Search for anomalous internal magnetic fields in high-Tc superconductors as evidence for broken time-reversal symmetry

Several recent models of high-Tc superconductivity predict static internal magnetic fields along the c axis due to spontaneously broken parity and time-reversal symmetries. Muon spin relaxation has been used to study the internal fields in c-axis-oriented samples of sintered YBa2Cu3O7 and of thick-film Bi2Sr2CaCu2O8. The magnitude, anisotropy, and temperature dependence of the observed fields favor a nuclear dipolar origin. Any anomalous fields at μ+ sites in bulk CuO2 superconductors must be ≲0.08 mT.