Triangular Mott-Hubbard Insulator Phases of Sn/Si(111) and Sn/Ge(111) Surfaces

TL;DR

This paper theoretically reexamines the ground states of Sn/Si(111) and Sn/Ge(111) surfaces, revealing a correlation-driven Mott-Hubbard insulating phase with magnetic properties and potential for superconductivity upon doping.

Contribution

It introduces a new theoretical analysis showing the stabilization of a Mott-Hubbard insulator phase in these surfaces due to electron correlations, supported by ab-initio calculations.

Findings

Correlations destabilize vertical buckling in Sn/Ge(111).

Surface becomes magnetic with a metal-insulator transition.

Evidence of a narrow gap Mott-Hubbard insulator consistent with experiments.

Abstract

The ground state of Sn/Si(111) and Sn/Ge(111) surface $\alpha$-phases is reexamined theoretically, based on $ab-initio$ calculations where correlations are approximately included through the orbital dependence of the Coulomb interaction (in the local density + Hubbard U approximation). The effect of correlations is to destabilize the vertical buckling in Sn/Ge(111) and to make the surface magnetic, with a metal-insulator transition for both systems. This signals the onset of a stable narrow gap Mott-Hubbard insulating state, in agreement with very recent experiments. Antiferromagnetic exchange is proposed to be responsible for the observed $\Gamma$-point photoemission intensity, as well asfor the partial metallization observed above above 60 K in Sn/Si(111). Extrinsic metallization of Sn/Si(111) by, $e.g.$ alkali doping, could lead to a novel 2D triangular superconducting state of this and similar surfaces.