Origin of the ESR linewidth in the normal phase of GdBa2Cu3O7.

Abstract

The ESR linewidth of the superconducting oxide ${\mathrm{GdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ in the normal state is studied experimentally at 300 K. Several factors that strongly affect its shape and width\char22{}skin effect, contribution of the ${\mathrm{\ensuremath{\omega}}}_{\mathrm{\ensuremath{-}}}$ resonance, and chemical preparation\char22{}are examined carefully, and in this context some experimental results on ${\mathrm{Gd}}_{2}$${\mathrm{O}}_{3}$ are presented. It is shown that Q-band spectra from both ${\mathrm{GdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ powders (Lorentzian line) and single crystals (isotropic spectra) are of particular interest for theoretical analysis. The g factor of ${\mathrm{Gd}}^{3+}$ in ${\mathrm{GdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ is found to be 2.01\ifmmode\pm\else\textpm\fi{}0.01. Experimental results are given that indicate the existence of an exchange-narrowed dipolar line, via the Van Vleck\char21{}Anderson mechanism and not the Plefka-Barnes mechanism. Both the second moment of the ESR dipolar line, and the contribution of exchange to the fourth moment for a powder and for selected field orientations in the case of a single crystal are calculated numerically. Comparison of the predictions of the Van Vleck\char21{}Anderson-Weiss model with experimental results suggests that the nonadiabatic situation is present even at the Q band, and the coupling constant between nearest neighbors is found to be 2J/k=-0.3 K. Finally, the effect of the coupling with the conduction electrons is discussed, and an experimental check is made in a dilute sample.