Resistivity and superconductivity from anharmonic phonons

Temperature dependence of alkali-metal rattling dynamics in the β-pyrochlores, AOs2O6 (A = K, Rb, Cs), from MD simulation

We investigate the temperature response of the alkali-metal rattling modes in β-pyrochlores, AOs2O6 (A = K, Rb, Cs), from the results of ab initio molecular dynamics (MD) simulations performed at 20 K, 100 K and 300 K. Our results show that the temperature response of the T1u mode is clearly different from that of the T2g mode for all three pyrochlores. In this regard, two features are of particular note for both K and Rb; (1) the T1u mode exhibits a distinctly stronger softening response with decreasing temperature compared to the T2g mode, and (2) the T1u mode becomes stronger and sharper with decreasing temperature. These two findings suggest that the T1u mode is significantly more anharmonic and sensitive to the cage dynamics than the T2g mode. Examination of the local potentials around the alkali-metal atoms reveals that K has the flattest and most anharmonic potential at all temperatures while Cs exhibits the narrowest potential. The temperature dependence of the local potentials reveals that, for K, the potential at a higher temperature is not a simple extrapolation to higher energy of that at a lower temperature. Instead, we find significant reconstruction of the potential at different temperatures. Finally, we explore the temperature response of the coupling between the alkali metals and find a complex temperature dependence which suggests that the origin of the coupling may be more complex than a pure Coulomb interaction. We also find an unexpected increase in the static disorder of the system at low temperatures for the K and Rb pyrochlores.

NMR relaxation and resistivity from rattling phonons in pyrochlore superconductors

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 T2 at low temperatures and following a square root T behavior at high temperatures. Both results are in qualitative agreement with recent observations on beta-pyrochlore oxide superconductors.