RESUMO
Polarized second-order Raman scattering spectra of CuO single crystals are reported. It is shown that for some scattering geometries the second-order processes dominate the inelastic light scattering spectra. Group-theoretical symmetry analysis of the selection rules for the first- and second-order scattering processes is performed and phonon dispersion relations are calculated within density functional theory. The main spectral features of the two-phonon spectra are assigned to overtones of the vibrational branches at various special points across the Brillouin zone.
RESUMO
Single crystals of the mixed valence compound Pb3Ni1.5Mn5.5O15 were successfully grown. Polarized Raman spectroscopy revealed that they belong to the point group D(3d), in accordance with single-crystal x-ray diffraction data which were refined within the trigonal space group P3c1 (No. 165), with lattice parameters a = 9.941(3) Å and c = 13.543(3) Å. Strongly anisotropic long range magnetic order is established below 65 K.
RESUMO
The A(1), E(1) and E(2) Raman active modes in hexagonal YMnO(3) and LuMnO(3) single crystals are studied as a function of temperature and compared with previous measurements. In addition to anharmonicity, some phonon frequencies show below T(N) anomalous temperature dependences that reflect the atomic displacements while some other phonon frequencies are more sensitive to the spin-phonon coupling.
RESUMO
A reentrant novel phase is observed in the hexagonal ferroelectric HoMnO3 in the presence of magnetic fields in the temperature range defined by a plateau of the dielectric constant anomaly. The plateau evolves with fields from a narrow dielectric peak at the Mn-spin rotation transition at 32.8 K in zero field. The anomaly appears both as a function of temperature and as a function of magnetic field without detectable hysteresis. This is attributed to the indirect coupling between the ferroelectric (FE) and antiferromagnetic (AFM) orders, arising from an FE-AFM domain wall effect.
