Anisotropic s-wave superconductivity: Comparison with experiments on single crystals

¹ Laboratorium voor Vaste-Stoffusica en Magnetisme, Katholieke Universiteit Leuven B-3001 Leuven, Belgium
² Max-Planck-Institute for the Chemical Physics of Solids Nöthnitzer Str. 40, D-01187 Dresden, Germany
³ Universität Tübingen, Institut für Theoretische Physik Auf der Morgenstelle 14, 72076 Tübingen, Germany
⁴ Max-Planck-Institute for Physics of Complex Systems Nöthnitzer Str. 38, D-01187 Dresden, Germany
⁵ Department of Physics and Astronomy, University of Southern California Los Angeles, CA 90089-0484, USA

Received: 4 April 2002
Accepted: 9 July 2002

Abstract

The recently discovered superconductivity in MgB₂ has captured world attention due to its simple crystal structure and relatively high superconducting transition temperature . It appears to be generally accepted that it is phonon-mediated s-wave BCS-like superconductivity. Surprisingly, the strongly temperature-dependent anisotropy of the upper critical field, observed experimentally in magnesium diboride single crystals, is still lacking a consistent theoretical explanation. We propose a simple single-gap anisotropic s-wave order parameter in order to compare its implications with the predictions of a multi-gap isotropic s-wave model. The quasiparticle density of states, thermodynamic properties, NMR spin-lattice relaxation rate, optical conductivity, and H_c2 anisotropy have been analyzed within this anisotropic s-wave model. We show that the present model can capture many aspects of the unusual superconducting properties of the MgB₂ compound, though more experimental data appear to be necessary from single-crystal MgB₂.