An interpretation of dlts spectra of Al/Si3N4/ultrathin SI/GaAs structures — Effect of quantum well or interface states?

We discuss DLTS andC-V measurements on Al/Si₃N₄/Si(2nm)/n-GaAs (≈ 5×10¹⁷ cm^?3) structures. Three discrete deep traps superimposed on a U-shaped interface-state continuum have been identified, with respective thermal energies:E _c?0.53 eV,E _c?0.64 eV, andE _v+0.69 eV. The second one (0.64 eV) is presented as an electric field sensitive level, its enhanced phonon-assisted emission resulting in a rapid shift of the corresponding DLTS peak to lower temperatures, as the applied (negative) reverse bias voltage increases. An interpretation through emission from the quantum well, introduced by means of the intermediate ultrathin Si layer, has failed.     

Superconductivity of (Sn1−z Pbz)1−x InxTe alloys

The temperature dependences of the electrical resistivity of (Sn_1?z Pb_z)_1?x In_xTe alloys with different concentrations of lead (z=0–0.60) and indium (x=0.03–0.20) were studied at temperatures T=0.4–4.2 K in magnetic fields from zero to H=15 kOe. A resistivity drop of no less than three-four orders of magnitude was observed in this range of alloy compositions. Application of a magnetic field above a critical level resulted in a recovery of the sample resistivity to the original value. The observed resistivity drop is identified with a superconducting transition. The critical parameters of the superconducting transition (T _c and H _c2) were determined at the drop to one half the normal resistivity level. Experimental dependences of the critical supercon-ducting-transition temperature T _c and of the second critical magnetic field H _c2 on the contents of lead (z) and indium (x) were measured. The data obtained confirm a strong localization of the In impurity states and are evidence of the extrinsic nature of superconductivity in the class of materials under study. It was established that as the Pb content in (Sn_1?z Pb_z)_1?x In_xTe increases, T _c and H _c2 decrease as the Fermi level E _F (fixed in the In impurity resonance band) leaves the Δ extremum and the superconductivity breaks down when E _F leaves the LΣ saddle point in the valence-band energy spectrum.     

Flicker noise of hot electrons in silicon at T = 78 K

From flicker-noise and current-voltage measurements performed on an n⁺nn⁺ silicon planar device at T = 78 K we calculated Hooge's parameter α as a function of the electric field strength, E₀. We found that α(E₀) = α(0)/[1 + (E₀/E_c)²]. E_c is a critical field where the drift velocity equals the sound velocity, indicating the connection of the observed effect with acoustical phonon scattering.     

Ionic thermocurrents in sodium fluoride thin film

Ionic thermocurrent (ITC) in NaF thin film deposited between gold electrodes has been investigated in situ in the temperature region 300–500 K. The thermally stimulated polarization current (TSPC) and thermally stimulated depolarization current (TSDC) obtained under low electric field pf polarization (E_p) at a lower temperature of polarization (T_p) exhibited current-peak temperatures (T_M) about 420 K and 450 K. The activation energies associated with the first and the second current peaks were 1.15 + 0.05 eV and 0.65 + 0.02 eV, which are assigned to cation vacancy blocking at the grain boundary barrier and the electrode contact interface barrier respectively. Polarization under a strong electric field (E_p > MV m₋₁) at T_p > 480 K causes a quasi-stable shift of the ITC peak positions to higher temperature. The current peak positions and magnitudes then depend upon the polarity of the electrode metal during polarization, and activation energies associated with the first peak positions are 0.90 + 0.02 eV and 0.79 + 0.02 eV for positive and negative bias at the counter electrode respectively, which may be attributed to the clustering of the cation vacancies and the dislocation networks at the interfaces.     

Effect of an external electric field on liquid crystalline polymers

The effect of an external electric field on the order parameter and on the isotropic-anisotropic phase transition temperature for semi-flexible liquid crystalline polymers is studied by a mean-field approximation. For the polymers whose electric dipole moments are parallel to the chain backbone, the critical transition temperature T_c is extensively changed by gDT ∼ ∥E∥², where E is the external electric field.     

