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Abstract

The transverse-optic (TO) phonons of zinc-blende-type CuCl
(gamma-CuCl) and the longitudinal-optic (LO) phonons of gamma-
CuBr exhibit striking anharmonic self-energy anomalies at the
center of the Brillouin zone. We investigate here by Raman
spectroscopy the dependence of the LO and TO phonons of gamma-
CuI on pressure, temperature, and isotopic mass of copper, in
the search for related effects. We find that the pressure
dependence of the TO phonon linewidth is qualitatively
different at low temperature and close to room temperature. A
model is developed to interpret these differences based on
anharmonic decay into two and three phonons (two different
channels). In order to make semiquantitative predictions we
used the experimental pressure dependence of the TO phonons and
that of phonons at the edge of the Brillouin zone that we
obtained by linear-response ab initio local-density
approximation calculations. We have also calculated the
pressure dependence of the zone-edge phonons of gamma-CuCl and
gamma-CuBr and compared it with the few existing experimental
results. In this process, interesting anomalies concerning the
transverse-acoustic phonons at the zone edge have been found.