Theoretical study of NMR relaxation due to rattling phonons

TL;DR

This paper provides a theoretical analysis of NMR relaxation caused by anharmonic rattling phonons, using a square-well potential model to derive temperature-dependent relaxation rates and compare with experimental data.

Contribution

It introduces an analytical model for anharmonic rattling phonons and predicts a peak in NMR relaxation rate temperature dependence, aligning with experimental observations.

Findings

NMR relaxation rate peaks at certain temperatures.

Analytical expressions for low and high temperature limits.

Qualitative agreement with experimental NMR data on KOs2O6.

Abstract

We calculate the NMR relaxation rate due to quadrupolar coupling of the nucleus to a local, strongly anharmonic phonon mode. As a model potential for a rattling motion we consider a square-well potential. We calculate the free phonon Green's function analytically and derive the low and high temperature limits of the NMR relaxation rate. It is shown that the temperature dependence of the NMR relaxation rate possesses a peak in contrast to harmonic phonons but in qualitative agreement with a recent NMR study on KOs2O6. We discuss the influence of phonon renormalization due to electron-phonon interaction.