The symmetry of phonon spectra in crystals with sublattices

A method is presented for investigation into the symmetry of phonon spectra of complex compounds composed of the Bravais sublattices. It is shown that quasi-degeneracy in the phonon spectra of crystals containing sublattices with the symmetry higher than that of the crystallographic group of the compound results from this high symmetry. The mechanism responsible for the translational quasi-degeneracy has peculiar features for various structures. For substitution structures in a highly symmetrical lattice, the translational quasi-degeneracy is caused by convolution of the total phonon spectrum of the highly symmetrical structure. The convolution of the phonon spectra in crystals containing sublattices of different Bravais types occurs separately for each sublattice. Quasi-degeneracy also occurs in the compounds containing sublattices whose point symmetry is higher than that of the crystal structure. The method is illustrated by the example of a one-dimensional structure containing sublattices. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 31–38, July, 2008.     

Density of states of disordered two-dimensional crystals with half-filled band

A diagrammatic method is applied to study the effects of commensurability in two-dimensional disordered crystalline metals by using the particle-hole symmetry with respect to the nesting vector P0 = +/-pi / a,pi / a for a half-filled electronic band. The density of electronic states (DoS) is shown to have nontrivial quantum corrections due to both nesting and elastic impurity scattering processes, and as a result the Van Hove singularity is preserved in the center of the band. However, the energy dependence of the DoS is significantly changed. A small offset from the middle of the band gives rise to the disappearance of quantum corrections to the DoS.     

Crystals with sublattices belonging to the cubic system and special features of their elementary excitation spectra

Genesis of spectra of elementary excitations of crystals comprising sublattices belonging to the cubic system is investigated. Using the methods of group theory, the Brillouin sublattice zones are transformed into the Brillouin crystal zones, and degeneration is established caused by the convolution of branches of the spectrum. The degeneration is removed with allowance for hybridization of sublattice states. The special features of the band spectra of some crystals belonging to the cubic system are discussed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 21–27, May, 2006.     

Valence band and core-level photoemission spectra of black phosphorus single crystals

Valence band structures of black phosphorus have been measured by synchrotron-radiation photoemission spectroscopy. The excitation photon-energy dependence of the spectra has provided information about the 3s-3p hybridization. As for inner core-levels, binding energies of 2p and 2s core-levels, $\text{2p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{?2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ splitting, and the bulk plasmon energy are estimated from X-ray photoemission spectroscopy.     

二维复式声子晶体中基元配置对声学能带结构的影响

研究了二维复式声子晶体中基元配置对其声学能带结构的影响,发现当声子晶体的基元配置改变时,声子晶体的不可约布里渊区也会改变,而且部分能带的极值不再在高对称线上.特别地,在某些基元配置下,不可约布里渊区扩大为整个第一布里渊区.因此,对于对称性较高的复式晶格声子晶体,可用通常的方法得到能带结构,而对于对称性较低的复式晶格结构声子晶体,只有采用对整个第一布里渊区进行研究的方法,才能获得可信的能带结构及带隙.     

Nutations of magnetizations of sublattices and their role in the formation of Mössbauer spectra of antiferromagnetic nanoparticles

A law determining the dispersion shift of the anti-Stokes band of the supercontinuum of a light bullet in a filament of a femtosecond laser pulse in transparent dielectrics has been established. The dispersion equation theoretically obtained for the anti-Stokes shift has been confirmed by spectroscopic studies of the filamentation of the near and middle infrared ranges in fused silica and fluorides.     

Optical spectra and energy band structure of AgAsS2 crystals

Ground and excited states of three exciton series are observed in the region of fundamental absorption edge of AgAsS₂ crystals. The contours of exciton reflection spectra are calculated and the main parameters of excitons and energy bands are determined in the center of Brillouin zone. The optical reflection spectra are investigated at 30 K in E∥c and E⊥c polarizations in AgAsS₂ crystals in the region of 2-6 eV. The optical functions are calculated from the reflection spectra and a scheme of electronic transitions responsible for peculiarities of reflection spectra deep into the absorption band is proposed.     

