Title: Valence Fluctuations and Unusual Correlations in Superconducting Pb1-xTlxTe.
When: Thursday, April 19, (2018), 12:00.
Place: Department of Condensed Matter Physics, Faculty of Sciences, Module 3, Seminar Room (5th Floor).
Speaker: Phil Walmsley, Geballe Laboratory for Advanced Materials, and Department of Applied Physics, McCullough Bldg. Stanford University, USA.
The narrow-band semiconductor PbTe superconducts with a Tc an order of magnitude greater than comparable low-density metals, but only when it is doped with valence-skipping elements. There is strong experimental evidence that upon hole-doping PbTe with Tl the onset of superconductivity occurs as the chemical potential is tuned to a point of degeneracy of the Tl 1+ and 3+ valences. At these compositions the Tl dopants can dynamically skip valence by filling and emptying their 6s shell of two electrons simultaneously, naturally raising the question of whether Tl dopants can act as a negative U center to produce the enhanced superconductivity seen in this compound.
Here we present a study of the quantum oscillations, electrical transport and specific heat of hole-doped PbTe in which we use a series of non-superconducting, non-valence skipping Na doped samples as a control system for the Tl doped series described above. We identify the presence of an unusual 2e resonant impurity state that is unique to the superconducting portion of the Tl series, and appears to be enhancing, or even introducing, the pairing interaction in this material. This realization of a pairing interaction that is introduced extrinsically to a host material could guide the design of improved superconducting materials.