Superconductivity in Tl-doped PbTe

Negative-U effects have long been discussed as a possible pairing mechanism for superconductivity, but there have been very few, if any, model systems to test these ideas. Valence skipping elements, such as Tl, provide experimental access to these effects in bulk materials.

PbTe is a small gap semiconductor. It can be doped to degeneracy by either Pb vacancies or third-element dopants, with typical carrier concentrations in the range of 1018 - 1020 cm-3. By comparison with similar semiconducting materials, such as SnTe, GeTe and InTe, it was previously anticipated that doped PbTe would only superconduct below approximately 0.01 K, if at all. This has been found to be the case for all dopants except thallium, for which superconductivity was observed with critical temperatures up to 1.5 K, two orders of magnitude higher than anticipated given the modest carrier concentrations. Given the anomalously high Tc values for Tl-doped PbTe, there has been considerable discussion as to the role of the Tl impurities in this material, whether they act as negative-U centers, and specifically whether such impurities can enhance an incipient tendency towards superconductivity.

In this talk I will present results from a systematic study of the thermodynamic and transport properties of single crystals of Tl-doped PbTe. I will discuss evidence that the Tl impurities do indeed act as negative-U centers, including evidence for an unconventional charge Kondo effect at low temperatures. These observations strongly support the notion that the negative-U character of the Tl impurities plays a significant role in the superconductivity of this material.