Item

Abstract

Low temperature photoluminescence spectra of synthetic PbS crystals
having the natural isotopic abundance (95% S-32) are compared with
crystals grown using 99% S-34. An anomalous decrease in the band gap
energy of 6.5 +/- 2 cm(-1) with increasing S mass was observed. The
unusual sign of this isotope shift can be related to the temperature
dependence of the band gap energy, which opposite to the behavior seen
for most common semiconductors, increases strongly between 2 K and 320
K. This temperature dependence was measured with improved accuracy
using absorption spectroscopy, and a fit to this data suggests that
there should be virtually no net renormalization of the band gap energy
due to zero point motion at low temperatures. This must result from a
cancellation of the contributions from S and Pb, but the predicted
"normal" isotope shift of the band gap energy with Pb mass is too small
to be measured.