Aleksandr Pishtshev

Excitons in Mg(OH)2 and Ca(OH)2 from ab initio calculations

Abstract By using ab initio calculations with the HSE06 hybrid functional and GW approximation combined with numerical solution of the Bethe Salpeter equation (GW–BSE) we predict the existence of diverse number of excitonic states in multifunctional hydroxides X(OH)2 (X=Mg and Ca) that were not previously reported experimentally or theoretically. The imaginary part of the dielectric function and the reflectivity spectra show very strong peaks corresponding to the electron–hole pair states of large binding energy. The origin of the excitons is attributed to strong localization of the hole and the electron associated with oxygen 2 p x , 2 p y occupied states as well as to oxygen and earth metal s empty states, respectively. The results have important implications for different applications of the materials in optoelectronic devices.

Evaluation of the electron–TO–phonon interaction in polar crystals from experimental data

Abstract The aim of this study was to determine the strengths of the coupling of electrons with the polar long-wavelength transverse optical (TO) vibrations from infrared spectroscopy data. This determination is made by means of a simple relationship between the electron–TO–phonon interaction constant and material parameters, based on a parametrization of the electron–TO–phonon coupling in terms of the long-range dipole–dipole interaction. The combination of experimental data employed here allowed us to calculate directly the relevant constants for a number of representative polar insulators and to show that in ferroelectrics the interband electron–TO–phonon interaction at the Γ point is essentially strong. In these calculations, infrared spectroscopy methods proved to be an effective tool for study of the main properties of electron interaction with polar long-wavelength TO phonons.

Structure-property relationships in cubic cuprous iodide: A novel view on stability, chemical bonding, and electronic properties

Based on the combination of density functional theory and theory-group methods, we performed systematic modeling of γ-CuI structural design at the atomistic level. Being started from the metallic copper lattice, we treated a crystal assembly as a stepwise iodination process characterized in terms of a sequence of intermediate lattice geometries. These geometries were selected and validated via screening of possible structural transformations. The genesis of chemical bonding was studied for three structural transformations by analyzing the relevant changes in the topology of valence electron densities. We determined structural trends driven by metal-ligand coupling. This allowed us to suggest the improved scenario of chemical bonding in γ-CuI. In particular, the unconventional effect of spatial separation of metallic and covalent interactions was found to be very important with respect to the preferred arrangements of valence electrons in the iodination process. We rigorously showed that useful electronic and optical properties of γ-CuI originate from the combination of two separated bonding patterns-strong covalency established in I-Cu tetrahedral connections and noncovalent interactions of copper cores is caused by the 3d10 closed-shell electron configurations. The other finding of ours is that the self-consistency of the GW calculations is crucial for correctly determining the dynamic electronic correlations in γ-CuI. Detail reinvestigation of the quasi-particle energy structure by means of the self-consistent GW approach allowed us to explain how p-type electrical conductivity can be engineered in the material.

Electronic and optical properties of magnesium and calcium hydroxides: the role of covalency and many-body effects

Magnesium and calcium hydroxides X(OH)2 (X = Mg, Ca) are multifunctional materials that have many important applications in industry, technology, and research. In solid-state electronics, the emerging applications of these compounds are related to photovoltaic devices. In the present paper we review electronic properties of X(OH)2, band gaps, work function, and features of chemical bonding and discuss theoretically predicted exciton effects.

Electronic excitations and self-trapping of electrons and holes in CaSO4

A first-principles study of the electronic properties of a CaSO4 anhydrite structural phase has been performed. A theoretical estimation for the fundamental band gap (p →s transitions) is Eg=9.6 eV and a proper threshold for p →d transitions is Epd=10.8 eV. These values agree with the data obtained for a set of CaSO4 doped with Gd 3+ ,D y 3+ ,T m 3+ and Tb 3+ ions using the methods of low-temperature highly sensitive luminescence and thermoactivation spectroscopy. The results are consistent with theoretical predictions of a possible low-temperature self-trapping of oxygen p-holes. The hopping diffusion of hole polarons starts above ∼40 K and is accompanied by a ∼50–60 K peak of thermally stimulated luminescence of RE 3+ ions caused due to the recombination of hole polarons with the electrons localized at RE 3+ . There is no direct evidence of the self-trapping of heavy d-electrons, however, one can argue that their motion rather differs from that of conduction s-electrons.

Contribution of long-wavelength transverse optical phonons to electron–phonon coupling in doped polar insulators

Abstract We estimate the contribution of the long-wavelength el–TO–ph interaction and discuss the effect it has on electron–phonon scatterings in doped polar systems like SrTiO3 and PbTe. The analytical and numerical results presented in the study indicate that the el–TO–ph interaction tends to contribute little to the total strength of electron–phonon coupling in these and related materials. To explain this fact we consider possible reasons why the effect of the polar long-wavelength transverse optical phonons on the coupling constant λ is far less than one might suppose.

Analytic and geometric properties of photoinduced effects in noncentrosymmetric crystals: Photovoltaic current and optical rectification

An original dispersion relation between the stationary coherent nonlinear optical responses by current and polarisation is obtained. The dispersion relation provides a new complementary tool that can be employed to study light-induced charge transport models and facilitate experimental data analysis. It is shown that the origin of the coherent current and the dc-polarisation induced in a noncentrosymmetric crystal under illumination is related to the theory of the Berry phase and can be represented in terms of the renormalised geometric potentials. This renormalisation originates from the extra phase difference acquired by a carrier in the light field on the quantum transition between the electronic bands. The gauge invariance of the corresponding expressions for the current and the polarisation is demonstrated.