Spectral and polarization dependencies of luminescence by hot carriers in graphene

The luminescence caused by the interband transitions of hot carriers in graphene is considered theoretically. The dependencies of emission in mid- and near-IR spectral regions versus energy and concentration of hot carriers are analyzed; they are determined both by an applied electric field and a gate voltage. The polarization dependency is determined by the angle between the propagation direction and the normal to the graphene sheet. The characteristics of radiation from large-scale-area samples of epitaxial graphene and from microstructures of exfoliated graphene are considered. The averaged over angles efficiency of emission is also presented.Comment: 6 pages, 5 figure

THE EFFECT OF THE ANISOTROPY DRILLING INDEX ON THE DEVIATION OF THE WELL AXIS FROM THE DESIGN PROFILE

Analytical studies of the interaction of the  load on the bottom and the bottom hole assembly of the drill string with the bottom itself and the wall of the well during drilling tilted formations have been  carried out. The correlation interrelation among the drilling index of anisotropy, zenith angle, the slope of the seams, geometric characteristics of the well, the bottom hole assembly of the drill string and the axial load on the bit has been proved.Analytical studies of the impact of bottom hole  assembly  of the drill string containing a centralizer on the bottom itself and the borehole wall have been performed. It has been found  that with an increase in the axial load on the bit and the gap between  the loaded drill pipes and the borehole wall from the bit to the point of contact, the column with the borehole wall distance decreases, and the centralizer in the bottomhole assembly of the drill string increases this distance, thus  increasing bit  load without the risk of the zenith angle deviation.Keywords: borehole wall, zenith angle, drill string, bit load, bottom hole assembly of the drill string, centralizer, dispersion index anisotropy, reservoir angle.кандидат технічних наук, доцент, Кочкодан Я. М., Васько А. І., Добруцький Р. Л. Вплив бурового індексу анізотропії на відхилення осі свердловини від проектного профілю/ Івано-Франківський національний технічний університет нафти і газу, Україна, Івано-ФранківськПроведено аналітичні дослідження взаємодії навантаження на вибій та компоновки низу бурильної колони з вибоєм та стінкою свердловини при бурінні у похило залеглих пластах. Показано взаємозв’язок між буровим індексом анізотропії, зенітним кутом, кутом нахилу пластів, геометричними характеристиками свердловини, компоновкою низу бурильної колони та осьовим навантаженням на долото.Аналітично  досліджено вплив нижньої частини бурильної колони з вибоєм та стінкою свердловини при наявності центратора. Встановлено, що зі збільшенням осьового навантаження на долото та зазору між обважненими бурильними трубами і стінкою свердловини відстань від долота до точки дотику колони зі стінкою свердловини зменшується, а наявність в компоновці низу бурильної колони центратора збільшує цю відстань, що дозволяє збільшити навантаження на долото без небезпеки росту зенітного кута.Ключові слова: стінка свердловини, зенітний кут, бурильна колона, навантаження на долото,компоновка низу бурильної колони, центратор, буровий індекс анізотропії, кут нахилу пласта

Kinetic models of tangential discontinuities in the solar wind

TN acknowledges financial support by the UK's Science and Technology Facilities Council (STFC) via Consolidated Grant ST/S000402/1. OA was supported by the Natural Environment Research Council (NERC) Highlight Topic Grant #NE/P017274/1 (Rad-Sat).Kinetic-scale current sheets observed in the solar wind are frequently approximately force-free despite the fact that their plasma β is of the order of one. In-situ measurements have recently shown that plasma density and temperature often vary across the current sheets, while the plasma pressure is approximately uniform. In many cases these density and temperature variations are asymmetric with respect to the center of the current sheet. To model these observations theoretically we develop in this paper equilibria of kinetic-scale force-free current sheets that have plasma density and temperature gradients. The models can also be useful for analysis of stability and dissipation of the current sheets in the solar wind.PostprintPeer reviewe

Energy spectrum, density of states and optical transitions in strongly biased narrow-gap quantum wells

We study theoretically the effect of an electric field on the electron states and far-infrared optical properties in narrow-gap lead salt quantum wells. The electron states are described by a two-band Hamiltonian. An application of a strong electric field across the well allows the control of the energy gap between the two-dimensional (2D) states in a wide range. A sufficiently strong electric field transforms the narrow-gap quantum well to a nearly gapless 2D system, whose electron energy spectrum is described by linear dispersion relations \epsilon_{\sigma} (k) ~\pm (k-k_{\sigma}), where k_{\sigma} are the field-dependent 2D momenta corresponding to the minimum energy gaps for the states with spin numbers \sigma. Due to the field-induced shift of the 2D subband extrema away from k=0 the density of states has inverse-square-root divergencies at the edges. This property may result in a considerable increase of the magnitude of the optical absorption and in the efficiency of the electrooptical effect.Comment: Text 18 pages in Latex/Revtex format, 7 Postscript figure

