ON THE ABSENCE OF EPR IN CUPRATE SUPERCONDUCTORS AND THEIR ANTIFERROMAGNETIC ANTECEDENTS

We propose here an explanation for the absence of intrinsic Electron Paramagnetic Resonance (EPR) in high-Tc ceramic cuprate superconductors in their normal state. This phenomenon is compared to and contrasted with the absence of a Cu2+ resonance in the related semiconducting, antiferromagnetic cuprates such as La2CuO4. We report structural data, and the absence of EPR, for the series La2Cu(1-x)Zn(x)O4 and Nd2Cu(1-x)Zn(x)O4 for x = 0 to 0.1. It has previously been reported that antiferromagnetic order is rapidly suppressed by the substitution of Zn onto Cu sites in La2CuO4. These Zn substituted materials (and the related Nd2Cu(1-x)Zn(x)O4 series) exhibit no EPR signals from 77K to room temperature. Most notably, the absence of EPR for these cuprates in which the Neel temperature is less than, or comparable to, the observation temperature reflects the strong 2D antiferromagnetic coupling within the CuO2 planes.