the paramagnetic (P), ferromagnetic
(F), and spin glass (SG) phases as well as the Almeida-Thouless line as
functions of the degree exponent , the mean degree , and the
fraction of ferromagnetic interactions . To reflect the inhomogeneity of
vertices, we modify the magnetization and the spin glass order parameter
with vertex-weights. The transition temperature () between the
P-F (P-SG) phases and the critical behaviors of the order parameters are found
analytically. When , and are infinite, and the
system is in the F phase or the mixed phase for , while it is in the
SG phase at . and decay as power-laws with increasing
temperature with different -dependent exponents. When ,
the and are finite and related to the percolation threshold. The
critical exponents associated with and depend on for () at the P-F (P-SG) boundary.Comment: Phys. Rev. E in pres
The sawtooth chain: From Heisenberg spins to Hubbard electrons
We report on recent studies of the spin-half Heisenberg and the Hubbard model
on the sawtooth chain. For both models we construct a class of exact
eigenstates which are localized due to the frustrating geometry of the lattice
for a certain relation of the exchange (hopping) integrals. Although these
eigenstates differ in details for the two models because of the different
statistics, they share some characteristic features. The localized eigenstates
are highly degenerate and become ground states in high magnetic fields
(Heisenberg model) or at certain electron fillings (Hubbard model),
respectively. They may dominate the low-temperature thermodynamics and lead to
an extra low-temperature maximum in the specific heat. The ground-state
degeneracy can be calculated exactly by a mapping of the manifold of localized
ground states onto a classical hard-dimer problem, and explicit expressions for
thermodynamic quantities can be derived which are valid at low temperatures
near the saturation field for the Heisenberg model or around a certain value of
the chemical potential for the Hubbard model, respectively.Comment: 16 pages, 6 figure, the paper is based on an invited talk on the XXXI
International Workshop on Condensed Matter Theories, Bangkok, Dec 2007;
notation of x-axis in Fig.6 corrected, references update
Deep Spin-Glass Hysteresis Area Collapse and Scaling in the Ising Model
We investigate the dissipative loss in the Ising spin glass in three
dimensions through the scaling of the hysteresis area, for a maximum magnetic
field that is equal to the saturation field. We perform a systematic analysis
for the whole range of the bond randomness as a function of the sweep rate, by
means of frustration-preserving hard-spin mean field theory. Data collapse
within the entirety of the spin-glass phase driven adiabatically (i.e.,
infinitely-slow field variation) is found, revealing a power-law scaling of the
hysteresis area as a function of the antiferromagnetic bond fraction and the
temperature. Two dynamic regimes separated by a threshold frequency
characterize the dependence on the sweep rate of the oscillating field. For
, the hysteresis area is equal to its value in the adiabatic
limit , while for it increases with the
frequency through another randomness-dependent power law.Comment: 6 pages, 6 figure
Positional, Reorientational and Bond Orientational Order in DNA Mesophases
We investigate the orientational order of transverse polarization vectors of
long, stiff polymer molecules and their coupling to bond orientational and
positional order in high density mesophases. Homogeneous ordering of transverse
polarization vector promotes distortions in the hexatic phase, whereas
inhomogeneous ordering precipitates crystalization of the 2D sections with
different orientations of the transverse polarization vector on each molecule
in the unit cell. We propose possible scenarios for going from the hexatic
phase, through the distorted hexatic phase to the crystalline phase with an
orthorhombic unit cell observed experimentally for the case of DNA.Comment: 4 pages, 2 figure
A renormalization-group analysis of the interacting resonant level model at finite bias: Generic analytic study of static properties and quench dynamics
Using a real-time renormalization group method we study the minimal model of
a quantum dot dominated by charge fluctuations, the two-lead interacting
resonant level model, at finite bias voltage. We develop a set of RG equations
to treat the case of weak and strong charge fluctuations, together with the
determination of power-law exponents up to second order in the Coulomb
interaction. We derive analytic expressions for the charge susceptibility, the
steady-state current and the conductance in the situation of arbitrary system
parameters, in particular away from the particle-hole symmetric point and for
asymmetric Coulomb interactions. In the generic asymmetric situation we find
that power laws can be observed for the current only as function of the level
position (gate voltage) but not as function of the voltage. Furthermore, we
study the quench dynamics after a sudden switch-on of the level-lead couplings.
