479 research outputs found
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
Lattice calculations for A=3,4,6,12 nuclei using chiral effective field theory
We present lattice calculations for the ground state energies of tritium,
helium-3, helium-4, lithium-6, and carbon-12 nuclei. Our results were
previously summarized in a letter publication. This paper provides full details
of the calculations. We include isospin-breaking, Coulomb effects, and
interactions up to next-to-next-to-leading order in chiral effective field
theory.Comment: 38 pages, 11 figures, final publication versio
Haemophilus influenzae type b reemergence after combination immunization
An increase in Haemophilus influenzae type b (Hib) in
British children has been linked to the widespread use of a
diphtheria/tetanus/acellular pertussis combination vaccine
(DTaP-Hib). We measured anti-polyribosyl-ribitol phos-
phate antibody concentration and avidity before and after a
Hib booster in 176 children 2–4 years of age who had
received 3 doses of DTP-Hib (either DT whole cell pertus-
sis-Hib or DTaP-Hib) combination vaccine in infancy. We
also measured pharyngeal carriage of Hib. Antibody con-
centrations before and avidity indices after vaccination
were low (geometric mean concentration 0.46μg/mL, 95%
confidence interval [CI] 0.36–0.58; geometric mean avidity
index 0.16, 95% CI 0.14–0.18) and inversely related to the
number of previous doses of DTaP-Hib (p = 0.02 and
p<0.001, respectively). Hib was found in 2.1% (95% CI
0.7%–6.0%) of study participants. Our data support an
association between DTaP-Hib vaccine combinations and
clinical Hib disease through an effect on antibody concen-
tration and avidit
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
Hyperbolic Deformation on Quantum Lattice Hamiltonians
A group of non-uniform quantum lattice Hamiltonians in one dimension is
introduced, which is related to the hyperbolic -dimensional space. The
Hamiltonians contain only nearest neighbor interactions whose strength is
proportional to , where is the lattice index and where
is a deformation parameter. In the limit the
Hamiltonians become uniform. Spacial translation of the deformed Hamiltonians
is induced by the corner Hamiltonians. As a simple example, we investigate the
ground state of the deformed Heisenberg spin chain by use of the
density matrix renormalization group (DMRG) method. It is shown that the ground
state is dimerized when is finite. Spin correlation function show
exponential decay, and the boundary effect decreases with increasing .Comment: 5 pages, 4 figure
Precision calculation of within chiral perturbation theory
The reaction is calculated up to order in
chiral perturbation theory, where denotes the ratio of the pion to the
nucleon mass. Special emphasis is put on the role of nucleon--recoil
corrections that are the source of contributions with fractional power in
. Using the known near threshold production amplitude for as the only input, the total cross section for
is described very well. A conservative estimate suggests that the theoretical
uncertainty for the transition operator amounts to 3 % for the computed
amplitude near threshold.Comment: 28 page
Product Wave Function Renormalization Group: construction from the matrix product point of view
We present a construction of a matrix product state (MPS) that approximates
the largest-eigenvalue eigenvector of a transfer matrix T, for the purpose of
rapidly performing the infinite system density matrix renormalization group
(DMRG) method applied to two-dimensional classical lattice models. We use the
fact that the largest-eigenvalue eigenvector of T can be approximated by a
state vector created from the upper or lower half of a finite size cluster.
Decomposition of the obtained state vector into the MPS gives a way of
extending the MPS, at the system size increment process in the infinite system
DMRG algorithm. As a result, we successfully give the physical interpretation
of the product wave function renormalization group (PWFRG) method, and obtain
its appropriate initial condition.Comment: 8 pages, 8 figure
The role of winding numbers in quantum Monte Carlo simulations
We discuss the effects of fixing the winding number in quantum Monte Carlo
simulations. We present a simple geometrical argument as well as strong
numerical evidence that one can obtain exact ground state results for periodic
boundary conditions without changing the winding number. However, for very
small systems the temperature has to be considerably lower than in simulations
with fluctuating winding numbers. The relative deviation of a calculated
observable from the exact ground state result typically scales as ,
where the exponent is model and observable dependent and the prefactor
decreases with increasing system size. Analytic results for a quantum rotor
model further support our claim.Comment: 5 pages, 5 figure
On absolute moments of characteristic polynomials of a certain class of complex random matrices
Integer moments of the spectral determinant of complex
random matrices are obtained in terms of the characteristic polynomial of
the Hermitian matrix for the class of matrices where is a
given matrix and is random unitary. This work is motivated by studies of
complex eigenvalues of random matrices and potential applications of the
obtained results in this context are discussed.Comment: 41 page, typos correcte
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