Instabilities of electron-hole plasma under impact ionization and microwave emission

The nonlinear stage of the evolution of electron—hole plasma instabilities in semiconductors under impact ionization (static differential conductivity σ_d > 0) is considered for the case of spatially uniform pertubations of density and electric field. If the differential mobility of carriers μ _d > 0, the instability arises only with allowance for the retardation of the process of impact ionization (linear theory of this effect was developed by M. Toda [4]). When μ _d < 0, the instability may appear in the absence of the retardation. Both these instabilities exhibit oscillating form and are due to h.f. negative dynamic differential conductivity. We determine time sweeps of the electric field under condition of fixed current for the case n-InSb at T = 77 K. The amplitude and the frequency (f>10¹⁰Hz) of the oscillations are evaluated and the conditions, when the shape of E(t)-oscillations is essentially non-harmonic, are determined. Microwave emission observed in semiconductors under impact ionization may have resulted from the instabilities at hand.     

Current fluctuations in negistor structures based on noncrystalline Ga-Te films

The results of investigation of low-frequency (up to 10⁶ Hz) and microwave (at a frequency of 10 GHz) electric current fluctuations in negistor structures of the type of glass carbon-noncrystalline Ga_{1 ? x} Te _x film (where x = 0.75 or 0.80)-glass carbon are presented. Low-frequency current fluctuations are studied in strong electric fields up to the emergence of electric instability, while microwave fluctuations are studied after the current pinching at low current sustaining the low-resistance state. It is shown that regularities governing such fluctuations in these structures substantially differ from the corresponding regularities in selenium film negistors studies earlier. According to the results of analysis, the processes initiating electrical instability and the current pinching in these structures are not associated with manifestations of heating of charge carriers by the electric field. In addition, it is found that the filament diameter virtually remains unchanged in the range of currents sustaining the low-resistance state, while the electrical conductivity of the pinching region shows a tendency to increase with current. Possible reasons for these phenomena are considered.     

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.     

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.     

Electric field properties at cationic sites in normal, incommensurate and commensurate K2ZnCl4 crystals

The local electric properties at K and Zn sites in the normal, incommensurate and commensurate phases of K₂ZnCl₄, as derived from a numerical computation of the lattice contributions to the electric potential V(r), electric field intensityE(r) and electric field gradient tensorV _αβ(r) are reported. The numerical data obtained at each cationic position were correlated with the experimental³⁹K NMR, Cu²⁺ and Mn²⁺ EPR and⁵⁷Fe Mössbauer results in pure and doped K₂ZnCl₄. A proportionality between crystal field and zero-field splitting was taken into account for Mn²⁺, whereas for K⁺, Cu²⁺ and Fe³⁺ ions the electric field gradient is directly related to the crystal field parameter. By this comparison, on computations done in the ionic fractional charge and relaxed lattice approximations, the insertion of probe-species of iron, copper and manganese ions on off-center Zn sites is proposed. The³⁹K electric field gradient tensor calculations in the incommensurate phase fit well with the NMR data reported recently.     

Influence of the surface electric field in ion-beam-surface interaction at grazing incidence

We have measured the polarization of light emitted after ion surface scattering at small angle of incidence. The measurements are carried out with H⁺-, H ₂ ⁺ - and He⁺-ions under UHV-conditions with mono- and polycrystalline targets. We explain the typical variation of the polarization as “post collision Stark interaction” (PCSI) in the surface electric field, which can force transitions between nearly degenerate terms. The electric field is composed of two different contributions, a strong but short range surface field which is “seen” by atomsand ions and a long range but weak field due to the image charge which is “seen” to first orderonly by ions. The influence of the electric field on H-Balmer radiation is negligible at typical survival distances r_s≧0.35nm. But in contrast to H-atoms He⁺-ions feel the additional influence of the image field leading to a strong alteration of the polarization of the emitted light. The polarization of the Balmer-radiation stemming from Coulomb exploding H ₂ ⁺ -beams is observed to be modified by the electric field of the “spectator proton”.     

Linear electric shift in the paramagnetic resonance of Mn2+ in ZnTe

The influence of an electric field on the EPR transition has been observed in ZnTe:Mn²⁺ at 29 K by applying an ac electric field. The value of the component of the third rank tensor describing the effect is R₁₄ = 164 HzV^-1 cm. A strong correlation between the magnitude of the component R₁₄ and the amount of covalency was found.     

