817 research outputs found
Meron-Cluster Solution of Fermion and Other Sign Problems
Numerical simulations of numerous quantum systems suffer from the notorious
sign problem. Important examples include QCD and other field theories at
non-zero chemical potential, at non-zero vacuum angle, or with an odd number of
flavors, as well as the Hubbard model for high-temperature superconductivity
and quantum antiferromagnets in an external magnetic field. In all these cases
standard simulation algorithms require an exponentially large statistics in
large space-time volumes and are thus impossible to use in practice.
Meron-cluster algorithms realize a general strategy to solve severe sign
problems but must be constructed for each individual case. They lead to a
complete solution of the sign problem in several of the above cases.Comment: 15 pages,LATTICE9
From Spin Ladders to the 2-d O(3) Model at Non-Zero Density
The numerical simulation of various field theories at non-zero chemical
potential suffers from severe complex action problems. In particular, QCD at
non-zero quark density can presently not be simulated for that reason. A
similar complex action problem arises in the 2-d O(3) model -- a toy model for
QCD. Here we construct the 2-d O(3) model at non-zero density via dimensional
reduction of an antiferromagnetic quantum spin ladder in a magnetic field. The
complex action problem of the 2-d O(3) model manifests itself as a sign problem
of the ladder system. This sign problem is solved completely with a
meron-cluster algorithm.Comment: Based on a talk by U.-J. Wiese, 6 pages, 12 figures, to be published
in computer physics communication
A many-fermion generalization of the Caldeira-Leggett model
We analyze a model system of fermions in a harmonic oscillator potential
under the influence of a dissipative environment: The fermions are subject to a
fluctuating force deriving from a bath of harmonic oscillators. This represents
an extension of the well-known Caldeira-Leggett model to the case of many
fermions. Using the method of bosonization, we calculate one- and two-particle
Green's functions of the fermions. We discuss the relaxation of a single extra
particle added above the Fermi sea, considering also dephasing of a particle
added in a coherent superposition of states. The consequences of the separation
of center-of-mass and relative motion, the Pauli principle, and the
bath-induced effective interaction are discussed. Finally, we extend our
analysis to a more generic coupling between system and bath, that results in
complete thermalization of the system.Comment: v3: fixed pdf problem; v2: added exact formula (Eq. 42) for Green's
function and discussion of equilibrium density matrix (new Fig. 2); 10
figures, 21 pages, see quant-ph/0305098 for brief version of some of these
result
Correlation Lengths in Quantum Spin Ladders
Analytic expressions for the correlation length temperature dependences are
given for antiferromagnetic spin-1/2 Heisenberg ladders using a finite-size
non-linear sigma-model approach. These calculations rely on identifying three
successive crossover regimes as a function of temperature. In each of these
regimes, precise and controlled approximations are formulated. The analytical
results are found to be in excellent agreement with Monte Carlo simulations for
the Heisenberg Hamiltonian.Comment: 5 pages LaTeX using RevTeX, 3 encapsulated postscript figure
Dynamical simulation of current fluctuations in a dissipative two-state system
Current fluctuations in a dissipative two-state system have been studied
using a novel quantum dynamics simulation method. After a transformation of the
path integrals, the tunneling dynamics is computed by deterministic integration
over the real-time paths under the influence of colored noise. The nature of
the transition from coherent to incoherent dynamics at low temperatures is
re-examined.Comment: 4 pages, 4 figures; to appear in Phys. Rev. Letter
Broken time-reversal symmetry in strongly correlated ladder structures
We provide, for the first time, in a doped strongly correlated system
(two-leg ladder), a controlled theoretical demonstration of the existence of a
state in which long-range ordered orbital currents are arranged in a staggered
pattern,coexisting with a charge density wave. The method used is the highly
accurate density matrix renormalization group technique.This brings us closer
to recent proposals that this order is realized in the enigmatic pseudogap
phase of the cuprate high temperature superconductors.Comment: The version accepted in Phys. Rev. Lett. 5 pages, 6 eps figures,
RevTex
Critical exponents of the quantum phase transition in a planar antiferromagnet
We have performed a large scale quantum Monte Carlo study of the quantum
phase transition in a planar spin-1/2 Heisenberg antiferromagnet with CaV4O9
structure. We obtain a dynamical exponent z=1.018+/-0.02. The critical
exponents beta, nu and eta agree within our errors with the classical 3D O(3)
exponents, expected from a mapping to the nonlinear sigma model. This confirms
the conjecture of Chubukov, Sachdev and Ye [Phys. Rev. B 49, 11919 (1994)] that
the Berry phase terms in the planar Heisenberg antiferromagnet are dangerously
irrelevant.Comment: 5 pages including 4 figures; revised version: some minor changes and
added reference
Pseudo-gap behavior in dynamical properties of high-Tc cuprates
Dynamical properties of 2D antiferromagnets with hole doping are investigated
to see the effects of short range local magnetic order on the temperature
dependence of the dynamical magnetic susceptibility. We show the pseudo-gap
like behavior of the temperature dependence of the NMR relaxation rate. We also
discuss implications of the results in relations to the observed spin gap like
behavior of low-doped copper oxide high- superconductors.Comment: 3 pages, Revtex, with 2 eps figures, to appear in J.Phys.Soc.Jpn.
Vol.67 No.
Nontrivial behavior of the Fermi arc in the staggered-flux ordered phase
The doping and temperature dependences of the Fermi arc in the
staggered-flux, or the d-density wave, ordered phase of the t-J model are
analyzed by the U(1) slave boson theory. Nontrivial behavior is revealed by the
self-consistent calculation. At low doped and finite-temperature region, both
the length of the Fermi arc and the width of the Fermi pocket are proportional
to and the area of the Fermi pocket is proportional to .
This behavior is completely different from that at the zero temperature, where
the area of the Fermi pocket becomes . This behavior should be
observed by detailed experiments of angle-resolved photoemission spectroscopy
in the pseudogap phase of high-T_c cuprates if the pseudogap phase is the
staggered-flux ordered phase.Comment: 4 pages, 4 figure
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