4,492 research outputs found
Pseudogap and the specific heat of high superconductors
The specific heat of a two dimensional repulsive Hubbard model with local
interaction is investigated. We use the two-pole approximation which exhibits
explicitly important correlations that are sources of the pseudogap anomaly.
The interplay between the specific heat and the pseudogap is the main focus of
the present work. Our self consistent numerical results show that above the
occupation , the specific heat starts to decrease due to the
presence of a pseudogap in the density of states. We have also observed a two
peak structure in the specific heat. Such structure is robust with respect to
the Coulomb interaction but it is significantly affected by the occupation
. A detailed study of the two peak structure is carried out in terms of
the renormalized quasi-particle bands. The role of the second nearest neighbor
hopping on the specific heat behavior and on the pseudogap, is extensively
discussed.Comment: 6 pages, 6 figures, accepted for publication in Solid State
Communication
A van Hemmen-Kondo model for disordered strongly correlated electron systems
We present here a theoretical model in order to describe the competition
between the Kondo effect and the spin glass behavior. The spin glass part of
the starting Hamiltonian contains Ising spins with an intersite exchange
interaction given by the local van Hemmen model, while the Kondo effect is
described as usual by the intrasite exchange . We obtain, for large
values, a Kondo phase and, for smaller values, a succession, with
decreasingComment: 14 pages, 4 figures, accepted for publication in Phys. Rev.
Antiferromagnetic Ising spin glass competing with BCS pairing interaction in a transverse field
The competition among spin glass (SG), antiferromagnetism (AF) and local
pairing superconductivity (PAIR) is studied in a two-sublattice fermionic Ising
spin glass model with a local BCS pairing interaction in the presence of an
applied magnetic transverse field . In the present approach, spins in
different sublattices interact with a Gaussian random coupling with an
antiferromagnetic mean and standard deviation . The problem is
formulated in the path integral formalism in which spin operators are
represented by bilinear combinations of Grassmann variables. The saddle-point
Grand Canonical potential is obtained within the static approximation and the
replica symmetric ansatz. The results are analysed in phase diagrams in which
the AF and the SG phases can occur for small ( is the strength of the
local superconductor coupling written in units of ), while the PAIR phase
appears as unique solution for large . However, there is a complex line
transition separating the PAIR phase from the others. It is second order at
high temperature that ends in a tricritical point. The quantum fluctuations
affect deeply the transition lines and the tricritical point due to the
presence of .Comment: 16 pages, 6 figures, accepted Eur. Phys. J.
An AGN Identification for 3EG J2006-2321
We present a multiwavelength analysis of the high-energy gamma-ray source 3EG
J2006-2321. The flux of this source above 100 MeV is shown to be variable on
time scales of days and months. Optical observations and careful examination of
archived radio data indicate that its radio counterpart is PMN J2005-2310, a
flat-spectrum radio quasar with a 5-GHz flux density of 260 mJy. Study of the
optical counterpart indicates a redshift of 0.833 and variable linear
polarization. No X-ray source has been detected near the position of PMN
J2005-2310, but an X-ray upper limit is derived from ROSAT data. This upper
limit provides for a spectral energy distribution with global characteristics
similar to those of known gamma-ray blazars. Taken together, these data
indicate that 3EG J2006-2321, listed as unidentified in the 3rd EGRET Catalog,
is a member of the blazar class of AGN. The 5-GHz radio flux density of this
blazar is the lowest of the 68 EGRET-detected AGN. The fact that EGRET has
detected such a source has implications for unidentified EGRET sources,
particularly those at high latitudes (), many of which may be
blazars.Comment: 22 pages, 6 figures. To appear in ApJ v569 n1, 10 April 200
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