Dependence of the low-temperature specific heat of YBa2Cu3O7-delta on delta and magnetic field: Concentrations of spin-2 and spin-1/2 paramagnetic centers

Specific heat data on two samples of YBa2Cu3O7-delta with relatively low concentrations of paramagnetic centers show the presence of both spin-2 and spin-1/2 moments. Measurements on one sample following incremental increases in delta show that for delta less than or equal to 0.2 approximately one spin-1/2 moment is produced for each 100 O vacancies, with no substantial increase in the linear term of the specific heat. For several reasons it is concluded that these O-vacancy related moments are different from those that have figured in previously reported correlations with parameters characteristic of the superconductivity.