Infrared transmission study of crystal-field excitations in Al- and Sr-doped Pr_(1+x)Ba_(2-x)Cu_3O_6

摘要

Absorption bands corresponding to the crystal-field excitations in Pr_(1.05)Ba_(1.88)Cu_(2.39)Al_(0.34)O_(6+y) and Pr_(0.93)ba_(1.93)Sr_(0.13)Mg_(0.05)Cu_(2.74)O_(6+y) (where y ～ 0) have been observed by Raman scattering and infrared transmission spectroscopy. These excitations are assigned to transitions occurring from the two lowest-energy levels of the ~3H_4 multiplet to the excited multiplets ~3H_5, ~3H_6, ~3F_2, and ~3F_3 of Pr~(3+) ions located in their D_(4h)-symmetry sites. The spectra exhibit additional bands consistent with the presence of Pr~(3+) ions in the C_(4v)-symmetry Ba sites, as well as various defects associated with disorder. This analysis confirms that the phenomenological crystal-field parameters follow the trends reported in NdBa_2Cu_3O_6 and SmBa_2Cu_3O_6. A spectral splitting of the Γ_5 levels is observed at low temperature and indicates Pr-Cu exchange interaction. Al doping results in the expected inversion of the Γ_5-Γ_1 symmetry sequence of the quasitriplet ground state, in accordance with the magnetic-susceptibility anisotropy measurements.