Manifestation of exciton Bose condensation in induced two-phonon emission and Raman scattering

The unusual two-photon emission by Bose-condensed excitons caused by simultaneous recombination of two excitons with opposite momenta leaving the occupation numbers of excitonic states with momenta $p nonequal to 0$ unchanged (below coherent two-exciton recombination) is investigated. Raman scattering accompanied by the analogous two-exciton recombination (or creation) is also analyzed. The excess momentum equal to the change of the electromagnetic field momentum in these processes can be transferred to phonons or impurities. The processes under consideration take place if there is Bose condensation in the interacting exciton system, and, therefore, can be used as a new method to reveal exciton Bose condensation. If the frequency of the incident light $\omega< 2\Omega$ ($\Omega$ is the frequency corresponding to the recombination of an exciton with p=0), the coherent two-exciton recombination with the excess momentum elastically transferred to impurities leads to the appearance of the spectral line $2\Omega-\omega$ corresponding to the induced two-photon emission. In this case the anti-Stokes line on frequency $\omega+2\Omega$ also appears in the Raman spectrum. If $\omega>2\Omega$, there are both Stokes and anti-Stokes lines on frequencies $\omega\pm2\Omega$ in the Raman spectrum. The induced two-photon emission is impossible in this case. The spectral lines mentioned above have phonon replicas on frequencies $|\omega\pm (2\Omega-n\omega^s_0)|$ corresponding to the transmission of the excess momentum (partially or as a whole) to optical phonons of frequency $\omega^s_0$ ($n$ is an integer number).Comment: 21 pages, 2 Postscript figures. Submitted to Phys. Rev. B (1998

Non-minimally Coupled Cosmological Models with the Higgs-like Potentials and Negative Cosmological Constant

We study dynamics of non-minimally coupled scalar field cosmological models with Higgs-like potentials and a negative cosmological constant. In these models the inflationary stage of the Universe evolution changes into a quasi-cyclic stage of the Universe evolution with oscillation behaviour of the Hubble parameter from positive to negative values. Depending on the initial conditions the Hubble parameter can perform either one or several cycles before to become negative forever.Comment: 22 pages, 6 figures, v4:Section 2 expanded, references added, accepted for publication in Class. Quant. Gra

Temperature effects on dislocation core energies in silicon and germanium

Temperature effects on the energetics of the 90-degree partial dislocation in silicon and germanium are investigated, using non-equilibrium methods to estimate free energies, coupled with Monte Carlo simulations. Atomic interactions are described by Tersoff and EDIP interatomic potentials. Our results indicate that the vibrational entropy has the effect of increasing the difference in free energy between the two possible reconstructions of the 90-degree partial, namely, the single-period and the double-period geometries. This effect further increases the energetic stability of the double-period reconstruction at high temperatures. The results also indicate that anharmonic effects may play an important role in determining the structural properties of these defects in the high-temperature regime.Comment: 8 pages in two-column physical-review format with six figure

Scanning Electron Microscopy Study of Drilling Cuttings in Tomsk Oblast Sites

The research is focused on determining mineralogical composition of drilling cuttings by scanning electron microscope as well as imaging the sample surface of high resolution that allows studying the structural characteristics of the site. In addition, a number of other techniques permit obtaining information on chemical composition of sample in near-surface layers. The study in drilling cuttings by means of scanning microscopy has revealed the presence of titanium, iron, zirconium oxides, iron sulphide, barium sulphate. The former is a mineral that concentrates rare-earth elements, presumably monocyte, as well as uranium silicate, etc. The results obtained confirm the data of previous X-ray structural analysis, i.e. the study samples consist of alumosilicate matrix. Apart from silicon and aluminium oxides, the matrix includes such elements as Na, K, Mg. Such a composition corresponds to rock-forming minerals: quartz, albite, microcline, clinochlore, muscovite, anorthoclase

Stable Exact Solutions in Cosmological Models with Two Scalar Fields

The stability of isotropic cosmological solutions for two-field models in the Bianchi I metric is considered. We prove that the sufficient conditions for the Lyapunov stability in the Friedmann-Robertson-Walker metric provide the stability with respect to anisotropic perturbations in the Bianchi I metric and with respect to the cold dark matter energy density fluctuations. Sufficient conditions for the Lyapunov stability of the isotropic fixed points of the system of the Einstein equations have been found. We use the superpotential method to construct stable kink-type solutions and obtain sufficient conditions on the superpotential for the Lyapunov stability of the corresponding exact solutions. We analyze the stability of isotropic kink-type solutions for string field theory inspired cosmological models.Comment: 23 pages, v3:typos corrected, references adde

