ESR in YBa2Cu3O7-δsingle crystals

Abstract
Reports ESR measurements in single crystals of the high-temperature superconductor YBa2Cu3O7- delta . Measurements were carried out both on a superconducting single crystal with Tc=89 K, and on a non-superconducting sample. No ESR signal was found in either sample at 300 K, nor at 4.2 K. These results show irrefutably that there are no unpaired localised spins in the 90 K superconductor YBa2Cu3O7- delta . It also implies that the earlier reported ESR signals in powders of the 90 K superconducting ceramics YBa2Cu3O7- delta come from an impurity phase.