Narrow Results

The perturbation formulae of EPR parameters g factors g_x, g_y and g_z for the lowest Kramers doublet of 4f¹ ion in orthorhombic symmetry are established. In these formulae, the contributions of the covalency effects, the admixture between J=7/2 and J=5/2 states as well as the second-order perturbation (which is not considered in the previous works) via crystal-field and orbital angular momentum interactions are all included. According to these formulae, and by using the superposition model, the g-factors g_x, g_y and g_z for Ce³⁺ center in YBa₂Cu₃O_6+x superconductor are calculated. The results are discussed.

The EPR g factors g_// and g_⊥ for the six-coordinated Cu²⁺(1) site in YBa₂Cu₃O_6+x are theoretically investigated from the perturbation formulas for a 3d⁹ ion in tetragonally elongated octahedra. The related tetragonal crystal-field parameters Ds and Dt are determined from the superposition model and the structural data for the Cu²⁺(1) site, by considering the effective elongation of the oxygen octahedron due to the Jahn-Teller effect of the Cu²⁺ cluster. Based on the calculations, the effective elongation (≈ 15%) of the ligand octahedron is obtained for the signals at 100K. As for the EPR signals at room temperature, the elongation declines to merely 4%, corresponding to the smaller anisotropy Δg (=g_// - g_⊥). The theoretical results for the g factors are in good agreement with the observed values, and the local tetragonal distortion due to the Jahn-Teller effect is discussed.