85 research outputs found
Bond excitations in the pseudogap phase of the Hubbard Model
Using the dynamical cluster approximation, we calculate the correlation
functions associated with the nearest neighbor bond operator which measure the
z component of the spin exchange in the two-dimensional Hubbard model with
equal to the bandwidth. We find that in the pseudogap region, the local bond
susceptibility diverges at T=0. This shows the existence of degenerate bond
spin excitation and implies quantum criticality and bond order formation when
long range correlations are considered. The strong correlation between
excitations on parallel neighboring bonds suggests bond singlet dimerization.
The suppression of divergence for implies that tor these
model parameters this is quantum critical point which separates the
unconventional pseudogap region characterized by bond order from a conventional
Fermi liquid.Comment: 5 pages, 5 figure
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
Electron-phonon interaction in correlated electronic systems: polarons and the formation of orbital ordering
The properties of a dilute electron gas, coupled to the lattice degrees of
freedom, are studied and compared with the properties of an electron gas at
half-filling, where spinless fermions with two orbitals per lattice site are
considered. The simplest model which includes both the local electron-lattice
interaction of the Jahn-Teller type and the electronic correlations is the
-Jahn-Teller-Hubbard model. We analyze the formation and
stability of Jahn-Teller polarons and bipolarons, respectively. Our approach is
based on a hopping expansion in the strong-coupling regime. The results are
compared with recently published findings for the Hubbard-Holstein model [1,2].
The special case of the Jahn-Teller-Hubbard model at half-filling is mapped on
a spin-1/2 Heisenberg model with phonon-dependent coupling constants. This has
been derived within a projection formalism that provides a continued-fraction
representation of the Green's function. We study the exact solution for two and
three particles and compare it with the effective theory on the infinite
lattice with one particle per site.Comment: 4 pages, 0 figures, submitted to Phonons2004, to appear in physica
status solid
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
Quantum Criticality and Incipient Phase Separation in the Thermodynamic Properties of the Hubbard Model
Transport measurements on the cuprates suggest the presence of a quantum
critical point hiding underneath the superconducting dome near optimal hole
doping. We provide numerical evidence in support of this scenario via a
dynamical cluster quantum Monte Carlo study of the extended two-dimensional
Hubbard model. Single particle quantities, such as the spectral function, the
quasiparticle weight and the entropy, display a crossover between two distinct
ground states: a Fermi liquid at low filling and a non-Fermi liquid with a
pseudogap at high filling. Both states are found to cross over to a marginal
Fermi-liquid state at higher temperatures. For finite next-nearest-neighbor
hopping t' we find a classical critical point at temperature T_c. This
classical critical point is found to be associated with a phase separation
transition between a compressible Mott gas and an incompressible Mott liquid
corresponding to the Fermi liquid and the pseudogap state, respectively. Since
the critical temperature T_c extrapolates to zero as t' vanishes, we conclude
that a quantum critical point connects the Fermi-liquid to the pseudogap
region, and that the marginal-Fermi-liquid behavior in its vicinity is the
analogous of the supercritical region in the liquid-gas transition.Comment: 18 pages, 9 figure
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