NMR Observation of Rattling Phonons in the Pyrochlore Superconductor KOs2O6

TL;DR

This study uses NMR to observe anharmonic rattling phonons in KOs2O6, revealing their behavior change with temperature and their role in strong electron-phonon coupling in this superconductor.

Contribution

It provides direct NMR evidence of rattling phonons and their temperature-dependent dynamics in KOs2O6, highlighting their impact on superconductivity.

Findings

Rattling phonons cause nuclear relaxation at K sites.

Crossover from overdamped to underdamped rattling with decreasing temperature.

Absence of Hebel-Slichter peak indicates strong electron-phonon coupling.

Abstract

We report nuclear magnetic resonance studies on the beta-pyrochlore oxide superconductor KOs2O6. The nuclear relaxation at the K sites is entirely caused by fluctuations of electric field gradient, which we ascribe to highly anharmonic low frequency oscillation (rattling) of K ions. A phenomenological analysis shows a crossover from overdamped to underdamped behavior of the rattling phonons with decreasing temperature and its sudden sharpening below the superconducting transition temperature Tc. Absence of the Hebel-Slichter peak in the relaxation rate at the O sites below Tc also indicates strong electron-phonon coupling.