Photoreflection and photoconduction spectra of CdS crystals: Excitons in the electric fields of surface states

The exciton photoreflection spectra of CdS crystals are studied. It is found that the form of the exciton photoreflection spectrum is determined by a Stark shift of the exciton energy in the electric field of surface states. The dependences of the exciton photoreflection spectrum on temperature on the intensity and wavelength of the modulating radiation, and on the processes by which the photoreflection signal relaxes is determined. An energy scheme is proposed for the surface states which explains the observed effects of photoinduced changes in the surface field. A correlation is established between the exciton photoreflection spectrum and the form of the fine structure in the photoconductivity. Fiz. Tverd. Tela (St. Petersburg) 40, 875–876 (May 1998)     

Mutual influence of sublattices in the Cu y Fe x TiSe2 cointercalate compound

The concentration dependence of parameters of the Cu _y Fe _x TiSe₂ cointercalate compound has been interpreted based on the statistical approach relating the structural characteristics of the material to the probability of implementation of various configurations of defects. The Vegard’s law approximation assuming the absence of interactions between the defects has been used.     

Long-lived excited states and photoluminescence excitation spectra in single crystals of fullerene C60

Photoluminescence, optical absorption spectra, and photoluminescence excitation spectra were measured on large (2–3 mm), very pure crystals of fullerene C₆₀ at 5 K. It is shown that the main contribution to the photoluminescence of these crystals is from singlet and triplet excitons captured on crystal defects. The concentration of these defects does not exceed 10¹⁸ cm⁻²³, and the lifetime of triplet excitons on these defects is about 3 ms. It is shown that the symmetry distortion of the C₆₀ molecules at the defects is rather large and causes the oscillator strength of the zero-phonon optical transitions to be comparable to the most intense optical transitions with the participation of intramolecular vibrations. Zh. éksp. Teor. Fiz. 113, 734–746 (February 1998)     

Comparison of site-specific valence band densities of states determined from Auger spectra and XPS-determined valence band spectra in GeS (001) and GeSe (001)

Auger lineshapes of the Ge M₁M_4,5V and M₃M_4,5V and Se _M1M_4,5V transitions in GeS (001) and GeSe (001) are measured and compared to XPS valence band spectra. Distortions in both types of spectra due to inelastic scattering, analyzer and source broadening, and core level lifetime broadening are removed by deconvolution techniques. The valence band consists of three main peaks at ?2 eV, ?8 eV, and ?13 eV. There is excellent agreement of peak positions in AES and XPS spectra. The Auger lineshapes can be interpreted in terms of site-specific densities of states. They indicate that the states at ～?8 eV and at ～?13 eV are associated with the cation and anion sites respectively. The bonding p-like states at the top of the valence band have both cation and anion character. The Auger lineshapes indicate that the states closest to the valence band maximum are preferentially associated with Ge.     

Bound and many-particle states in polariton Raman spectra of NH4Cl crystals

The properties of polariton Raman spectra for the internal vibration region of NH₄Cl crystal are discussed. These spectra have been recorded by means of a copper laser ( $\text{λ = 5105 and 5782}\text{A}\text{?}$) for sample temperature T=80 and 10°K.According to the theory a polariton singularities in the region of a polar bound state has been obtained. The two-particle bands have been studied and a very complicate structure inside of these bands has been revealed. Moreover, multiphonon singularities at polariton curves have been observed. The experimental polariton law has been compared to the theory and a conclusion has been made as to the necessity of a further development of the theory.     

