Spectroscopic evidence for temperature-dependent convergence of light and heavy hole valence bands of PbQ (Q=Te, Se, S)

TL;DR

This study uses temperature-dependent ARPES to reveal how light and heavy hole valence bands in PbQ compounds converge with increasing temperature, shedding light on their thermoelectric properties.

Contribution

It provides direct experimental evidence of band convergence in PbQ materials and details the temperature-dependent behavior of valence bands.

Findings

Observation of light and heavy hole valence bands in PbQ.

Temperature-dependent decrease in energy separation between bands.

Band convergence correlates with thermoelectric performance.

Abstract

We have conducted temperature dependent Angle Resolved Photoemission Spectroscopy (ARPES) study of the electronic structures of PbTe, PbSe and PbS. Our ARPES data provide direct evidence for the \emph{light} hole upper valence bands (UVBs) and hitherto undetected \emph{heavy} hole lower valence bands (LVBs) in these materials. An unusual temperature dependent relative movement between these bands leads to a monotonic decrease in the energy separation between their maxima with increasing temperature, which is referred as band convergence and has long been believed to be the driving factor behind extraordinary thermoelectric performances of these compounds at elevated temperatures.