28 research outputs found

    Consistent low-energy reduction of the three-band model for copper oxides with O-O hopping to the effective t-J model

    Full text link
    A full three-band model for the CuO2_{2} plane with inclusion of all essential interactions - Cu-O and O-O hopping, repulsion at the copper and oxygen and between them - is considered. A general procedure of the low-energy reduction of the primary Hamiltonian to the Hamiltonian of the generalized tt-tt'-JJ model is developed. An important role of the direct O-O hopping is discussed. Parameters of the effective low-energy model (the hopping integral, the band position and the superexchange constant JJ are calculated. An analysis of the obtained data shows that the experimental value of JJ fixes the charge transfer energy Δ=(ϵpϵd)\Delta =(\epsilon_{p}-\epsilon_{d}) in a narrow region of energies.Comment: 32 pp. (LATEX), two figures (PostScript) appende

    Range of the t--J model parameters for CuO2_{2} plane: experimental data constraints

    Full text link
    The t-J model effective hopping integral is determined from the three-band Hubbard model for the charge carriers in CuO2_{2} plane. For this purpose the values of the superexchange constant JJ and the charge-transfer gap EgapE_{gap} are calculated in the framework of the three-band model. Fitting values of JJ and EgapE_{gap} to the experimental data allows to narrow the uncertainty region of the three-band model parameters. As a result, the t/Jt/J ratio of the t-J model is fixed in the range 2.4÷2.72.4 \div 2.7 for holes and 2.5÷3.02.5 \div 3.0 for electrons. Formation of the Frenkel exciton is justified and the main features of the charge-transfer spectrum are correctly described in the framework of this approach.Comment: 20pp., REVTEX 3.0, (11 figures), report 66

    High Power Electric Propulsion: MARS plus EUROPA – Already Beyond 2025!

    Get PDF
    It’s mid-term realization plus global strategic investments: the results of the European Russian DEMOCRITOS project (Horizon 2020) related to the MW class INPPS (International Nuclear Power and Propulsion System) flagship will be described. INPPS flagship includes high power electric thrusters cluster, supplied electric power by the nuclear reactor (successfully tested in Russia) and a solar power ring. Two INPPS versions were studied – the wide and arrow wing versions. Both versions have a futuristic design with standardized interfaces for several flagship subsystems. Especially the high payload mass of INPPS allows the transport of – for example – up to 12 t to JUPITER moon EUROPA and about 18 t to MARS – as a function of specific impulse of electric thrusters. INPPS flagship not only allows scientific, but especially commercial and communication payloads as well. This means industrial-scale production of space flight systems for robotic and human space exploration. International cooperation related to INPPS realization are necessary within an International High Power Space Transportation program to realize the DEMOCRITOS core, ground and space components until 2025. DEMOCRITOS project included partners from Europe, Russia and a Brazilian guest observer and received several inputs from NASA Cleveland and JAXA Tokyo

    Lifshitz quantum phase transitions and Fermi surface transformation with hole doping in high-TcT_c superconductors

    Full text link
    We study the doping evolution of the electronic structure in the normal phase of high-TcT_c cuprates. Electronic structure and Fermi surface of cuprates with single CuO2_2 layer in the unit cell like La2x_{2-x}Srx_xCuO4_4 have been calculated by the LDA+GTB method in the regime of strong electron correlations (SEC) and compared to ARPES and quantum oscillations data. We have found two critical concentrations, xc1x_{c1} and xc2x_{c2}, where the Fermi surface topology changes. Following I.M. Lifshitz ideas of the quantum phase transitions (QPT) of the 2.5-order we discuss the concentration dependence of the low temperature thermodynamics. The behavior of the electronic specific heat δ(C/T)(xxc)1/2\delta(C/T) \sim (x - x_c)^{1/2} is similar to the Loram and Cooper experimental data in the vicinity of xc10.15x_{c1} \approx 0.15.Comment: 8 pages, 4 figure

    Geography of “Archival continents” of Russian Diaspora: history of formation

    No full text
    The article analyzes the process of the establishment of archival continents of the Russian diaspora and the causes of the emergence of foreign archival Rossica. The article reveals the influence of migrations on this process. Authors show the diversity of archival materials on the Russian history which are kept outside Russia and often scattered in different archival collections, private and public archives and libraries. The emergence of communities of the Russian diaspora in several countries of the world led to the formation of extensive archival heritage. The study of this heritage is one the most urgent contemporary problems. The activities related to the discovery, restitution to Russia and use of this heritage have significantly increased in the last decades which is connected with the change of the political situation, researchers’ gaining access to the archival collections of the Russian diaspora. The relevance of this fundamental research problem is enhanced by the necessity of eliminating the existing information gap at the current stage of the Russian history development - to reveal the information concerning foreign archival Rossica, the documents of the Russian diaspora and their locations in foreign archives and compile them in the unified information data base. The identification of the location of different documents of this group is aimed at the reconstruction of the integrity of the scattered archival collections by means of both restitution, exchange or copying of the documents and by means of virtual unification, for example, in the form of digital archive. These activities will also contribute to solving the problems related to the restitution of the archival heritage

    Investigation of discharge channel wall material influence on lifetime of hall effect thruster with high specific impulse

    No full text
    Results of 500-hour life tests of the 900-watt Hall-thruster laboratory model with the specific impulse of 2000 s are presented. The thruster discharge channel walls were manufactured from 60% BN + 40% SiO2 and >90% BN hot-pressed ceramics. The predicted total lifetime was ∼3000 h for both wall materials in spite of greater erosion resistance of pure BN in comparison with BN-SiO2 mixture. To clarify the accompanying phenomena, the following diagnostics were carried out. The surface microstructure and composition insulators were investigated by means of electron microscopy and X-ray fluorescence analysis and nearwall plasma parameters were measured with flat Langmuir probes. The obtained distributions of plasma parameters were compared with the results of stationary one-dimensional (1D) hydrodynamic modeling of discharge channel
    corecore