Spin dynamics at planar sites in oxygen-deficient YBa2Cu306+x have been investigated by nuclear spin-lattice relaxation measurements using ^^'^^Cu nuclear quadrupole resonance and ^^Y NMR. A sharp decrease of ^^Cu nuclear relaxation below^ 100 K in YBaiCusOe? suggests the possible onset of spin pairing in individual planes well above the transition to three-dimensional bulk superconductivity. Comparison of the relaxation rates for the 60-K superconductor YBa2Cu306.7 with those of YBa2Cu307.o in the range 100-300 K shows that the planar Cu(2) spin dynamics are strongly affected by changes in carrier concentration.

We study the sawtooth lattice of a coupled spin 1/2 Heisenberg system, a variant of the railroad trestle lattice. The ground state of this system is two-fold degenerate with periodic boundary conditions and supports kink antikink excitations, which are distinct in this case, unlike the railroad trestle lattice. The resulting low temperature thermodynamics is compared with the recently discovered Delafossites Y CuO 2.5 .

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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