4,352 research outputs found
The Boson-Hubbard Model on a Kagome Lattice with Sextic Ring-Exchange Terms
High order ring-exchange interactions are crucial for the study of quantum
fluctuations on highly frustrated systems. We present the first exact quantum
Monte Carlo study of a model of hard-core bosons with sixth order ring-exchange
interactions on a two-dimensional kagome lattice. By using the Stochastic Green
Function algorithm, we show that the system becomes unstable in the limit of
large ring-exchange interactions. It undergoes a phase separation at all
fillings, except at 1/3 and 2/3 fillings for which the superfluid density
vanishes and an unusual mixed valence bond and charge density ordered solid is
formed.Comment: 4 pages, 7 figure
Complex phases in the doped two-species bosonic Hubbard Model
We study a two-dimensional bosonic Hubbard model with two hard-core species
away from half filling using Quantum Monte Carlo simulations. The model
includes a repulsive interspecies interaction and different nearest-neighbor
hopping terms for the two species. By varying the filling we find a total of
five distinct phases, including a normal liquid phase at higher temperature,
and four different phases at lower temperature. We find an
anti-ferromagnetically ordered Mott insulator and a region of coexistent
anti-ferromagnetic and superfluid phases near half filling. Further away from
half filling the phase diagram displays a superfluid phase and a novel phase
inside the superfluid region at even lower temperatures. In this novel phase
separated region, the heavy species has a Mott behavior with integer filling,
while the lighter species shows phase separated Mott and superfluid behaviors.Comment: 5 pages, 4 figure
Phase diagram of the Bose-Hubbard model on a ring-shaped lattice with tunable weak links
Motivated by recent experiments on toroidal Bose-Einstein condensates in
all-optical traps with tunable weak links, we study the one-dimensional
Bose-Hubbard model on a ring-shaped lattice with a small region of weak hopping
integrals using quantum Monte Carlo simulations. Besides the usual Mott
insulating and superfluid phases, we find a phase which is compressible but non
superfluid with a local Mott region. This `local Mott' phase extends in a large
region of the phase diagram. These results suggest that the insulating and
conducting phases can be tuned by a local parameter which may provide a new
insight to the design of atomtronic devices.Comment: 5 pages, 5 figure
Periodic Anderson model with Holstein phonons for the description of the Cerium volume collapse
Recent experiments have suggested that the electron-phonon coupling may play
an important role in the volume collapse transition
in Cerium. A minimal model for the description of such transition is the
periodic Anderson model. In order to better understand the effect of the
electron-phonon interaction on the volume collapse transition, we study the
periodic Anderson model with coupling between Holstein phonons and electrons in
the conduction band. We find that the electron-phonon coupling enhances the
volume collapse, which is consistent with experiments in Cerium. While we start
with the Kondo Volume Collapse scenario in mind, our results capture some
interesting features of the Mott scenario, such as a gap in the conduction
electron spectra which grows with the effective electron-phonon coupling.Comment: 8 pages, 6 figure
Parallel Tempering Simulation of the three-dimensional Edwards-Anderson Model with Compact Asynchronous Multispin Coding on GPU
Monte Carlo simulations of the Ising model play an important role in the
field of computational statistical physics, and they have revealed many
properties of the model over the past few decades. However, the effect of
frustration due to random disorder, in particular the possible spin glass
phase, remains a crucial but poorly understood problem. One of the obstacles in
the Monte Carlo simulation of random frustrated systems is their long
relaxation time making an efficient parallel implementation on state-of-the-art
computation platforms highly desirable. The Graphics Processing Unit (GPU) is
such a platform that provides an opportunity to significantly enhance the
computational performance and thus gain new insight into this problem. In this
paper, we present optimization and tuning approaches for the CUDA
implementation of the spin glass simulation on GPUs. We discuss the integration
of various design alternatives, such as GPU kernel construction with minimal
communication, memory tiling, and look-up tables. We present a binary data
format, Compact Asynchronous Multispin Coding (CAMSC), which provides an
additional speedup compared with the traditionally used Asynchronous
Multispin Coding (AMSC). Our overall design sustains a performance of 33.5
picoseconds per spin flip attempt for simulating the three-dimensional
Edwards-Anderson model with parallel tempering, which significantly improves
the performance over existing GPU implementations.Comment: 15 pages, 18 figure
Locally self-consistent embedding approach for disordered electronic systems
We present a new embedding scheme for the locally self-consistent method to
study disordered electron systems. We test this method in a tight-binding basis
and apply it to the single band Anderson model. The local interaction zone is
used to efficiently compute the local Green's function of a supercell embeded
into a local typical medium. We find a quick convergence as the size of the
local interaction zone which reduces the computational costs as expected. This
method captures the Anderson localization transition and accurately predicts
the critical disorder strength. The present work opens the path towards the
development of a typical medium embedding scheme for the multiple
scattering methods.Comment: 7 pages, 5 figure
Evidence that low endocytic activity is not directly responsible for human serum resistance in the insect form of African trypanosomes.
