Vibronic interaction in crystals with the Jahn-Teller centers in the elementary energy bands concept

The order-disorder type phase transition caused by the vibronic interaction (collective Jahn-Teller effect) in a monoclinic CuInP$_2$S$_6$ crystal is analyzed. For this purpose, a trigonal protostructure model of CuInP$_2$S$_6$ is created, through a slight change in the crystal lattice parameters of the CuInP$_2$S$_6$ paraelectric phase. In parallel to the group-theoretical analysis, the DFT-based {\it ab initio} band structure calculations of the CuInP$_2$S$_6$ protostructure, para-, and ferriphases are performed. Using the elementary energy bands concept, a part of the band structure from the vicinity of the forbidden energy gap, which is created by the $d$-electron states of copper, has been related with a certain Wyckoff position where the Jahn-Teller's centers are localized. A construction procedure of the vibronic potential energy matrix is generalized for the case of crystal using the elementary energy bands concept and the group theoretical method of invariants. The procedure is illustrated by the creation of the adiabatic potentials of the $\Gamma_5$-$\Gamma_5$ vibronic coupling for the protostructure and paraphase of the CuInP$_2$S$_6$ crystal. A structure of the obtained adiabatic potentials is analyzed, followed by conclusions on their transformation under a phase transition and the discussion on the possibility for the spontaneous polarization to arise in this crystal.Comment: 17 pages, 7 figure

Electron-Phonon Interaction as a Mechanism of Phase Transition in the CuInP2S6CuInP_{2}S_{6} Crystal

Studies on the phases of $CuInP_{2}S_{6}$ crystal exhibiting phase transitions of the order-disorder type are performed. A possibility for the cooperative Jahn-Teller effect to occur in this crystal is analyzed. For this purpose, the procedure of constructing the potential energy matrix is generalized for the case of crystal and it is illustrated by the construction of the adiabatic potentials for $Γ_{5}$-$Γ_{5}$ vibronic coupling for the proto- and paraelectric phases of $CuInP_{2}S_{6}$. The structure of the obtained potentials is analyzed together with the discussion on the appearance of spontaneous polarization in this crystal

Band Structure and Optical Properties οf the Layered Hg3TeCl4Hg_{3}TeCl_{4} Crystal

First experimental investigations on absorption and photoluminescence of the novel $Hg_{3}TeCl_{4}$ monocrystals grown by the Bridgman method are reported. A comparison of the measurement results with theoretical band structure calculations of the $Hg_{3}TeCl_{4}$ crystal confirmed that $Hg_{3}TeCl_{4}$ is a wide-band-gap photoconductor ($E_{g}$ = 3.64 eV at 24 K) with the effective masses of charge carriers characteristic for semiconductors. Energetic position of the main photoluminescence peak and its temperature dependence indicates the presence of an additional energy level in the energy gap which takes part in the radiative recombination process and whose origin was discussed

The In4\text{}_{4}Se3\text{}_{3} Crystal as a Three-Dimensional Imitative Model of Phenomena in One-Dimensional Crystals

For three-dimensional charge carriers described by the dispersion law with quartic terms of the wave vector, the density of states function similar as in the one-dimensional case was determined. This similarity allows the Pekar and Dejgen condenson states in the continuum approximation to exist. The calculated phonon spectrum reveals optical vibrations of a very low frequency, which favours the electron-phonon interaction and creation of the condenson states

Gain Spectrum for the In4\text{}_{4}Se3\text{}_{3} Crystal with a Non-Standard Dispersion Law of Charge Carriers

Based upon the ab initio band structure calculations results and the density of states function of the orthorhombic In$\text{}_{4}$ Se$\text{}_{3}$ crystal as well as the experimental data concerning its radiative recombination, it was shown that the Bernard-Durafour condition is fulfilled for this crystal. The absorption coefficient α that exhibits a negative value in the given energy range and for the given concentrations of non-equilibrium charge carriers, was calculated

Dispersion Law with a Low-Energy Non-Parabolicity for the Charge Carriers in the In_{4}Se_{3} Crystal and Related Effects

The influence of parameters of the dispersion law exhibiting a low-energy non-parabolicity for the $In_{4}Se_{3}$ crystal on the plasma-electric effect occurring in the non-equilibrium plasma of this semiconductor was investigated under the circumstances that a longitudinal plasma wave propagates in its non-equilibrium plasma