Hironori Sakai

Superconductivity in a pyrochlore oxide, Cd2Re2O7

We make the first report that a metallic pyrochlore oxide, Cd2Re2O7, exhibits type II superconductivity at 1.1xa0K. 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 0xa0K, is estimated as about 0.8xa0T, 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.

Physical properties of new superconductor Cd2Re2O7 with pyrochlore-type structure

Abstract 111 Cd NMR study has been performed in the normal state of superconducting pyrochlore oxide Cd2Re2O7. A large orbital susceptibility is identified from K–χ analysis. The sharp decreases of both K and T1−1 below T ∗ suggests a decrease in density of states which is apparently caused by a developing partial energy gap at the Fermi surface.

Superconductivity in pyrochlore oxide Cd2Re2O7

Abstract The pyrochlore oxide Cd 2 Re 2 O 7 shows superconductivity at 1.1 K. The upper critical field H C2 extrapolated to 0 K is estimated to be about 0.8 T. The electronic specific heat coefficient γ is 14.7 mJxa0K −2 /Re mol. Our experimental results indicate that the Cooper pairs are composed of rather heavy quasiparticles.

Synthesis and physical properties of pyrochlore iridium oxides Pb2−xCaxIr2O7−y

Abstract Pyrochlore iridium oxides Pb2−xCaxIr2O7−y (x=0.0,0.2,0.4,0.6,0.8) were prepared by solid state reaction. The electrical resistivity and the magnetic susceptibility were measured between 4.2 and 300 K . These compounds show metallic conductivity and obey the Curie–Weiss law with temperature independent term. The metallic conductivity of Pb2Ir2O7 is suppressed by the Ca substitution for Pb site.

Thermoelectric power of the superconducting pyrochlore Cd2Re2O7

We report the thermoelectric power (S) of the recently discovered superconducting pyrochlore Cd2Re2O7 with Tc = 1.4?K. In the normal state, this compound shows a continuous structural phase transition at T* = 200?K. S is negative from room temperature down to 1.4?K. Above T*, S shows small absolute values and approximatively linear dependence on temperature. S(295?K) = -2.2??V?K-1. The absolute value of S increases rapidly below T*. A kink and a minimum were observed round 125 and 60?K, respectively. S(60?K) = -8.2??V?K-1. In the low-temperature range 1.4?K<T<20?K, another linear portion was observed. As regards the T2-dependent electrical resistivity in this temperature range, a Fermi liquid ground state is suggested for this compound. No change of S in a magnetic field up to 2?T was observed at 77 and 295?K.

Ga and Pt NMR study of UPtGa5

Abstract 69,71 Ga and 195 Pt NMR/NQR measurements have been carried out for the 5f-antiferromagnet, UPtGa 5 . From a NMR study using a single crystal sample, Knight shift measurements are reported for two different Ga sites and for Pt. The principal axes of the electrical field gradient tensor at both Ga sites have been determined and the values of the splitting parameter ν Q and the asymmetry parameter η have been evaluated. The hyperfine coupling constants with the external field along various directions are also reported for the two Ga sites and Pt.

99,101Ru NMR study of EuSr2RuCu2O8

The copper-ruthenium oxide, EuSr 2 RuCu 2 O 8 has been studied by means of nuclear magnetic resonance (NMR) and nuclear spin-lattice relaxation rate (T -1 1 ) measurements. This compound shows the ferromagnetic-like transition at T M ∼ 130 K and the superconducting transition at T onset ∼ 30 K. 99,101 Ru NMR signals under zero applied field have been observed at high frequencies around 60 MHz and around 130 MHz, both of which could be identified as ferromagnetic resonances even below T onset c . These frequency-swept spectra around 60 MHz and around 130 MHz are ascribed to 99,101 Ru nuclei of Ru 4+ (S = 1) and to those of Ru 5+ (S = ), respectively. The nuclear spin-lattice relaxation rate T -1 1 for 101 Ru nuclei decreases just below T onset c due to the formation of a superconducting gap. Our NMR results are supported microscopically that coexistence of the ferromagnetism and superconductivity is realized in this compound.

NMR study of metallic pyrochlore ruthenium oxides, Bi2Ru2O7 and Pb2Ru2O7−δ

Abstract To elucidate the magnetism of metallic pyrochlore oxides, the Knight shift and nuclear spin–lattice relaxation rate have been measured in the metallic pyrochlore oxides, Bi 2 Ru 2 O 7 and Pb 2 Ru 2 O 7− δ . The temperature dependence on T 1 −1 obeys a modified Korringa relation. The correlation factors in the Korringa relation indicate the existence of antiferromagnetic spin fluctuations in the metallic state of these compounds.

NMR study of Na3−xRu4O9

Abstract Low-dimensional conductor Na3−xRu4O9 has been studied by means of 23Na nuclear magnetic resonance measurement in order to clarify the electronic state of this compound. The magnetic susceptibility χ is almost constant from 100 to 300xa0K, then increases with temperature, and shows a kink at 584xa0K. The temperature dependence of the Knight shift K is almost in proportion to χ. From the result of the temperature dependence of spin–lattice relaxation time, it has been found that this compound has an energy gap, Δ, which is closely correlated with the gap-like behavior of χ. Consequently, we consider that there exists a spin-singlet state in all or a part of RuO6 chains in Na3−xRu4O9.