Temperature-induced electron phonon coupling behaviour of AgGaS₂ probed by Brillouin spectroscopy

Abstract

Low-temperature Brillouin scattering studies on Chalcopyrite AgGaS2 have been carried out between the temperatures ranges 20-300 K. We observe the Brillouin modes for AgGaS₂ as follows: for crossed polarization (VH) the two modes are around 5.4 and 18 GHz, labelled as RSW1 (Rayleigh surface mode), LA (longitudinal) modes and for parallel polarization (HH), we see the modes are around 4.5 and 11 GHz, labelled as RSW2, TA (transverse acoustic). According to this acoustic phonon mode behaviour with temperature, we do see phase transition at 80 K for AgGaS₂. The phase change observed at 80 K is attributed to a strong electron-phonon coupling between the acoustic modes, which modulates the acoustic mode frequency 10-15 %. The strong electron-phonon coupling between acoustic modes (LA and TA), which is seen from the anomalous behaviour below 80 K, shows the electronic transition to be of the type direct to indirect band gap. The sound velocity and elastic constants have been calculated using Brillouin modes, which are in good agreement with previous reports.