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.     

Effect of ionization of impurity centres by electric field on the conductivity of superlattice

The electric fieldE₀effect on ionization on impurity centres and on the conductivity of superlattices (SLs) has been studied theoretically. It is observed that as the fieldE₀increases the current rises, reaches a maximum, then falls off, i.e. show a negative differential conductivity (NDC). Further increase inE₀leads to an exponential rise of the current. This occurs aroundE₀=3×10⁴ V cm⁻¹. Hence the current density field shows a ‘N’ shape characteristics as against the ‘n’ shape characteristics in the absence of impurity.     

Poole-Frenkel effect in chalcogenide semiconductors with various crystalline structures

The results of investigation of electrical conductivity of a large group of chalcogenide semiconductors having the layered, cubic, and orthorhombic structures in strong electric fields up to 10⁵ V/cm are presented. The revealed increase in electrical conductivity σ in strong electric fields has been explained by the Frenkel thermionic ionization. This has made it possible, along with other parameters (for example, activation energy and trap concentration, charge-carrier mean free path, permittivity), to evaluate the concentration and mobility of charge carriers in semiconductors under study. It has been shown that in strong electric fields in semiconductors, when the thermionic ionization of the traps occurs, their permittivity ?, which is caused by the electron polarization, is determined by the simple formula ? = n ², where n is the refractive index of light.     

Effect of pressure anisotropy on magnetorotational instability

It is shown that two new instabilities of hybrid type can occur in a rotating magnetized plasma with anisotropic pressure, i.e., the rotational firehose instability and the rotational mirror instability. In the case of β_∥ > β_⊥, where β_∥ and β_⊥ are the ratios of the parallel and perpendicular plasma pressure to the magnetic field pressure, the pressure anisotropy tends to suppress both new instabilities; in the case β_⊥ > β_∥, it leads to their strengthening. In the latter case, the perturbations considered can be unstable even if the Velikhov instability criterion is not satisfied. The text was submitted by the authors in English.     

Instability of electromagnetic waves in transversely magnetised semiconductor-plasmas

Using the hydrodynamic model of plasmas the general dispersion relation is derived in the collisiondominated regime when a d.c. magnetic field is applied (Y-axis) transversly to the propagation vector k (Z-axis), and the d.c. electric field is inclined to the Z-axis in the X-Z plane. The dispersion relation is solved for intrinsic and extrinsic semiconductors to explore the possibility of wave instability. The threshold conditions of wave oscillations are obtained. In n-InSb the frequency of the oscillation attains a maximum value when the electron cyclotron frequency is equal to the electron collision frequency. In intrinsic InSb instability is possible only in the long wavelength region for E₀ ? 10 kVm^?1 when B₀> 0.2 T, while for lower values of B₀, E₀ should be greater 20kVm^?1. The energy dependent collision frequency has a significant effect on the threshold frequency of oscillation.     

Appearance of the dielectric instability and its coexistence with the superconductivity in ultrathin metallic filaments with decreasing diameter

Some dielectric instability appears before the superconducting (s/c) transition of Hg, In, Ga, Sn filaments when their diameters are decreased to 20–30 Å. Above the s/c transition the differential conductivity of the filaments may be approximated by the formula σ = σ_a + σ_b exp [-E₀(T)/E], where the electric field E₀ is equal toabout 10–60 mV cm^-1, and falls abruptly before the s/c transition. There are conductivity oscillations along E in the s/c transition region. The results are interpreted in terms of Peierls instability and charge-density wave (CDW) conductivity.     

Electrical conductivity of cobalt-titanium substituted SrCaM hexaferrites

A series of polycrystalline M-type hexagonal ferrites with the composition Sr_0.5Ca_0.5Co_xTi_xFe_12−2xO₁₉ (where x=0.0-0.8) were prepared by the conventional ceramic technique. The electrical conductivity has been measured from 300 to 590 K. The dc conductivity, σ_dc, exhibited a semiconductor behavior. The negative sign of thermoelectric power coefficient S reveals that all samples are n-type semiconductors. Both σ_dc and mobility, μ_d, increases with the substitution of Co²⁺ and Ti⁴⁺ ions, reach maximum at x=0.4 and start decreasing at x>0.4. Many conduction mechanisms were discussed to explain the electric conduction in the system. It was found that the hopping conduction is the predominant conduction mechanism. For samples with compositional parameter x=0.0 and 0.8, the band conduction mechanism shares in electric conduction beside the hopping process.     

