Tunneling and electronic structure of the two-gap superconductorMgB2

J A Silva-Guillén,Y Noat,T Cren,P Ordejón,W Sacks,E Canadell

doi:10.1103/physrevb.92.064514

Abstract

A combined experimental (superconductor-insulator-superconductor tunneling spectra) and theoretical (density functional theory) study of the two-gap superconductor ${\mathrm{MgB}}_{2}$ is reported. The calculations confirm that the small gap is associated with a $\ensuremath{\pi}$ band mostly based on the boron ${p}_{z}$ orbitals leading to the three-dimensional band component of the Fermi surface. This channel almost completely dominates the tunneling images and spectra for $c$-axis-oriented samples and not the two-dimensional $\ensuremath{\sigma}$ band. The origin of this effect is due to the faster decay of the electronic states associated with the boron ${p}_{x}$ and ${p}_{y}$ orbitals compared to those associated with the boron ${p}_{z}$ orbitals, together with the symmetry properties of the wave functions. The calculated tunneling channels and partial density of states for each band agree with the values deduced from precise fits of experimental tunneling spectra. The present approach provides a framework for the understanding of tunneling spectra and the nature of superconducting gaps of other multigap superconductors.

Highlights

MgB2 has been known for a long time [1,2], but interest in this material was boosted by the discovery of superconductivity with a high critical temperature (Tc = 39 K). [3] It exhibits the AlB2-type crystal structure [4], where hexagonal layers of graphenelike boron atoms alternate with hexagonal layers of magnesium atoms sitting on top of the center of the boron hexagons
We present a first-principles density functional theory (DFT) [25,26] study of the electronic structure in the normal state, from which we identify the character of the electronic states associated with the two-gap superconductivity
We focus on the experimental fact that the tunneling process is dominated by the contribution of the small gap

Summary

INTRODUCTION

We present a first-principles density functional theory (DFT) [25,26] study of the electronic structure in the normal state, from which we identify the character of the electronic states associated with the two-gap superconductivity These calculations allow us to explain the ratio of the interband coupling parameters deduced from the fit of various STS experiments of the literature, including ours, with the McMillan equations. We calculate using DFT the tunneling selectivity arising from the wave function at the MgB2 surface, allowing a full understanding of the tunneling spectra This combined experimental-theoretical approach is useful to understand other multigap superconductors such as 2H -NbSe2, Ba8Si46, the FeSe-based superconductors, and so on

EXPERIMENTAL TUNNELING SPECTRA

TUNNELING IMAGES AND SELECTIVITY

CONCLUSIONS