23 research outputs found
A superconductor with 4-fermion attraction perturbed by magnetic impurities
A superconductor with 4-fermion attraction, considered by Ma\'{c}kowiak and
Tarasewicz is modified by adding to the Hamiltonian a long-range magnetic
interaction between conduction fermions and localized distinguishable spin
1/2 magnetic impurities. has the form of a reduced s-d interaction. An
upper and lower bound to the system's free energy density is
derived and the two bounds are shown to coalesce in the thermodynamic limit.
The resulting mean-field equations for the gap and a parameter ,
characterizing the impurity subsystem are solved and the solution minimizing
is found for various values of magnetic coupling constant and impurity
concentration. The phase diagrams of the system are depicted with five distinct
phases: the normal phase, unperturbed superconducting phase, perturbed
superconducting phase with nonzero gap in the excitation spectrum, perturbed
gapless superconducting phase and impurity phase with completely suppressed
superconductivity.Comment: 21 pages, 3 figure
Reentrant behavior of superconducting alloys
A dirty BCS superconductor with magnetic impurities is studied. Asymptotic
solution of the thermodynamics of such superconductor with spin and
magnetic impurities, is found. To this end, the system's free energy is bounded from above and below by mean-field type bounds, which are
shown to coalesce almost exactly in the thermodynamic limit, provided the
impurity concentration is sufficiently small. The resulting mean-field
equations for the gap and a parameter , characterizing the
impurity subsystem, are solved and the solution minimizing is found for
various values of magnetic coupling constant and impurity concentration
. The phase diagrams of the system are depicted with five distinct phases:
the normal phase, unperturbed superconducting phase, perturbed superconducting
phase with nonzero gap in the excitation spectrum, perturbed gapless
superconducting phase and impurity phase with completely suppressed
superconductivity. Furthermore, evidence of reentrant superconductivity and
Jaccarino-Peter compensation is found. The credibility of the theory is
verified by testing the dependence of the superconducting transition
temperature on . Very good quantitative agreement with
experimental data is obtained for several alloys: (LaCe)Al,
(LaGd)Al and (LaY)Ce. The theory
presented improves earlier developments in this field.Comment: 15 pages, 8 figures, full length articl