Observation of a semimetal-semiconductor phase transition in the intermetallic ZrTe5

Temperature dependent high-resolution angle-resolved photoelectron spectroscopy has been performed on the quasi-two-dimensional compound ZrTe5, a metal at low temperatures (T less than or equal to 4.2 K) that exhibits a maximum resistivity at a temperature (T-C), concomitant with a sign change of the thermopower. A semiconducting gap has been observed in the photoemission spectra, where the valence band maximum shifts upward from 82 meV (75 K) to 40 meV (170 K) as a function of temperature. The band shifts are accompanied by small band distortions. Based on the photoemission experiments, in conjunction with the metallic character of ZrTe5 at low temperatures, we have modelled the thermopower of ZrTe5 by treating it as a metal at low temperatures and a semiconductor at elevated temperatures.