We calculate the temperature dependence of the NMR relaxation rate and electrical resistivity for coupling to a local, strongly anharmonic phonon mode. We argue that the two-phonon Raman process is dominating NMR relaxation. Due to the strong anharmonicity of the phonon an unusual temperature dependence is found having a low temperature peak and becoming constant towards higher temperatures. The electrical resistivity is found to vary like $T^2$ at low temperatures and following a $\sqrt{T}$ behavior at high temperatures. Both results are in qualitative agreement with recent observations on $\beta$-pyrochlore oxide superconductors.

• Received 13 June 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.187003