Large and homogeneous mass enhancement in the rattling-induced   superconductor KOs$_2$O$_6$
  We have determined the Fermi surface in KOs$_2$O$_6$ ($T_c$ = 9.6 K and $B_{c2} \sim$ 32 T) via de Haas-van Alphen (dHvA) oscillation measurements and a band structure calculation. We find effective masses up to 26(1) $m_e$ ($m_e$ is the free electron mass), which are unusually heavy for compounds where the mass enhancement is mostly due to electron-phonon interactions. Orbit-resolved mass enhancement parameters $\lambda_{dHvA}$ are large but fairly homogeneous, concentrated in the range 5 -- 8. We discuss origins of the large homogeneous mass enhancement in terms of rattling motion of the K ions. 
Taichi Terashima, Nobuyuki Kurita, Andhika Kiswandhi, Eun-Sang Choi,   James S. Brooks, Kota Sato, Jun-ichi Yamaura, Zenji H...
cond-mat.supr-con cond-mat.str-el