Voltage and current characteristics of pulsed hollow cathodedischarges in hydrogen

Volt-ampere characteristics of a hollow-cathode, hollow-anode discharge in hydrogen were measured by using a very fast di/dt probe. The large bandwidth of the probe enabled us to observe fast oscillations occurring at the onset of the current pulse and also at the moment of its quenching. Three different modes of operation are observed, the first with smooth voltage and current waveforms. The second mode occurs for voltages higher than 1500 V and is characterized by strong oscillations and the occurrence of current quenching. The third, mode occurs with a sharp transition and in it the current rises more rapidly to a much higher maximum value, while the oscillations are less pronounced. This mode has identical characteristics with the superdense glow mode of pseudospark discharges. At the point of transition a sudden change in the frequency of oscillations, of the impedance, and of the maximum transmitted power occur. Current quenching is observed in the second and in the third mode. The development of the superdense glow changes the character of the waveforms and current quenching while the mechanism leading to the superdense glow may be associated with the available voltage     

Space charge dynamics in a semiconductor superlattice affected by titled magnetic field and heating

The transition between different modes of current oscillations in a semiconductor superlattice, from close-to-harmonic (near the generation onset) to relaxation oscillations, has been investigated. The transition type is shown to change with an increase in temperature. A period-doubling bifurcation is observed at low temperatures. With an increase in temperature, the period-doubling bifurcation is observed at increasingly larger values of the voltage across the superlattice. The doubling bifurcation ceases to be observed at voltages at which the generation of oscillations of the current through the semiconductor superlattice is suppressed.     

Intersubband transport in quantum wells in strong magnetic fields mediated by single- and two-electron scattering

We show theoretically that in quantum wells subjected to a strong magnetic field the intersubband current peaks at magnetic field values, which reveal the underlying specific intersubband scattering mechanism. We have designed and grown a superlattice structure in which such current oscillations are clearly visible, and in which the transition from the purely single-electron to the mixed single- and two-electron scattering regimes can be observed by tuning the applied voltage bias. The measurements were conducted in ultrahigh magnetic fields (up to 45 T) to obtain the full spectrum of the current oscillations.     

Excess noise peaks in porous silicon-based diode structures

Results of an experimental observation of the voltage oscillations associated with a discrete tunneling of holes in porous silicon at room temperature are presented. The noise characteristics of diode structures with a porous silicon interlayer formed on heavily boron-doped silicon single crystals are studied. Peaks of excessive noise are observed at frequencies of ～1 MHz, at which single-electron oscillations should be expected. The peak noise power is found to increase with current according to the ～2.5 power law and, at a current density of 0.15 A/cm², to exceed the noise power of the receiver by three to four orders of magnitude. The complex shape of the noise spectrum and its extension to the higher frequency region with increasing current are explained by the three-dimensionality of the system of nanometer-sized silicon grains embedded in insulating silicon dioxide of porous silicon.     

Low-frequency current oscillations in a linear hot-cathode discharge

Low-frequency (ω ⪡ ω_pi) plasma oscillations in the transition regime between the high and the low current mode of a thermionic hot-cathode discharge are investigated experimentally. This type of current oscillation often shows chaotic dynamics. The current oscillations are related to nonlinear short wavelength potential structures which are identified as ion bunches formed by a fluctuating ionization front. These ion bunches are separated by ion holes and move at ion thermal speed rather than ion acoustic speed. By entering the negative space charge region of the cathode sheath, the ion bunches trigger electron current fluctuations that provide the required feedback mechanism for the observed wave train formation.     

Tunneling current oscillations excited by DC voltage

Investigating a polycrystalline gold layer on glass by a scanning tunneling microscope in air, tunneling current oscillations were found, which are excited by the DC voltage across the gap. The oscillation amplitude is dependent on the place on the surface of the sample and correlates with its topography. The frequency spectra of these oscillations are influenced by resonances of the mechanical system (z-piezo/sample holder/transducer/sample). A piezoelectric transducer is able to detect alternating forces originating from the tunnel junction. The resonances in the spectrum of the AC tunneling current and the mechanical resonances of the STM system seem to be related. Trapping and subsequent delayed desorption of charge carriers at localized surface states could play a role in generating the observed time-dependent forces across the gap and thereby creating tunneling current oscillations.     

Diamagnetic domains and magnetostriction in beryllium

Magnetostriction was for the first time studied under the conditions of formation of diamagnetic domains (Condon domains). Transverse magnetostriction oscillations on a beryllium single crystalline plate oriented normally to magnetic field were measured in magnetic fields up to 7 T at temperatures down to 1.5 K. The relative amplitude of oscillations increased almost as the square of magnetic field and reached 10^?5. The signal had a sawtoothed shape corresponding to alternation of homogeneous and inhomogeneous (domain) states in the region of the existence of magnetic domains. The arising of domains was accompanied by singularities in the observed signals which is explained by an anomalous increase in the compressibility coefficient of the domain state: coefficient oscillations were more than 100 times larger than the value predicted by the standard theory. The observed relation between magnetization current and deformation led us to conclude that the compressibility of the metal was fully determined by conduction electrons. Magnetostriction then exactly compensated Fermi level oscillations. The position of the Fermi level therefore remained constant under magnetic field variations. In addition, the domain wall thickness had to increase as the plate grew thicker.     

