Defect formation in tellurium in various gravity conditions

The effect of gravity (from micro-to a rather high gravity, 5g ₀) acting during tellurium crystallization on the concentration of neutral (N _D ) and electrically active (N _AD ) acceptor-type structural defects in samples grown both under complete remelting of the starting ingot and under directed seed recrystallization of an ingot was studied. The concentrations N _AD and N _D and their distribution over the sample length were derived from the electrical characteristics (conductivity and the Hall effect) measured along the ingots in the temperature range 1.6–300 K. The contributions of various mechanisms to hole scattering were found from an analysis of the temperature behavior of the mobility. The results obtained were compared with the characteristics of samples grown following a similar program under normal conditions. The presence of N _AD defects is characteristic of the initial crystallization stage of all samples. N _AD is substantially lower (N _AD ～ 10¹⁵ cm^?3) than N _D ～ 10¹⁸ cm^?3 and decreases exponentially in the course of sample crystallization. Complete remelting under microgravity revealed indications of strong supercooling and spontaneous crystallization, as well as spatial oscillations of the electrical resistivity over the sample length caused by N _D modulation. These observations are related to the specific features of the melting and crystallization in zero gravity, namely, the melt breaking away from the wall of the ampoule and the increasing role of Marangoni convection.     

The role of orbital ordering in the formation of electron structure in undoped LaMnO3 manganites in the regime of strong electron correlations

The electron structure of undoped LaMnO₃ and slightly doped La_1?x Sr_xMnO₃ manganites has been calculated within the framework of a generalized tight binding method with explicit allowance for strong intraatomic electron correlations. According to the results of these calculations, the ground state in orbitally disordered undoped LaMnO₃ ferromagnets would be metallic despite the Mott-Hubbard correlation gap in the spectrum of quasiparticles. Owing to the orbital ordering, the insulating state is stabilized in both antiferromagnetic and paramagnetic phases. In-gap states of a polaron nature with a spectral weight proportional to the dopant concentration have been found near the top of the valence band in La_1?x Sr_xMnO₃. As the doping level increases, a metal state appears in the ferromagnetic phase, which has a metallic character for one spin subband and an insulating character for the other subband (representing the so-called half-metallic state).