Revealing puddles of electrons and holes in compensated topological insulators

TL;DR

This study uncovers electron-hole puddles in the bulk of compensated topological insulators through optical conductivity measurements, revealing their temperature-dependent behavior and underlying Coulomb potential fluctuations.

Contribution

It demonstrates the presence of bulk electron-hole puddles in topological insulators and explains their temperature evolution via non-linear screening effects from thermally activated carriers.

Findings

Electron-hole puddles exist in the bulk at low temperatures.

Puddles diminish rapidly and nearly vanish around 40 K.

Monte Carlo simulations support Coulomb interaction-driven puddle destruction.

Abstract

Three-dimensional topological insulators harbour metallic surface states with exotic properties. In transport or optics, these properties are typically masked by defect-induced bulk carriers. Compensation of donors and acceptors reduces the carrier density, but the bulk resistivity remains disappointingly small. We show that measurements of the optical conductivity in BiSbTeSe$_2$ pinpoint the presence of electron-hole puddles in the bulk at low temperatures, which is essential for understanding DC bulk transport. The puddles arise from large fluctuations of the Coulomb potential of donors and acceptors, even in the case of full compensation. Surprisingly, the number of carriers appearing within puddles drops rapidly with increasing temperature and almost vanishes around 40 K. Monte Carlo simulations show that a highly non-linear screening effect arising from thermally activated carriers destroys the puddles at a temperature scale set by the Coulomb interaction between neighbouring dopants, explaining the experimental observation semi-quantitatively. This mechanism remains valid if donors and acceptors do not compensate perfectly.