Absorption and emission of terahertz radiation in doped GaAs/AlGaAs quantum wells

Spontaneous emission of terahertz radiation from structures with GaAs/AlGaAs quantum wells in a longitudinal magnetic field has been studied. It is shown that some bands in the emission spectrum can be related to radiative electron transitions between resonant and localized impurity states, as well as to the transitions with participation of subband states. The temperature dependence of the equilibrium intraband absorption of terahertz radiation and its modulation in a longitudinal electric field in GaAs/AlGaAs quantum wells has been investigated.     

Classification of explicit three-stage symplectic difference schemes for the numerical solution of natural Hamiltonian systems: A comparative study of the accuracy of high-order schemes on molecular dynamics problems

The natural Hamiltonian systems (systems with separable Hamiltonians) are considered. The variety of explicit three-stage symplectic schemes is described. A classification of the third-order accurate schemes is given. All fourth-order schemes are found (there are seven of them). It is proved that there are no fifth-order schemes. The schemes with improved properties, such as invertibility and optimality with respect to the phase error, are listed. Numerical results that demonstrate the properties of these schemes are presented, and their comparative analysis with respect to the accuracy–efficiency criterion is given. The disbalance of total energy is used as the accuracy criterion.     

Electronic Structure and Chemical Bonds in Titanium Dicarbide with Disordered C2Dimers

A quantum–chemical zone and cluster approaches were used to model the electronic energy spectrum and to study the chemical bond in a hypothetical phase of titanium dicarbide (TiC₂) composed of C₂dimers as structural units. The effect of orientational disorder of the C₂dimers on the electronic structure and chemical stability of the dicarbide was investigated.     

Electronic structure and stability of polymorphous modifications of titanium monocarbide

The electronic structures and stabilities of five polymorphous modifications (PM) of titanium monocarbide, TiC, with the NaCl (Bl), CsCl (B2), zinc blende (B3), würtzite (B4), and NiAs (B8) structural types. were studied by the non-empirical, self-consistent linear “muffin-tin” orbital-tight-binding (LMTO-TB) method. The equilibrium unit cell volumes. latice parameters, and the densities of electronic states were calculated. Depending on the structural type (coordination number, CN), the PM of TiC possess essentially different conducting properties (semimetallic for CN 6 (B1, B8), metallic for CN 8 (B2), and semiconducting for CN 4 (B3, B4)). Based on the results of calculations of the total band energies, it was found that the stabilities of these PM of TiC decrease in the order B1>B8>B3>≈B4>B2. The nature of chemical bonding in the five PM of TiC was analyzed in the tight-binding approximation with EHT parametrization. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1821–1824, October, 2000.     

Intrinsic point defects and electron-energetic characteristics of hexagonal monoxide of beryllium

Using the zone method of a linear combination of plane waves for the supercell model, there have been studied energy and electron states of beryllium monoxide BeO with intrinsic point defects: the vacancy in the sub-lattice of beryllium + interstitial atom of Be {V_Be + Be_i}. The most stable among the considered configurations of defects are the vacancies and interstitial atoms of beryllium Be _i ^o occupying octahedral positions at R(V_Be-Be _i ^o ) ～ 3.15–3.35 Å distances from each other. Depending on the configuration of defects, the forbidden gap of beryllium oxide can change by ～0.4–2.4eV.     

(+)-Dimethyl tartrate in the synthesis of optically active bis-arylphosphonodithioic acids

Russian Journal of Organic Chemistry -     

Nucleophilic thiylation of carvone

The reaction of carvone with 2-mercaptoethanol and an S-phenoxybenzylisothiuronium salt in the presence of sodium ethoxide was found to proceed exclusively at the endocyclic double bond. Under these same conditions, an S-allylisothiuronium salt reacted to produce a bis-sulfide with the menthene structure. __________ Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 44–46, January–February, 2007.