449 research outputs found
Superconductivity and antiferromagnetism in a hard-core boson spin-1 model in two dimensions
A model of hard-core bosons and spin-1 sites with single-ion anisotropy is
proposed to approximately describe hole pairs moving in a background of
singlets and triplets with the aim of exploring the relationship between
superconductivity and antiferromagnetism. The properties of this model at zero
temperature were investigated using quantum Monte Carlo techniques. The most
important feature found is the suppression of superconductivity, as long range
coherence of preformed pairs, due to the presence of both antiferromagnetism
and excitations. Indications of charge ordered and other phases are
also discussed.Comment: One figure, one reference, adde
On the Finite Size Scaling in Disordered Systems
The critical behavior of a quenched random hypercubic sample of linear size
is considered, within the ``random-'' field-theoretical mode, by
using the renormalization group method. A finite-size scaling behavior is
established and analyzed near the upper critical dimension and
some universal results are obtained. The problem of self-averaging is clarified
for different critical regimes.Comment: 21 pages, 2 figures, submitted to the Physcal Review
Crossover and self-averaging in the two-dimensional site-diluted Ising model
Using the newly proposed probability-changing cluster (PCC) Monte Carlo
algorithm, we simulate the two-dimensional (2D) site-diluted Ising model. Since
we can tune the critical point of each random sample automatically with the PCC
algorithm, we succeed in studying the sample-dependent and the sample
average of physical quantities at each systematically. Using the
finite-size scaling (FSS) analysis for , we discuss the importance of
corrections to FSS both in the strong-dilution and weak-dilution regions. The
critical phenomena of the 2D site-diluted Ising model are shown to be
controlled by the pure fixed point. The crossover from the percolation fixed
point to the pure Ising fixed point with the system size is explicitly
demonstrated by the study of the Binder parameter. We also study the
distribution of critical temperature . Its variance shows the power-law
dependence, , and the estimate of the exponent is consistent
with the prediction of Aharony and Harris [Phys. Rev. Lett. {\bf 77}, 3700
(1996)]. Calculating the relative variance of critical magnetization at the
sample-dependent , we show that the 2D site-diluted Ising model
exhibits weak self-averaging.Comment: 6 pages including 6 eps figures, RevTeX, to appear in Phys. Rev.
Two-Dimensional Quantum XY Model with Ring Exchange and External Field
We present the zero-temperature phase diagram of a square lattice quantum
spin 1/2 XY model with four-site ring exchange in a uniform external magnetic
field. Using quantum Monte Carlo techniques, we identify various quantum phase
transitions between the XY-order, striped or valence bond solid, staggered Neel
antiferromagnet and fully polarized ground states of the model. We find no
evidence for a quantum spin liquid phase.Comment: 4 pages, 4 figure
Qubits as Parafermions
Qubits are neither fermions nor bosons. A Fock space description of qubits
leads to a mapping from qubits to parafermions: particles with a hybrid
boson-fermion quantum statistics. We study this mapping in detail, and use it
to provide a classification of the algebras of operators acting on qubits.
These algebras in turn classify the universality of different classes of
physically relevant qubit-qubit interaction Hamiltonians. The mapping is
further used to elucidate the connections between qubits, bosons, and fermions.
These connections allow us to share universality results between the different
particle types. Finally, we use the mapping to study the quantum computational
power of certain anisotropic exchange Hamiltonians. In particular, we prove
that the XY model with nearest-neighbor interactions only is not
computationally universal. We also generalize previous results about universal
quantum computation with encoded qubits to codes with higher rates.Comment: 17 pages, no figures. v3: This version to appear in J. Math. Phys.,
special issue on quantum computatio
Wang-Landau study of the 3D Ising model with bond disorder
We implement a two-stage approach of the Wang-Landau algorithm to investigate
the critical properties of the 3D Ising model with quenched bond randomness. In
particular, we consider the case where disorder couples to the nearest-neighbor
ferromagnetic interaction, in terms of a bimodal distribution of strong versus
weak bonds. Our simulations are carried out for large ensembles of disorder
realizations and lattices with linear sizes in the range . We apply
well-established finite-size scaling techniques and concepts from the scaling
theory of disordered systems to describe the nature of the phase transition of
the disordered model, departing gradually from the fixed point of the pure
system. Our analysis (based on the determination of the critical exponents)
shows that the 3D random-bond Ising model belongs to the same universality
class with the site- and bond-dilution models, providing a single universality
class for the 3D Ising model with these three types of quenched uncorrelated
disorder.Comment: 7 pages, 7 figures, to be published in Eur. Phys. J.
Destruction of diagonal and off-diagonal long range order by disorder in two-dimensional hard core boson systems
We use quantum Monte Carlo simulations to study the effect of disorder, in
the form of a disordered chemical potential, on the phase diagram of the hard
core bosonic Hubbard model in two dimensions. We find numerical evidence that
in two dimensions, no matter how weak the disorder, it will always destroy the
long range density wave order (checkerboard solid) present at half filling and
strong nearest neighbor repulsion and replace it with a bose glass phase. We
study the properties of this glassy phase including the superfluid density,
energy gaps and the full Green's function. We also study the possibility of
other localized phases at weak nearest neighbor repulsion, i.e. Anderson
localization. We find that such a phase does not truly exist: The disorder must
exceed a threshold before the bosons (at weak nn repulsion) are localized. The
phase diagram for hard core bosons with disorder cannot be obtained easily from
the soft core phase diagram discussed in the literature.Comment: 7 pages, 10 eps figures include
Transcriptional Responses of Resistant and Susceptible Fish Clones to the Bacterial Pathogen Flavobacterium psychrophilum
Flavobacterium psychrophilum is a bacterial species that represents one of the most important pathogens for aquaculture worldwide, especially for salmonids. To gain insights into the genetic basis of the natural resistance to F. psychrophilum, we selected homozygous clones of rainbow trout with contrasted susceptibility to the infection. We compared the transcriptional response to the bacteria in the pronephros of a susceptible and a resistant line by micro-array analysis five days after infection. While the basal transcriptome of healthy fish was significantly different in the resistant and susceptible lines, the transcriptome modifications induced by the bacteria involved essentially the same genes and pathways. The response to F. psychrophilum involved antimicrobial peptides, complement, and a number of enzymes and chemokines. The matrix metalloproteases mmp9 and mmp13 were among the most highly induced genes in both genetic backgrounds. Key genes of both pro- and anti-inflammatory response such as IL1 and IL10, were up-regulated with a greater magnitude in susceptible animals where the bacterial load was also much higher. While higher resistance to F. psychrophilum does not seem to be based on extensive differences in the orientation of the immune response, several genes including complement C3 showed stronger induction in the resistant fish. They may be important for the variation of susceptibility to the infection
- âŠ