Spectroscopic evidence for the convergence of lower and upper valence   bands of PbQ (Q=Te, Se, S) with rising temperature
  We have conducted temperature dependent Angle Resolved Photoemission Spectroscopy (ARPES) study of the electronic structure of n-, p- type PbTe, PbSe and PbS, which are pre- mier thermoelectric materials. Our ARPES measurements on them provide direct evidence for the light hole upper valence bands (UVBs) and the so-called heavy hole lower valence bands (LVBs), and an unusual temperature dependent relative movement between their band maxima leading to a monotonic decrease in the energy separation between LVBs and UVBs with increase in temperature. This enables convergence of these valence bands and consequently, an effective increase in the valley degeneracy in PbQ at higher temperatures, which has long been speculated to be the driving factor behind their extraordinary thermoelectric performance. 
J. Zhao, C. D. Malliakas, D. Bugaris, N. Appathurai, V. Karlapati, D.   Y. Chung, M. G. Kanatzidis, U. Chatterjee
cond-mat.mtrl-sci