The curious case of the Cu²⁺ electron paramagnetic resonance in high-T_c superconductors and related antiferromagnets

Possible reasons behind the total absence of EPR signals from divalent copper ions in copper oxide superconductors and related "two-dimensional" antiferromagnetic insulators are explored. The non-observability of the signal in the metallic states can plausibly be attributed to short spin-lattice relaxation times due to spinon-holon scatterings. However, the absence of resonance in the insulators at high temperatures suggests that either the correlation lengths have been grossly underestimated even for compounds with very low Neel temperatures, or straightforward application of the conventional theory of antiferromagnetism is not valid. © 1989.