Role of electronic correlations in the Fermi surface formation of Nax_xCoO2_2

Band structure of metallic sodium cobaltate Na$_x$CoO$_2$ ($x$=0.33, 0.48, 0.61 0.72) has been investigated by local density approximation+Hubbard $U$ (LDA+$U$) method and within Gutzwiller approximation for the Co-$t_{2g}$ manifold. Correlation effects being taken into account results in suppression of the $e'_g$ hole pockets at the Fermi surface in agreement with recent angle-resolved photo-emission spectroscopy (ARPES) experiments. In the Gutzwiller approximation the bilayer splitting is significantly reduced due to the correlation effects. The formation of high spin (HS) state in Co $d$-shell was shown to be very improbable.Comment: 6 pages, 2 figure

Two-instanton approximation to the Coulomb blockade problem

We develop the two-instanton approximation to the current-voltage characteristic of a single electron transistor within the Ambegaokar-Eckern-Sch\"on model. We determine the temperature and gate voltage dependence of the Coulomb blockade oscillations of the conductance and the effective charge. We find that a small (in comparison with the charging energy) bias voltage leads to significant suppression of the Coulomb blockade oscillations and to appearance of the bias-dependent phase shift

Charge relaxation resistance in the Coulomb blockade problem

We study the dissipation in a system consisting of a small metallic island coupled to a gate electrode and to a massive reservoir via single tunneling junction. The dissipation of energy is caused by a slowly oscillating gate voltage. We compute it in the regimes of weak and strong Coulomb blockade. We focus on the regime of not very low temperatures when electron coherence can be neglected but quantum fluctuations of charge are strong due to Coulomb interaction. The answers assume a particularly transparent form while expressed in terms of specially chosen physical observables. We discovered that the dissipation rate is given by a universal expression in both limiting cases.Comment: 21 pages, 12 figure

Basing factors for young innovative companies resulting from localization of foreign corporations

The relevance of our research is due to the fact that foreign corporations’ technologies transfer is an efficient tool of achieving a high pace of economic development in Russia and its regions. Its success depends on the ability of economic actors to embrace innovations and their high innovative activity. The purpose of this study was the consideration of start-up factors of young innovative companies resulting from the localization of foreign corporations’ technologies and the identification of conditions conducive to accelerating the diffusion of innovation in local markets. During the study of the specifics of Russian start-ups, with the use of the methods such as analysis of documents, content analysis, observation, expert interviews, the following hypothesis was identified and justified: the emergence of new markets for an already developed and functioning innovation makes it possible to intensify the process of establishing new enterprises in the domestic environment. The novelty of the scientific problem is to identify and substantiate the main conditions and factors affecting the creation of a non-pioneer young innovative company in the Russian regional economy based on the experience and technologies of foreign corporations, including learning state of the investment climate and area’s business ecosystem. The article highlights the stages of transformation of business ideas of such enterprise into functioning business in a context of its design tools, such as: monitoring emerging consumer needs and turning them into business opportunities and business ideas; building business models, business planning. At this point the creation of a new organization is considered in conjunction with factors of regional economic environment. As a result of the survey there was developed theoretical and methodological basis of companies’ localization based on the concept of lean production; there was proposed a stepwise mechanism involving a sequence of steps that must be made by young enterprises in order to effectively perform the diffusion of innovation.Актуальность темы исследования обусловлена тем, что трансфер технологий зарубежных корпораций является эффективным средством, позволяющим достичь высоких темпов экономического развития России и регионов. Но его успешность определяется способностью экономических субъектов воспринимать инновации и их высокой инновационной активностью. Целью настоящего исследования стало рассмотрение факторов основания молодых инновационных компаний, возникших в результате локализации технологий зарубежных корпораций и выявление условий, способствующих ускорению диффузии инноваций на региональных рынках. В процессе исследования специфики российских стартапов с помощью таких методов, как анализ документов, контент-анализ, экспертный опрос авторами была выявлена и обоснована следующая гипотеза: в отечественных условиях именно появление новых рынков сбыта для уже освоенных и функционирующих инноваций позволяет активизировать процесс создания молодых предприятий. Новизна научной проблемы заключается в выявлении и обосновании основных условий и факторов, оказывающих влияние на создание молодых не пионерских инновационных компаний в экономике России и регионов на основе использования опыта и технологий зарубежных корпораций, с учетом инвестиционного климата и предпринимательской экосистемы территории. В статье выделены стадии трансформации предпринимательской идеи такого предприятия в функционирующий бизнес в контексте инструментов его проектирования, таких как мониторинг возникающих потребностей потребителей и превращение их в предпринимательские возможности и бизнес-идеи; построение бизнес-моделей, бизнес-планирование. При этом этапы создания новой организации рассматриваются во взаимосвязи с факторами региональной экономической среды. По итогам исследования были разработаны теоретико-методологические основы локализации компаний на основе концепции бережливого производства; предложен ее пошаговый механизм, включающий последовательность этапов, которые необходимо пройти молодому предприятию для того, чтобы эффективно совершить диффузию инновации; раскрыты условия, позволяющие эффективно создавать новые предприятия за счет диффузии инноваций

Transformations in the grain boundary ensemble of M1 copper subjected to equal-channel angular pressing during recrystallization annealing

