Nonthermal optical transients from relativistic fireballs

A general upper bound is derived on the total energy in incoherent nonthermal transients at frequency nu from relativistic fireballs with bulk Lorentz factors gamma and observed duration Deltat, and shown to be about 10(-2)[gammanuDeltat](3) ergs. It is suggested that detection in the optical can be achieved with the next generation of ground based gamma ray telescopes and/or small optical telescopes. Phenomena within the Galaxy such as accretion disk flares and neutron star magnetospheric discharges might be discovered in this way.     

Shakeup excitation during optical tunnel ionization

Shakeup of a two-electron system is investigated in the strong infrared laser field limit, both theoretically and experimentally. During tunnel ionization the electron shakes up a second electron to an excited bound state. Theoretically, a complete analytical theory of shakeup in intense laser fields is developed. We predict that shakeup produces one excited sigma(u) D(+)(2) state in approximately 10(5) ionization events. Shakeup is measured experimentally by using the molecular clock provided by the internuclear motion. The number of measured events is found to be in excellent agreement with theory.     

Oscillations of tunnel magnetoresistance in ferromagnet-insulator-ferromagnet structures

A model of spin-dependent electron transport in ferromagnet-insulator-ferromagnet structures that takes into account the potential relief, applied voltage, image forces, and scattering by the potential relief is developed. It is shown that tunnel magnetoresistance oscillations are caused by the nonmonotonic energy dependence of transmission coefficients and their split in the molecular field of the ferromagnet. Conditions are found under which the tunnel magnetoresistance is virtually independent of scattering by the interfaces.     

A study of the ultrafast optical response of magnetized GaAs/AlGaAs quantum wire structures

We have studied analytically the ultrafast optical response of GaAs/AlGaAs quantum wire subjected to a moderately strong transverse magnetic field. The energy dispersion relations have been numerically calculated and show a significant deviation from parabolic behaviour as the magnetic field is increased. The effective semiconductor Bloch equation technique is used to calculate the induced polarization and differential transmission spectra in the quantum wire. The calculated induced polarization is used to study the optical coherent transient phenomenon of optical nutation. The analysis demonstrates that the magnetic field effectively alters the optical response of the semiconductor quantum wire nanostructures. It is observed that the nutating signal frequency enhances with an increasing magnetic field. The results are useful to explain magnetic field effects on the transient optical properties of semiconductor nanostructures.     

Magnetic tunnel structures: Transport properties controlled by bias,magnetic field,and microwave and optical radiation

Different phenomena that give rise to a spin-polarized current in some systems with magnetic tunnel junctions are considered. In a manganite-based magnetic tunnel structure in CIP geometry, the effect of current-channel switching was observed, which causes bias-driven magnetoresistance, rf rectification, and the photoelectric effect. The second system under study, ferromagnetic/insulator/semiconductor, exhibits the features of the transport properties in CIP geometry that are also related to the current-channel switching effect. The described properties can be controlled by a bias, a magnetic field, and optical radiation. At last, the third system under consideration is a cooperative assembly of magnetic tunnel junctions. This system exhibits tunnel magnetoresistance and the magnetic-field-driven microwave detection effect.     

Inelastic resonance tunneling in S-Sm-S tunnel structures

Inelastic resonance tunneling through junctions with an amorphous interlayer and superconducting electrodes is studied. The form of the current-voltage characteristic I(V) at low temperature and the temperature dependence of the conductance G(0) at low bias are calculated and are found to be much different from the analogous dependences of structures with normal electrodes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 2, 159–163 (25 January 1997)     

The resonance tunnel effect in sandwich structures

The influence of the resonance tunnel effect on the leakage current in sandwich cathodes is investigated. The aim of this study is to find the conditions for cathode parameters that must be fulfilled in order that the resonance current could create a measurable part of the leakage current. It is shown that at low voltages and for reasonable cathode parameters the resonance current would be much greater than the tunnel one, and a model is described which makes the calculations simpler.     

Theory of coupled optical PT-symmetric structures

Starting from Lagrangian principles we develop a formalism suitable for describing coupled optical parity-time symmetric systems.     

Subharmonic gap structures in asymmetric superconducting tunnel junctions

Tunneling in the asymmetric superconducting junctions of PbIn and PbTl shows sharp current steps at voltages eV = Δ₁ + Δ₂,$\text{2Δ}{}_2\text{2n}\text{and}\text{2Δ}{}_1\text{2n}$ (structures for n > 1 not readily observed); 2Δ₁ and 2Δ₂ are the energy gaps and n is an integer. We obtain new results for the relative magnitude of the structures, where the larger of the two structures is observed at the larger of the two voltages $\text{Δ}{}_1\text{ρ}\text{and}\text{Δ}{}_2\text{ρ}$. These results, in contrast to other available experimental results on different superconductors, seem to exclude self-coupling theory as the mechanism.     

