4,499 research outputs found
Exact duality and dual Monte-Carlo simulation for the Bosonic Hubbard model
We derive the exact dual to the Bosonic Hubbard model. The dual variables
take the form of conserved current loops (local and global). Previously this
has been done only for the very soft core model at very high density. No such
approximations are made here. In particular, the dual of the hard core model is
shown to have a very simple form which is then used to construct an efficient
Monte Carlo algorithm which is quite similar to the World Line algorithm but
with some important differences. For example, with this algorithm we can
measure easily the correlation function of the order parameter (Green
function), a quantity which is extremely difficult to measure with the standard
World Line algorithm. We demonstrate the algorithm for the one and two
dimensional hardcore Bosonic Hubbard models. We present new results especially
for the Green function and zero mode filling fraction in the two dimensional
hardcore model.Comment: 14 pages, 15 figures include
Solitons in Trapped Bose-Einstein condensates in one-dimensional optical lattices
We use Quantum Monte Carlo simulations to show the presence and study the
properties of solitons in the one dimensional soft-core bosonic Hubbard model
with near neighbor interaction in traps. We show that when the half-filled
Charge Density Wave (CDW) phase is doped, solitons are produced and quasi long
range order established. We discuss the implications of these results for the
presence and robustness of this solitonic phase in Bose-Einstein Condensates
(BEC) on one dimensional optical lattices in traps and study the associated
excitation spectrum. The density profile exhibits the coexistence of Mott
insulator, CDW, and superfluid regions.Comment: 5 pages, Latex with figure
Single-particle versus pair condensation of hard-core bosons with correlated hopping
We investigate the consequences of correlated hopping on the ground state
properties of hard-core bosons on a square lattice as revealed by extensive
exact diagonalizations and quantum Monte Carlo simulations. While for non
interacting hard-core bosons the effective attraction induced by the correlated
hopping leads to phase separation at low density, we show that a modest
nearest-neighbor repulsion suppresses phase separation, leading to a remarkable
low-density pairing phase with no single particle Bose-Einstein condensation
but long-range two-particle correlations, signaling a condensation of pairs. We
also explain why the unusual properties of the pairing phase are a real
challenge for standard one-worm quantum Monte Carlo simulations.Comment: 8 pages, 7 figure
Magnetic and Transport Properties of a Coupled Hubbard Bilayer with Electron and Hole Doping
The single band, two dimensional Hubbard Hamiltonian has been extensively
studied as a model for high temperature superconductivity. While Quantum Monte
Carlo simulations within the dynamic cluster approximation are now providing
considerable evidence for a d-wave superconducting state at low temperature,
such a transition remains well out of reach of finite lattice simulations
because of the "sign problem". We show here that a bilayer Hubbard model, in
which one layer is electron doped and one layer is hole doped, can be studied
to lower temperatures and exhibits an interesting signal of d-wave pairing. The
results of our simulations bear resemblance to a recent report on the magnetic
and superconducting properties of BaCaCuOF which contains
both electron and hole doped CuO planes. We also explore the phase diagram
of bilayer models in which each sheet is at half-filling
A Survey on Ear Biometrics
Recognizing people by their ear has recently received significant attention in the literature. Several reasons account for this trend: first, ear recognition does not suffer from some problems associated with other non contact biometrics, such as face recognition; second, it is the most promising candidate for combination with the face in the context of multi-pose face recognition; and third, the ear can be used for human recognition in surveillance videos where the face may be occluded completely or in part. Further, the ear appears to degrade little with age. Even though, current ear detection and recognition systems have reached a certain level of maturity, their success is limited to controlled indoor conditions. In addition to variation in illumination, other open research problems include hair occlusion; earprint forensics; ear symmetry; ear classification; and ear individuality. This paper provides a detailed survey of research conducted in ear detection and recognition. It provides an up-to-date review of the existing literature revealing the current state-of-art for not only those who are working in this area but also for those who might exploit this new approach. Furthermore, it offers insights into some unsolved ear recognition problems as well as ear databases available for researchers
Quantum Monte Carlo Study of an Interaction-Driven Band Insulator to Metal Transition
We study the transitions from band insulator to metal to Mott insulator in
the ionic Hubbard model on a two dimensional square lattice using determinant
Quantum Monte Carlo. Evaluation of the temperature dependence of the
conductivity demonstrates that the metallic region extends for a finite range
of interaction values. The Mott phase at strong coupling is accompanied by
antiferromagnetic (AF) order. Inclusion of these intersite correlations changes
the phase diagram qualitatively compared to dynamical mean field theory.Comment: 4 pages, 6 figure
Penetration and cratering experiments of graphite by 0.5-mm diameter steel spheres at various impact velocities
Cratering experiments have been conducted with 0.5-mm diameter AISI 52100 steel spherical projectiles and 30-mm diameter, 15-mm long graphite targets. The latter were made of a commercial grade of polycrystalline and porous graphite named EDM3 whose behavior is known as macroscopically isotropic. A two-stage light-gas gun launched the steel projectiles at velocities between 1.1 and 4.5 km s 1. In most cases, post-mortem tomographies revealed that the projectile was trapped, fragmented or not, inside the target. It showed that the apparent crater size and depth increase with the impact velocity. This is also the case of the crater volume which appears to follow a power law significantly different from those constructed in previous works for similar impact conditions and materials. Meanwhile, the projectile depth of penetration starts to decrease at velocities beyond 2.2 km s 1. This is firstly because of its plastic deformation and then, beyond 3.2 km s 1, because of its fragmentation. In addition to these three regimes of penetration behavior already described by a few authors, we suggest a fourth regime in which the projectile melting plays a significant role at velocities above 4.1 km s 1. A discussion of these four regimes is provided and indicates that each phenomenon may account for the local evolution of the depth of penetration
A study of 2 GHz electromagnetic wave propagation over optical paths in three geographical regions of the United States
Statistical correlation between optical microwave propagation reliability, fade margin, path length, and geographic locatio
Depletion induced isotropic-isotropic phase separation in suspensions of rod-like colloids
When non-adsorbing polymers are added to an isotropic suspension of rod-like
colloids, the colloids effectively attract each other via depletion forces. We
performed Monte Carlo simulations to study the phase diagram of such
rod-polymer mixture. The colloidal rods were modelled as hard spherocylinders;
the polymers were described as spheres of the same diameter as the rods. The
polymers may overlap with no energy cost, while overlap of polymers and rods is
forbidden.
Large amounts of depletant cause phase separation of the mixture. We
estimated the phase boundaries of isotropic-isotropic coexistence both, in the
bulk and in confinement. To determine the phase boundaries we applied the grand
canonical ensemble using successive umbrella sampling [J. Chem. Phys. 120,
10925 (2004)], and we performed a finite-size scaling analysis to estimate the
location of the critical point. The results are compared with predictions of
the free volume theory developed by Lekkerkerker and Stroobants [Nuovo Cimento
D 16, 949 (1994)]. We also give estimates for the interfacial tension between
the coexisting isotropic phases and analyse its power-law behaviour on approach
of the critical point
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