1,686 research outputs found
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
Gap States in Dilute Magnetic Alloy Superconductors
We study states in the superconducting gap induced by magnetic impurities
using self-consistent quantum Monte Carlo with maximum entropy and formally
exact analytic continuation methods. The magnetic impurity susceptibility has
different characteristics for T_{0} \alt T_{c0} and T_{0} \agt T_{c0}
(: Kondo temperature, : superconducting transition temperature)
due to the crossover between a doublet and a singlet ground state. We
systematically study the location and the weight of the gap states and the gap
parameter as a function of and the concentration of the
impurities.Comment: 4 pages in ReVTeX including 4 encapsulated Postscript 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
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
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