Vibronic ferroelectrics in the field of a strong electromagnetic wave

The effect of a strong electromagnetic field on the phase transition in narrow-band ferroelectric-semiconductors is discussed in the framework of the vibronic theory of ferroelectricity.     

Vibronic structure and the anisotropy of excitons in cubic LiH single crystals

The shape of one-phonon sidebands of exciton in LiH is studied both in reflection and edge luminescence spectra. The main structure of the shape is found to reflect the phonon density of states. This fact is interpreted as a result of a high anisotropy of the exciton band.     

Topological nature of polarization and charge pumping in ferroelectrics

Electric polarization in insulators is represented by the transferred charge through a shift of the Bloch wave functions induced by an adiabatic change of external parameters Q-->. It is found that this covalent/quantum contribution is determined nonlocally by the topological structure in the Q--> space. The condition for the charge pumping for a cyclic change of Q--> is also obtained. Applications of this picture to various organic ferroelectrics and BaTiO3 are discussed.     

Dielectric properties of microcomposite ferroelectrics

The dielectric response in two-phase composites of ferroelectric and dielectric materials was studied within the quasistatic approximation. Using different effective medium theories (Maxwell-Garnet theory, the effective medium approximation and the Bergman representation) the dielectric function was represented as a distribution of polar modes. One of the main results concerns the existence of modes due to geometric resonances which among other effects can lead to new low-frequency absorption peaks near the percolation threshold of the ferroelectric material. These peaks appear always higher than the transverse polar mode frequencies of the bulk components but below their corresponding longitudinal mode frequencies. The soft mode with renormalized mode strength but unchanged frequency exists only in the composite with percolated ferroelectric clusters. When the ferroelectric clusters are only finite the soft mode does not soften completely.     

Symmetry and anisotropic optical spectrum of charge-transfer excitons in the model of CuO2 plane

A method of group-theoretical classification of excitonic states with a charge transfer from a unit cell to neighboring ones is suggested in a tetragonal two-dimensional model of a CuO₂ plane. The orientation (anisotropy) and polarization (pleochroism) dependences of the intensities of excitonic dipole-forbidden optical transitions are determined. The phenomenon of initiating dipole-forbidden transitions in an external electric field is theoretically considered. General dependences of the probabilities of such transitions on the value and direction of the electric field and on the light wave polarization are found. The dependences obtained can be used to identify weak absorption bands in dielectric cuprates containing CuO₂ planes.     

Spectroscopy of excitons and shallow impurities in isotopically enriched silicon—electronic properties beyond the virtual crystal approximation

Recent high resolution spectroscopic studies of excitonic and impurity transitions in high-quality samples of isotopically enriched Si have dramatically expanded our understanding of the effects of isotopic composition on the electronic properties of semiconductors. Prior to these studies on Si, the results for other semiconductors, focusing mainly on the isotopic dependence of the electronic band gap energy E_G in the T→0 limit, could all be explained within the virtual crystal approximation (VCA), in which only the average isotopic mass was relevant. Remarkably, not only were the effects of isotopic randomness observable in natural Si (when compared to enriched ²⁸Si), but the random isotopic distribution present in natural Si was found to be the true source of what had been thought of as ‘fundamental’ spectroscopic limits in Si, including the linewidths of bound exciton emission lines and impurity absorption transitions, and the ‘intrinsic’ acceptor ground state splitting. Many of these transitions are far narrower in highly enriched ²⁸Si than in the most perfect natural Si, challenging existing spectroscopic methods, and opening up new possibilities for precision measurements, and for the observation of new physics.     

Elastic properties of cubic relaxor ferroelectrics

Comparative analysis of the elastic properties of PbMg_1/3Nb_2/3O₃ (PMN) and PbMg_1/3Ta_2/3O₃ (PMT) crystals has been performed. The temperature dependences of the elastic moduli C ₁₁, C ₁₂, and C ₄₄ are presented. These curves exhibit anomalies near the Burns temperature for both crystals. Additional anomalies are observed in the vicinity of the suppressed ferroelectric transition (T _C ～ 210 K) in PMN. Nontrivial frequency dispersion of the elastic moduli has been revealed for PMN.     