BACKGROUND: In Trypanosoma brucei, the African trypanosome, endocytosis is developmentally regulated and substantially more active in all known mammalian infective stages. In both mammalian and insect stages endocytic activity is likely required for nutrient acquisition, but in bloodstream forms increased endocytosis is involved in recycling the variant surface glycoprotein and removing host immune factors from the surface. However, a rationale for low endocytic activity in insect stages has not been explored. Here we asked if endocytic down-regulation in the procyclic form was associated with resistance to innate trypanolytic immune factors in the blood meal or tsetse fly midgut. FINDINGS: Using a well-characterized procyclic parasite with augmented endocytic flux mediated via TbRab5A overexpression, we found that insect stage parasites were able to grow both in the presence of trypanosome lytic factor (TLF) provided in human serum, and also in tsetse flies. Additionally, by placing blood stage parasites in restricted glucose medium, we observed that enlargement of the flagellar pocket, a key morphology associated with defective endocytosis, manifests in parallel with loss of cellular ATP levels. CONCLUSIONS: These observations suggest that a high rate of endocytosis per se is insufficient to render insect form parasites sensitive to TLF or tsetse-derived trypanocidal factors. However, the data do suggest that endocytosis is energetically burdensome, as endocytic activity is rapidly compromised on energy depletion in bloodstream stages. Hence an important aspect of endocytic modulation in the nutrient-poor tsetse midgut is likely energetic conservation.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are
Deep learning on the 2-dimensional Ising model to extract the crossover region with a variational autoencoder
The 2-dimensional Ising model on a square lattice is investigated with a variational autoencoder in the non-vanishing field case for the purpose of extracting the crossover region between the ferromagnetic and paramagnetic phases. The encoded latent variable space is found to provide suitable metrics for tracking the order and disorder in the Ising configurations that extends to the extraction of a crossover region in a way that is consistent with expectations. The extracted results achieve an exceptional prediction for the critical point as well as agreement with previously published results on the configurational magnetizations of the model. The performance of this method provides encouragement for the use of machine learning to extract meaningful structural information from complex physical systems where little a priori data is available
Periodic Anderson model with electron-phonon correlated conduction band
This paper reports dynamical mean field calculations for the periodic
Anderson model in which the conduction band is coupled to phonons. Motivated in
part by recent attention to the role of phonons in the -
transition in Ce, this model yields a rich and unexpected phase diagram which
is of intrinsic interest. Specifically, above a critical value of the
electron-phonon interaction, a first order transition with two coexisting
phases develops in the temperature-hybridization plane, which terminates at a
second order critical point. The coexisting phases display the familiar Kondo
screened and local moment character, yet they also exhibit pronounced polaronic
and bipolaronic properties, respectively.Comment: 4 pages, 6 figure
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