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.