Splitting of the resonance levels of double-barrier structures in a high microwave electric field

The dependence of the width and shape of the resonance levels of asymmetric double-barrier resonance-tunneling structures with high narrow barriers on the amplitude of the resonance microwave field and specific features of the electron transport in the case of ballistic passage of electrons near the resonance levels has been investigated. It has been found that a resonance level can be first broadened significantly and be further split into two absolutely transparent levels with an increase in the microwave field amplitude. The conditions have been formulated under which nonresonance scattering channels near the resonance levels also become absolutely transparent.     

Dynamical behavior of electron transport in AlGaAs/GaAs double-barrier structures under a high-frequency radiation field

We investigate the time-dependent dynamical behavior of electron transport in AlGaAs/GaAs double-barrier structures under a high-frequency radiation field. The effects of the radiation field with different amplitude and frequency on the real-time and mean current-voltage curves are taken into account. We find that the amplitude and frequency of the radiation field affect the final stable state current-voltage (I-V) behaviors, which leads to the switching between different current states at a smaller bias than that of the absence of the radiation field, and both current hysteresis and resonant peaks are suppressed by the external radiation field. The high radiation field strength can make the resonant peak of current split and the hysteresis of current disappear. This effect provides the potential to use double-barrier structure as a THz photoelectric switch.     

Effects of virtual transitions in a high-frequency field on electron transport in triple-barrier structures

High-frequency conductivity has been calculated for triple-barrier nanostructures at resonance diagonal transitions. It is shown that the neglect of transitions to side nonresonance quasi-levels in a high-frequency electric field leads in some cases to an overestimation of the maximum conductivity by 60%, an increase in the line width of more than 40%, and an increase in the integral conductivity by a factor of almost 2.5. In these cases, the transmission coefficient calculated with allowance made for side satellites is approximately 0.6 rather than 1.     

High quantum efficiency of intersubband transitions in coherent tunneling of electrons through asymmetric double-barrier structures

A converging perturbation series that can be summed analytically has been obtained for intersubband transitions of electrons coherently tunneling through the middle of a dimensionally quantized level in an asymmetric double-barrier structure in a high-frequency terahertz electric field. The possibility of a substantial increase in tunneling current accompanied by either absorption or emission of a photon has been demonstrated. The quantum efficiency of radiative transitions between dimensionally quantized levels can be up to 66%. Zh. éksp. Teor. Fiz. 112, 237–245 (July 1997)     

Effect of a random deformation-potential field on interband optical transitions

We have calculated the correlation function for the random field caused by the deformation potential due to spherical clusters randomly distributed throughout the sample. We evaluate the characteristic energy which determines the rate at which the interband optical absorption coefficient decreases as the frequency decreases in the neighborhood of the optical tail.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 3–6, February, 1984.     

Effect of disorder on structures due to interband transitions in silicon

The optical response of silicon near the E₁ structure, due to interband transitions, has been studied as a function of disorder by means of electroreflectance (ER). It is found that crystal grains smaller than the Rutherford backscattering (RBS) spatial resolution can still give rise to a measurable signal. The ER lineshape is briefly discussed and some important features are stressed.     

Two-photon interband transitions in semiconductors in a static magnetic field. Nonlocal effects

Summary It is shown that in the presence of a static magnetic field, the electronic states in the effective-mass approximation are described by a Hamiltonian containing a nonlocal potential. In order to preserve the gauge invariance of the transition rates, the interaction between the electrons and the radiation has to be modified. For ZnS, the transition rate between two particular levels is evaluated by showing a difference of more than one order of magnitude when compared with the transition rate previously calculated. Partially supported by the Italian Research Council (C.N.R.) through G.N.S.M.     

Force-extension curve of a polymer in a high-frequency electric field

We study theoretically the conformation and force-extension curve of a semiflexible polymer in a spatially uniform ac electric field. The polymer backbone minimizes its energy by aligning along one of two orientations parallel to the field. In a strong ac field, hairpin kinks develop between regions of opposite alignment. These kinks are mathematically described as sine-Gordon solitons. We calculate the equation of state of the one-dimensional kink gas, which yields the force-extension curve of the polymer. A sufficiently strong ac field causes the polymer to extend spontaneously to almost its full contour length. The theory is applied to recent experiments on dielectrophoretic stretching of DNA.     

The influence of multiphonon transitions on interband optical absorption in semiconductors with a Coulomb-type random field

The temperature dependence of the optical absorption coefficient in the tail region is studied in semiconductors with a random Coulomb-type field. The results of numerical calculation are compared with data for chalcogenide glasses.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 63–67, September, 1981.The author is grateful to V. L. Bonch-Bruevich for his attention to the work.     

Two-photon interband transitions in crossed electric and magnetic fields in semiconductors. Non-local effects

Summary It is shown that in the presence of a crossed electric and magnetic field the electronic states of a semiconductor in the effective-mass approximation are described by a Hamiltonian containing a non-local potential. In order to preserve the gauge invariance of the two-photon interband transition rate, the interaction Hamiltonian between the electrons and the incident radiation has to be generalized. For GaAs, the transition rate between two particular levels is evaluated in both length and velocity gauges demonstrating the correctness of our approach. Work partially supported by the European Community Programme ESPRIT, Basic research, action n. 6878, EASI.     

Inversion-asymmetry-induced interband magneto-optical transitions in HgTe

The first evidence of linear-k term effects in HgTe is reported. New resonance structures observed in interband Γ₆ → Γ₈ magnetoabsorption spectra, for H ∥ [111], can be unambiguously identified with inversion asymmetry- induced transitions.     

Excitonic transitions in GaAs---GaAlAs superlattices under a weak electric field

We report a study of the photoluminescence spectra of GaAs---Ga_0.65Al_0.35As superlattice under an electric field of about 10 kVcm⁻¹, from 9 K up to 80 K; the ratio of the intensities of the −1 and 0 peaks of the Wannier-Stark ladder varies with the temperature T. To fit these variations, we assume for both transitions, a trapped exciton density of states with a gaussian distribution of eigenenergies, and a two dimensional density of states for free excitons. We show that above about 50 K there is a thermal equilibrium between direct and crossed free excitons whereas at low temperatures, the equilibrium statistics are not respected: the excitons trap into the direct state rather than to the crossed one.     

Role of electric field polarisation in rotational transitions

The molecular dynamics of polar diatomic molecule, interacting with linearly and circularly polarised laser field is studied. Non-perturbative quasi-energy technique is used to determine the rotational field surfaces. Degeneracy in different M-levels is found to be removed in the presence of circularly polarised electromagnetic radiation. In this paper, we present the calculations exhibiting the dynamics of rotational excitation of a linear molecule and characteristic features of spectra which become observable in considered fields. Received 22 March 2002 / Received in final form 15 July 2002 Published online 6 November 2002 RID="a" ID="a"e-mail: nsinghal@physics.du.ac.in RID="b" ID="b"e-mail: vnautiyal@himalaya.du.ac.in RID="c" ID="c"e-mail: manmohan@physics.du.ac.in     

The dielectric constant associated with direct interband transitions in germanium

An expression for the complex dielectric constant associated with direct interband transitions in germanium has been obtained using a simple model. The model consists of four damped oscillators, and a fit to experimental results for the complex dielectric constant is made to obtain the model parameters, i.e., the oscillator frequencies and damping constants. The results show good agreement with experimental data over a broad range of frequencies.