We make the first report that a metallic pyrochlore oxide, Cd2Re2O7, exhibits type II superconductivity at 1.1 K. The pyrochlore oxide is known to be a geometrically frustrated system, which includes a tetrahedral network of magnetic ions. A large number of compounds are classified in the family of pyrochlore oxides, and these compounds exhibit a wide variety of physical properties ranging from insulator through semiconductor and from bad metal to good metal. Until now, however, no superconductivity has been reported for frustrated pyrochlore oxides. The bulk superconductivity of this compound is confirmed by measurements of the resistivity and the alternating-current magnetic susceptibility. The upper critical field Hc2, which is extrapolated to 0 K, is estimated as about 0.8 T, using the resistivity measurements under an applied field. The plot of Hc2 versus T indicates that the Cooper pairs are composed of rather heavy quasiparticles. This fact suggests that frustrated heavy electrons become superconducting in this compound.

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We have performed the 125 Te-nuclear magnetic resonance (NMR) measurement in the field along the b axis on the newly discovered superconductor UTe 2 , which is a candidate of a spin-triplet superconductor. The nuclear spin-lattice relaxation rate divided by temperature 1/T 1 T abruptly decreases below a superconducting (SC) transition temperature T c without showing a coherence peak, indicative of UTe 2 being an unconventional superconductor. It was found that the temperature dependence of 1/T 1 T in the SC state cannot be understood by a single SC gap behavior but can be explained by a two SC gap model. The Knight shift, proportional to the spin susceptibility, decreases below T c , but the magnitude of the decrease is much smaller than the decrease expected in the spin-singlet pairing. Rather, the small Knight-shift decrease as well as the absence of the Pauli-depairing effect can be interpreted by the spin triplet scenario.

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