Anisotropic s-wave superconductivity: comparison with experiments on MgB2 single crystals

TL;DR

This paper proposes an anisotropic s-wave superconductivity model for MgB2, explaining its unusual properties and anisotropy in critical fields, aligning with several experimental observations.

Contribution

It introduces a simple anisotropic s-wave model to explain MgB2's superconducting properties, contrasting with multi-gap models.

Findings

The model reproduces key thermodynamic and spectroscopic features.

It explains the temperature-dependent anisotropy of Hc2.

The model aligns with several experimental measurements.

Abstract

The recently discovered superconductivity in MgB2 has captured world attention due to its simple crystal structure and relatively high superconducting transition temperature Tc=39K. 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 prediction of a multi-gap isotropic s-wave model. The quasiparticle density of states, thermodynamic properties, NMR spin-lattice relaxation rate, optical conductivity, and Hc2 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 MgB2 compound, though more experimental data appear to be necessary from single crystal MgB2.