Strong electron-electron interactions in a dilute weakly-localized metal near a metal-to-insulator transition

TL;DR

This study investigates the metal-insulator transition in ultra-thin dopant layers on silicon, revealing dominant electron-electron interactions near the transition that challenge conventional localization interpretations.

Contribution

It demonstrates the realization of a two-dimensional disordered Hubbard model in silicon dopant layers and uncovers the dominance of electron-electron interactions over weak localization effects near the MIT.

Findings

Electron-electron interactions dominate near the MIT.

Weak localization effects are replaced by Zeeman-scaling electron-electron contributions.

Negative conductance contributions challenge Kondo regime interpretations.

Abstract

Because it is easily switched from insulator to metal either via chemical doping or electrical gating, silicon is at the core of modern information technology and remains a candidate platform for quantum computing. The metal-to-insulator transition in this material has therefore been one of the most studied phenomena in condensed matter physics, and has been revisited with considerable profit each time a new fabrication technology has been introduced. Here we take advantage of recent advances in creating ultra-thin layers of Bohr-atom-like dopants to realize the two-dimensional disordered Hubbard model at half-filling and its metal-to-insulator transition (MIT) as a function of mean distance between atoms. We use gas-phase dosing of dopant precursor molecules on silicon to create arsenic and phosphorus $\delta$-layers as thin as 0.4~nm and as dilute as 10$^{13}$~cm$^{-2}$. On approaching the insulating state, the conventional weak localization effects, prevalent at high dopant densities and due to orbital motion of the electrons in the plane, become dominated by electron-electron interaction contributions which obey a paramagnetic Zeeman scaling law. The latter make a negative contribution to the conductance, and thus cannot be interpreted in terms of an emergent Kondo regime near the MIT.