Deep impurity-center ionization by far-infrared radiation

An analysis is made of the ionization of deep impurity centers by high-intensity far-infrared and submillimeter-wavelength radiation, with photon energies tens of times lower than the impurity ionization energy. Within a broad range of intensities and wavelengths, terahertz electric fields of the exciting radiation act as a dc field. Under these conditions, deep-center ionization can be described as multiphonon-assisted tunneling, in which carrier emission is accompanied by defect tunneling in configuration space and electron tunneling in the electric field. The field dependence of the ionization probability permits one to determine the defect tunneling times and the character of the defect adiabatic potentials. The ionization probability deviates from the field dependence e(E) ∝ exp(E ²/E _c ² ) (where E is the wave field, and E _c is a characteristic field) corresponding to multiphonon-assisted tunneling ionization in relatively low fields, where the defects are ionized through the Poole-Frenkel effect, and in very strong fields, where the ionization is produced by direct tunneling without thermal activation. The effects resulting from the high radiation frequency are considered and it is shown that, at low temperatures, they become dominant. Fiz. Tverd. Tela (St. Petersburg) 39, 1905–1932 (November 1997)     

On the nature of energy bands in tetracyanoplatinates

Optical data and band calculations are presented for a series of tetracyanoplatinates with varying Pt—Pt-distance R. The band gap energies decrease according to R^-3 with decreasing R. The energy bands which determine the optical and electrical properties for E6 c originate from (Pt5d_z², 6s) and (Pt6p_z, CNπ¹) hybrid molecular states.     

Mean field theory and ϵ-expansion for Anderson localization

We present a field theoretic formulation of Anderson localization of an electron in a random potential. In mean field theory we find a mobility edge at energy E_c separating a region with no states from one with conducting states. When the nearest neighbor hopping is a random variable with variance σ, E_c²=4zσ², where z is the coordination number. We study this mobility edge using the ?-expansion. We find an upper critical dimension of eight near which this mobility transition is in the same universality class as the statistics of dilute branched polymers (lattice animals).     

The size effect of Ba0.6Sr0.4TiO3 thin films on the ferroelectric properties

Barium strontium titanate (BST) thin films were prepared by RF magnetron sputtering. The dielectric constant-voltage curves and the hysteresis loops of BST thin films with different grain sizes and film thicknesses were investigated. When the grain size increases from 12 nm to 35 nm, remarkable increases in dielectric constant and tunability were observed. Above 12 nm, the BST films exhibited size effects, i.e. a decrease in maximal polarization (P_m) and an increase in coercive electric field (E_c) with reduction in grain size. In our investigation, the dielectric constant, tunability and maximal polarization increased as the film thickness increased. Furthermore, the size dependence of the dielectric constant and tunability of Ba_0.6Sr_0.4TiO₃ thin films is determined by that of the maximal polarization and the coercive electric field.     

Pressure experiment determination of the direct-indirect transition in the quarternary In1−xGaxP1−zAsz

Spontaneous and laser emission from In_1-xGa_xP_1-zAs_z double heterojunction diodes near the direct-indirect crossover (E_Γ = E_X, x ≡ x_c, z ≡ z_c) are studied at 77°K as a function of hydrostatic pressure up to 6 kbar. The pressure coefficients of the spontaneous emission peaks and of the laser modes are ～- 10.5 × 10^-6 eV/bar which is characteristics of the Γ band edge in III–V semiconductors. Laser threshold current is found to rise rapidly as pressure is applied owing to the decreasing Γ-X separation and the resultant carrier transfer to the X minima. Experimental lower limits for the direct-indirect crossover at three points in the In_1-xGa_xP_1-zAs_z quaternary system are determined. These three points and the established crossover in GaAs_1-yP_y (y_c ≈ 0.46, 77°K) give for the quaternary crossover (77°K) x_c ? 0.52z_c = 0.72. and the value x_c ≈ 0.72 for the limiting case of In_1-xGa_xP. Band edge bowing effects along the direct-indirect crossover in the In_1-xGa_xP_1-zAs_z system are discussed. The highest energy laser (77°K) for this quaternary system is estimated from pressure measurements to be ～ 2.155 eV (5752 Å).     