Transition between synchronization and chaos in doped GaAs/AlAs superlattices

The transition between chaotic and periodic regimes in spontaneous current oscillations of weakly coupled, doped GaAs/AlAs superlattices has been observed by varying the external d.c. bias. The chaotic current oscillations are observed for voltage ranges, which exhibit a large negative differential conductance in the time-averaged I–V characteristic. Since this system can be described by a spatially distributed, non-linear system with many degrees of freedom, the coupling between the degrees of freedom in the chaotic windows is repulsive, while in the periodic windows it is attractive.     

Nonlinear dynamics of breathing current filaments inn-GaAs andp-Ge

A novel model is presented for spatio-temporal pattern formation in semiconductors. It leads to self-generated nonlinear current oscillations due to “breathing” current filaments in the regime of impurity impact ionization. The four qualitatively different regimes which have been observed in Ge with increasing current are consistently explained as: a stationary nonconducting state; bulkdominated oscillations; breathing filaments; stable filaments. The physical origin of the breathing oscillations is impact ionization coupled with transverse diffusion and longitudinal dielectric relaxation. A method is developed to derive simple nonlinear dynamic equations for the filament radius and the position of the peak transverse electric field by a nonlinear mode expansion.     

Current-induced voltage oscillations in superconducting Y<Subscript>1</Subscript>Ba<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7-δ</Subscript>

We investigated influence of bidirectional square wave current with long periods and dc current on the evolution of the voltage-time (V - t) curves in superconducting polycrystalline bulk Y₁Ba₂Cu₃O_7-δ (YBCO) material at the temperatures near the critical temperature. In a well-defined range of amplitudes and periods of driving current, and temperatures, novel type of non-linear dynamic responses was observed by means of the V-t curves. It was seen that such a non-linear response to bidirectional square wave current sometimes reflects itself as regular sinusoidal-like voltage oscillations. The sinusoidal-like and non-sinusoidal oscillations were discussed mainly in terms of the dynamic competition between pinning and depinning and significant relaxation effects which appear in this competing process. The density fluctuations associated with the current induced self-magnetic flux (SMF) lines and semi-elastic coupling of SMF lines with the pinning centers were also considered as possible physical mechanisms in the interpretation of the experimental results.     

Magnetotransport in a modulated two-dimensional electron gas

We propose a semiclassical theory of dc magnetotransport in a two-dimensional electron gas modulated along one direction with weak electrostatic modulations. We show that oscillations of the magnetoresistivity ρ_∥ corresponding to the current driven along the modulation lines observed at moderately low magnetic fields can be explained as commensurability oscillations.     

FOURIER ANALYSIS OF TEMPORAL AND SPATIAL OSCILLATIONS OF TUNNELING CURRENT IN SCANNING TUNNELING MICROSCOPY

Partially oxidized Si(111) surfaces and surfaces of highly oriented pyrolytic graphite (HOPG) were studied by two different ultrahigh vacuum scanning tunneling microscope (UHV-STM) systems and by an STM system working under ambient conditions, respectively. The STM current images of partially oxidized Si(111) surfaces and HOPG surfaces were analyzed by one/two-dimensional fast Fourier transformation (1D-FFT/2D-FFT). The phenomenon of temporal oscillations of tunneling current on the partially oxidized Si(111) surfaces was detected with both UHV-STM systems. Temporal as well as spatial oscillations of tunneling current appeared in highly resolved STM current images of the Si(111) surfaces simultaneously, but both kinds of oscillations could be discriminated according to their different influence on the 2D-FFT spectra of the current images, while varying the scanning range and rate. On clean HOPG surfaces only spatial oscillations of tunneling current induced by the surface structure were observed.     

The burning voltage of a high-current vacuum arc in a six-cap rodelectrode system

This paper studies the voltage-current characteristics of the six-gap rod electrode system and their dependence on the shape and material of the electrodes and interelectrode gap value. The investigations were carried out for currents up to 200 kA with the rate of its rise up to ~5·10⁹ A/s. A number of features were observed concerning arc-burning voltage dynamics depending on the value and rate of rise of the arc current, namely, a step-like voltage change at an initial discharge burning stage, and excitation of HF voltage oscillations for the discharge current wavefront. The duration and intensity of the unstable discharge burning phase accompanied by HF voltage oscillations depend strongly on the electrode system parameters. Power dissipated in the vacuum gaps and full dissipated energy were calculated as a function of the switching current peak. Results of the investigations give the possibility of designing a sealed-off triggered vacuum switch with minimum power losses and long service life evaluated by a transferred charge value >10⁶ C     

On the question of short-period oscillations of the collector voltage during transverse electron focusing