Combined breakdown in bismuth—Antimony semiconductor alloys

S-shaped current-voltage characteristics for Bi_1?xSb_x alloys are studied theoretically and experimentally. The phenomenon is shown to occur due to the combined interband breakdown, the impact ionization being caused both by the external electric field and the Hall field. The latter is governed by the proper magnetic field of the plasma current. The negative differential resistance (NDR) occurs for the range of currents where the impact ionization is growing intensively but the pinch effect is not yet developed to full extent. The phenomenon is enhanced if the impact ionization rate in the Hall field is greater than in the applied one.     

Heating of magnons by hot charge carriers and electrical properties of HgCr2Se4

Strong variations of the conductivity and the Hall coefficient as a function of the magnetic and electric field strengths are discovered in the ferromagnetic semiconductor HgCr₂Se₄. The nature of these phenomena is discussed in connection with its electronic structure and the strong interaction between the electric and magnetic subsystems in this magnetic semiconductor. The results can be interpreted within a model of ordinary semiconductors with consideration of the strong electron-magnon interaction specific to magnetic semiconductors, the heating of magnons by hot charge carriers, and the trapping of charge carriers (the formation of ferrons) due to the s-d exchange interaction. Fiz. Tverd. Tela (St. Petersburg) 39, 664–667 (April 1997)     

向列相液晶电流体不稳定性——偏置电场效应

本文考虑了外电场与液晶盒玻璃片法线成倾角θ₀时,沿面排列的向列相液晶(θ_α<0,σ_α>0)的电流体不稳定性现象。在Dubois-Violette等人的一维理论的基础上,我们获得了直流、正弦波和矩形波三种电场激发下的失稳条件。重要结果包括:(1)电荷弛豫时间随θ₀单调上升。(2)对于MBBA液晶,存在一个临界角θ_c(ω)(ω是外场频率),当θ₀>θ_c(ω) 关键词：     

Evolution of electron-hole avalanches and streamers in indirect gap semiconductors

A numerical simulation of the origination and evolution of streamers in semiconductors has been performed using the diffusion-drift approximation including the impact and tunnel ionization. It is assumed that an external electric field E ₀ is static and uniform, an avalanche and a streamer are axisymmetric, background electrons and holes are absent, and all their kinetic coefficients are identical. The linear evolution of an electron-hole avalanche, an avalanche-to-streamer transition, and two successive stages of the evolution of the streamer—intermediate “diffusion” and main exponentially self-similar—have been examined in detail. It has been shown that a streamer is similar to a dumbbell with conical weights. The bases of these cones, streamer fronts, are thin shells, which contain almost the entire streamer charge and are close in shape to the halves of ellipsoids of revolution. A front propagates so that its shape and the shape of the weight of the dumbbell, the maximum field on the front, and the electron-hole plasma density in weights remain unchanged. The field strength behind the front is much smaller than E ₀, but increases with approaching the bar of the dumbbell whose diameter increases with the time t owing to the transverse diffusion. The electron and hole densities in the bar increase due to the impact ionization in an almost uniform field, which is only slightly lower than E ₀. At the diffusion stage, the length of the streamer and the curvature radius of its front increase with constant rates, which are determined not only by the impact ionization and drift, but also by diffusion. In relatively low fields (E ₀ ≲ 0.4 MV/cm for silicon) this stage ends due to the appearance of the instability of the front. In higher fields, the tunnel ionization is manifested before the appearance of instability and gives rise to the appearance of a new-type streamer. Its main feature is the stable exponential increase in all spatial scales with the same response time t _R , so that the charge-carrier density and field strength at large times t depend only on one vector variable $ \hat R $ \hat R = Rexp(−t/t _R ). This means that the solution of a Cauchy problem describing the evolution of the streamer in the uniform field is asymptotically exponentially self-similar.     

