Phonon and magnon Raman scattering in CuB_2O_4

摘要

The Brillouin-zone-center phonons and two-magnon excitations in CuB_2O_4 are studied experimentally by polarized Raman spectroscopy. Most of the expected modes are clearly pronounced and their symmetry unambiguously identified from the polarization selection rules. The experimentally observed transverse optical phonon frequencies are in good agreement with those obtained by means of density functional theory. The two-magnon scattering band is centered at 82 cm~(-1) and is clearly identified at temperatures below the Neel temperature T_N. The spectral shape of the two-magnon band confirms the existing theoretical models of magnon dispersion in the commensurate phase of CuB_2O_4 and suggests an exchange integral of J = 33 cm~(-1) (48 K) between the nearest-neighbor Cu(A) ions. The quantitative line-shape analysis of the two-magnon band evidences for additional magnon self-energy contributions below the temperature T* of the commensurate-to-incommensurate phase transition.