Dynamics of magnetic insulation failure and self-organization of an electron flow in a magnetron diode

The dynamics of back cathode bombardment (BCB) instability in a magnetron diode (a coaxial diode in a magnetic field, B ≡ B _0z ≡ B ₀) is numerically simulated. The quasi-stationary regime of electron leakage across the high magnetic field (B ₀/B _cr > 1.1, where B _cr is the insulation critical field) is realized. An electron beam in the electrode gap is split into a series of bunches in the azimuthal direction and generates the electric field component E _θ(r, θ, t), which accelerates some of the electrons. Having gained an extra energy, these electrons bombard the cathode, causing secondary electron emission. The rest of the electrons lose kinetic energy and move toward the anode. Instability is sustained if the primary emission from the cathode is low and the secondary emission coefficient k _se=I _se/I _{e, BCB} is greater than unity. The results of numerical simulation are shown to agree well with experimental data. A physical model of back-bombardment instability is suggested. Collective oscillations of charged flows take place in the gap with crossed electric and magnetic fields (E × B field) when the electrons and E × B field exchange momentum and energy. The self-generation and self-organization of flows are due to secondary electron emission from the cathode.     

Measurement of the surface density of states by field ionization

It is shown that recent measurements of field ion energy distributions from clean tungsten surfaces probe the density of metal states in the vicinity of the surface. We find j(ω) = (2π/kh)Σ_m| ∫ d³rψ_m(r)γ_z|²δ(ω??_m), where j(ω) is the ion current a ω, ψ_mand ?_m are electronic metal eigenfunctions and eigenvalues in the presence of the external electric field used in field ionization and γ(z) is a function which is large near the noble gas atom. An explicit expression for γ(z) is given in the text. It is estimated that tungsten metal states with values of k₆ at least as large as 0.5 Å^?1 make an appreciable contribution to j(ω) where k₆ is the electron momentum parallel to the surface.     

Effect of an electric field on the I–V curves of DyBa2Cu3−x Oy/1 wt % Pt HTSC ceramics

This paper reports on an experimental study of the effect of a magnetic field, B≤70 G, and an electric field, E=120 MV/m, on the critical current I _c and I–V curves of DyBa₂Cu_3?x O_y HTSC ceramics (x=0 and 0.2), both undoped and doped with 1 wt % Pt. It has been established that, in stoichiometric ceramics (x=0) at 77 K, I _c drops sharply (by more than an order of magnitude) already at very low B<1 G. In copper-deficient ceramics (x=0.2), I _c decreases with increasing B slowly, with Pt-doped samples exhibiting [on the dropping I _c (B) dependence] a peak effect, i.e., an increase rather than decrease of I _c at B≈10 G. As for the effect of an electric field on I _c and the I–V curves (the E effect), it is not observed in ceramics of a stoichiometric composition. DyBa₂Cu_2.8O _y samples acted upon by an electric field reveal a substantial increase in I _c and a decrease in the resistance R for I>I _c . In the case of DyBa₂Cu_2.8O_y/Pt, the electric field practically does not affect I _c but R decreases for I>I _c . In a sample placed in a magnetic field, the magnitude of the E effect is observed to correlate with the I _c (B) dependence. In particular, in Pt-doped samples, the E effect decreases with increasing magnetic field B not gradually but with a maximum appearing at B ≈10 G, i.e., in the region of the peak effect in the I _c (B) dependence. The data obtained suggest the conclusion that the electric-field effect in ceramics exhibiting weak links of the superconductor-insulator-superconductor (SIS) type correlates with magnetic vortex pinning.     

Acoustic emission from domain wall motion in ferroelectric lead germanate

Simultaneous visual observation and monitoring of the ultrasonic signals show that acoustic emission (AE) produced as Pb₅Ge₃O₁₁c-plate crystals undergo the ferroelectric hysteresis cycle results from domain wall nucleation and collapse. AE activity takes place predominantly at applied electric fields large enough to bring a crystal substantially towards its saturation polarisation. A notable feature observed in Pb₅Ge₃O₁₁ and Pb_5?xBa_xGe₃O₁₁ alloys is the existence of an abrupt threshold electric field, denoted by E_∞ in the high gain limit, for production of AE: as the electric field is increased beyond the threshold value, the amount of AE observed increases by several orders of magnitude. E_∞ depends linearly on the inverse of the c-plate sample thickness (1/d) and also upon sample temperature—falling to a minimum at about 70°C for Pb₅Ge₃O₁₁ and then rising steeply as the Curie temperature (178°C) is approached. E_∞ is also dependent on the rate of change of the applied electric field and as this is increased extrapolates to the high frequency limit of the coercive field. Measurements of crystal polarisation indicate a one-to-one correspondence between AE and the electrical Barkhausen pulses which occur during polarisation reversal—further evidence for domain nucleation and collapse as the source of AE. Optical and AE studies made simultaneously on gadolinium molybdate add confirmation that these particular processes are responsible for the AE produced by ferroelectric crystals.     