Atomic-scale modeling of the deformation of nanocrystalline metals

Nanocrystalline metals, i.e. metals with grain sizes from 5 to 50 nm, display technologically interesting properties, such as dramatically increased hardness, increasing with decreasing grain size. Due to the small grain size, direct atomic-scale simulations of plastic deformation of these materials are possible, as such a polycrystalline system can be modeled with the computational resources available today. We present molecular dynamics simulations of nanocrystalline copper with grain sizes up to 13 nm. Two different deformation mechanisms are active, one is deformation through the motion of dislocations, the other is sliding in the grain boundaries. At the grain sizes studied here the latter dominates, leading to a softening as the grain size is reduced. This implies that there is an ``optimal'' grain size, where the hardness is maximal. Since the grain boundaries participate actively in the deformation, it is interesting to study the effects of introducing impurity atoms in the grain boundaries. We study how silver atoms in the grain boundaries influence the mechanical properties of nanocrystalline copper.Comment: 10 pages, LaTeX2e, PS figures and sty files included. To appear in Mater. Res. Soc. Symp. Proc. vol 538 (invited paper). For related papers, see http://www.fysik.dtu.dk/~schiotz/publist.htm

ГЕНЕТИЧЕСКИЕ БИОЛОГИЧЕСКИЕ МАРКЕРЫ ГЛИАЛЬНЫХ ОПУХОЛЕЙ ГОЛОВНОГО МОЗГА: МУТАЦИИ В ГЕНАХ ИЗОЦИТРАТДЕГИДРОГЕНАЗ 1 И 2

Introduction. Mutations in the isocytrate dehydrogenase 1 and 2 genes (IDH1 and IDH2) are considered driver genetic events in gliomas. Their frequency reaches 7080 % in low-grade gliomas and in secondary glioblastomas, and their oncogenic effect is realized by accumulation of the metabolite 2-hydroxyglutarate, which disrupts DNA and protein methylation processes. The aim of the study was to analyze the associations between the presence of IDH1/2 mutations and clinical and morphological parameters of glial tumors. Material and Methods. The study included 147 patients with glial brain tumors. Associations between IDH1/2 status and tumor histological type, age of disease onset, tumor localization, and clinical manifestations were investigated. Results. Gliomas containing IDH1/2 mutations were characterized by a younger age at diagnosis (mean: 39.5 years) compared to IDH-negative cases (47.2 years) (p&lt;0.01). IDH1/2-mutated tumors were more often localized in the frontal (53.4 %) and parietal lobes (61.3 %) than in the other areas of the brain (p&lt;0.05). It was demonstrated that the incidence of epilepsy was significantly higher among patients with IDH1/2 genetic defects (69.2 % vs. 48.2 %, p&lt;0.05). Patients with IDH1/2 mutations had more favorable course of the disease. Among individuals with a combination of these factors (localization of the tumor in the frontal or parietal lobe, presence of epilepsy, age younger than 39 years), the frequency of IDH1/2 mutations reached 21/27 (77.8 %), which was significantly higher than that in all other patients (44/119 (37.0 %), OR = 5.97, 95 % CI: 2.2415.91, p&lt;0.001). Conclusion. The presence of IDH1/2 genetic defects is associated with localization of glial tumors in the frontal and parietal lobes of the brain, earlier age at disease onset and the presence of epileptic syndrome.Актуальность. Мутации в генах изоцитратдегидрогеназ 1 и 2 (IDH1 и IDH2) относятся к «драйверным» генетическим событиям при глиомах. Их частота достигает 70–80 % в глиомах низкой степени злокачественности и во вторичных глиобластомах, а онкогенный эффект реализуется путем избыточного накопления метаболита 2-гидроксиглутарата, нарушающего нормальные процессы метилирования ДНК и белков в клетках. Цель исследования – проанализировать ассоциации между наличием мутаций IDH1/2 и важнейшими клинико-морфологическими параметрами глиальных опухолей. Материал и методы. В исследование вошло 147 пациентов с глиальными опухолями головного мозга. Была изучена связь мутаций IDH1/2 с гистологическим типом, возрастом начала заболевания, локализацией опухоли, клиническими проявлениями глиом. Результаты. Установлено, что для cодержащих мутации IDH1/2 глиом характерен более молодой возраст установки диагноза (39,5 лет) по сравнению с IDH-негативными случаями (47,2 года) (p&lt;0,01). Обнаружено, что опухоли с повреждениями IDH1/2 значительно чаще локализуются в лобной (53,4 %) и теменной долях (61,3 %), чем в других областях головного мозга (p&lt;0,05). Продемонстрировано, что частота развития эпилепсии значительно выше среди пациентов с IDH1/2-мутациями (69,2 % vs 48,2 %, p&lt;0,05). У больных с мутациями IDH1/2 также зафиксировано более благоприятное течение заболевания. При сочетании перечисленных факторов (локализация опухоли в лобной или теменной доле, наличие эпилепсии, возраст моложе 39 лет) частота мутаций IDH1/2 достигла 21/27 (77,8 %) и оказалась намного выше, чем у остальных пациентов (44/119 (37,0 %), OR=5,97, 95 % CI: 2,24–15,91, p&lt;0,001). Заключение. Присутствие генетических дефектов IDH1/2 ассоциировано с локализацией глиальных опухолей в лобной и теменной долях головного мозга и с клиническими прогностическими факторами, такими как возраст начала заболевания и наличие эпилептического синдрома