Synthesis of superconducting pyrochlore RbOs2O6

RbOs2O6, the third superconducting pyrochlore oxide (known so far), has been synthesized by encapsulation and by high pressure techniques. Suitable post chemical treatment of the as-prepared sample allowed us to eliminate the impurity phases. Bulk superconductivity with Tc=6.4 K was observed in magnetisation and specific heat measurements. The transition temperature of RbOs2O6 was found to be the same for both preparation methods. Structural investigations showed that Rb atoms occupy the 8b site in the pyrochlore lattice with a lattice parameter of 10.1137(1) A.Comment: 12 pages, 5 figures, correction to one author name and Figure

Thermal Conductivity of the Pyrochlore Superconductor KOs2O6: Strong Electron Correlations and Fully Gapped Superconductivity

To elucidate the nature of the superconducting ground state of the geometrically frustrated pyrochlore KOs2O6 (Tc=9.6K), the thermal conductivity was measured down to low temperatures (~Tc/100). We found that the quasiparticle mean free path is strikingly enhanced below a transition at Tp=7.5K, indicating enormous electron inelastic scattering in the normal state. In a magnetic field the conduction at T ->0K is nearly constant up to ~0.4Hc2, in contrast with the rapid growth expected for superconductors with an anisotropic gap. This unambiguously indicates a fully gapped superconductivity, in contrast to the previous studies. These results highlight that KOs2O6 is unique among superconductors with strong electron correlations.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. Let

NMR Observation of Rattling Phonons in the Pyrochlore Superconductor KOs2O6

We report nuclear magnetic resonance studies on the beta-pyrochlore oxide superconductor KOs2O6. The nuclear relaxation at the K sites is entirely caused by fluctuations of electric field gradient, which we ascribe to highly anharmonic low frequency oscillation (rattling) of K ions. A phenomenological analysis shows a crossover from overdamped to underdamped behavior of the rattling phonons with decreasing temperature and its sudden sharpening below the superconducting transition temperature Tc. Absence of the Hebel-Slichter peak in the relaxation rate at the O sites below Tc also indicates strong electron-phonon coupling.Comment: 4 pages, 3 figure

Unprecedented Superconductivity in the beta-Pyrochlore Osmate KOs2O6

Superconductivity in the potassium osmium oxide KOs2O6 crystallizing in the beta-pyrochlore structure is studied by means of electrical resistivity, magnetic susceptibility and specific heat. It is the second superconductor in the family of pyrochlore oxides, following the alfa-type pyrochlore oxide Cd2Re2O7 which is believed to be a conventional s-wave superconductor. The superconducting transition temperature Tc of KOs2O6 is 9.6 K, almost one order higher than the Tc = 1.0 K of Cd2Re2O7. Moreover, the superconductivity of KOs2O6 is remarkably robust under high magnetic fields, with a large upper critical magnetic field Hc2 of about 38 T, which seems to exceed Pauli's limit expected for conventional superconductivity. This is also in contrast to the case of Cd2Re2O7, in which the Hc2 is 0.29 T, much smaller than the corresponding Pauli's limit. These distinct contrasts strongly suggest that the mechanism of superconductivity is essentially different between the two pyrochlore oxides.Comment: This paper has been withdrawn by the authors, due to a crucial error in the magnitude of magnetic fields used in the experiment