Ginzburg-Landau theory and effects of pressure on a two-band superconductor : application to MgB2

TL;DR

This paper develops a pressure-dependent Ginzburg-Landau model for two-band superconductors, applied to MgB2, revealing how pressure influences band interactions, potential topological transitions, and superconductivity mechanisms.

Contribution

It introduces a pressure-dependent two-band Ginzburg-Landau model and analyzes pressure effects on MgB2, including scenarios for band behavior and possible topological transitions.

Findings

Pressure reduces band splitting in one scenario, possibly enabling lower band participation in superconductivity.

Two distinct scenarios for band evolution under pressure are identified, affecting superconducting properties.

Experimental signatures for both scenarios are discussed, linking theory to observable phenomena.

Abstract

We present a model of pressure effects of a two-band superconductor based on a Ginzburg-Landau free energy with two order parameters. The parameters of the theory are pressure as well as temperature dependent. New pressure effects emerge as a result of the competition between the two bands. The theory then is applied to MgB2. We identify two possible scenaria regarding the fate of the two $\sigma$ subbands under pressure, depending on whether or not both subbands are above the Fermi energy at ambient pressure. The splitting of the two subbands is probably caused by the E2g distortion. If only one subband is above the Fermi energy at ambient pressure (scenario I), application of pressure diminishes the splitting and it is possible that the lower subband participates in the superconductivity. The corresponding crossover pressure and Gruneisen parameter are estimated. In the second scenario both bands start above the Fermi energy and they move below it, either by pressure or via the substitution of Mg by Al. In both scenaria, the possibility of electronical topological transition is emphasized. Experimental signatures of both scenaria are presented and existing experiments are discussed in the light of the different physical pictures.