4 research outputs found
Heavy fermion d-wave superconductivity: a X-boson approach
From an extension of the periodic Anderson model (PAM) in the
limit taking into account the effect of a nearest neighbor attractive
interaction between -electrons, we compare the obtained superconducting
phase diagram of a two dimensional d-wave superconductor with the results
obtained for an isotropic s-wave superconductor employing the X-boson method.Comment: Submitted to the Proceeding of the ICM 2003-Rome. Requires
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BCS Model in Tsallis' Statistical Framework
We show that there is an effect of nonextensivity acting upon the BCS model
for superconductors in the ground state that motivates its study in the
Tsallis' statistical framework. We show that the weak-coupling limit
superconductors are well described by , where q is a real parameter
which characterizes the degree of nonextensivity of the Tsallis' entropy.
Nevertheless, small deviations with respect to q = 1 provide better agreement
when compared with experimental results. To illustrate this point, making use
of an approximated Fermi function, we show that measurements of the specific
heat, ultrasonic attenuation and tunneling experiments for tin (Sn) are better
described with q = 0.99.Comment: 13 pages, amssym
Superconductivity in graphene stacks: from the bilayer to graphite
We study the superconducting phase transition, both in a graphene bilayer and
in graphite. For that purpose we derive the mean-field effective potential for
a stack of graphene layers presenting hopping between adjacent sheets. For
describing superconductivity, we assume there is an on-site attractive
interaction between electrons and determine the superconducting critical
temperature as a function of the chemical potential. This displays a
dome-shaped curve, in agreement with previous results for two-dimensional Dirac
fermions. We show that the hopping between adjacent layers increases the
critical temperature for small values of the chemical potential. Finally, we
consider a minimal model for graphite and show that the transition temperature
is higher than that for the graphene bilayer for small values of chemical
potential. This might explain why intrinsic superconductivity is observed in
graphite
Quantum Criticality and Superconductivity in Quasi-Two-Dimensional Dirac Electronic Systems
We present a theory describing the superconducting (SC) interaction of Dirac
electrons in a quasi-two-dimensional system consisting of a stack of N planes.
The occurrence of a SC phase is investigated both at T=0 and T\neq 0, in the
case of a local interaction, when the theory must be renormalized and also in
the situation where a natural physical cutoff is present in the system. In both
cases, at T=0, we find a quantum phase transition connecting the normal and SC
phases at a certain critical coupling. The phase structure is shown to be
robust against quantum fluctuations. The SC gap is determined for T=0 and T\neq
0, both with and without a physical cutoff and the interplay between the gap
and the SC order parameter is discussed. Our theory qualitatively reproduces
the SC phase transition occurring in the underdoped regime of the high-Tc
cuprates. This fact points to the possible relevance of Dirac electrons in the
mechanism of high-Tc superconductivity.Comment: To be published in Nuclear Physics, Section B. 24 pages, 4 figure