Direct spectroscopic evidence for mixed-valence Tl in the low carrier-density superconductor Pb1−xTlxTe

Abstract

Upon doping with Tl the narrow band-gap semiconductor PbTe exhibits anomalously high-temperature superconductivity despite a very low carrier-density as well as signatures of the Kondo effect despite an absence of magnetic moments. These phenomena have been explained by invoking $2e$ fluctuations of the valence of the Tl dopants, but a direct measurement of the mixed valency implied by such a mechanism has not been reported to date. In this work we present the unambiguous observation of multiple valences of Tl in Tl-doped PbTe via photoemission spectroscopy measurements. It is shown via a quantitative analysis that the suppression of the carrier density in compositions exhibiting superconductivity and Kondo-like behavior can be accounted for by mixed valency, thus arguing against a self-compensation scenario proposed elsewhere for this material and strengthening the case for valence fluctuation models. In addition to the identification of ${\mathrm{Tl}}^{+}$ and ${\mathrm{Tl}}^{3+}$ a possible third intermediate local charge-density is suggested by full fits to the data, the origins of which are also discussed but remain unclear.