Crystal field and microwave absorption spectra in tetragonal HoBa 2 Cu 3 O x ( x = 6.0 and 6.3)

The microwave resonance absorption at wavelengths 871, 406, 305, and 118 mu m (frequencies 344, 739, 984, and 2540 GHz) in tetragonal layered perovskites HoBa2Cu3Ox (x approximate to 6.0,6.3) in pulsed magnetic fields up to 40 T has been observed. This absorption is caused by the electronic transitions between low-lying levels of the Ho3+ ion in the crystal field. The positions and intensities of the tetragonal symmetry allowed absorption lines for the crystal with x=6.0 in the magnetic field oriented along the principal tetragonal axes are adequately described in terms of the available crystal field parameters. To explain the symmetry forbidden absorption lines, it is necessary to take into account the effect of the inhomogeneous orthorhombic and monoclinic components of the crystal field as well as an effective exchange field from the ordered Cu2+ subsystem at the Ho3+ sites. The origin of the local deviations from the ideal tetragonal symmetry are ascribed to disorder in the oxygen subsystem This disorder and consequently also its impact on the absorption spectra is more pronounced in the crystal with x=6.3.