Electronic and structural properties of superconducting diborides and calcium disilicide in the $\rm AlB_2$ structure

TL;DR

This study uses density functional theory to analyze the electronic and structural properties of superconducting diborides and calcium disilicide, revealing their three-dimensional nature and the role of two-dimensional bonding bands.

Contribution

It provides detailed computational insights into the electronic structure and phonon properties of various diborides and calcium disilicide within the AlB2 structure, including effects of substitution and pressure.

Findings

Strong two-dimensional $\sigma$-bonding bands contribute at the Fermi level.

Metal substitution effects cannot be fully captured by a rigid band model.

Pressure influences the stability and properties of calcium disilicide.

Abstract

We report a detailed study of the electronic and structural properties of the 39K superconductor \mgbtwo and of several related systems of the same family, namely \mgalbtwo, \bebtwo, \casitwo and \cabesi. Our calculations, which include zone-center phonon frequencies and transport properties, are performed within the local density approximation to the density functional theory, using the full-potential linearized augmented plane wave (FLAPW) and the norm-conserving pseudopotential methods. Our results indicate essentially three-dimensional properties for these compounds; however, strongly two-dimensional $\sigma$-bonding bands contribute significantly at the Fermi level. Similarities and differences between \mgbtwo and \bebtwo (whose superconducting properties have not been yet investigated) %it is not yet know if it is a superconductor) are analyzed in detail. Our calculations for \mgalbtwo show that metal substitution cannot be fully described in a rigid band model. \casitwo is studied as a function of pressure, and Be substitution in the Si planes leads to a stable compound similar in many aspects to diborides.