Electrical instability of LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript> crystals

The temperature behavior of I-U curves and the field and temperature dependences of the electrical resistivity and dielectric permittivity of crystals of the LiCu₂O₂ phase have been studied. It was established that the crystals belong to p-type semiconductors and that their static resistivity in the range 80–260 K follows the Mott law ρ=Aexp(T₀/T)^1/4 describing variable-range hopping over localized states. At comparatively low electric fields, the crystals exhibit threshold switching and characteristic S-shaped I-U curves containing a region of negative differential resistivity. In the critical voltage region, jumps in the conductivity and dielectric permittivity are observed. Possible mechanisms of the disorder and electrical instability in these crystals are discussed.     

Crystalline electric field effects on the resistivity of PrAl2

Calculations in the mean field approximation of the magnetic contribution to the resistivity of a J = 4 ion in a cubic lattice, considering the effects of the crystalline electric field, are presented for both T >T_c and T < T_c (excluding the critical region). The magnetic contribution to the resistivity of ferromagnetic PrAl₂ is discussed in terms of the above calculations and the total crystalline electric field splitting of this system is determined to be 100 K. Results are also presented for the critical behavior of d?/dT.     

Plasma self-oscillations in semiconductors within submillimetre frequency range

In strong electric fields, when the impact ionization becomes intensive and the electron drift velocity may decrease with the growing field, self-oscillating processes arise in semiconductors at very high frequencies up to the submillimetre range. These processes are due to negative dynamic differential conductivity of plasma within the corresponding frequency region. The highest frequency branch results from the excitation of plasma oscillations.     

Effects of internal viscous damping on the stability of a rotating shaft driven through a universal joint

A rotating flexible shaft, with both external and internal viscous damping, driven through a universal joint is considered. The mathematical model consists of a set of coupled, linear partial differential equations with time-dependent coefficients. Use of Galerkin's technique leads to a set of coupled linear differential equations with time-dependent coefficients. Using these differential equations some effects of internal viscous damping on parametric and flutter instability zones are investigated by the monodromy matrix technique. The flutter zones are also obtained on discarding the time-dependent coefficients in the differential equations which leads to an eigenvalue analysis. A one-term Galerkin approximation aided this analysis. Two different shafts (“automotive” and “lab”) were considered. Increasing internal damping is always stabilizing as regards to parametric instabilities. For flutter type instabilities it was found that increasing internal damping is always stabilizing for rotational speeds v below the first critical speed, v₁. For v>v₁, there is a value of the internal viscous damping coefficient, C_iv, which depends on the rotational speed and torque, above which destabilization occurs.The value of C_iv (“critical value”) at which the unstable zone first enters the practical range of operation was determined. The dependence of C_iv critical on the external damping was investigated. It was found for the automotive case that a four-fold increase in external damping led to an increase of about 20% of the critical value. For the lab model an increase of two orders of magnitude of the external damping led to an increase of critical value of only 10%.For the automotive shaft it was found that this critical value also removed the parametric instabilities out of the practical range. For the lab model it is not always possible to completely stabilize the system by increasing the internal damping. For this model using C_iv critical, parametric instabilities are still found in the practical range of operation.     

Effect of thickness on the structure, morphology and optical properties of sputter deposited Nb2O5 films

Nb₂O₅ films with the thickness (d) ranging from 55 to 2900 nm were deposited on BK-7 substrates at room temperature by a low frequency reactive magnetron sputtering system. The structure, morphology and optical properties of the films were investigated by X-ray diffraction, atomic force microscopy and spectrophotometer, respectively. The experimental results indicated that the thickness affects drastically the structure, morphology and optical properties of the film. There exists a critical thickness of the film, d_cri =2010 nm. The structure of the film remains amorphous as d < d_cri. However, it becomes crystallized as d > d_cri. The root mean square of surface roughness increases with increasing thickness as d > 1080 nm. Widths and depths of the holes on film surface increase monotonously with increasing thickness, and widths of the holes are larger than 1000 nm for the crystalline films. Refractive index increases with increasing thickness as d < d_cri, while it decreases with increasing thickness as d > d_cri. In addition, the extinction coefficient increases with increasing thickness as d > d_cri.     