Effective optical constants and effective optical properties of ultrathin trilayer structures

This work presents an extension of the characteristic effective medium approximation (CEMA) to ultrathin trilayer systems. The extension has been carried out analytically and is supported by corresponding calculations of the effective optical constants of Cu-Au-Cu and Ag-SiO-Ag trilayer systems using the CEMA approximation. This work is in essence a generalization of the characteristic effective medium approximation introduced earlier for ultrathin bilayer structures. This method is used to derive the effective optical constants of a trilayer system, consisting of three thin layers with each constituent layer of thickness much less than the wavelength of the incident radiation. Within this regime a trilayer system is viewed as one effective layer referred to as an effective stack (ES) with well defined effective optical constants, which can be used to calculate the optical properties of the trilayer stack within a specified wavelength range. The CEMA based calculations of the effective optical constants are applied to two trilayer systems with a total of five stacks. Three are Cu-Au-Cu and two are Ag-SiO-Ag stacks. The thicknesses of the parent layers in the Cu-Au-Cu stack range from 3 to 30 nm for Cu and 4 to 40 nm for Au; in the Ag-SiO-Ag stack the constituent layers are 6 nm for Ag, but range from 5 to 10 nm for SiO. This study is for normal or near normal incidence spectroscopy in a wavelength range that extends from visible to near infrared. The agreement between CEMA based ES stack results and those of the standard CMT technique is very satisfactory.     

Isolated structures in two-dimensional optical superlattice

Overlaying commensurate optical lattices with various configurations called superlattices can lead to exotic lattice topologies and, in turn, a discovery of novel physics. In this study, by overlapping the maxima of lattices, a new isolated structure is created, while the interference of minima can generate various “sublattice” patterns. Three different kinds of primitive lattices are used to demonstrate isolated square, triangular, and hexagonal “sublattice” structures in a two-dimensional optical superlattice, the patterns of which can be manipulated dynamically by tuning the polarization, frequency, and intensity of laser beams. In addition, we propose the method of altering the relative phase to adjust the tunneling amplitudes in “sublattices”. Our configurations provide unique opportunities to study particle entanglement in “lattices” formed by intersecting wells and to implement special quantum logic gates in exotic lattice geometries.     

Resonance optical spectroscopy of long-period quantum-well structures

Theory of specular light reflection from long-period quantum-well structures taking into account the exciton contribution to dielectric polarization has been developed for an arbitrary relation between the background refractive index in the well, n _a, and barrier-material refractive index nb. General expressions for the optical reflection and transmission coefficients for a structure with N equidistant quantum wells are derived with the use of the Green’s function and transfer matrix methods. Normal and oblique light reflectance spectra from II-VI-based heterostructures were found to reveal a bright interference pattern caused by the difference between n _a and n _b. A comparison of the theory with experiment has yielded the dispersion of n _a and n _b within a broad wavelength range and the parameters of the quasi-two-dimensional heavyhole exciton (e1-hh1), namely, the resonant frequency and the radiative and nonradiative damping rates. Reflectance spectra from resonant Bragg and quasi-Bragg structures with real exciton parameters are calculated, and the effect on these spectra of the refractive-index difference and the deviation from the Bragg condition is analyzed. Fiz. Tverd. Tela (St. Petersburg) 39, 2072–2078 (November 1997)     

Nonthermal production of gravitinos and inflatinos

We explicitly calculate nonthermal gravitino production during the preheating period in the inflationary Universe. Contrary to earlier investigations, we consider a two-field model to separate the mechanisms of supersymmetry breaking and inflation. We show that the superpartner of the inflaton is significantly generated, while the gravitino production is considerably smaller. Nonthermal production of gravitinos seems thus less worrisome than recently claimed.     

Nonthermal radiation of rotating black holes

The nonthermal radiation of a Kerr black hole is considered as the tunneling of particles being produced through an effective Dirac gap. In the leading semiclassical approximation, this approach is also applicable to bosons. Our semiclassical results for photons and gravitons are consistent with those obtained previously. For neutrinos, the result of our complete quantum-mechanical calculation is about twice as large as the previous one.     

Nonthermal effect of dilatonic black holes

The quantum nonthermal effect of the spherically symmetric and rotating dilatonic black holes is studied. A crossing of the positive and negative Dirac energy of particles occurs near dilatonic black holes. We find that the dilaton coupling parameter α affects the energy of spontaneous radiant particles. The energy of particles decreases when the coupling parameter α increases.     

New fluxon resonant mechanism in annular Josephson tunnel structures

A novel dynamical state has been observed in the dynamics of a perturbed sine-Gordon system. This resonant state has been experimentally observed as a singularity in the dc current-voltage characteristic of an annular Josephson tunnel junction, excited in the presence of a magnetic field. In this respect it can be assimilated to self-resonances known as Fiske steps. Differently from these, however, we demonstrate, on the basis of numerical simulations, that its detailed dynamics involves rotating fluxon pairs, a mechanism associated, so far, to self-resonances known as zero-field steps. This occurs because the size of nonlinear excitations is comparable with that of the system.