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To assess electron correlation and electron-phonon coupling in the recentlydiscovered beta-pyrochlores KOs2O6 and RbOs2O6, we have performed specific heatmeasurements in magnetic fields up to 14 T. We present data from high qualitysingle crystalline KOs2O6, showing that KOs2O6 is a strong couplingsuperconductor with a coupling parameter lambda_ep \approx 1.0 to 1.6 (RbOs2O6:lambda_ep \approx 1). The estimated Sommerfeld coefficient of KOs2O6, gamma=76to 110 mJ/(mol K^2), is twice that of RbOs2O6 [gamma=44 mJ/(mol K^2)]. Usingstrong-coupling corrections, we extract useful thermodynamic parameters ofKOs2O6. Quantifying lambda_ep allows us to determine the mass enhancement overthe calculated band electronic density of states. A significant contribution inaddition to the electron-phonon term of lambda_c=1.7 to 4.3 is deduced. In aneffort to understand the origin of the enhancement mechanism, we alsoinvestigate an unusual energetically low-lying phonon. There are three phononmodes per RbOs2O6, suggestive of the phonon source being the rattling motion ofthe alkali ion. This dynamic instability of the alkali ions causes largescattering of the charge carriers which shows up in an unusual temperaturedependence of the electrical resistivity.Comment: Accepted for publication in PR

physical review. b, condensed matter and materials physics

Mass enhancement, correlations, and strong-coupling superconductivity in the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>β</mml:mi></mml:math>-pyrochlore<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="normal">K</mml:mi><mml:msub><mml:mi mathvariant="normal">Os</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:mrow></mml:math>

Mass enhancement, correlations, and strong-coupling superconductivity in theβ-pyrochloreKOs2O6