Electronic properties of metal induced gap states at insulator/metal interfaces -- dependence on the alkali halide and the possibility of excitonic mechanism of superconductivity

TL;DR

This study uses first-principles calculations to analyze metal induced gap states at insulator/metal interfaces, revealing their long-range nature and potential role in excitonic superconductivity, with implications for material design.

Contribution

It provides a detailed theoretical analysis of MIGS at alkali halide/metal interfaces, highlighting their spatial characteristics and potential for mediating superconductivity.

Findings

MIGS have long tails on halogen sites with p_z character.

Penetration depth of MIGS depends on the lattice constant, not carrier density.

MIGS may facilitate exciton-mediated superconductivity.

Abstract

Motivated from the experimental observation of metal induced gap states (MIGS) at insulator/metal interfaces by Kiguchi {\it et al.} [Phys. Rev. Lett. {\bf 90}, 196803 (2003)], we have theoretically investigated the electronic properties of MIGS at interfaces between various alkali halides and a metal represented by a jellium with the first-principles density functional method. We have found that, on top of the usual evanescent state, MIGS generally have a long tail on halogen sites with a $p_z$-like character, whose penetration depth ($\lambda$) is as large as half the lattice constant of bulk alkali halides. This implies that $\lambda$, while little dependent on the carrier density in the jellium, is dominated by the lattice constant (hence by energy gap) of the alkali halide, where $\lambda_{\rm LiF} < \lambda_{\rm LiCl} < \lambda_{\rm LiI}$. We also propose a possibility of the MIGS working favorably for the exciton-mediated superconductivity.