Gain without inversion in a biased superlattice

Intersubband transitions in a superlattice under homogeneous electric field is studied within the tight-binding approximation. Since the levels are equi-populated, the non-zero response appears beyond the Born approximation. Calculations are performed in the resonant approximation with scattering processes exactly taken into account. The absorption coefficient is equal zero for the resonant excitation while a negative absorption (gain without inversion) takes place below the resonance. A detectable gain in the THz spectral region is obtained for the low-doped $GaAs$-based superlattice and spectral dependencies are analyzed taking into account the interplay between homogeneous and inhomogeneous mechanisms of broadening.Comment: 6 pages, 4 figure

Trion dynamics in coupled double quantum wells. Electron density effects

We have studied the coherent dynamics of injected electrons when they are either free or bounded both in excitons and in trions (charged excitons). We have considered a remotely doped asymmetric double quantum well where an excess of free electrons and the direct created excitons generate trions. We have used the matrix density formalism to analyze the electron dynamics for different concentration of the three species. Calculations show a significant modification of the free electron inter-sublevel oscillations cWe have studied the coherent dynamics of injected electrons when they are aused by electrons bound in excitons and trions. Based on the present calculations we propose a method to detect trions through the emitted electromagnetic radiation or the current density.Comment: 14 pages, 13 figure

Voltage and temperature dependencies of conductivity in gated graphene

The resistivity of gated graphene is studied taking into account electron and hole scattering by short- and long-range structural imperfections the characteristics of disorder were taken from the scanning tunneling microscopy data and by acoustic phonons. The calculations are based on the quasiclassical kinetic equation with the normalization condition fixed by surface charge. The gate-voltage and temperature effects on the resistance peak, which is centered at the point of intrinsic conductivity, are found to be in agreement with the transport measurements.Comment: 4 pages, 4 Fig

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

Accurate quadratic-response approximation for the self-consistent pseudopotential of semiconductor nanostructures

Quadratic-response theory is shown to provide a conceptually simple but accurate approximation for the self-consistent one-electron potential of semiconductor nanostructures. Numerical examples are presented for GaAs/AlAs and InGaAs/InP (001) superlattices using the local-density approximation to density-functional theory and norm-conserving pseudopotentials without spin-orbit coupling. When the reference crystal is chosen to be the virtual-crystal average of the two bulk constituents, the absolute error in the quadratic-response potential for Gamma(15) valence electrons is about 2 meV for GaAs/AlAs and 5 meV for InGaAs/InP. Low-order multipole expansions of the electron density and potential response are shown to be accurate throughout a small neighborhood of each reciprocal lattice vector, thus providing a further simplification that is confirmed to be valid for slowly varying envelope functions. Although the linear response is about an order of magnitude larger than the quadratic response, the quadratic terms are important both quantitatively (if an accuracy of better than a few tens of meV is desired) and qualitatively (due to their different symmetry and long-range dipole effects).Comment: 16 pages, 20 figures; v2: new section on limitations of theor

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

Small molecule activation of apurinic/apyrimidinic endonuclease 1 reduces DNA damage induced by cisplatin in cultured sensory neurons

Although chemotherapy-induced peripheral neuropathy (CIPN) affects approximately 5-60% of cancer patients, there are currently no treatments available in part due to the fact that the underlying causes of CIPN are not well understood. One contributing factor in CIPN may be persistence of DNA lesions resulting from treatment with platinum-based agents such as cisplatin. In support of this hypothesis, overexpression of the base excision repair (BER) enzyme, apurinic/apyrimidinic endonuclease 1 (APE1), reduces DNA damage and protects cultured sensory neurons treated with cisplatin. Here, we address stimulation of APE1's endonuclease through a small molecule, nicorandil, as a means of mimicking the beneficial effects observed for overexpression of APE1. Nicorandil, was identified through high-throughput screening of small molecule libraries and found to stimulate APE1 endonuclease activity by increasing catalytic efficiency approximately 2-fold. This stimulation is primarily due to an increase in kcat. To prevent metabolism of nicorandil, an approved drug in Europe for the treatment of angina, cultured sensory neurons were pretreated with nicorandil and daidzin, an aldehyde dehydrogenase 2 inhibitor, resulting in decreased DNA damage but not altered transmitter release by cisplatin. This finding suggests that activation of APE1 by nicorandil in cisplatin-treated cultured sensory neurons does not imbalance the BER pathway in contrast to overexpression of the kinetically faster R177A APE1. Taken together, our results suggest that APE1 activators can be used to reduce DNA damage induced by cisplatin in cultured sensory neurons, although further studies will be required to fully assess their protective effects

Resonant transmission of normal electrons through Andreev states in ferromagnets

Giant oscillations of the conductance of a superconductor - ferromagnet - superconductor Andreev interferometer are predicted. The effect is due to the resonant transmission of normal electrons through Andreev levels when the voltage $V$ applied to the ferromagnet is close to $2h_0/e$ ($h_0$ is the spin-dependant part of the electron energy). The effect of bias voltage and phase difference between the superconductors on the current and the differential conductance is presented. These efects allow a direct spectroscopy of Andreev levels in the ferromagnet.Comment: 4 pages, 4 figure