Rabi oscillations under ultrafast excitation of graphene

We study coherent nonlinear dynamics of carriers under ultrafast interband excitation of an intrinsic graphene. The Rabi oscillations of response appear with increasing of pumping intensity. The photoexcited distribution is calculated versus time and energy taking into account the effects of energy relaxation and dephasing. Spectral and temporal dependencies of the response on a probe radiation (transmission and reflection coefficients) are considered for different pumping intensities and the Rabi oscillations versus time and intensity are analyzed.Comment: 6 pages, 6 figure

Frequency dependence of induced spin polarization and spin current in quantum wells

Dynamic response of two-dimensional electron systems with spin-orbit interaction is studied theoretically on the basis of quantum kinetic equation, taking into account elastic scattering of electrons. The spin polarization and spin current induced by the applied electric field are calculated for the whole class of electron systems described by p-linear spin-orbit Hamiltonians. The absence of nonequilibrium intrinsic static spin currents is confirmed for these systems with arbitrary (nonparabolic) electron energy spectrum. Relations between the spin polarization, spin current, and electric current are established. The general results are applied to the quantum wells grown in [001] and [110] crystallographic directions, with both Rashba and Dresselhaus types of spin-orbit coupling. It is shown that the existence of the fixed (momentum-independent) precession axes in [001]-grown wells with equal Rashba and Dresselhaus spin velocities or in symmetric [110]-grown wells leads to vanishing spin polarizability at arbitrary frequency of the applied electric field. This property is explained by the absence of Dyakonov-Perel-Kachorovskii spin relaxation for the spins polarized along these precession axes. As a result, a considerable frequency dispersion of spin polarization at very low frequency in the vicinity of the fixed precession axes is predicted. Possible effects of extrinsic spin-orbit coupling on the obtained results are discussed.Comment: 14 pages, 6 figures; published with minor corrections in Phys. Rev.

Coherent oscillations of electrons in tunnel-coupled wells under ultrafast intersubband excitation

Ultrafast intersubband excitation of electrons in tunnell-coupled wells is studied depending on the structure parameters, the duration of the infrared pump and the detuning frequency. The temporal dependencies of the photoinduced concentration and dipole moment are obtained for two cases of transitions: from the single ground state to the tunnel-coupled excited states and from the tunnel-coupled states to the single excited state. The peculiarities of dephasing and population relaxation processes are also taken into account. The nonlinear regime of the response is also considered when the splitting energy between the tunnel-coupled levels is renormalized by the photoexcited electron concentration. The dependencies of the period and the amplitude of oscillations on the excitation pulse are presented with a description of the nonlinear oscillations damping.Comment: 8 pages, 12 figure

Inhomogeneous broadening of tunneling conductance in double quantum wells

The lineshape of the tunneling conductance in double quantum wells with a large-scale roughness of heterointerfaces is investigated. Large-scale variations of coupled energy levels and scattering due to the short-range potential are taken into account. The interplay between the inhomogeneous broadening, induced by the non-screened part of large-scale potential, and the homogeneous broadening due to the scattering by short-range potentials is considered. It is shown that the large inhomogeneous broadening can be strongly modified by nonlocal effects involved in the proposed mechanism of inhomogeneity. Related change of lineshape of the resonant tunneling conductance between Gaussian and Lorentzian peaks is described. The theoretical results agree quite well with experimental data.Comment: 11 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

Spin torque due to non-uniform Rashba spin orbit effect

Following the early theoretical descriptions of the spin-orbit-induced spin torque [S.G. Tan et al., arXiv:0705.3502 (2007); S.G.Tan et al., Ann. Phys.326, 207 (2011)], the first experimental observation of such effect was reported by L. M. Miron et al., Nature Mater, 9, 230 (2010). We present in this article three additional spin torque terms that arise from the non-uniformity in magnetization space of the Rashba spin-orbit effect. We propose a simple Rashba gradient device which could potentially lower switching current by n orders of magnitude, where large n measures a small magnetization change.Comment: 11 pages, 1 figur

Magneto-gyrotropic effects in semiconductor quantum wells (review)

Magneto-gyrotropic photogalvanic effects in quantum wells are reviewed. We discuss experimental data, results of phenomenological analysis and microscopic models of these effects. The current flow is driven by spin-dependent scattering in low-dimensional structures gyrotropic media resulted in asymmetry of photoexcitation and relaxation processes. Several applications of the effects are also considered.Comment: 28 pages, 13 figure

Spin Accumulation in Diffusive Conductors with Rashba and Dresselhaus Spin-Orbit Interaction

We calculate the electrically induced spin accumulation in diffusive systems due to both Rashba (with strength $\alpha)$ and Dresselhaus (with strength $\beta)$ spin-orbit interaction. Using a diffusion equation approach we find that magnetoelectric effects disappear and that there is thus no spin accumulation when both interactions have the same strength, $\alpha=\pm \beta$. In thermodynamically large systems, the finite spin accumulation predicted by Chaplik, Entin and Magarill, [Physica E {\bf 13}, 744 (2002)] and by Trushin and Schliemann [Phys. Rev. B {\bf 75}, 155323 (2007)] is recovered an infinitesimally small distance away from the singular point $\alpha=\pm \beta$. We show however that the singularity is broadened and that the suppression of spin accumulation becomes physically relevant (i) in finite-sized systems of size $L$, (ii) in the presence of a cubic Dresselhaus interaction of strength $\gamma$, or (iii) for finite frequency measurements. We obtain the parametric range over which the magnetoelectric effect is suppressed in these three instances as (i) $|\alpha|-|\beta| \lesssim 1/mL$, (ii)$|\alpha|-|\beta| \lesssim \gamma p_{\rm F}^2$, and (iii) |\alpha|-|\beta| \lesssiM \sqrt{\omega/m p_{\rm F}\ell} with $\ell$ the elastic mean free path and $p_{\rm F}$ the Fermi momentum. We attribute the absence of spin accumulation close to $\alpha=\pm \beta$ to the underlying U (1) symmetry. We illustrate and confirm our predictions numerically

Intrinsic electric polarization in spin-orbit coupled semiconductor heterostructures

We present Maxwell equations with source terms for the electromagnetic field interacting with a moving electron in a spin-orbit coupled semiconductor heterostructure. We start with the eight--band ${\bm k}{\bm p}$ model and derive the electric and magnetic polarization vectors using the Gordon--like decomposition method. Next, we present the ${\bm k}{\bm p}$ effective Lagrangian for the nonparabolic conduction band electrons interacting with electromagnetic field in semiconductor heterostructures with abrupt interfaces. This Lagrangian gives rise to the Maxwell equations with source terms and boundary conditions at heterointerfaces as well as equations for the electron envelope wave function in the external electromagnetic field together with appropriate boundary conditions. As an example, we consider spin--orbit effects caused by the structure inversion asymmetry for the conduction electron states. We compute the intrinsic contribution to the electric polarization of the steady state electron gas in asymmetric quantum well in equilibrium and in the spin Hall regime. We argue that this contribution, as well as the intrinsic spin Hall current, are not cancelled by the elastic scattering processes.Comment: 12 pages, 3 figure