Influence of an electron-phonon subsystem on specific heat and two-photon absorption of the semimagnetic semiconductorsPb1−xYbxX(X=S,Se,Te)near the semiconductor-isolator phase transformation

Abstract

Specific heat, transport, and two-photon absorption (TPA) effects were studied in single crystalline magnetic semiconductors ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Yb}}_{x}X$ ($X=\mathrm{S},\phantom{\rule{0.3em}{0ex}}\mathrm{Se},\mathrm{Te}$ at $x=1--3%$) near semiconductor-isolator phase transitions. It was shown that the TPA may be used as a sensitive tool for investigations of electron-phonon interactions near low temperature semiconductor-isolator phase transformation. Comparison with other methods of phase transition studies has shown more sharplike temperature dependence of the TPA. Particularly, comparison with piezo-optical and dilatometric methods shows on a better temperature resolution of the TPA with respect to the phase transitions. It was established as a correlation between the density of low-temperature phonon modes and values of the TPA. Particularly with increasing of the electron-phonon interactions defined by the temperature-dependent Debye term (parameter $\ensuremath{\beta}$) from the equation $C=\ensuremath{\gamma}T+\ensuremath{\beta}{T}^{3}+{\ensuremath{\sum}}_{i=1}^{n}{\ensuremath{\delta}}_{i}{T}^{2i+3}$, one can observe an increase of the TPA oscillator strengths. It was shown that only the low-temperature TPA behaviors might be determined by the features of the low-energy phonon modes related to the observed phase transformation. We also have discovered that low-temperature dependence of the specific heat ${C}_{p}$ in ${\mathrm{Pb}}_{1\ensuremath{-}x}{R}_{x}\mathrm{Te}$ $(R=\mathrm{Yb},\mathrm{Te})$ with $x=2.6%$ and 1.65%, respectively, exhibits a nonmagnetic ordering caused by large value of electron-phonon contribution. Applying magnetic fields up to $2\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ substantially modifies the temperature features of several phonon low-frequency modes, which may indicate a contribution of magnons.