Electric-field-induced orientation of iron tetragonal centers in KTaO3

Fe _K ³⁺ -O _i ^2? impurity centers in a KTaO₃ sample to which a dc electric field E=75 kV/cm is applied are shown to be oriented at temperatures T≥120 K. In these conditions, the effective local field acting on the electric dipole moment of a center exceeds the applied field by a factor 7.6.     

Electrical properties of highly (111)-oriented lead zirconate thin films

Antiferroelectric PbZrO₃ thin films were grown on Pt/Ti/SiO₂/Si substrates with predominant (111) orientation using a sol-gel process. The Pt/PbZrO₃/Pt film capacitor showed well-saturated hysteresis loops at an applied voltage of 5 V with remanent polarisation (P_r) and coercive electric field (E_c) values of 8.97 μC/cm² and 162 kV/cm, respectively. The leakage current density of the highly (111)-oriented PbZrO₃ film was less than 1.0×10⁻⁷ A/cm² over electric field ranges from 0 to 105 kV/cm. The conduction current depended on the voltage polarity. The PbZrO₃/Pt interface forms a Schottky barrier at electric fields from 20 to 160 kV/cm. The dielectric relaxation current behaviour of Pt/PbZrO₃/Pt capacitor obeys the well-known Curie-Von Schweidler law at electric field of 20-80 kV/cm, the currents have contributions of both dielectric relaxation current and leakage current.     

Hot́ hole anisotropic effect in silicon and germanium

Anisotropic effect of the hole drift velocity in silicon and germanium has been investigated with the time of flight technique by applying the electric field parallel to the <100 and <111 crystallographic axis. The measurements were performed for electric fields ranging from 10 to 3 × 10⁴V/cm and temperatures from 40 to 200°K. The results indicate that the anisotropic effect v_d<100/v_d<111 increases with decreasing temperature and increasing electric field, and reaches a saturation value at high electric fields (? 10⁴V/cm). The maximum anisotropic effect for Ge is 1.25 at 40°K and for Si is 1.2 at 45°K. A qualitative analysis of the experimental data indicates that the anisotropic effect is due to the warped heavy-valence-band shape.     

Ion cyclotron drift instability driven by cross-field current

An ion cyclotron drift instability is excited by a cross field current due to E_r × B drift of electrons and ion heating is observed in a plasma column in a magnetic field when a dc radial electric field E_r is applied.     

Field-independence of thermal emission rate in Au-doped silicon

The thermal emission rate of holes, e^t_p, has been measured from dark leakage currents for the Au acceptor level in silicon diodes at different electric fields and found to be field independent for average electric fields below 10⁵V/cm.     

Doppelresonanzuntersuchung des Einflusses eines elektrischen Feldes auf die Zeeman-Aufspaltung des 5s 5p 3P1-Terms im SrI-Spektrum

Under the simultaneous action of external electric and magnetic fields the 5s 5p ³ P ₁-level of the even Sr-isotopes splits into three non-equidistant Zeeman-sublevels. In an atomic beam experiment the spacings between the sublevels were investigated by the double resonance method at a magnetic field strength of ca. 22 Oe and at electric field strengths up to ca. 17 kV/cm. From the r.f.-resonance signals the tensor polarizability of the 5s 5p ³ P ₁-level was deduced to be α_ten(³ P ₁)=6.1(8) kHz/(kV/cm)². This value may be used to get an estimate of the oscillator strength of the infrared transitions between the multiplets 5s 5p ³P and 5s 4d ³ D. Taking into account oscillator strengths of electric dipole transitions to other low lying levels one obtainsf(³ P→³ D)≈0.09.     

Field electron emission from HfNxOy thin films deposited by direct current sputtering

HfN_xO_y thin films were deposited on Si substrates by direct current sputtering at room temperature. The samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). SEM indicates that the film is composed of nanoparticles. AFM indicates that there are no sharp protrusions on the surface of the film. XRD pattern shows that the films are amorphous. The field electron emission properties of the film were also characterized. The turn-on electric field is about 14 V/μm at the current density of 10 μA/cm², and at the electric field of 24 V/μm, the current density is up to 1 mA/cm². The field electron emission mechanism of the HfN_xO_y thin film is also discussed.     

