6533b81ffe1ef96bd1276f22

RESEARCH PRODUCT

Effects of pressure and temperature on the dielectric constant of GaS, GaSe, and InSe:  Role of the electronic contribution

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In this work we report on direct measurements of the temperature and pressure dependences of the low-frequency dielectric constant along c axis $({\ensuremath{\varepsilon}}_{\ensuremath{\parallel}})$ of GaS, GaSe, and InSe. The temperature dependence of both the ordinary and extraordinary refractive indexes is also presented. A large increase of ${\ensuremath{\varepsilon}}_{\ensuremath{\parallel}}$ under pressure has been observed. In the framework of a rigid ion model, the lattice contribution to ${\ensuremath{\varepsilon}}_{\ensuremath{\parallel}}$ is shown to increase slightly under pressure, due to the change of the angle between the anion-cation bond and the layer plane. Consequently, the pressure behavior of ${\ensuremath{\varepsilon}}_{\ensuremath{\parallel}}$ is proposed to arise from a large increase of the electronic contribution to ${\ensuremath{\varepsilon}}_{\ensuremath{\parallel}}.$ This fact is explained through a decrease of the Penn gap for polarization parallel to the c axis, whose energy and pressure coefficient are shown to scale with those of the indirect band gap in these compounds. A supplementary and reversible step increase of ${\ensuremath{\varepsilon}}_{\ensuremath{\parallel}}$ is observed at 1.6 GPa in GaS, which is associated with a phase transition that has been already observed by other authors.