The effect of Ca and Th substitution on the crystal-field spectrum of the high-T_c superconductor HoBa₂Cu₃O_x

The inelastic neutron scattering technique was employed to study the transitions between the low-lying levels of the ground-state J-multiplet of the Ho³⁺ ions in Ho_1-yCa_yBa₂Cu₃O_x (x ≈ 6.3, 7.0; y ≈ 0.1, 0.2) and Ho_1-yTh_yBa₂Cu₃O_x (x ≈ 6.3; y ≈ 0.07, 0.13) split by the crystalline electric field. For oxygen-rich Ca-doped samples, the observed energy spectra exhibit a fine structure, which reflects the behaviour of the local hole density near the doping centre. For the first time this experimental observation unambiguously confirms that the charge inhomogeneity, which was found to be a characteristic feature of the underdoped cuprates, still exists in the deeply overdoped regime with T_c = 56 K. On the other hand, the modification of the crystal-field spectra observed for the oxygen-deficient samples with both Ca²⁺ and Th⁴⁺ substitutions for Ho³⁺ is found to result from distortions of the copper-oxygen polyhedron around the Ho³⁺ ion in the direct neighbourhood of the implanted ions, rather than from the doping-induced variation of the carrier concentration in the CuO₂ planes.