Transverse effective charge and its pressure dependence in GaN single crystals

The pressure dependence of the ${A}_{1}(\mathrm{TO})$ and ${A}_{1}(\mathrm{LO})\mathrm{}$ phonon modes of bulk, single crystals of wurtzite GaN has been measured by Raman scattering in a diamond-anvil cell up to 40 GPa. The measured phonon frequencies have been used to determine the LO and TO phonon-mode Gr\"uneisen parameters and also the value of effective transverse charge ${e}_{T}^{*}=2.6$ and its pressure derivative ${\mathrm{de}}_{T}^{*}/dP=\ensuremath{-}2.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3} {\mathrm{GPa}}^{\ensuremath{-}1}$. The experimentally obtained values are compared with results of calculations by means of tight-binding formalism combined with ab initio linear muffin-tin orbital calculations.