1,279 research outputs found
Quantum Monte Carlo scheme for frustrated Heisenberg antiferromagnets
When one tries to simulate quantum spin systems by the Monte Carlo method,
often the 'minus-sign problem' is encountered. In such a case, an application
of probabilistic methods is not possible. In this paper the method has been
proposed how to avoid the minus sign problem for certain class of frustrated
Heisenberg models. The systems where this method is applicable are, for
instance, the pyrochlore lattice and the Heisenberg model. The method
works in singlet sector. It relies on expression of wave functions in dimer
(pseudo)basis and writing down the Hamiltonian as a sum over plaquettes. In
such a formulation, matrix elements of the exponent of Hamiltonian are
positive.Comment: 19 LaTeX pages, 6 figures, 1 tabl
On the Sign Problem in the Hirsch-Fye Algorithm for Impurity Problems
We show that there is no fermion sign problem in the Hirsch and Fye algorithm
for the single-impurity Anderson model. Beyond the particle-hole symmetric case
for which a simple proof exists, this has been known only empirically. Here we
prove the nonexistence of a sign problem for the general case by showing that
each spin trace for a given Ising configuration is separately positive. We
further use this insight to analyze under what conditions orbitally degenerate
Anderson models or the two-impurity Anderson model develop a sign.Comment: 2 pages, no figure; published versio
Neutron-proton analyzing power at 12 MeV and inconsistencies in parametrizations of nucleon-nucleon data
We present the most accurate and complete data set for the analyzing power
Ay(theta) in neutron-proton scattering. The experimental data were corrected
for the effects of multiple scattering, both in the center detector and in the
neutron detectors. The final data at En = 12.0 MeV deviate considerably from
the predictions of nucleon-nucleon phase-shift analyses and potential models.
The impact of the new data on the value of the charged pion-nucleon coupling
constant is discussed in a model study.Comment: Six pages, four figures, one table, to be published in Physics
Letters
Efficiency of symmetric targeting for finite-T DMRG
Two targeting schemes have been known for the density matrix renormalization
group (DMRG) applied to non-Hermitian problems; one uses an asymmetric density
matrix and the other uses symmetric density matrix. We compare the numerical
efficiency of these two targeting schemes when they are used for the finite
temperature DMRG.Comment: 4 pages, 3 Postscript figures, REVTe
Experimental implementation of an adiabatic quantum optimization algorithm
We report the realization of a nuclear magnetic resonance computer with three
quantum bits that simulates an adiabatic quantum optimization algorithm.
Adiabatic quantum algorithms offer new insight into how quantum resources can
be used to solve hard problems. This experiment uses a particularly well suited
three quantum bit molecule and was made possible by introducing a technique
that encodes general instances of the given optimization problem into an easily
applicable Hamiltonian. Our results indicate an optimal run time of the
adiabatic algorithm that agrees well with the prediction of a simple
decoherence model.Comment: REVTeX, 5 pages, 4 figures, improved lay-out; accepted for
publication in Physical Review Letter
Monte Carlo Study of the Separation of Energy Scales in Quantum Spin 1/2 Chains with Bond Disorder
One-dimensional Heisenberg spin 1/2 chains with random ferro- and
antiferromagnetic bonds are realized in systems such as . We have investigated numerically the thermodynamic properties of a
generic random bond model and of a realistic model of by the quantum Monte Carlo loop algorithm. For the first time we
demonstrate the separation into three different temperature regimes for the
original Hamiltonian based on an exact treatment, especially we show that the
intermediate temperature regime is well-defined and observable in both the
specific heat and the magnetic susceptibility. The crossover between the
regimes is indicated by peaks in the specific heat. The uniform magnetic
susceptibility shows Curie-like behavior in the high-, intermediate- and
low-temperature regime, with different values of the Curie constant in each
regime. We show that these regimes are overlapping in the realistic model and
give numerical data for the analysis of experimental tests.Comment: 7 pages, 5 eps-figures included, typeset using JPSJ.sty, accepted for
publication in J. Phys. Soc. Jpn. 68, Vol. 3. (1999
Continuity of Local Time: An applied perspective
Continuity of local time for Brownian motion ranks among the most notable
mathematical results in the theory of stochastic processes. This article
addresses its implications from the point of view of applications. In
particular an extension of previous results on an explicit role of continuity
of (natural) local time is obtained for applications to recent classes of
problems in physics, biology and finance involving discontinuities in a
dispersion coefficient. The main theorem and its corollary provide physical
principles that relate macro scale continuity of deterministic quantities to
micro scale continuity of the (stochastic) local time.Comment: To appear in: "The fascination of Probability, Statistics and Their
Applications. In honour of Ole E. Barndorff-Nielsen on his 80th birthday
Transport in the 3-dimensional Anderson model: an analysis of the dynamics on scales below the localization length
Single-particle transport in disordered potentials is investigated on scales
below the localization length. The dynamics on those scales is concretely
analyzed for the 3-dimensional Anderson model with Gaussian on-site disorder.
This analysis particularly includes the dependence of characteristic transport
quantities on the amount of disorder and the energy interval, e.g., the mean
free path which separates ballistic and diffusive transport regimes. For these
regimes mean velocities, respectively diffusion constants are quantitatively
given. By the use of the Boltzmann equation in the limit of weak disorder we
reveal the known energy-dependencies of transport quantities. By an application
of the time-convolutionless (TCL) projection operator technique in the limit of
strong disorder we find evidence for much less pronounced energy dependencies.
All our results are partially confirmed by the numerically exact solution of
the time-dependent Schroedinger equation or by approximative numerical
integrators. A comparison with other findings in the literature is additionally
provided.Comment: 23 pages, 10 figure
Path integral Monte Carlo simulations of silicates
We investigate the thermal expansion of crystalline SiO in the --
cristobalite and the -quartz structure with path integral Monte Carlo
(PIMC) techniques. This simulation method allows to treat low-temperature
quantum effects properly. At temperatures below the Debye temperature, thermal
properties obtained with PIMC agree better with experimental results than those
obtained with classical Monte Carlo methods.Comment: 27 pages, 10 figures, Phys. Rev. B (in press
Metamagnetism in the 2D Hubbard Model with easy axis
Although the Hubbard model is widely investigated, there are surprisingly few
attempts to study the behavior of such a model in an external magnetic field.
Using the Projector Quantum Monte Carlo technique, we show that the Hubbard
model with an easy axis exhibits metamagnetic behavior if an external field is
turned on. For the case of intermediate correlations strength , we observe a
smooth transition from an antiferromagnetic regime to a paramagnetic phase.
While the staggered magnetization will decrease linearly up to a critical field
, uniform magnetization develops only for fields higher than .Comment: RevTeX 5 pages + 2 postscript figures (included), accepted for PRB
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