Phase transition to the conducting state in a system of charge-transfer excitons at a donor-acceptor interface

We discuss the phase transition to the conducting state in a system of 2D charge-transfer excitons (CTEs) at a donor-acceptor interface. The phase transition arises due to strong dipole-dipole repulsion between CTEs which stimulates the population of free carriers in higher energy states even at low temperature. We use the computer simulations with the random distribution of excitons, with finite lifetime explicitly taken into account. The critical concentration of CTEs and their energy distribution are calculated. We also discuss the possibility of observing the predicted phenomena. Fiz. Tverd. Tela (St. Petersburg) 41, 781–784 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

On the nature of emission bands of self-trapped excitons in solid xenon

The paper presents the analysis of experimental data on the temperature dependence of luminescence spectra of solid xenon. The mechanisms of exciton self-trapping down to quasi-molecular states under different conditions are discussed. A new treatment of spectral distribution of intensity through quasi-molecular luminescence bands is proposed; according to the treatment, at T < 55 K the emission is associated with the transition from the lowest vibrational relaxation excited state ³Σ_u⁺, while at T 60 K from the term ¹Σ_u⁺. Spectral redistribution of intensities in the 60–150 K range is due to an increase in the rate of vibrational relaxation in the state ¹Σ_u⁺ with increasing the crystal temperature.     

Vibronic properties of 2-azaanthracene and its derivatives

We report an experimental and theoretical (PPP method) study of the UV absorption spectra of 2-azaanthracene and four of its methyl-substituted derivatives. We find that the p bands of these compounds consist of two different vibronic ^* bands. Our experimental and theoretical results suggest a rule for evaluating the effect of methyl groups on ^* transition energies as a function of the charge on the atoms. We interpret the reasons for the different effects of a polar solvent and hydrogen bonding on the ^* bands of these compounds.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 7–15, June, 1977.     

Possible nature of the pseudogap anomalies in HTSC

An HTSC model, in which the interaction of valence-band electrons with diatomic negative U centers is assumed to be responsible for the anomalous properties of HTSC compounds, is proposed and used to explain the nature of the pseudogap and pseudogap anomalies (including the giant Nernst effect, the anomalous diamagnetism above T _c, the “transfer” of the optical spectral weight). For YBa₂Cu₃O_{6 + δ}, the pseudogap opening temperature T* and T _c are calculated as functions of the degree of doping δ. The calculated dependences agree quantitatively with the experimental dependences without using scale fitting parameters. The good agreement between the calculated and experimental results can serve as an argument for the proposed HTSC model.     

Spatial and orientational ordering of charge-transfer excitons at intense pumping. Absorption and fluorescence spectra

The non-linear properties of charge-transfer exciton dynamics at intense pumping are investigated. The shift of an absorption band of an arbitrary cell, induced by the electric field of the available excitons, results in spatial and orientational ordering of the excitons. The side bands appear in the absorption and fluorescence spectra.     

Self-trapped excitons in silicon dioxide: mechanism and properties

Irradiating silica produces self-trapped excitons (STEs) that spontaneously create atomic-scale distortions on which they localize themselves. Despite enduring interest in STEs and subsequent defects in this key technological material, the trapping mechanism and geometry remain a mystery. Our ab initio study of STEs in alpha-quartz using a many-electron Green's function approach answers both questions. The STE comprises a broken O-Si bond with the hole localized on the defected oxygen and the electron on the defected silicon atom in a planar sp2 conformation. The results further explain quantitatively the measured STE spectra.     

Spectral properties of molecular charge-transfer probe QMOM

The spectral characteristics of solutions of a dye with dual fluorescence, 1-methyl-2-(4-methoxy)phenyl-3-hydroxy-4(1H)-quinolone, in acetonitrile are studied upon selective excitation. This dye is a structural analogue of 3-hydroxyflavone and also exhibits excited-state proton transfer, which, as well as in the case of 3-hydroxyflavone, has a kinetic nature. The fluorescence spectra are studied upon excitation by photons of various energies, and the excitation spectra are recorded at wavelengths of different fluorescence bands. It is found that the intensity ratio of the emission of the normal and tautomeric forms (at wavelength of 415 and 518 nm, respectively) is almost the same (0.23–0.25) for excitation in the regions of the main and the second absorption bands. At the same time, in the case of excitation between these bands, this ratio decreases to 0.19. The second interesting feature is the existence of a third latent emission band peaked at about 480 nm, which is reliably detected upon excitation at wavelengths in the region of 400–450 nm. This study shows that this emission belongs to the anionic form of the dye. This form is also responsible for a decrease in the intensity ratio of the emission of the two main forms in the case of excitation between the first and second absorption bands.