Giant short-period oscillations during transverse electron focusing at high emitter voltages have been observed in bismuth samples with a superlattice on the surface. A model is proposed which explains the onset of the oscillations, their position on the magnetic-field scale, and their intensification and shift along the magnetic-field scale with increasing current (this shift depending on the direction of current flow) and which can also account for the absence of oscillations in fields which are multiples of the field of the first oscillation. In this model the oscillations are attributed to the appearance of resonant surface (edge) states and their contribution to the electron transport. Pis'ma Zh. éksp. Teor. Fiz. 68, No. 12, 887–892 (25 December 1998)     

Three-phase-boundary dynamics at the Ni/ScYSZ interface

Chronoamperometry using a three-electrode cell configuration was undertaken with a nickel point-electrode acting as the working electrode on a polished ScYSZ electrolyte in a hydrogen atmosphere at 750–850 °C. High anodic overpotentials resulted in the occurrence of distinct sawtooth oscillation patterns in the measured current signal. The current oscillations indicated that a dynamic electrode process was taking place. Decreasing the water content in the measurement atmosphere as well as lowering the applied anodic overpotential had the effect of lowering the frequency and the amplitude of the current oscillations. A mechanism accounting for the observed phenomena and possible implications for solid oxide fuel cell operation are presented.     

Experimente zum Abreißstrom des Vakuumbogens

The theory of the chopping current of a vacuum arc developed by Ecker assumes that in the local circuit vacuum arc — wire inductivity — capacity connected in parallel oscillations arise on account of the falling characteristic of the vacuum arc at small currents. These oscillations cause the vacuum arc to chop. If this theory is correct, the chopping current can be reduced by damping these oscillations. The aim of this paper is to proof this consideration experimentally, and it appears that a strong reduction of chopping current can be obtained. The chopping current never falls short of a lower limit, which is probably caused by the available voltage.

     

Radiative recombination in ZnTe-CdSe and ZnSe-CdTe heterojunctions

Electroluminescence in II–VI heterojunctions has been investigated. Three types of ZnTe-CdSe heterojunctions were studied depending on the preparation method: Red diodes obtained by doping with O₂, yellow Cu-doped diodes and green undoped heterojunctions. Radiation was observed only in the forward biased junctions. At 80 K the external quantum efficiency is about 1.3% for red ZnTe-CdSe heterojunctions and decreases by one order of magnitude at room temperature. The radiation intensity for the other heterojunctions is the same at 80 K but at 150 K the luminescence disappears. The band diagram and the electroluminescence spectra show that the two-directional injection takes place in the ZnTe-CdSe heterojunctions. The blue electroluminescence for ZnSe-CdTe heterojunctions is due to the injection of hot holes in ZnSe from the high-resistivity layer at the interface and the recombination of these holes with the free electrons through an accepto level at 0.124 eV from the valence band. At 80 K slow periodic current oscillations accompanied by in-phase oscillations of the luminescence have been observed in ZnSe-CdTe heterojunctions.     

Spontaneous resistance oscillations inp-germanium at low temperatures and their spatial correlation

Spontaneous oscillations and chaotic behavior of the electric resistance can be observed inp-doped germanium during avalanche breakdown at liquid-He temperatures. By a series of different ohmic contacts attached to the specimen surface, the spatial coupling between the resistance oscillations in different sections of the same single-crystalline Ge sample can be studied. Using a geometry where coupling via charge carriers can be ignored, we have investigated the case where the lattice heat conductivity of the Ge crystal represents the dominant coupling mechanism. If two coupled sample sections were operated in the post-breakdown regime, a distinct phase shift developed between the current oscillations in both sections eventually reaching exact phase reversal. These results show a striking similarity to the dynamics of a coupled Rashevsky-Turing reaction-diffusion system.     

Resonance imaging of dynamical filamentary current structures in a semiconductor

The spatially resolved observation of the nonlinear dynamical behavior of spontaneous current oscillations obtained during low-temperature avalanche breakdown of homogeneously p-doped germanium is reported. Stationary current filaments developing in the breakdown regime through impurity impact ionization were observed two-dimensionally by means of a scanning electron microscope equipped with a liquid-helium cryostage. Further, spontaneous current oscillations showing typical transitions to chaos were localized two-dimensionally by means of a novel resonance imaging technique, which provides spatially resolved analysis of the nonlinear dynamical behavior. From these measurements different oscillation frequencies were clearly identified as spatially separated oscillation centers localized along the stationary current filaments. The electron beam was demonstrated to act as an exemplary control parameter, which can be manipulated both spatially and temporally.     

One-electron spin-dependent transport in split-gate structures containing self-organized InAs quantum dots

The paper presents the results obtained in a study of electron transport in split-gate structures prepared from heterostructures with self-organizing InAs quantum dots situated close to a two-dimensional electron gas. Coulomb oscillations of current through InAs quantum dots depending on the voltage on the gate were observed. Coulomb current oscillations persisted up to about 20 K. The Coulomb energy ΔE _C = 12.5 meV corresponding to theoretical estimates for the p-states of quantum dots in our structures was determined.