Large and homogeneous mass enhancement in the rattling-induced superconductor KOs$_2$O$_6$

TL;DR

This study reveals unusually large and homogeneous mass enhancements in KOs$_2$O$_6$, attributed to rattling phonons, through Fermi surface analysis and band structure calculations, shedding light on its superconducting properties.

Contribution

It provides the first detailed Fermi surface and effective mass measurements in KOs$_2$O$_6$, highlighting the role of rattling phonons in mass enhancement.

Findings

Effective masses up to 26 m_e observed.

Mass enhancement parameters are large and homogeneous.

Rattling motion of K ions likely causes the mass enhancement.

Abstract

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.