The effect of Ca and Thsubstitution on the crystal-field spectrum of the high-Tcsuperconductor HoBa2Cu3Ox

Abstract

The inelastic neutron scattering technique was employed to study thetransitions between the low-lying levels of the ground-stateJ-multiplet of the Ho3+ ions inHo1-yCayBa2Cu3Ox (x≈6.3, 7.0; y≈0.1,0.2) and Ho1-yThyBa2Cu3Ox (x≈6.3;y≈0.07, 0.13) split by the crystalline electric field.For oxygen-rich Ca-doped samples, the observed energy spectraexhibit a fine structure, which reflects the behaviour of the localhole density near the doping centre. For the first time thisexperimental observation unambiguously confirms that the chargeinhomogeneity, which was found to be a characteristic feature of theunderdoped cuprates, still exists in the deeply overdoped regimewith Tc = 56 K. On the other hand, the modification of thecrystal-field spectra observed for the oxygen-deficient samples withboth Ca2+ and Th4+ substitutions for Ho3+ is foundto result from distortions of the copper-oxygen polyhedron aroundthe Ho3+ ion in the direct neighbourhood of the implantedions, rather than from the doping-induced variation of the carrierconcentration in the CuO2 planes.