There is a number of metallic materials in which atoms can make very extended oscillations around their equilibrium position. Such a situation occurs in particular in materials in which certain atoms are surrounded by an oversized "cage structure" of other atoms. In these cases the atomic vibrations are highly anharmonic and one speaks of a "rattling" motion. In the picture above the effective potential for an atom is illustrated for a conventional harmonic case (left) and a strongly rattling type case with a hard wall potential (right). Such materials are promising as high efficiency, thermoelectric materials, because they have high electrical conductivity, but low thermal conductivity due to the rattling.
The anharmonic motion strongly influences the nuclear spin-lattice relaxation rate and the electrical resistivity of such materials. We have studied the anomalous properties of one of these materials, KOs₂O₆, due to the rattling motion and were able to reproduce the anomalous temperature dependence of the nuclear spin-lattice relaxation rate and the electrical resistivity [1,2]. Our theory predicts that there should be a strong temperature dependence of the phonon spectral function, which was confirmed by inelastic neutron scattering experiments (see H. Mutka et al, Phys. Rev. B 78, 104307 (2008)).