Low-frequency instabilities of electron-hole plasmas in crossed fields

Using local point-contact probes, we observed two types of low-frequency instabilities inn-InSb at 85 K if the samples were exposed to crossed fields. One is a local density instability with threshold frequencies off = 1 20 Mc, the other a more turbulent current instability. The threshold values ofU ₀ andB for the onset of these instabilities and the dependence of their amplitudes on the fields have been measured.If a rectangular semiconductor slab is placed in crossed fields, regions of high electric field strength at opposite edges of the contacts are caused by the distortion of the Hall field, giving rise to the generation of electron-hole plasmas by impact ionization. These plasmas are the sources of the observed instabilities. This is especially evident in the case of the local density instability, which originates at the anode high field corner. Several possible reasons for the development of the instabilities are discussed.     

Propagation and spreading of unstable perturbations in electron-hole plasmas subject to crossed fields

A model is developed for electrostatic drift instabilities which arise in inhomogeneous electron-hole plasmas subject to crossed fieldsE ₀B ₀. The instabilities are initiated by gradients in the equilibrium plasma densityn ₀. Using two-fluid magnetohydrodynamics and linear perturbation theory the dispersion relation of local density oscillations is calculated for arbitrary inhomogeneous equilibrium distributions and plasma densities. For cases wheren ₀B ₀ andn ₀ ⁺ n ₀ ^- it is found that the propagation direction of maximal gain, , is the bisectrix of the angle between (–n ₀) and (E ₀ ×B ₀) and that stable and unstable configurations are distinguished by the angle between ₀ andn ₀. A local density perturbation built by superpositions of the plane waves, and initially chosen radially symmetric, broadens unisotropically. In the direction transverse to the broadening is anomaleously enhanced, as compared to the broadening by diffusion in the stable case. The results are referred to experimental observations of low-frequency instabilities reported in [1].     

Diffusion of stable electron-hole plasmas in magnetic fields

The transport of a semiconductor plasma, composed of electrons and holes, can be described with the aid of moments relativ to the conditional one particle density function. To calculate these moments, an operator equation is given which was derived with the generalized Stratonovich method. This equation is extended by taking recombination and scattering with a lattice into account. The coefficient functions are determined for stable and homogeneous systems in a self-consistent manner. In particular, the spatial diffusion of test particles in a strong magnetic field is considered, neglecting quantum effects. The anomalous contribution to the diffusion across the magnetic field is calculated explicitly and compared with that for a high temperature plasma. It is shown, that anomalous diffusion is possible also in stable systems, provided the time dependence of the stochastic electric field is determined by the polarization due to the test particle.     

Transport of degenerate electron-hole plasmas in Si and Ge

At low crystal temperatures, pulsed-laser excitation of Si and Ge can produce a mobile electron-hole plasma with a Fermi energy much larger than k_BT. The motion of this degenerate plasma away from the excitation surface depends intimately on its interactions with high-frequency phonons. Momentum damping and phonon-wind forces are principal factors which determine the plasma motion on nanosecond and longer time scales. A variety of luminescence and heat-pulse experiments are reviewed here which characterize the transport behavior of photo-generated electron-hole plasma in these indirect-gap semiconductors.     

Renormalized diffusion in systems with periodic variables

The well-known results for the effective or renormalized diffusion of a Brownian particle in a periodic potential is generalized for arbitrary systems subject to Fokker-Planck equations with coefficients that are time-independent, periodic in at least one variable and such that no average motion persists. In presence of a bare diffusion the conservative and the dissipative drifts play a different role, and the renormalizing term results from the time integral of a combination of auto- and cross-correlations of both drifts. Nevertheless, the numerical evaluation of the result is essentially limited to solving the homogeneous and time-independent forward and backward equation within one period.Work supported by the Swiss National Science Foundation     

Ordered structure formation in 2D mass asymmetric electron-hole plasmas

We study strong Coulomb correlations in dense two-dimensional electron-hole plasmas by means of direct path integral Monte Carlo simulations. In particular, the formation and dissociation of bound states, such as excitons, bi-excitons and many particle clusters, is analyzed and the density-temperature regions of their occurrence are identified. At high density, the Mott transition to the fully ionized state (electron-hole hexatic liquid) is detected. Particular attention is paid to the influence of the hole to electron mass ratio M on the properties of the plasma. For high enough values of M we observed the formation of Coulomb hole crystal-like structures.     