The effect of high pressure on the electric field gradients at two inequivalent lattice sites of tantalum in ω-zirconium

The pressure variation of the electric field gradients at substitutional tantalum atoms on the two lattice sites (A and B) of ω-zirconium have been determined to be (? lnq ^A/?P)_T = ?1.05(26)10^?3 kbar^?1 and (? lnq ^B/?P)_T = +6.03(40)10^?3 kbar^?1. In addition the thermal expansion coefficients of the unit cell of ω-Zr were found to be α_‖ = +3.5(3)10^?6 K^?6 and α_⊥ = +7.9(4)10^?6 K^?1. A combined analysis of the pressure dependence and the previously determined temperature dependence of the electric field gradients supports the assumption of two different bonding types at the A and B sites.     

Limiting stable states of high-Tc superconductors in the alternating current modes

The limiting current-carrying capacity of high-T _c superconductor and superconducting tape has been studied in the alternating current states. The features that are responsible for their stable formation have been investigated under the conduction-cooled conditions when the operating peak values of the electric field and the current may essentially exceed the corresponding critical values of superconductor. Besides, it has been proved that these peak values are higher than the values of the electric field and the current, which lead to the thermal runaway phenomenon when the current instability onset occurs in the operating modes with direct current. As a result, the stable extremely high heat generation exists in these operating states, which can be called as overloaded states. The limiting stable peak values of charged currents and stability conditions have been determined taking into account the flux creep states of superconductors. The analysis performed has revealed that there exist characteristic times defining the corresponding time windows in the stable development of overloaded states of the alternating current. In order to explain their existence, the basic thermo-electrodynamics mechanisms have been formulated, which have allowed to explain the high stable values of the temperature and the induced electric field before the onset of alternating current instability. In general, it has been shown that the high-T _c superconductors may stably operate in the overloaded alternating current states even under the not intensive cooling conditions at a very high level of heat generation, which is not considered in the existing theory of losses.     

Equilibrium and stability of the charged particles’ fluxes drifting crosswise to the strong magnetic field at the expense of the Hall charge separation

The current equilibrium is investigated, where the generation of the Hall electric field on the magnetic Debye radius r _B = B ₀/(4πen _e) is considered by the drifting of the relativistic electrons crosswise to the strong magnetic field. In this case, the electron propagation is possible at the distance d that is essentially larger than the electron radius of the backward reflection in the magnetic field r ₀ ? m _e v _z c/(eB ₀). The instability of the joint drift motion of ions and electrons is investigated for the frequency oscillation w much higher than the ion cyclotron frequency w _Bi and by 4π n _i m _i c ² ? B ₀ ² and (k · B ₀) = 0. It is shown that the resonance effects by the ion beam’s plasma frequency w ? kv ₀ = w _pi leads to the generation of the nonpotential perturbations with the characteristic increment Imw ～ 10^?1 ÷ 10^{? 2} w _pi. Estimates show that the instability, associated with the propagation of the high-energy ion beam through the strong magnetic field, can essentially be like the edge-localized mode in tokamaks.     

The electric field gradient in heavy rare earth metals

Estimates of the electric field gradient in heavy rare earth metals have been evaluated from experimental hyperfine interaction data. In addition, the magnetic hyperfine fields are analyzed. In the metals the effective radial integrals 〈r ^?3〉_4f of the magnetic and quadrupole hyperfine interaction are reduced at most by 10% compared with the free ion values. The electric field gradients due to the crystalline field have been found to be 200 times larger than those predicted from point charge calculations. This antishielding effect can be explained by an enhanced conduction electron density at the interstitial sites and an increase of the Sternheimer factor γ_∞ in the metallic environment.     

Coherent signal beam amplification in two-wave mixing experiments with photorefractive Bi12SiO20 crystals

The amplification of the input signal beam in two-wave mixing experiments with photorefractive Bi₁₂SiO₂₀ crystals is achieved when an additional phase shift is established between the photoinduced index modulation (phase volume hologram) and the incident fringe pattern. This stationary phase shift is introduced by either moving the crystal or the interference fringes at a constant speed. The transferred intensity is measured versus the applied electric field, fringe spacing and crystal velocity. The crystallographic orientation and the relative displacement with respect to the applied electric field polarity determine the amplitude of the energy transfer. For the first time in this crystal, signal beam amplification is reached for an applied field E₀ > 8 kV cm^?1 and a crystal or fringe displacement speed around 5 μm s^?1 at the green line (λ = 514 nm) of an argon laser.