Charge Kondo Effect Induced by Valence Skipping Dopants in Pb_1−xTl_xTe and Pb_1−xNa_xTe Probed by ¹²⁵Te-NMR

Abstract

We have performed schematic 125 Te-NMR measurements in Pb 1−x Tl x Te (x = 0, 0.35, 1.0%) and Pb 1−x Na x Te (x = 0.46, 1.45%). Superconductivity occurs above x ∼ 0.3% in Pb 1−x Tl x Te and superconducting temperature T c reaches about 1 K at x = 1.0%. In Pb 0.99 Tl 0.01 Te, the 125 Te nuclear spin relaxation rate (1/T 1 T) for Te sites near Tl dopants is unexpectedly enhanced in the normal state below a characteristic temperature of ∼ 10 K, below which the resistivity experiences an upturn. In contrast, no enhancement of 1/T 1 T is observed at low temperatures for Te sites both near and far from non-valence-skipping Na dopants in Pb 1−x Na x Te with x = 1.45%. These results suggest the existence of valence fluctuations associated with the charge Kondo effect arising from Tl dopants in the superconducting sample Pb 1−x Tl x Te with x = 1.0%.

Valence skipping phenomena, charge Kondo effect, and superconductivity

“…Motivated by these bulk experiments and theoretical insights, the local electronic states around Tl dopants were investigated microscopically by means of NMR probes. In this subsection, we review 125 Te-NMR studies on Pb 1−x Tl x Te [5,26,27], revealing the unusual character of the local electronic states introduced by Tl dopants through systematic measurements of the Knight shift (K) and the nuclear spin relaxation rate (1/T 1 T). Figure 10a shows the 125 Te-NMR spectra for x = 0, 0.35, and 1.0%.…”

“…In this subsection, we review the experimental results on Tl-doped PbTe that exhibits superconductivity and Kondo-like effect, which is one of the promising candidates for novel superconducting (SC) mechanism related to the valence skipping phenomena [3,5,[23][24][25][26][27].…”