Theory for the first-order spin-wave instability threshold in ferromagnetic insulating thin films

A theory for the first-order Suhl and the parallel pumping instability in thin films is presented. Significant differences for the critical microwave field and wave vector to former calculations occur, which discuss the problem in terms of bulk spin-waves neglecting boundary conditions. A coupling matrixC _kk′ is introduced, which describes the couplings between the modes and the driving microwave field. For bulk standing spin-wavesC _kk′ is always diagonal. For the true discrete standing modes of a thin filmC _kk′ changes only in case of 1. Suhl instability and if the wavevector has a non vanishing component perpendicular to the film plane. Here the diagonal bulk couplings have to be replaced in part by off diagonal terms, describing couplings between modes, which perpendicular wave vector componentk _⊥ differs byπ/d (d=film thickness). The decisive quantity, which decides if the finite thickness of the film is of importance or if the film can be treated as a bulk system, is the frequency difference δω _k of the coupled modes. For δω _k much smaller than the spin-wave damping η _k the bulk approximation is correct. For \(\delta \omega _k > > \eta _k \) two experimental situations for 1. Suhl instability are discussed: For a perpendicular to the film plane magnetized film the critical microwave field is by π/2 bigger than in the bulk case. In an in-plane magnetized film the critical spin-waves propagate always in the film plane, as only hereC _kk′ remains identical to the bulk case.     

Z-pinch dynamics under the combined breakdown in bismuth-antimony alloys

Time scans of the electric field in n-Bi_0.9Sb_0.1 samples are calculated under the impact ionization for various field orientations with respect to the crystal axes (the conditions of current given). The strong anisotropy of the pinch characteristics in the electron-hole plasma of such alloys is shown to be caused by the joint effect of the hole mobility anisotropy in the T-valley and the combined breakdown, the impact ionization being produced both by the external electric field and the Hall field. The magnitude of the latter is governed by the proper magnetic field of the current. The results explain the experimental data of Brandt et al. [1] who have observed the dynamical characteristics of the pinch in the above mentioned alloys to be strongly anisotropic.     

Anfachung und Dämpfung von Ionisationswellen in MHD-Kanälen und ihr Einfluß auf die effektive elektrische Leitfähigkeit,den effektiven Hall-Parameter und die mittlere Elektronentemperatur

The phase velocity, the amplification rate and the critical Hall parameter are theoretically determined for ionization waves in a weakly ionized plasma streaming across a strong external magnetic field and bearing a current flowing perpendicular to both the magnetic field and the stream velocity. The investigations hold for seeded rare gases at any degree of seed ionization. The critical Hall parameter β_c depends on the degree of ionization, the ionization energy and the temperatures of electron gas T₀ and neutral gas T_g · β_c is always greater than one, if 0 < T₀ — T_g ? T₀ holds. The three-dimensional treatment indicates the existence of waves with a nonvanishing wave vector component in the direction of the magnetic field. The influence of ionization waves on mean current density, mean Hall field intensity and mean electron temperature is determined up to second order terms in the relative fluctuations of the electron temperature. The amplification of ionization waves reduces the effective electric conductivity, the effective Hall parameter and the mean electron temperature compared to the undisturbed state. Similar results are also obtained for steady state homogeneous isotropic turbulence and a special case of axially symmetric turbulence. Furthermore, a component of the electric field in direction or in opposite direction to the magnetic field vector may be generated by non isotropic and non homogeneous turbulence.     

Hysteresis phenomena in the course of polarization evolution in a sinusoidal electric field in lead magnesium niobate crystals

The processes of polarization evolution in single crystals of the PbMg_1/3Nb_2/3O₃ model ferroelectric relaxor in a sinusoidal electric field are investigated at temperatures near and above the temperature T _d 0 of destruction of the induced ferroelectric state upon heating in zero electric field. The polarization switching current loops are measured in the ac electric field applied along the 〈111〉 and 〈110〉 pseudocubic directions. The electroluminescence intensity loops are obtained under the combined action of ac and dc electric fields applied along the 〈100〉 direction. In a certain temperature range above T _d 0 and the freezing temperature T _f in lead magnesium niobate, there are electric current anomalies, that correspond to the dynamic formation and subsequent destruction of the ferroelectric macroregions throughout each half-cycle of the ac electric field. The measurements of electroluminescence hysteresis loops demonstrate that the observed depolarization delay (related to the ac electric field amplitude) increases with an increase in the dc electric field and decreases as the ac field amplitude increases. The nature of the observed phenomena is discussed.