The grain structure of M1 copper subjected to equal-channel angular pressing (ECAP) and subsequent annealing at 593K for 1 h is studied by means of transmission electron microscopy and scanning electron microscopy with the diffraction of backscattered electron

Itinerant in-plane magnetic fluctuations and many-body correlations in Nax_xCoO2_2

Based on the {\it ab-initio} band structure for Na$_x$CoO$_2$ we derive the single-electron energies and the effective tight-binding description for the $t_{2g}$ bands using projection procedure. Due to the presence of the next-nearest-neighbor hoppings a local minimum in the electronic dispersion close to the $\Gamma$ point of the first Brillouin zone forms. Correspondingly, in addition to a large Fermi surface an electron pocket close to the $\Gamma$ point emerges at high doping concentrations. The latter yields the new scattering channel resulting in a peak structure of the itinerant magnetic susceptibility at small momenta. This indicates dominant itinerant in-plane ferromagnetic fluctuations above certain critical concentration $x_m$, in agreement with neutron scattering data. Below $x_m$ the magnetic susceptibility shows a tendency towards the antiferromagnetic fluctuations. We further analyze the many-body effects on the electronic and magnetic excitations using various approximations applicable for different $U/t$ ratio.Comment: 10 page

Quasiparticle states of the Hubbard model near the Fermi level

The spectra of the t-U and t-t'-U Hubbard models are investigated in the one-loop approximation for different values of the electron filling. It is shown that the four-band structure which is inherent in the case of half-filling and low temperatures persists also for some excess or deficiency of electrons. Besides, with some departure from half-filling an additional narrow band of quasiparticle states arises near the Fermi level. The dispersion of the band, its bandwidth and the variation with filling are close to those of the spin-polaron band of the t-J model. For moderate doping spectral intensities in the new band and in one of the inner bands of the four-band structure decrease as the Fermi level is approached which leads to the appearance of a pseudogap in the spectrum.Comment: 8 pages, 7 figure

Electronic theory for itinerant in-plane magnetic fluctuations in Nax_xCoO2_2

Starting from {\it ab-initio} band structure for Na$_x$CoO$_2$, we derive the single-electron energies and the effective tight-binding description for the $t_{2g}$ bands using a projection procedure. We find that due to the presence of the next-nearest-neighbor hoppings a local minimum in the electronic dispersion close to the $\Gamma$ point of the first Brillouin zone forms. Therefore, in addition to a large Fermi surface an electron pocket close to the $\Gamma$ point emerges at high doping concentrations. The latter yields the new scattering channel resulting in a peak structure of the itinerant magnetic susceptibility at small momenta. This indicates itinerant in-plane ferromagnetic state above certain critical concentration $x_m$, in agreement with neutron scattering data. Below $x_m$ the magnetic susceptibility shows a tendency towards the antiferromagnetic fluctuations. We estimate the value of $0.58 < x_m < 0.7$ within the rigid band model and within the Hubbard model with infinite on-site Coulomb repulsion consistent with the experimental phase diagram.Comment: 4 pages, 4 figures; LDA calculations were done with Na in the symmetric 2d position contrary to the 6h position in a previous version of this pape

The problem of "macroscopic charge quantization" in single-electron devices

In a recent Letter by the authors [I.S. Burmistrov and A.M.M. Pruisken, Phys. Rev. Lett. 101, 056801 (2008)] it was shown that single-electron devices (single electron transistor or SET) display "macroscopic charge quantization" which is completely analogous to the quantum Hall effect observed on very different electron systems. In this investigation we present more detail on these new findings. Based on the Ambegaokar-Eckern-Schoen (AES) theory of the Coulomb blockade we introduce a general response theory that probes the sensitivity of SET to changes in the boundary conditions. This response theory defines a new set of physical observables and we establish the contact with the standard results obtained from ordinary linear response theory. The response parameters generally define the renormalization behavior of the SET in the entire regime from weak coupling with large values of the tunneling conductance all the way down to the strong coupling phase where the system displays the Coulomb blockade. We introduce a general criterion for charge quantization that is analogous to the Thouless criterion for Anderson localization. We present the results of detailed computations on the weak coupling side of the theory, i.e. both perturbative and non-perturbative (instantons). Based on an effective theory in terms of quantum spins we study the quantum critical behavior of the AES model on the strong coupling side. Consequently, a unifying scaling diagram of the SET is obtained. This diagram displays all the super universal topological features of the theta-angle concept that previously arose in the theory of the quantum Hall effect.Comment: RevTex, 22 pages, 10 figure

Parameters of the Effective Singlet-Triplet Model for Band Structure of High-TcT_c Cuprates by Different Approaches

The present paper covers the problem of parameters determination for High-$T_c$ superconductive copper oxides. Different approaches, {\it ab initio} LDA and LDA+U calculations and Generalized Tight-Binding (GTB) method for strongly correlated electron systems, are used to calculate hopping and exchange parameters of the effective singlet-triplet model for $CuO_2$-layer. The resulting parameters are in remarkably good agreement with each other and with parameters extracted from experiment. This set of parameters is proposed for proper quantitative description of physics of hole doped High-$T_c$ cuprates in the framework of effective models.Comment: PACS 74.72.h; 74.20.z; 74.25.Jb; 31.15.A