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

Abstract

Muon spin rotation and relaxation (μSR) techniques have been applied to high‐T_C and related systems. In the antiferromagnets La₂CuO_4−y and YBa₂Cu₃O_6.0–6.4, the muon precession frequency in zero field indicates that the microscopic static ordered moments do not change much despite drastic decrease of the Néel 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_1.85Sr_0.15CuO₄ and YBa₂Cu₃O_6.5–7.0, 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_C∝σ∝1/λ²∝n_s/m* between the transition temperature T_C and the carrier density n_s, which suggests a high‐energy scale for the pairing interaction.