306 research outputs found
The electron gas with a strong pairing interaction: Three particle correlations and the Thouless instability
We derive simplified Faddeev type equations for the three particle T-matrix
which are valid in the Hubbard model where only electrons with opposite spins
interact. Using the approximation of dynamical mean field theory these
equations are partially solved numerically for the attractive Hubbard model. It
is shown that the three particle T-matrix contains a term vanishing
at the Thouless (or BCS) instability where the two-particle T-matrix diverges.
Based on the three particle term we further derive the low density - strong
coupling extension for the two-particle vertex function. We therefore
understand our equations as a step towards a systematic low density expansion
from the weak coupling BCS theory towards the stronger coupling limit.Comment: 14 pages, 9 figure
Level statistics and localization in a 2D quantum percolation problem
A two dimensional model for quantum percolation with variable tunneling range
is studied. For this purpose the Lifshitz model is considered where the
disorder enters the Hamiltonian via the nondiagonal elements. We employ a
numerical method to analyze the level statistics of this model. It turns out
that the level repulsion is strongest around the percolation threshold. As we
go away from the maximum level repulsion a crossover from a GOE type behavior
to a Poisson like distribution is indicated. The localization properties are
calculated by using the sensitivity to boundary conditions and we find a strong
crossover from localized to delocalized states.Comment: 4 pages, 4 figure
On the theory of light scattering in molecular liquids
The theory of light scattering for a system of linear molecules with
anisotropic polarizabilities is considered. As a starting point for our theory,
we express the result of a scattering experiment in VV and VH symmetry as
dynamic correlation functions of tensorial densities with
and . , denote indices of spherical harmonics. To account for all
observed hydrodynamic singularities, a generalization of the theory of
Schilling and Scheidsteger \cite{schilling97} for these correlation functions
is presented, which is capable to describe the light scattering experiments
from the liquid regime to the glassy state. As a microscopic theory it fulfills
all sum rules contrary to previous {\em phenomenological} theories. We
emphasize the importance of the helicity index for the microscopic theory
by showing, that only the existence of components lead to the well known
Rytov dip in liquids and to the appearance of transversal sound waves in VH
symmetry in the deeply supercooled liquid and the glass. Exact expressions for
the phenomenological frequency dependent rotation translation coupling
coefficients of previous theories are derived.Comment: 30 pages including 15 figures, submitted to EPJ B, revised version a
detailed discussion on the depolarization ratio is added and the discussion
on the hydrodynamic analysis is enlarge
Spectral Properties of the Attractive Hubbard Model
Deviations from Fermi liquid behavior are well documented in the normal state
of the cuprate superconductors, and some of these differences are possibly
related to pre-formed pairs appearing at temperatures above T_c. In order to
test these ideas we have investigated the attractive Hubbard model within a
self-consistent, conserving ladder approximation. In this version of the
theory, no feature is present which can be related to the pseudo gap found in
the high-T_c materials. Further, the interactions between two-particle bound
states change the physics of the superconducting instability in a profound
fashion, and lead to a completely different phenomenology that one predicts
based on the non-self-consistent version of the same theory.Comment: 4 pages including 2 figures, to appear in the proceedings of the
SNS'9
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