Motion of electron-hole drops in low magnetic fields

The effect of magnetic fields on the motion of electron-hole drops in germanium is studied. A non-uniform strain is used to provide a known and controllable driving force for drop motion in the sample plane. Contrary to the results of earlier experiments in which drop motion was normal to the sample plane, the results are consistent with conventional models of drop-phenomenon interaction and weak magnetic fields have no observable effects on this motion.     

Radiation emission coefficients for sulfur hexafluoride arc plasmas

Calculations are made of the total spectral absorptivities of plasmas of sulfur hexafluoride for wavelengths of 100 to 15,500 Å. Both line and continuum radiation are considered. For the contributions from line radiation, we have calculated the strengths, widths and shifts of both neutral, singly and doubly ionized atoms of S and F, except that we used experimental line strengths where they are available. The theory used was that of Griem, which assumes LS coupling. Curves are given for the emission coefficient of radiation appropriate to the arc center for isothermal cylindrical plasmas of various radii for pressures of 1 and 10 atm and temperatures from 5000 to 35,000°K. It is found that at 1 atm line radiation can be an order of magnitude higher than continuum radiation and radiation >2000 Å is less than 10% of the total radiation for temperatures greater than 15,000°K. Predictions are given of volt-ampere characteristics and central temperatures for arcs of various radii in SF₆ at pressures at 1 and 10 atm.     

Synergism in electron gas in crossed fields

An electron gas in crossed electric and magnetic fields of arbitrary strength is considered in the framework of nonequilibrium statistical mechanics. It is known that more than one independent frequency existing in a system, which are of comparable magnitude, can generate coherent synergic radiation. In the present system three independent frequencies exist: viz the synchrotron frequency due to the magnetic field, the hopping frequency due to the electric field, and the plasma frequency. It is shown that all these can combine to generate a new synergic coherent radiation. The results also show the possibility of interpreting the quantum nature of Hall conductance as due to density function alone. Besides these, the solution admits the Schubnikov-de Haas oscillation of the electrical conductivity due to change in the fields.     

Cooling of hot electron-hole plasmas in the presence of screened electron-phonon interactions

We have computed for several semiconductors, (GaAs, CdSe, CdS), the energy loss-rate in hot electron-hole plasmas, at high density, when occurs the screening of the electron-phonon interactions. We show that the long range interactions (piezoelectrical and polar) are quickly reduced when the plasma density is raised, and that the energy loss-rate diminishes therefore significantly (one order of magnitude).     

Decay instability of plasma in crossed fields

In this paper it is shown that in plasma placed in crossed electric and magnetic fields, oscillations are excited with the frequency close to the drift frequencyω ₀ =k _? υ _? . Decay of the oscillations into two other oscillations, for which the selection rulesω ₀ =ω ₂ ?ω ₁ andm ₀=m ₂?m ₁ hold, where the frequencyω ₁ is close to the Langmuir frequency of ions, was observed.     

Confined ferrofluid droplet in crossed magnetic fields

When a ferrofluid drop is trapped in a horizontal Hele-Shaw cell and subjected to a vertical magnetic field, a fingering instability results in the droplet evolving into a complex branched structure. This fingering instability depends on the magnetic field ramp rate but also depends critically on the initial state of the droplet. Small perturbations in the initial droplet can have a large influence on the resulting final pattern. By simultaneously applying a stabilizing (horizontal) azimuthal magnetic field, we gain more control over the mode selection mechanism. We perform a linear stability analysis that shows that any single mode can be selected by appropriately adjusting the strengths of the applied fields. This offers a unique and accurate mode selection mechanism for this confined magnetic fluid system. We present the results of numerical simulations that demonstrate that this mode selection mechanism is quite robust and “overpowers” any initial perturbations on the droplet. This provides a predictable way to obtain patterns with any desired number of fingers.     

Electron impact excitation coefficients for laboratory and astrophysical plasmas

Electron impact excitation rate coefficients have been obtained for a number of transitions in highly ionized ions of interest to astrophysical and laboratory plasmas. The calculations were done using the method of distorted waves. Results are presented for various transitions in highly ionized Ne, Na, Al, Si, A, Ca, Ni and Fe.