2 research outputs found
Effects of Boson Dispersion in Fermion-Boson Coupled Systems
We study the nonlinear feedback in a fermion-boson system using an extension
of dynamical mean-field theory and the quantum Monte Carlo method. In the
perturbative regimes (weak-coupling and atomic limits) the effective
interaction among fermions increases as the width of the boson dispersion
increases. In the strong coupling regime away from the anti-adiabatic limit,
the effective interaction decreases as we increase the width of the boson
dispersion. This behavior is closely related with complete softening of the
boson field. We elucidate the parameters that control this nonperturbative
region where fluctuations of the dispersive bosons enhance the delocalization
of fermions.Comment: 14 pages RevTeX including 12 PS figure
Variational Monte Carlo Study of Spin-Gapped Normal State and BCS-BEC Crossover in Two-Dimensional Attractive Hubbard Model
We study properties of normal, superconducting (SC) and CDW states for an
attractive Hubbard model on the square lattice, using a variational Monte Carlo
method. In trial wave functions, we introduce an interspinon binding factor,
indispensable to induce a spin-gap transition in the normal state, in addition
to the onsite attractive and intersite repulsive factors. It is found that, in
the normal state, as the interaction strength increases, a first-order
spin-gap transition arises at (: band width) from a
Fermi liquid to a spin-gapped state, which is conductive through hopping of
doublons. In the SC state, we confirm by analysis of various quantities that
the mechanism of superconductivity undergoes a smooth crossover at around
|U_{\ma{co}}|\sim |U_{\rm c}| from a BCS type to a Bose-Einstein condensation
(BEC) type, as increases. For |U|<|U_{\ma{co}}|, quantities such as
the condensation energy, a SC correlation function and the condensate fraction
of onsite pairs exhibit behavior of , as expected from the
BCS theory. For |U|>|U_{\ma{co}}|, quantities such as the energy gain in the
SC transition and superfluid stiffness, which is related to the cost of phase
coherence, behave as , as expected in a bosonic
scheme. In this regime, the SC transition is induced by a gain in kinetic
energy, in contrast with the BCS theory. We refer to the relevance to the
pseudogap in cuprate superconductors.Comment: 14 pages, 22 figures, submitted to Journal of the Physical Society of
Japa