125Te-NMR Study in Novel Superconductor Pb1−xTlxTe with Valence Skipping Dopants
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Pb1−xTlxTe is an anomalous low carrier density superconductor for which the origin of the superconductivity is not understood. Thallium is the only dopant to cause superconductivity in PbTe, suggesting that these specific impurities have a unique effect on the electronic states. Systematic 125Te-NMR studies on single-crystallines Pb1−xTlxTe (x = 0, 0.35, 1.0 at%) reveal that the Tl dopants induce spatially inhomogeneous electronic states around the Tl dopants. In the superconducting sample at x = 1.0 at%, 125Te nuclear spin relaxation rate (1/T1T) in the vicinity of the Tl dopants is unexpectedly enhanced below \(\sim \)10 K. This coincides with the temperature below which the resistivity exhibits an upturn. By contrast, such anomalies were not detected in the non-superconducting sample at x = 0.35 at%. These observations can be consistently explained by charge Kondo effect, in which two nearly degenerate valence states of the Tl dopant form a resonating valence state upon cooling below 10 K. Based on these observations, we suggest that the coherent hopping of 6s-electron pair may provide the pairing interaction (i.e., a negative-U scenario), leading to the anomalously high Tc found for Pb1−xTlxTe.
KeywordsSuperconductivity Charge Kondo effect Valence skipper NMR
We thank H. Matsuura for valuable comments.
This work was supported by Izumi Science and Technology Foundation, Toyota Riken Scolar, and JSPS KAKENHI (Grant Nos. 26400356, 26610102, 16H04013, 25400369, 17K05555, and 18K18734). The work at Stanford University was supported by AFOSR Grant No. FA9550-09-1-0583.