Modeling vibronic spectra and excited states of polyenes with a parametric method

The structural-dynamic models of excited states and vibronic structure of absorption spectra of linear polyenes (R-(CH=CH)_n-R, R=H, CH₃, n=4, 5, 7) are calculated using the parametric method of the theory of vibronic spectra of such molecules. Good agreement is obtained between the calculated and experimental spectra. It is shown that the system of parameters of the method for the polyenes has a high degree of transferability in the series of related polyenes. The constructed models adequately reflect the real structure of the molecules in excited states and allow one to predict quantitatively the fine vibrational structure of the spectra, including relatively weak effects related to the methyl substitution. The dynamic of structural changes of the molecules upon their excitation is studied in the series of polyenes.     

非Bragg禁带中的缺陷态

通过在周期柱状声波导中引入缺陷结构,研究了非Bragg禁带中的缺陷态。与传统Bragg禁带中的缺陷态相比,非Bragg禁带中的缺陷态具有完全不同的特性,所透射的缺陷模包含复杂的高阶横向模式,其形成与波导中的非Bragg共振密切相关。数值模拟表明缺陷态的频率与引入缺陷的几何尺寸有关,当缺陷长度变长时,非Bragg缺陷态与Bragg缺陷态一样向低频移动。而缺陷的半径变化时,非Bragg缺陷态却表现出完全不同的特性,随缺陷半径的变大而向高频移动。非Bragg缺陷态的这一反常变化与其模式结构密切相关,对非Bragg缺陷态的研究工作必将为声选频、滤波等器件的研制提供重要参考。     

The distribution of the valence electron density in predominantly ionic crystals with different Bravais sublattices

Based on the theory of the local-density functional, self-consistent valence electron densities are calculated for CaO with a rock-salt lattice, CaF₂ with a fluorite lattice, K₂O with an antifluorite lattice, and for their constituent sublattices. It is shown that in the crystals with different Bravais sublattices, the anionic sublattice is a framework with covalent bonds containing a metal sublattice inside of them. The coupling between the sublattices is characterized by the density difference, which is deflned as the difference between the total electron density and the densities of the individual sublattices. The density difference is found to be an order of magnitude smaller than the crystal and sublattice densities, which is evidence of weak hybridization of the sublattices and of the predominately ionic character of the bonding between them.     

Modeling trajectories and radiation spectra of relativistic particles in crystals. Radiation polarization characteristics

A method is developed for calculation of the polarization characteristics of radiation based on computer tracking of relativistic particle trajectories. The dependence of the Stokes parameter which defines the polarization properties of the radiation upon photon frequency is determined for characteristic trajectories. Formation of full spectra and the degree of linear polarization for averaging over possible trajectories are considered, and results are compared with available experiments.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 63–68, November, 1986.The authors are indebted to S. A. Vorob'ev for his support in the study and evaluation, to M. I. Podgoretskii and A. P. Potyltsyn for their valuable discussion, and to O. V. Boyarko for assistance in formulating the plan of the study.     

Effects of Poisson's ratio on the band gaps and defect states in two-dimensional vacuum/solid porous phononic crystals

The effects of the Poisson’s ratio of the solid host on the band gaps and point defect states of the mixed elastic wave modes in two-dimensional vacuum/solid porous PNCs are studied by numerical simulations. Four typical systems are considered. The four systems are, respectively, (I) the system with a square lattice and circular pores, (II) the system with a hexagonal lattice and circular pores, (III) the system with a square lattice and square pores and (IV) the system with a hexagonal lattice and regular-hexagonal pores. In the latter two systems, with respect to the outer boundaries of the Wigner-Seitz unit cell, the pores rotate 45° and 30°, respectively. Some observable effects of the Poisson’s ratio are found in the numerical results. Especially, the variations of the band gap boundaries with the Poisson’s ratio exhibit relatively consistent behaviors. With the increase of the Poisson’s ratio, the normalized frequency of a band gap boundary generally increases, except that in system (III) the normalized frequency of the upper boundary of the first band gap remains almost unchanged. Detailed interpretations on this phenomenon are given.