A. A. Andronov

Kinematic vortices and phase slip lines in the dynamics of the resistive state of narrow superconductive thin film channels

Abstract The resistive state of a superconducting thin film channel (bridge) of intermediate width W (ξ⪡ W ⪡λ eff , ξ is the coherence length and λ eff is the effective magnetic penetration depth) - this case is relevant to the nanoscale high- T c superconductor thin film bridges - is studied by simulations of two-dimensional time-dependent Ginzburg-Landau equations. It is found that in the uniform narrow channel at high dimensionless conductivity Σ the dynamical behavior is due to appearance of the phase slip lines , but at lower Σ vortices appear in the resistive state. In inhomogeneous channels vortices appear at any Σ. We call these vortices kinematic vortices because, unlike the Abrikosov vortices, they can not be treated as quasiparticles. They move with any velocity depending on the current distribution over the bridge (the bridge inhomogeneity). The more uniform the current distribution, the higher the vortex velocity. A kinematic vortex with infinite velocity is a phase slip line.

Charge transport in superlattices with low-strength barriers and the problem of a terahertz bloch oscillator

Charge-transport properties of superlattices with low-strength barriers and the possibility of designing a Bloch oscillator based on these superlattices are discussed. A terahertz Bloch oscillator based on n-GaAs-GaAlAs structures with low-strength barriers is suggested. Because of interminiband tunneling, the current is an increasing function of electric-field strength, so that domains cannot be formed. At the same time, tunneling and Bloch oscillations give rise to dynamic negative electrical conductivity in the terahertz region. Monte Carlo simulations show that dynamic negative conductivity exists in the frequency range of 1–7 THz for superlattices with moderate charge-carrier mobility at 77 K. A Bloch oscillator should include a superlattice with 350–700 periods of 150-Å, with this superlattice being sandwiched between contact regions, which are in fact strip-line sections (the oscillator cavity). Presumably, such an oscillator can operate at 77 K in the continuous-wave mode.

Tunable hot hole FIR lasers and CR masers

Abstract Basic principles for arranging population inversion and stimulated emission in hot hole systems at low temperature are discussed. Recent experimental results on observation of emissions from intersubband lasers, CR NEMAGs and CR lasers made from p-Ge are given.

Tunable fir lasers in semiconductors using hot holes

Abstract It is shown that active systems based on inter-subband transitions of hot holes could provide tunable FIR sources. Liquid helium cooled Ge is the most promising system—and some results are quoted—while for higher temperatures (with higher E and H fields) Si is the most likely material.

Transport in GaAs/AlxGa1−xAs superlattices with narrow minibands: Effects of interminiband tunneling

The results of experimental investigations for the I–V characteristics are presented for superlattices based on GaAs/AlxGa1−xAs with thin barriers in the electric-field region with an intense interminiband tunneling. The regular features for the I–V characteristics are found in the voltage range adequate to the static positive differential conductivity in the superlattice. On the basis of calculations for the Wannier-Stark levels, it is established that the observed features are associated with the resonant tunneling between these levels belonging to quantum wells located at a distance from 6–13 superlattice periods from each other. It is noted that the similar resonant delocalization of Wannier-Stark wave functions can lead to the existence of dynamic negative differential conductivity for the laser type of an appreciable value in such superlattices.

Stimulated radiation of optically pumped Cd xHg1 − x Te-Based heterostructures at room temperature

The experimental observation of stimulated radiation of optically pumped CdxHg1 − x Te-based heterostructures in the wavelength range of 1.4–4.5 μm is reported. In the experiments, graded-gap Cd xHg1 − x Te samples grown on GaAs and Si substrates by molecular beam epitaxy were used. Superluminescence of such structures was observed at 77–300 K under the pulsed pumping of the samples by a Nd:YAG laser at a wavelength of 1.064 μm. At room temperature, stimulated radiation was observed at wavelengths of 1.4–1.7 μm. The obtained experimental data are the first results on the observation of stimulated radiation from graded-gap Cd x Hg1 − x Te structures on Si and GaAs substrates at these wavelengths at room temperature.

Superluminescence from optically pumped CdxHg1-xTe heterostructures on GaAs and Si substrates

We report the experimental observation of superluminescence in the wavelength range 1 .4 - 4.5 μm from epitaxial structures Cdx4Hg1-xTe under pulsed optical pumping by Nd:YAG laser. The samples CdxHg1-xTe were grown on GaAs and Si substrates by the Molecular Beam Epitaxy. The stimulated emission has been observed from the structures at temperatures 77 - 200 K. Details of the samples, observation scheme, the super luminescence spectra are discussed.

Spontaneous and stimulated emission from CdxHg1−x Te semiconductor films

The experimental data on observation of spontaneous and stimulated emission from thin epitaxial CdxHg1−x Te films optically pumped by Nd: YAG laser radiation are reported. A simple theoretical model is suggested to describe the initiation of population inversion under these conditions. The parameters realized under the experimental conditions are theoretically estimated.

Stimulated emission at transitions between Wannier–Stark ladders in semiconductor superlattices

New intraband semiconductor lasers—Wannier–Stark lasers—based on simple GaAs (150 Å, quantum well)/GaAlAs (19 Å with an aluminum fraction of 12%, barrier) superlattices have been demonstrated. The amplification mechanism in these lasers is based on population inversion between the ground Wannier–Stark level in the superlattice quantum wells and the weakly populated upper Wannier–Stark level in the wells two, three, or four periods down in the applied potential. Multiple regions of intense stimulated microwave emission near voltages of 8, 13, and 20 V (i.e., in the vicinity of resonances between these Wannier–Stark levels of the superlattice) have been discovered in the laser chips. The stimulated emission emerges in the circuit formed by the chip and its wiring. The emission from one of the chips at a temperature of up to 150 K (near 20 V applied to the chip) occurs at a frequency of about 7.3 GHz and has an estimated power of up to 1 W. It has been shown that the negative conductivity responsible for the emission still persists at 300 K but the emission is unseen owing to high losses in the circuit at this temperature. The superlattice wafer has been grown by metalorganic chemical vapor deposition. It consists of 1000 periods and a stop layer, to produce a metal–superlattice–metal terahertz resonator. Terahertz radiation has not been observed owing to a low amplification, as compared to losses in the resonator. According to the performed experiments, calculations, and discussions, such superlattices as radiation sources in gigahertz, terahertz, and higher frequency ranges could compete with quantum cascade lasers under appropriate optimization of their parameters.

Transport in GaAs/AlxGa1−xAs superlattices with narrow forbidden minibands: Low-frequency negative differential conductivity and current oscillations

Current-voltage characteristics have been measured and low-frequency current instabilities have been studied for GaAs/AlxGa1−xAs superlattices with narrow forbidden minibands. At relatively low electric fields, a saw-like structure for current-voltage characteristics with alternating portions of positive and negative differential conductivity and spontaneous generation of low-frequency current oscillations with a complex frequency spectrum (varying from discrete to continuous) are observed. It is shown that the observed specific features of electron transport are caused by the spatial-temporal dynamics of electric-field domains (dipoles and monopoles). The effects of the bifurcation, hysteresis, and multistability of current-voltage characteristics are also observed. At high fields, regular features are observed and identified in the current-voltage characteristics; these features are caused by resonance tunneling of electrons between the levels of the Wannier-Stark ladders belonging to quantum wells separated by several periods.