F. Mehran

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

Abstract 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.

Comparison of the EPR of the antiferromagnets Y2BaCuO5 and Gd2BaCuO5 and the high Tc superconductors YBa2Cu3O7 and GdBa2Cu3O7

Electron paramagnetic resonance and magnetic susceptibility measurements on Gd3+ and Cu2+ ions in the antiferromagnetic “green phase” compounds Gd2BaCuO5 and Y2BaCuO5 are reported and their relationships to corresponding measurements in the “black phase” high Tc superconductors GdBa2Cu3O7 and YBa2Cu3O7 are described.

Absence of excited triplets in the EPR of the high Tc superconductors and the antiferromagnetic insulator CuO

Abstract We have been unable to observe electron paramagnetic resonance signals at temperatures of up to ∼ 570K from either the S = 1 2 Cu 2+ ions or the S = 1 excited states of the Cu2+ - Cu2+ pairs in the new Bi2Sr2CaCu2Ox (2212) high Tc ∼80K superconductors, in the YBa2Cu3Ox (123) high Tc∼90K superconductors, or in the antiferromagnetic insulator CuO with TN∼ 230K. No present theory of the finite spin susceptibility in the paramagnetic state is compatible with the absence of EPR. Short-range resonating-valence-bond (RVB) theory gives zero spin susceptibility at low temperatures, but the absence of the S = 1 excited states of pairs appears to eliminate that theory as well.

Observation by electron spin resonance of a pseudo-cubic site in YBa2Cu3O7−x.

Abstract We report electron paramagnetic resonance experiments in single crystals of the high Tc superconductor YBa2Cu3O7−x. No EPR signal is observed in the spectra of the properly annealed crystals with Tc≅90K. However, we observe a strong, almost isotropic Cu2+ signal in the spectra of the as-grown (unannealed) single crystals with Tc≅40K. The observed signal has a slightly anisotropic g-factor which becomes totally isotropic at T≅90K. The linewidth, dominated by the hyperfine interaction, remains anisotropic at all temperatures, showing that the signal originates from a site which does not have real cubic symmetry.

Electron paramagnetic resonance of the “reentrant” insulating spin-glass EuxSr1-xS

Abstract Electron paramagnetic resonance is reported in the paramagnetic region of the insulating spin-glass Eu x Sr 1-x S for concentration ranges that exhibit both ferromagnetic and spin-glass phases. From the temperature dependence of the line positions and line widths, the low temperature phases can be anticipated and explained in terms of the energy level distributions of the total spin states.