The time evolution of the dot occupation and current is governed by exponential
relaxation accompanied by voltage-dependent oscillations and characteristic
algebraic decay.Comment: 24 pages, 13 figures; revised versio
Ground-state topology of the Edwards-Anderson +/-J spin glass model
In the Edwards-Anderson model of spin glasses with a bimodal distribution of
bonds, the degeneracy of the ground state allows one to define a structure
called backbone, which can be characterized by the rigid lattice (RL),
consisting of the bonds that retain their frustration (or lack of it) in all
ground states. In this work we have performed a detailed numerical study of the
properties of the RL, both in two-dimensional (2D) and three-dimensional (3D)
lattices. Whereas in 3D we find strong evidence for percolation in the
thermodynamic limit, in 2D our results indicate that the most probable scenario
is that the RL does not percolate. On the other hand, both in 2D and 3D we find
that frustration is very unevenly distributed. Frustration is much lower in the
RL than in its complement. Using equilibrium simulations we observe that this
property can be found even above the critical temperature. This leads us to
propose that the RL should share many properties of ferromagnetic models, an
idea that recently has also been proposed in other contexts. We also suggest a
preliminary generalization of the definition of backbone for systems with
continuous distributions of bonds, and we argue that the study of this
structure could be useful for a better understanding of the low temperature
phase of those frustrated models.Comment: 16 pages and 21 figure
Phonons in the multiferroic langasite BaNbFeSiO : evidences for symmetry breaking
The chiral langasite BaNbFeSiO is a multiferroic
compound. While its magnetic order below T=27 K is now well characterised,
its polar order is still controversial. We thus looked at the phonon spectrum
and its temperature dependence to unravel possible crystal symmetry breaking.
We combined optical measurements (both infrared and Raman spectroscopy) with ab
initio calculations and show that signatures of a polar state are clearly
present in the phonon spectrum even at room temperature. An additional symmetry
lowering occurs below 120~K as seen from emergence of softer phonon modes in
the THz range. These results confirm the multiferroic nature of this langasite
and open new routes to understand the origin of the polar state
Quantum Optimization for Combinatorial Searches
I propose a "quantum annealing" heuristic for the problem of combinatorial
search among a frustrated set of states characterized by a cost function to be
minimized. The algorithm is probabilistic, with postselection of the
measurement result. A unique parameter playing the role of an effective
temperature governs the computational load and the overall quality of the
optimization. Any level of accuracy can be reached with a computational load
independent of the dimension {\it N} of the search set by choosing the
effective temperature correspondingly low. This is much better than classical
search heuristics, which typically involve computation times growing as powers
of log({\it N})Comment: Revised, published versio
Zero-temperature phase of the XY spin glass in two dimensions: Genetic embedded matching heuristic
For many real spin-glass materials, the Edwards-Anderson model with
continuous-symmetry spins is more realistic than the rather better understood
Ising variant. In principle, the nature of an occurring spin-glass phase in
such systems might be inferred from an analysis of the zero-temperature
properties. Unfortunately, with few exceptions, the problem of finding
ground-state configurations is a non-polynomial problem computationally, such
that efficient approximation algorithms are called for. Here, we employ the
recently developed genetic embedded matching (GEM) heuristic to investigate the
nature of the zero-temperature phase of the bimodal XY spin glass in two
dimensions. We analyze bulk properties such as the asymptotic ground-state
energy and the phase diagram of disorder strength vs. disorder concentration.
For the case of a symmetric distribution of ferromagnetic and antiferromagnetic
bonds, we find that the ground state of the model is unique up to a global O(2)
rotation of the spins. In particular, there are no extensive degeneracies in
this model. The main focus of this work is on an investigation of the
excitation spectrum as probed by changing the boundary conditions. Using
appropriate finite-size scaling techniques, we consistently determine the
stiffness of spin and chiral domain walls and the corresponding fractal
dimensions. Most noteworthy, we find that the spin and chiral channels are
characterized by two distinct stiffness exponents and, consequently, the system
displays spin-chirality decoupling at large length scales. Results for the
overlap distribution do not support the possibility of a multitude of
thermodynamic pure states.Comment: 18 pages, RevTex 4, moderately revised version as publishe
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