1,834 research outputs found
Absence of a Slater Transition in The Two-Dimensional Hubbard Model
We present well-controlled results on the metal to insulator transition (MIT)
within the paramagnetic solution of the dynamical cluster approximation (DCA)
in the two-dimensional Hubbard model at half-filling. In the strong coupling
regime, a local picture describes the properties of the model; there is a large
charge gap . In the weak-coupling regime, we find a symbiosis
of short-range antiferromagnetic correlations and moment formation cause a gap
to open at finite temperature as in one dimension. Hence, this excludes the
mechanism of the MIT proposed by Slater long ago.Comment: 4 pages, 5 figure
A Maximum Entropy Method of Obtaining Thermodynamic Properties from Quantum Monte Carlo Simulations
We describe a novel method to obtain thermodynamic properties of quantum
systems using Baysian Inference -- Maximum Entropy techniques. The method is
applicable to energy values sampled at a discrete set of temperatures from
Quantum Monte Carlo Simulations. The internal energy and the specific heat of
the system are easily obtained as are errorbars on these quantities. The
entropy and the free energy are also obtainable. No assumptions as to the
specific functional form of the energy are made. The use of a priori
information, such as a sum rule on the entropy, is built into the method. As a
non-trivial example of the method, we obtain the specific heat of the
three-dimensional Periodic Anderson Model.Comment: 8 pages, 3 figure
A novel FLEX supplemented QMC approach to the Hubbard model
This paper introduces a novel ansatz-based technique for solution of the
Hubbard model over two length scales. Short range correlations are treated
exactly using a dynamical cluster approximation QMC simulation, while
longer-length-scale physics requiring larger cluster sizes is incorporated
through the introduction of the fluctuation exchange (FLEX) approximation. The
properties of the resulting hybrid scheme are examined, and the description of
local moment formation is compared to exact results in 1D. The effects of
electron-electron coupling and electron doping on the shape of the
Fermi-surface are demonstrated in 2D. Causality is examined in both 1D and 2D.
We find that the scheme is successful if QMC clusters of are used
(with sufficiently high temperatures in 1D), however very small QMC clusters of
lead to acausal results
Effect of long-range hopping on Tc in a two-dimensional Hubbard-Holstein model of the cuprates
We study the effect of long-range hoppings on Tc for the two-dimensional (2D)
Hubbard model with and without Holstein phonons using parameters evaluated from
band-structure calculations for cuprates. Employing the dynamical cluster
approximation (DCA) with a quantum Monte Carlo (QMC) cluster solver for a
4-site cluster, we observe that without phonons, the long-range hoppings, t'
and t'', generally suppress Tc. We argue that this trend remains valid for
larger clusters. In the presence of the Holstein phonons, a finite t' enhances
Tc in the under-doped region for the hole-doped system, consistent with
local-density approximation (LDA) calculations and experiment. This is
interpreted through the suppression of antiferromagnetic (AF) correlations and
the interplay between polaronic effects and the antiferromagnetism.Comment: 5 pages, 4 figure
Transport Properties of the Infinite Dimensional Hubbard Model
Results for the optical conductivity and resistivity of the Hubbard model in
infinite spatial dimensions are presented. At half filling we observe a gradual
crossover from a normal Fermi-liquid with a Drude peak at in the
optical conductivity to an insulator as a function of for temperatures
above the antiferromagnetic phase transition. When doped, the ``insulator''
becomes a Fermi-liquid with a corresponding temperature dependence of the
optical conductivity and resistivity. We find a -coefficient in the low
temperature resistivity which suggests that the carriers in the system acquire
a considerable mass-enhancement due to the strong local correlations. At high
temperatures, a crossover into a semi-metallic regime takes place.Comment: 14 page
The isotope effect in the Hubbard model with local phonons
The isotope effect (IE) in the two-dimensional Hubbard model with Holstein
phonons is studied using the dynamical cluster approximation with quantum Monte
Carlo. At small electron-phonon (EP) coupling the IE is negligible. For larger
EP coupling there is a large and positive IE on the superconducting temperature
that decreases with increasing doping. A significant IE also appears in the
low-energy density of states, kinetic energy and charge excitation spectrum. A
negligible IE is found in the pseudogap and antiferromagnetic (AF) properties
at small doping whereas the AF susceptibility at intermediate doping increases
with decreasing phonon frequency . This IE stems from increased
polaronic effects with decreasing . A larger IE at smaller doping
occurs due to stronger polaronic effects determined by the interplay of the EP
interaction with stronger AF correlations. The IE of the Hubbard-Holstein model
exhibits many similarities with the IE measured in cuprate superconductors
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