Isotopic investigation of the lattice dynamics in CuBr - art. no. 045201

We present a detailed investigation of the Raman spectra of isotopically ta ilored CuBr at low temperature. The transverse optic (TO) phonon of CuBr ex hibits an almost perfect Lorentzian line shape, whereas the longitudinal op tic (LO) phonon displays a complex broad structure. The change of the TO fr equency with the variation of the isotope composition can be well described within the virtual crystal approximation (VCA), which corresponds to a ome ga proportional to mu (-1/2) dependence on the reduced mass mu. Slight devi ations from this general trend are attributed to anharmonic renormalization and agree semiquantitatively with results extracted from previous measurem ents of the temperature dependence of the Raman spectra. In the LO case, th e broad structure is resolved into three separate features, A, B, and C. Wh ile A and B are rather broad, C is a narrow peak located at the high-energy side of the LO structure. Two different trends are observed when analyzing the evolution of the LO structure with isotope substitution: peak B shows proportional to mu (-1/2) behavior, analogous to that of the TO phonon, whe reas peaks A and C shift almost only with the copper mass. The LO line shap e is explained in terms of the Fermi resonance (FR) model, i.e., an interac tion between the LO mode and a combination band of two acoustic phonons. We have performed a shell model calculation, with parameters taken from inela stic neutron scattering measurements, in order to obtain the one- and two-p honon densities of states (DOS). This calculation yields Raman line shapes in remarkable agreement with the experimental observations.