A narrow-gap semiconductor PbTe exhibits superconductivity by a small amount of substitution of Tl for Pb, when x exceeds xc ~ 0.3% in Pb1-xTlxTe[1]. Its superconducting Temperature(Tc) reaches 1 K for x ~ 1.0%. The dopant (Tl) is known to take either Tl1+(6s2) or Tl3+(6s0), but to skip an intermediate valence Tl2+(6s1). In Pb1-xTlxTe, the hole density np, that is evaluated by Hall-coefficient measurement, shows a linear increase up to x≦ xc, indicating the low valence substitution of Tl1+ for Pb2+. As x increases further, however, the increase in np is gradually saturated above xc, indicating that two states of Tl1+(hole-doping) and Tl3+(electron-doping) are nearly degenerate [1-3]. The fact that a logarithmic upturn in resistivity at low temperature below 10 K is observed for x > xc reminds us of the “spin” Kondo effect [1]. It is possible that two degenerate charge states of Tl1+(2e-) and Tl3+ (0e-) form a resonating valence state, which has been theoretically accounted for by “charge” Kondo effect in analogy with two degenerate spin states in “spin" Kondo effect. It should be noted that the “charge" Kondo effect is observed only in the superconducting samples with x > xc, possibly leading to the discovery of a new unconventional superconducting mechanism in terms of the “negative-U" model that introduces seemingly an on-site attractive interaction [3-6].
The previous 125Te-NMR measurements in Pb1-xTlxTe (x = 0, 0.35, 1.0 %) has investigated local electronic states around valence skipping Tl dopants from microscopic points of view [7]. The 125Te nuclear spin relaxation rate (1/T1T) near Tl dopants in the superconducting sample with Tc ~ 1 K at x = 1.0% is unexpectedly enhanced below 10 K, which coincides with the temperature below which the resistivity experiences an upturn, suggesting the existence of two nearly degenerate valence states of valence skipping dopants with Tl1+(6s2) and Tl3+(6s0). Here we present the 125Te-NMR measurements in Pb1-xNaxTe (x = 1.45%) with non-valence-skipping dopant (Na1+) in order to clarify the role of valence-skipping Tl dopants for the upturn of 1/T1T at low temperature below 10 K, which is possibly associated with the “charge" Kondo effect. In the Na-doped sample, 125Te-1/T1T stays almost constant below 50 K down to 2 K. Moreover, the extremely large enhancement of 1/T1T below 10 K in Pb0.99Tl0.01Te has been directly observed by the on-site 205Tl-NMR measurement, demonstrating that the upturn of 1/T1T below 10 K is induced by valence-skipping Tl dopants. These microscopic experimental evidences are consistent with a model of "charge" Kondo effect that is theoretically proposed.
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[6] H. Matsuura and K. Miyake, J. Phys. Soc. Jpn. 81, 113705 (2012).
[7] H. Mukuda et al. , J. Phys. Soc. Jpn. 87, 023706 (2018).
Keywords: Charge Kondo effect, Superconductivity, NMR