Effect of pressure on the Raman anomaly of zinc-blende CuBr and Raman spectra of high-pressure phases

Abstract

Raman spectra of isotopically pure CuBr ( 63 Cu 81 Br) were measured under hydrostatic pressures up to 10 GPa at 10 K. The anomalous line shape of the longitudinal-optic ͑LO͒ scattering in the zinc-blende phase ͑CuBr-III, 0-4 GPa͒, consisting of a broad structure between 155 and 177 cm Ϫ1 , exhibits a continuous change with pressure and develops into a narrow LO phonon peak near 4 GPa. The disappearance of the LO Raman anomaly as well as the pronounced broadening of the TO mode with pressure are explained in terms of pressure-dependent third-order anharmonic interactions with two-phonon states. A Fermi resonance model, which is based on a shell model fit of available phonon dispersion data, fully accounts for the changes in Raman line shapes under pressure. Low-temperature Raman spectra of the tetragonal CuBr-IV and cubic CuBr-V high-pressure phases are also reported. An assignment of the observed Raman modes of these phases is proposed.

Comparative study of the pressure dependence of optical-phonon transverse-effective charges and linewidths in wurtzite InN

“…Furthermore, with the help of ab-initio calculations we studied the pressure dependence of the Raman linewidths. For instance, we observed that the E high 2 mode exhibits a characteristic Fano-like broadening due to large phonon anharmonicities, which can be understood in terms of the pressure-tuning of a Fermi resonance, as reported for other semiconductors [14][15][16] .…”

Pressure and temperature dependence of the Raman phonons in isotopic γ-CuI

“…This off-center model has been discarded rather conclusively in recent work with isotopically substituted samples. 5,6,8,9 The fact that this feature appears at high temperatures ͑see Fig. 3͒, and the low frequencies reported by inelastic neutron scattering for the transverse phonons at the zone boundaries have led us to assign this feature to two phonon difference modes.…”

“…IV B for room temperature by scaling with the temperature dependence of the different phonons in germanium. The fit procedure has been described elsewere 5,8,9,50 and yields for ͉V 3 ͉ 2 a value of Ϸ6 cm Ϫ2 . The (TAϩLA) L singularity has been enhanced ad hoc ͓dotted line in Fig.…”

“…7 Recently, however, a similar anomaly has been found for the corresponding longitudinal-optic ͑LO͒ Raman phonon of ␥-CuBr. 8,9 The shift of the anomaly from the TO phonons in ␥-CuCl to the LO phonons in ␥-CuBr can be qualitatively understood with a rather simple argument: The TO and LO phonons of ␥-CuCl shift down in frequency when chlorine is replaced by bromine, because of the strong increase in reduced mass. However, the acoustic modes which give rise to the two-phonon DOS required for the Fermi anomaly are hardly shifted by this substitution since their reduced mass is mainly that of copper.…”

Dependence of phonon widths on pressure and isotopic mass: ZnO

“…The diamond anvil cell is an excellent tool for such investigations [2] which yield detailed information about the anharmonic decay mechanisms responsible for linewidths and lineshapes. The reason for the drastic lineshape changes observed by Raman spectroscopy in CuCl [3], CuBr [4], GaP [5] and, to a lesser extent Si [6] and CuI [7] is that, in these cases, the anharmonic self-energy is determined by decay into two phonons close to Van Hove singularities at the edge of the Brillouin zone. The fact that the corresponding two-phonon density of states (DOS) has strong singularities near Raman LO (CuBr) or TO (CuCl, GaP) phonons, together with strong anharmonic coupling constants, gives rise to complex lineshapes which are determined not only by the imaginary part but also by the real part of the selfenergy.…”