623 research outputs found
Exact ground state Monte Carlo method for Bosons without importance sampling
Generally ``exact'' Quantum Monte Carlo computations for the ground state of
many Bosons make use of importance sampling. The importance sampling is based,
either on a guiding function or on an initial variational wave function. Here
we investigate the need of importance sampling in the case of Path Integral
Ground State (PIGS) Monte Carlo. PIGS is based on a discrete imaginary time
evolution of an initial wave function with a non zero overlap with the ground
state, that gives rise to a discrete path which is sampled via a Metropolis
like algorithm. In principle the exact ground state is reached in the limit of
an infinite imaginary time evolution, but actual computations are based on
finite time evolutions and the question is whether such computations give
unbiased exact results. We have studied bulk liquid and solid 4He with PIGS by
considering as initial wave function a constant, i.e. the ground state of an
ideal Bose gas. This implies that the evolution toward the ground state is
driven only by the imaginary time propagator, i.e. there is no importance
sampling. For both the phases we obtain results converging to those obtained by
considering the best available variational wave function (the Shadow wave
function) as initial wave function. Moreover we obtain the same results even by
considering wave functions with the wrong correlations, for instance a wave
function of a strongly localized Einstein crystal for the liquid phase. This
convergence is true not only for diagonal properties such as the energy, the
radial distribution function and the static structure factor, but also for
off-diagonal ones, such as the one--body density matrix. From this analysis we
conclude that zero temperature PIGS calculations can be as unbiased as those of
finite temperature Path Integral Monte Carlo.Comment: 11 pages, 10 figure
The value of improved (ERS) information based on domestic distribution effects of U.S. agriculture crops
The value of improving information for forecasting future crop harvests was investigated. Emphasis was placed upon establishing practical evaluation procedures firmly based in economic theory. The analysis was applied to the case of U.S. domestic wheat consumption. Estimates for a cost of storage function and a demand function for wheat were calculated. A model of market determinations of wheat inventories was developed for inventory adjustment. The carry-over horizon is computed by the solution of a nonlinear programming problem, and related variables such as spot and future price at each stage are determined. The model is adaptable to other markets. Results are shown to depend critically on the accuracy of current and proposed measurement techniques. The quantitative results are presented parametrically, in terms of various possible values of current and future accuracies
Many-Body Theory of the Electroweak Nuclear Response
After a brief review of the theoretical description of nuclei based on
nonrelativistic many-body theory and realistic hamiltonians, these lectures
focus on its application to the analysis of the electroweak response. Special
emphasis is given to electron-nucleus scattering, whose experimental study has
provided a wealth of information on nuclear structure and dynamics, exposing
the limitations of the shell model. The extension of the formalism to the case
of neutrino-nucleus interactions, whose quantitative understanding is required
to reduce the systematic uncertainty of neutrino oscillation experiments, is
also discussed.Comment: Lectures delivered at the DAE-BRNS Workshop on Hadron Physics.
Aligarh Muslim University, Aligarh (India), February 18-23, 200
Excited states of quantum many-body interacting systems: A variational coupled-cluster description
We extend recently proposed variational coupled-cluster method to describe
excitation states of quantum many-body interacting systems. We discuss, in
general terms, both quasiparticle excitations and quasiparticle-density-wave
excitations (collective modes). In application to quantum antiferromagnets, we
reproduce the well-known spin-wave excitations, i.e. quasiparticle magnons of
spin . In addition, we obtain new, spin-zero magnon-density-wave
excitations which has been missing in Anserson's spin-wave theory. Implications
of these new collective modes are discussed.Comment: 17 pages, 4 figure
Dynamic Many-Body Theory. II. Dynamics of Strongly Correlated Fermi Fluids
We develop a systematic theory of multi-particle excitations in strongly
interacting Fermi systems. Our work is the generalization of the time-honored
work by Jackson, Feenberg, and Campbell for bosons, that provides, in its most
advanced implementation, quantitative predictions for the dynamic structure
function in the whole experimentally accessible energy/momentum regime. Our
view is that the same physical effects -- namely fluctuations of the wave
function at an atomic length scale -- are responsible for the correct
energetics of the excitations in both Bose and Fermi fluids. Besides a
comprehensive derivation of the fermion version of the theory and discussion of
the approximations made, we present results for homogeneous He-3 and electrons
in three dimensions. We find indeed a significant lowering of the zero sound
mode in He-3 and a broadening of the collective mode due to the coupling to
particle-hole excitations in good agreement with experiments. The most visible
effect in electronic systems is the appearance of a ``double-plasmon''
excitation.Comment: submitted to Phys. Rev.
Static Response Function for Longitudinal and Transverse Excitations in Superfluid Helium
The sum rule formalism is used to evaluate rigorous bounds for the density
and current static response functions in superfluid helium at zero temperature.
