Resolution of the exponent puzzle for the Anderson transition in doped semiconductors

TL;DR

This paper uses ab initio and tight-binding models to study the Anderson metal-insulator transition in doped semiconductors, providing new insights into the critical exponent and resolving the longstanding exponent puzzle.

Contribution

It introduces a realistic model of doped semiconductors and employs multifractal finite-size scaling to clarify the critical exponent of the Anderson transition.

Findings

Estimate of the critical exponent $ u$ for doped silicon.

Identification of hybridization effects explaining the exponent puzzle.

Phase diagram of the metal-insulator transition in doped semiconductors.

Abstract

The Anderson metal-insulator transition (MIT) is central to our understanding of the quantum mechanical nature of disordered materials. Despite extensive efforts by theory and experiment, there is still no agreement on the value of the critical exponent $\nu$ describing the universality of the transition --- the so-called "exponent puzzle". In this work, going beyond the standard Anderson model, we employ ab initio methods to study the MIT in a realistic model of a doped semiconductor. We use linear-scaling DFT to simulate prototypes of sulfur-doped silicon (Si:S). From these we build larger tight-binding models close to the critical concentration of the MIT. When the dopant concentration is increased, an impurity band forms and eventually delocalizes. We characterize the MIT via multifractal finite-size scaling, obtaining the phase diagram and estimates of $\nu$. Our results suggest an explanation of the long-standing exponent puzzle, which we link to the hybridization of conduction and impurity bands.