Scattering Theory of Current-Induced Spin Polarization

We construct a novel scattering theory to investigate magnetoelectrically induced spin polarizations. Local spin polarizations generated by electric currents passing through a spin-orbit coupled mesoscopic system are measured by an external probe. The electrochemical and spin-dependent chemical potentials on the probe are controllable and tuned to values ensuring that neither charge nor spin current flow between the system and the probe, on time-average. For the relevant case of a single-channel probe, we find that the resulting potentials are exactly independent of the transparency of the contact between the probe and the system. Assuming that spin relaxation processes are absent in the probe, we therefore identify the local spin-dependent potentials in the sample at the probe position, and hence the local current-induced spin polarization, with the spin-dependent potentials in the probe itself. The statistics of these local chemical potentials is calculated within random matrix theory. While they vanish on spatial and mesoscopic average, they exhibit large fluctuations, and we show that single systems typically have spin polarizations exceeding all known current-induced spin polarizations by a parametrically large factor. Our theory allows to calculate quantum correlations between spin polarizations inside the sample and spin currents flowing out of it. We show that these large polarizations correlate only weakly with spin currents in external leads, and that only a fraction of them can be converted into a spin current in the linear regime of transport, which is consistent with the mesoscopic universality of spin conductance fluctuations. We numerically confirm the theory.Comment: Final version; a tunnel barrier between the probe and the dot is considered. To appear in 'Nanotechnology' in the special issue on "Quantum Science and Technology at the Nanoscale

Kinetic model of force-free current sheets with non-uniform temperature

The kinetic model of a one-dimensional force-free current sheet (CS) developed recently by Harrison and Neukirch [Phys. Rev. Lett. 102(13), 135003 (2009)] predicts uniform distributions of the plasma temperature and density across the CS. However, in realistic physical systems, inhomogeneities of these plasma parameters may arise quite naturally due to the boundary conditions or local plasma heating. Moreover, as the CS spatial scale becomes larger than the characteristic kinetic scales (the regime often referred to as the MHD limit), it should be possible to set arbitrary density and temperature profiles. Thus, an advanced model has to allow for inhomogeneities of the macroscopic plasma parameters across the CS, to be consistent with the MHD limit. In this paper, we generalise the kinetic model of a force-free current sheet, taking into account the inhomogeneity of the density and temperature across the CS. In the developed model, the density may either be enhanced or depleted in the CS central region. The temperature profile is prescribed by the density profile, keeping the plasma pressure uniform across the CS. All macroscopic parameters, as well as the distribution functions for the protons and electrons, are determined analytically. Applications of the developed model to current sheets observed in space plasmas are discussed

Density Enhancement Streams in The Solar Wind

This letter describes a new phenomenon on the Parker Solar Probe of recurring plasma density enhancements that have $\Delta$n/n ~10% and that occur at a repetition rate of ~5 Hz. They were observed sporadically for about five hours between 14 and 15 solar radii on Parker Solar Probe orbit 12 and they were also seen in the same radial range on both the inbound and outbound orbits 11. Their apparently steady-state existence suggests that their pressure gradient was balanced by the electric field. The EX electric field component produced from this requirement is in good agreement with that measured. This provides strong evidence for the measurement accuracy of the density fluctuations and the X- and Y-components of the electric field (the Z-component was not measured). The electrostatic density waves were accompanied by an electromagnetic low frequency wave which occurred with the electrostatic harmonics. The amplitudes of these electrostatic and electromagnetic waves at $\ge$ 1 Hz were greater than the amplitude of the Alfvenic turbulence in their vicinity so they can be important for the heating, scattering, and acceleration of the plasma. The existence of this pair of waves is consistent with the observed plasma distributions and is explained by a magneto-acoustic wave theory that produces a low frequency electromagnetic wave and electrostatic harmonics.Comment: 9 pages including 5 figure

Depairing currents in the superconductor/ferromagnet proximity system Nb/Fe

We have investigated the behaviour of the depairing current J_{dp} in ferromagnet/superconductor/ferromagnet (F/S/F) trilayers as function of the thickness d_s of the superconducting layers. Theoretically, J_{dp} depends on the superconducting order parameter or the pair density function, which is not homogeneous across the film due to the proximity effect. We use a proximity effect model with two parameters (proximity strength and interface transparency), which can also describe the dependence of the superconducting transition temperature T_c on d_s. We compare the computations with the experimentally determined zero-field critical current J_{c0} of small strips (typically 5~ \mu m wide) of Fe/Nb/Fe trilayers with varying thickness d_{Nb} of the Nb layer. Near T_c the temperature dependence J_{c0}(T) is in good agreement with the expected behaviour, which allows extrapolation to T = 0. Both the absolute values of J_{c0}(0) and the dependence on d_{Nb} agree with the expectations for the depairing current. We conclude that J_{dp} is correctly determined, notwithstanding the fact that the strip width is larger than both the superconducting penetration depth and the superconducting coherence length, and that J_{dp}(d_s) is correctly described by the model.Comment: 10 pages, 5 figures, submitted to PR