Pair creation by a photon in an electric field

The process of pair creation by a photon in a constant and homogeneous electric field is investigated basing on the polarization operator in the field. The total probability of the process is found in a relatively simple form. At high energy the quasiclassical approximation is valid. The corrections to the standard quasiclassical approximation (SQA) are calculated. In the region of relatively low photon energies, where SQA is unapplicable, the new approximation is used. It is shown that in this energy interval the probability of pair creation by a photon in electric field exceeds essentially the corresponding probability in a magnetic field. This approach is valid at the photon energy much larger than the “vacuum” energy in electric field: ω?eE/m. For smaller photon energies the low energy approximation is developed. At ω?eE/m the found probability describes the absorption of soft photon by the particles created by an electric field.     

I–V characteristics and the electric-field effect in melt-grown YBa2Cu3Ox/Y2BaCuO5 HTSC crystals

The effect of an electric field E=120 MV/m in the electrode-insulator-superconductor system on I–V curves obtained at 77 K on two types of single-crystal samples cut from monolithic superconducting YBa₂Cu₃O_x/Y₂BaCuO₅ has been studied. The Y211 nonsuperconducting phase in the ingot was in the form of precipitates ≈1 μm in size. It has been found that an electric field applied to samples with a comparatively low Y211 content (volume fraction 8%) does not affect the critical current I _c while reducing the resistance R at currents slightly above I _c . In samples containing more than 35% Y211 phase, electric field results in an increase of I _c and a decrease of R for I>I _c . Data on the critical temperature T _c and the temperature dependence of I _c have also been obtained. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 7, 2000, pp. 1172–1175. Original Russian Text Copyright ? 2000 by Smirnov, Orlova, Sengupta, Goretta.     

The Weiss model and the Landau–Khalatnikov model for the switching of ferroelectrics

It is shown that the molecular field theory by P. Weiss formally leads to the switching kinetics of ferroelectrics, which is described by the well-known Landau–Khalatnikov equation. The switching has a critical character, taking place only at E_a>E_c (E_a: external field, E_c: coercive field). The results are checked by computer simulations.     

Mass-energy in static gravitational fields

The mass-energy equation in static gravitational fields is shown to beE =g ₄₄ mc ², which agrees with the expressionE =m ₀ c ²(dx/ds)ξ_μ for the energy in a gravitational field possessing a timelike Killing vector ξ. For the Schwarzschild field this leads toE _s ?m ₀ c ² + 1/2m ₀ v ² ?km ₀ M/r. For the Reissner-Nordström field an additional term describing the interaction between the mass and the charge is found to be 2πkm ₀ Q ²/c ² r ². In the Kerr-Newman case more terms are found due to the central rotating gravitating mass.     

2D-Electron Heating in Electric and Magnetic Crossed Fields

2D-electron heating in a potential well of a single n-(AlAs) _x (GaAs)_1–x /i-GaAs (x = 0.28) heterojunction is studied for the cases of a classical (weak) magnetic field B and constant and pulsed electric fields at fixed temperatures 77 and 4.2 K. It is shown that the heating of two-dimensional electrons is similar to that of the bulk ones. The magnetic field cools electrons, and this is manifested in the shifts of the characteristic critical electric fields E _{c 1} and E _{c 2} and in the regions of nonlinearity of voltage-current characteristics. The dependence of the effective electron temperature on the electric field T _e(E)_B is determined.     

Detection and study of Rb I Rydberg states

We describe the details of an experiment using an atomic beam of rubidium which allowed us to detect by field ionization techniques the np Rydberg states from n = 28 up to n = 78, to detect also ns and nd states using a Stark mixing, and for all of these detected states to check the classical law E_c = [16n^*4]^?1 concerning the critical ionizing electric field E_c. It turns out that for n as high as 65 this law is quite well verified.