# Anomalous ^{125}Te Nuclear Spin Relaxation Coincident with Charge Kondo Behavior in Superconducting Pb_{1-x}Tl_{x}Te

## TL;DR

This study uses ^{125}Te-NMR to investigate charge fluctuations and Kondo behavior in Pb_{1-x}Tl_{x}Te, revealing a link between valence fluctuations and superconductivity in doped semiconductors.

## Contribution

It provides microscopic evidence of charge Kondo behavior and valence fluctuations associated with Tl doping, connecting these phenomena to superconductivity in Pb_{1-x}Tl_{x}Te.

## Key findings

- Enhanced Knight shift with increasing Tl doping
- Anomalous increase in ^{125}Te relaxation rate near Tl in superconducting samples
- Correlation between resistivity upturn and nuclear spin relaxation anomalies

## Abstract

We report a ^{125}Te-NMR study of single-crystalline Pb_{1-x}Tl_{x}Te for x= 0 (undoped non-superconducting "parent"), 0.35 at% (doped but on the borderline of superconductivity), and 1.0 at% (superconducting, with a critical temperature Tc ~ 1.0 K). The Knight shift in the normal state is enhanced as x increases, corresponding to an increase in the average density of states (DOS). The NMR line-width also increases significantly with increasing x, indicative of a strong spatial variation in the local DOS surrounding each Tl dopant. Remarkably, for the superconducting composition, the ^{125}Te nuclear spin relaxation rate (1/T1T) for Te ions that are close to the Tl dopants is unexpectedly enhanced in the normal state below a characteristic temperature of ~10 K. This temperature coincides with the temperature below which the normal state resistivity experiences an upturn. Such a simultaneous upturn in both the resistivity and (1/T1T) is highly anomalous, and puts Pb_{1-x}Tl_{x}Te in a distinct new class of doped semiconductors. These observations provide microscopic evidence for dynamical charge fluctuations in the absence of paramagnetism, and are consistent with expectations for charge Kondo behavior associated with the Tl dopant ions. In contrast, such anomalies were not detected in the non-superconducting samples (x=0 and 0.35 at%), suggesting a connection between dynamical valence fluctuations and the occurrence of superconductivity in Pb_{1-x}Tl_{x}Te.