Both lower and upper bounds are considered. The bounds are expressed in terms
of ground state properties (density and current correlation funtions) and of
the interatomic potential. The results for the density static response
significantly improve the Feynman approximation and turn out to be close to the
experimental (neutron scattering) data. A quantitative prediction for the
transverse current response is given. The role of one-phonon and multi-particle
excitations in the longitudinal and transverse channels is discussed.
(Phys.Rev.B, in press)Comment: 19 pages (plain TeX) and 3 Figures (postscript), UTF-26
Comparison of Variational Approaches for the Exactly Solvable 1/r-Hubbard Chain
We study Hartree-Fock, Gutzwiller, Baeriswyl, and combined
Gutzwiller-Baeriswyl wave functions for the exactly solvable one-dimensional
-Hubbard model. We find that none of these variational wave functions is
able to correctly reproduce the physics of the metal-to-insulator transition
which occurs in the model for half-filled bands when the interaction strength
equals the bandwidth. The many-particle problem to calculate the variational
ground state energy for the Baeriswyl and combined Gutzwiller-Baeriswyl wave
function is exactly solved for the~-Hubbard model. The latter wave
function becomes exact both for small and large interaction strength, but it
incorrectly predicts the metal-to-insulator transition to happen at infinitely
strong interactions. We conclude that neither Hartree-Fock nor Jastrow-type
wave functions yield reliable predictions on zero temperature phase transitions
in low-dimensional, i.e., charge-spin separated systems.Comment: 23 pages + 3 figures available on request; LaTeX under REVTeX 3.
Collective and single-particle excitations in 2D dipolar Bose gases
The Berezinskii-Kosterlitz-Thouless transition in 2D dipolar systems has been
studied recently by path integral Monte Carlo (PIMC) simulations [A. Filinov et
al., PRL 105, 070401 (2010)]. Here, we complement this analysis and study
temperature-coupling strength dependence of the density (particle-hole) and
single-particle (SP) excitation spectra both in superfluid and normal phases.
The dynamic structure factor, S(q,omega), of the longitudinal excitations is
rigorously reconstructed with full information on damping. The SP spectral
function, A(q,omega), is worked out from the one-particle Matsubara Green's
function. A stochastic optimization method is applied for reconstruction from
imaginary times. In the superfluid regime sharp energy resonances are observed
both in the density and SP excitations. The involved hybridization of both
spectra is discussed. In contrast, in the normal phase, when there is no
coupling, the density modes, beyond acoustic phonons, are significantly damped.
Our results generalize previous zero temperature analyses based on variational
many-body wavefunctions [F. Mazzanti et al., PRL 102, 110405 (2009), D. Hufnagl
et al., PRL 107, 065303 (2011)], where the underlying physics of the excitation
spectrum and the role of the condensate has not been addressed.Comment: 27 pages, 15 figures, 7 table
High-quality variational wave functions for small 4He clusters
We report a variational calculation of ground state energies and radii for
4He_N droplets (3 \leq N \leq 40), using the atom-atom interaction HFD-B(HE).
The trial wave function has a simple structure, combining two- and three-body
correlation functions coming from a translationally invariant
configuration-interaction description, and Jastrow-type short-range
correlations. The calculated ground state energies differ by around 2% from the
diffusion Monte Carlo results.Comment: 5 pages, 1 ps figure, REVTeX, submitted to Phys. Rev.
Dynamics and scaling in the periodic Anderson model
The periodic Anderson model (PAM) captures the essential physics of heavy
fermion materials. Yet even for the paramagnetic metallic phase, a practicable
many-body theory that can simultaneously handle all energy scales while
respecting the dictates of Fermi liquid theory at low energies, and all
interaction strengths from the strongly correlated Kondo lattice through to
weak coupling, has remained quite elusive. Aspects of this problem are
considered in the present paper where a non-perturbative local moment approach
(LMA) to single-particle dynamics of the asymmetric PAM is developed within the
general framework of dynamical mean-field theory. All interaction strengths and
energy scales are encompassed, although our natural focus is the Kondo lattice
regime of essentially localized -spins but general conduction band filling,
characterised by an exponentially small lattice coherence scale .
Particular emphasis is given to the resultant universal scaling behaviour of
dynamics in the Kondo lattice regime as an entire function of , including its dependence on conduction band filling,
-level asymmetry and lattice type.A rich description arises, encompassing
both coherent Fermi liquid behaviour at low- and the crossover
to effective single-impurity scaling physics at higher energies -- but still in
the -scaling regime, and as such incompatible with the
presence of two-scale `exhaustion' physics, which is likewise discussed.Comment: 22 pages in EPJB format, 14 figures; accepted for publication in
EPJB; (small change in the comments section, no change in manuscript
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