Muon spin relaxation studies on high-T/sub C/ superconductors and related antiferromagnets (invited)

Muon spin rotation and relaxation (μSR) techniques have been applied to high-T/sub C/ and related systems. In the antiferromagnets La2CuO/sub 4-//sub y/ and YBa2Cu3O/sub 6.0//sub --//sub 6.4/, the muon precession frequency in zero field indicates that the microscopic static ordered moments do not change much despite drastic decrease of the Neel temperature with increasing oxygen content. In the family systems of the new three-dimensional superconductor (BaK)BiO3, no signature of static magnetic order was found by μSR, in sharp contrast to the two-dimensional systems with CuO planes. In the superconducting specimens La/sub 1.85/Sr/sub 0.15/CuO4 and YBa2Cu3O/sub 6.5//sub --//sub 7.0/, the magnetic field penetration depth λ was determined from the muon spin relaxation rate σ measured in transverse external magnetic fields. The temperature dependence of λ indicates a predominantly finite energy gap. The systematic variation of λ leads to an approximately linear relation T/sub C/proportionalσproportional1/λ2proportionaln/sub s//m* between the transition temperature T/sub C/ and the carrier density n/sub s/, which suggests a high-energy scale for the pairing interaction