Intrinsic thermodynamic properties of the pyrochlore superconductor RbOs2O6 extracted by condensation energy analysis -: art. no. 214517

We develop a general procedure for the analysis of bulk thermodynamic data of a superconductor for samples containing a metallic nonsuperconducting second phase. The method is based on the condensation energy and it allows the extraction of the intrinsic properties of a superconductor even for nonideal samples. Applying this procedure to the recently discovered geometrically frustrated beta-pyrochlore superconductor RbOs2O6 (T-c=6.4 K) yields a Sommerfeld coefficient as high as 79 mu J/g/K-2 (44 mJ/mol(f.u.)/K-2). RbOs2O6 is inferred to be a strong type-II superconductor [kappa(T-c)=23] in the intermediate-coupling regime similar to niobium (lambda(ep)approximate to 1). From the upper critical field mu H-0(c2)approximate to 6 T at 0 K, we estimate a Ginzburg-Landau coherence length xi approximate to 74 A. The condensation energy is 860 mu J/g (483 mJ/mol(f.u.)) resulting in 1/(8 pi)center dot(gamma T-1(c)2)/Delta U-1 approximate to 0.15, a value well in the range of conventional phonon-mediated superconductors. The superconducting electronic specific heat indicates conventional s-wave pairing. The experimental Sommerfeld coefficient of 44 mJ/mol(f.u.)/K-2 is about four times larger than the one found in band structure calculations. Together with the electron-phonon coupling constant lambda(ep)approximate to 1 this leaves an additional lambda(add)approximate to 2.1 for enhancement due to other mechanisms.