277 research outputs found
Anisotropic Superconductivity in the Induced Pairing Model
The model of local electron pairs and itinerant fermions coupled via charge
exchange mechanism, which mutually induces superconductivity in both subsystems
is studied for anisotropic pairing symmetry. The phase diagram is presented and
the phase fluctuations effects are analyzed within the Kosterlitz-Thouless
scenario.Comment: 4 pages, 2 figures. Physica B (in press), Proceedings of the
International Conference on Strongly Correlated Electron Systems, Ann Arbor,
Michigan, August 6-10, 200
Superconductivity in Inhomogeneous Hubbard Models
We present a controlled perturbative approach to the low temperature phase
diagram of highly inhomogeneous Hubbard models in the limit of small coupling,
, between clusters. We apply this to the dimerized and checkerboard models.
The dimerized model is found to behave like a doped semiconductor, with a
Fermi-liquid groundstate with parameters ({\it e.g.} the effective mass) which
are smooth functions of the Hubbard interaction, . By contrast, the
checkerboard model has a nodeless d-wave superconducting state (preformed pair
condensate, -BEC) for , which smoothly crosses over to an
intermediate BCS-like superconducting phase (-BCS), also with no nodal
quasi-particles, for , which gives way to a
Fermi liquid phase at large .Comment: 7 pages, a sign error in Eq.(3) has been corrected and its
consequence has been discussed with updated figure
Effects of Disorder on Superconductivity of Systems with Coexisting Itinerant Electrons and Local Pairs
We study the influence of diagonal disorder (random site energy) of local
pair (LP) site energies on the superconducting properties of a system of
coexisting local pairs and itinerant electrons described by the (hard-core)
boson-fermion model. Our analysis shows that the properties of such a model
with s-wave pairing can be very strongly affected by the diagonal disorder in
LP subsystem (the randomness of the LP site energies). This is in contrast with
the conventional s-wave BCS superconductors, which according to the Anderson's
theorem are rather insensitive to the diagonal disorder (i.e. to nonmagnetic
impurities). It has been found that the disorder effects depend in a crucial
way on the total particle concentration n and the LP level position DELTA_o and
depending on the parameters the system can exhibit various types of
superconducting behaviour, including the LP-like, intermediate (MIXED)and the
'BCS'-like. In the extended range of {n,DELTA_o} the superconducting ordering
is suppressed by the randomness of the LP site energies and the increasing
disorder induces a changeover from the MIXEDlike behaviour to the BCS-like one,
connected with abrupt reduction of T_c and energy gap to zero. However, there
also exist a definite range of {n,DELTA_o} in which the increasing disorder has
a quite different effect: namely it can substantially enhance T_c or even lead
to the phenomenon which can be called disorder induced superconductivity.
Another interesting effect is a possibility of a disorder induced bound pair
formation of itinerant electrons, connected with the change-over to the LP-like
regime.Comment: 18 pages, 12 figure
Real space inhomogeneities in high temperature superconductors: the perspective of two-component model
The two-component model of high temperature superconductors in its real space
version has been solved using Bogoliubov-de Gennes equations. The disorder in
the electron and boson subsystem has been taken into account. It strongly
modifies the superconducting properties and leads to local variations of the
gap parameter and density of states. The assumption that the impurities mainly
modify boson energies offers natural explanation of the puzzling positive
correlation between the positions of impurities and the values of the order
parameter found in the scanning tunnelling microscopy experiments.Comment: 19 pages, IOPP style include
The effects of the next-nearest-neighbour density-density interaction in the atomic limit of the extended Hubbard model
We have studied the extended Hubbard model in the atomic limit. The
Hamiltonian analyzed consists of the effective on-site interaction U and the
intersite density-density interactions Wij (both: nearest-neighbour and
next-nearest-neighbour). The model can be considered as a simple effective
model of charge ordered insulators. The phase diagrams and thermodynamic
properties of this system have been determined within the variational approach,
which treats the on-site interaction term exactly and the intersite
interactions within the mean-field approximation. Our investigation of the
general case taking into account for the first time the effects of
longer-ranged density-density interaction (repulsive and attractive) as well as
possible phase separations shows that, depending on the values of the
interaction parameters and the electron concentration, the system can exhibit
not only several homogeneous charge ordered (CO) phases, but also various phase
separated states (CO-CO and CO-nonordered). One finds that the model considered
exhibits very interesting multicritical behaviours and features, including
among others bicritical, tricritical, critical-end and isolated critical
points.Comment: 12 pages, 7 figures; final version, pdf-ReVTeX; corrected typos in
reference; submitted to Journal of Physics: Condensed Matte
On the Superconductivity in the Induced Pairing Model
The two component model of coexisting local electron pairs and itinerant
fermions coupled via charge exchange mechanism, which mutually induces
superconductivity in both subsystems, is discussed. The cases of isotropic
s-wave and anisotropic pairing of extended s and d_{x^2-y^2} -wave symmetries
are analyzed for a 2D square lattice within the BCS-mean field approximation
and the Kosterlitz-Thouless theory. We determined the phase diagrams and
superconducting characteristics as a function of the position of the local pair
(LP) level and the total electron concentration. The model exhibits several
types of interesting crossovers from BCS like behavior to that of LP's. Some of
our results are discussed in connection with a two-component scenario of
preformed pairs and unpaired electrons for exotic superconductors.Comment: Proceedings of the 3rd Polish-US Workshop on Magnetism and
Superconductivity of Advanced Materials, July 14-19, 2002, Ladek Zdroj
(Poland) to appear in Physica
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