Comparative studies of microwave absorption in the singlet paramagnets HoVO4 and HoBa2Cu3O x in strong pulsed magnetic fields

Abstract

Microwave absorption in the tetragonal singlet paramagnets HoVO4 (zircon structure) and HoBa2Cu3Ox (x ≈ 6, layered perovskite structure) is studied and compared in pulsed magnetic fields up to 40 T at low temperatures. These paramagnets are characterized by a singlet-doublet scheme of the low-lying levels of the Ho3+ ion in a crystal field. In a magnetic field directed along the tetragonal axis, HoVO4 exhibits resonance absorption lines at wavelengths of 871, 406, and 305 µm, which correspond to electron transitions between the low-lying levels of the Ho3+ ion in the crystal field. The positions and intensities of these absorption lines in HoVO4 are well described in terms of the crystal-field formalism with the well-known interaction parameters. The absorption spectra of HoBa2Cu3Ox at a wavelength of 871 µm exhibit broad resonance absorption lines against the background of strong nonresonance absorption. The effects of low-symmetry (orthorhombic, monoclinic) crystal-field components, the deviation of a magnetic field from a symmetry axis, and various pair interactions on the absorption spectra of the HoVO4 and HoBa2Cu3Ox crystals are discussed.