1,166 research outputs found
Optimized parallel tempering simulations of proteins
We apply a recently developed adaptive algorithm that systematically improves
the efficiency of parallel tempering or replica exchange methods in the
numerical simulation of small proteins. Feedback iterations allow us to
identify an optimal set of temperatures/replicas which are found to concentrate
at the bottlenecks of the simulations. A measure of convergence for the
equilibration of the parallel tempering algorithm is discussed. We test our
algorithm by simulating the 36-residue villin headpiece sub-domain HP-36
wherewe find a lowest-energy configuration with a root-mean-square-deviation of
less than 4 Angstroem to the experimentally determined structure.Comment: 22 pages, 7 figure
Quantum Monte Carlo Simulation of the Trellis Lattice Heisenberg Model for SrCuO and CaVO
We study the spin-1/2 trellis lattice Heisenberg model, a coupled spin ladder
system, both by perturbation around the dimer limit and by quantum Monte Carlo
simulations. We discuss the influence of the inter-ladder coupling on the spin
gap and the dispersion, and present results for the temperature dependence of
the uniform susceptibility. The latter was found to be parameterized well by a
mean-field type scaling ansatz. Finally we discuss fits of experimental
measurements on SrCuO and CaVO to our results.Comment: 7 pages, 8 figure
Topological Phases: An Expedition off Lattice
Motivated by the goal to give the simplest possible microscopic foundation
for a broad class of topological phases, we study quantum mechanical lattice
models where the topology of the lattice is one of the dynamical variables.
However, a fluctuating geometry can remove the separation between the system
size and the range of local interactions, which is important for topological
protection and ultimately the stability of a topological phase. In particular,
it can open the door to a pathology, which has been studied in the context of
quantum gravity and goes by the name of `baby universe', Here we discuss three
distinct approaches to suppressing these pathological fluctuations. We
complement this discussion by applying Cheeger's theory relating the geometry
of manifolds to their vibrational modes to study the spectra of Hamiltonians.
In particular, we present a detailed study of the statistical properties of
loop gas and string net models on fluctuating lattices, both analytically and
numerically.Comment: 38 pages, 22 figure
Spectral properties of the three-dimensional Hubbard model
We present momentum resolved single-particle spectra for the
three-dimensional Hubbard model for the paramagnetic and antiferromagnetically
ordered phase obtained within the dynamical cluster approximation. The
effective cluster problem is solved by continuous-time Quantum Monte Carlo
simulations. The absence of a time discretization error and the ability to
perform Monte Carlo measurements directly in Matsubara frequencies enable us to
analytically continue the self-energies by maximum entropy, which is essential
to obtain momentum resolved spectral functions for the N'eel state. We
investigate the dependence on temperature and interaction strength and the
effect of magnetic frustration introduced by a next-nearest neighbor hopping.
One particular question we address here is the influence of the frustrating
interaction on the metal insulator transition of the three-dimensional Hubbard
model.Comment: 16 pages, 14 figure
Self-consistent simulation of quantum wires defined by local oxidation of Ga[Al]As heterostructures
We calculate the electronic width of quantum wires as a function of their
lithographic width in analogy to experiments performed on nanostructures
defined by local oxidation of Ga[Al]As heterostructures. Two--dimensional
simulations of two parallel oxide lines on top of a Ga[Al]As heterostructure
defining a quantum wire are carried out in the framework of Density Functional
Theory in the Local Density Approximation and are found to be in agreement with
measurements. Quantitative assessment of the influence of various experimental
uncertainties is given. The most influential parameter turns out to be the
oxide line depth, followed by its exact shape and the effect of background
doping (in decreasing order).Comment: 5 pages, 6 figures; revised figures, clarified tex
Interacting classical dimers on the square lattice
We study a model of close-packed dimers on the square lattice with a nearest
neighbor interaction between parallel dimers. This model corresponds to the
classical limit of quantum dimer models [D.S. Rokhsar and S.A. Kivelson, Phys.
Rev. Lett.{\bf 61}, 2376 (1988)]. By means of Monte Carlo and Transfer Matrix
calculations, we show that this system undergoes a Kosterlitz-Thouless
transition separating a low temperature ordered phase where dimers are aligned
in columns from a high temperature critical phase with continuously varying
exponents. This is understood by constructing the corresponding Coulomb gas,
whose coupling constant is computed numerically. We also discuss doped models
and implications on the finite-temperature phase diagram of quantum dimer
models.Comment: 4 pages, 4 figures; v2 : Added results on doped models; published
versio
Quantum Monte Carlo Loop Algorithm for the t-J Model
We propose a generalization of the Quantum Monte Carlo loop algorithm to the
t-J model by a mapping to three coupled six-vertex models. The autocorrelation
times are reduced by orders of magnitude compared to the conventional local
algorithms. The method is completely ergodic and can be formulated directly in
continuous time. We introduce improved estimators for simulations with a local
sign problem. Some first results of finite temperature simulations are
presented for a t-J chain, a frustrated Heisenberg chain, and t-J ladder
models.Comment: 22 pages, including 12 figures. RevTex v3.0, uses psf.te
Universal critical temperature for Kosterlitz-Thouless transitions in bilayer quantum magnets
Recent experiments show that double layer quantum Hall systems may have a
ground state with canted antiferromagnetic order. In the experimentally
accessible vicinity of a quantum critical point, the order vanishes at a
temperature T_{KT} = \kappa H, where H is the magnetic field and \kappa is a
universal number determined by the interactions and Berry phases of the thermal
excitations. We present quantum Monte Carlo simulations on a model spin system
which support the universality of \kappa and determine its numerical value.
This allows experimental tests of an intrinsically quantum-mechanical universal
quantity, which is not also a property of a higher dimensional classical
critical point.Comment: 5 pages, 4 figure
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