Mass enhancement, correlations, and strong coupling superconductivity in the beta-pyrochlore KOs2O6

TL;DR

This study investigates electron correlations and strong coupling superconductivity in beta-pyrochlore KOs2O6 through specific heat measurements, revealing significant mass enhancement and low-lying phonon modes linked to alkali ion rattling.

Contribution

The paper provides the first detailed thermodynamic analysis of KOs2O6, quantifying electron-phonon coupling and mass enhancement, and identifies low-energy phonons related to alkali ion rattling.

Findings

KOs2O6 is a strong coupling superconductor with lambda_ep between 1.0 and 1.6.

The Sommerfeld coefficient gamma is significantly higher in KOs2O6 than in RbOs2O6.

Unusual low-energy phonon modes suggest alkali ion rattling as a scattering mechanism.

Abstract

To assess electron correlation and electron-phonon coupling in the recently discovered beta-pyrochlores KOs2O6 and RbOs2O6, we have performed specific heat measurements in magnetic fields up to 14 T. We present data from high quality single crystalline KOs2O6, showing that KOs2O6 is a strong coupling superconductor with a coupling parameter lambda_ep \approx 1.0 to 1.6 (RbOs2O6: lambda_ep \approx 1). The estimated Sommerfeld coefficient of KOs2O6, gamma=76 to 110 mJ/(mol K^2), is twice that of RbOs2O6 [gamma=44 mJ/(mol K^2)]. Using strong-coupling corrections, we extract useful thermodynamic parameters of KOs2O6. Quantifying lambda_ep allows us to determine the mass enhancement over the calculated band electronic density of states. A significant contribution in addition to the electron-phonon term of lambda_c=1.7 to 4.3 is deduced. In an effort to understand the origin of the enhancement mechanism, we also investigate an unusual energetically low-lying phonon. There are three phonon modes per RbOs2O6, suggestive of the phonon source being the rattling motion of the alkali ion. This dynamic instability of the alkali ions causes large scattering of the charge carriers which shows up in an unusual temperature dependence of the electrical resistivity.