Superconductivity in the β-pyrochlore osmates

To elucidate the effect of the mass renormalization on the superconducting state in AOs2O6, we have performed thermodynamic and transport measurements on KxRb1-xOs2O6 with x = 0-1. We show that KOs2O6 (RbOs2O6) is an intermediate to strong-coupling superconductor with a coupling parameter λep ∼ 1.2 (λep ∼ 1). Quantifying λep allows us to determine the mass enhancement over the calculated band electronic density of states in addition to the electron-phonon term. We correlate the additional enhancement with the upper critical field slope which is determined by orbital magnetism. In the normal state, the charge carriers scatter at a low-energetic phonon. This rattling motion appears as an Einstein contribution with three modes per Rb in the RbOs2O6 specific heat data, suggesting the rattler being the alkali-ion