Quasioptical superconducting hot electron bolometer for submillmeter waves

We report on a superconducting hot electron bolometer coupled to radiation via a broadband antenna. The bolometer, a structured NbN film, was patterned on a thin dielectric membrane between terminals of a gold slotline antenna. We investigated the response to submillimeter radiation (wave-lengths 0.1 mm to 0.7 mm) in the fundamental Gaussian mode. We found that the directivity of the antenna was constant within a factor of 2.5 through the whole experimental range. The noise equivalent power of the bolometer at 119 µm was 3 · 10^–13 W/Hz^1/2; a time constant of 160 ps was estimated.     

Energy relaxation of two-dimensional electrons in the quantum Hall effect regime

The mm-wave spectroscopy with high temporal resolution is used to measure the energy relaxation times τ_e of 2D electrons in GaAs/AlGaAs heterostructures in magnetic fields B=0–4 T under quasi-equilibrium conditions at T=4.2 K. With increasing B, a considerable increase in τe from 0.9 to 25 ns is observed. For high B and low values of the filling factor ν, the energy relaxation rate τ _e ^?1 oscillates. The depth of these oscillations and the positions of maxima depend on the filling factor ν. For ν>5, the relaxation rate τ _e ^?1 is maximum when the Fermi level lies in the region of the localized states between the Landau levels. For lower values of ν, the relaxation rate is maximum at half-integer values of τ _e ^?1 when the Fermi level is coincident with the Landau level. The characteristic features of the dependence τ _e ^?1 (B) are explained by different contributions of the intralevel and interlevel electron-phonon transitions to the process of the energy relaxation of 2D electrons.     

Kinetic mechanism of ClONO2 uptake on polycrystalline film of NaCl

Kinetic studies and the mechanism determination of ClONO2 uptake on polycrystalline NaCl were carried out using a coated-insert flow tube reactor combined with high-resolution, low-energy electron-impact mass spectrometer under the following conditions: p = 1-2 Torr, linear flow velocity v = 3.5-75 m s(-1), T = 293 and 387 K, [ClONO2] = (0.5-25) x 10(12) molecules cm(-3). The salt was deposited as a film from nonsaturated aqueous solution on the sliding rod. The temporal dependences of the uptake coefficient and the partial uptake coefficients leading to a formation of the prime Cl2 and HOCl products were measured for different ClONO2 concentrations. These dependences are established to be described by gamma = gamma0 exp(-t/tau) + gamma(s), gamma(0,s)(-1) = a(0,s) + b(0,s)[ClONO2]. In the framework of the proposed kinetic model, the data are explained and the main elementary kinetic parameters of the uptake are evaluated. The model is based on a combination of Langmuir adsorption, formation of surface complexes on initial active sites, Z(ch), followed by their unimolecular decomposition. Decomposition is proposed to proceed concurrently in two channels, one of which is a released surface site that conserves the properties of the initial site. In the other channel, the initial Z(ch) transforms into Z(ph) followed by steady-state uptake and reproduction of final Z(ph). The model gives an analytical expression for experimental parameters gamma0, gamma(s), and tau in terms of elementary rate constants and the reactant volume concentration. The final objective of the proposed model is the extrapolation of gamma0, gamma(s), and tau parameters to real tropospheric conditions.     

The kinetics and mechanisms of elementary NF2 reactions with O and N atoms

A combined EPR–LMR spectrometer with a fast-flow system has been used to investigate the kinetics and mechanisms of NF₂ reactions with O and N atoms at 298 K. The overall rate constants of these reactions are: k₀ = (2.8 ± 0.4) × 10^?11 cm³/s and k_N = (5.7 ± 0.8) × 10^?11 cm³/s. The stoichiometry of the reactions with respect to O, N, NF₂, F, and NO has been determined. The statistical theory of bimolecular reactions has been used for interpretation of the results obtained.     

Experimental study of the D + H2(ν = 1) reaction

The D + H₂(ν = 1) reaction, D + H₂(ν = 1) → K_a HD(ν = 1) + H, → K_n HD(ν = 0) + H, → K_r D + H₂(ν = 0) has been studied. The measurements were made in a flow-tube apparatus at 300 K. Vibrationally excited H₂ was generated in a furnace and D atoms in a microwave discharge. EPR and thermometric techniques were used for the detection of D and H atoms and H₂(ν = 1). The product branching rate constants (in CM³/Molecule s) were found to be K_a = (10.7 ± 4.1) × 10^?13. K_n = (5.4 ± 2.7) × 10^?13, K_r, < 2.7 × 10^?13.     

Magnetic field effects on the nonohmic impurity conduction of uncompensated crystalline silicon

The impurity conduction of a series of crystalline silicon samples with the concentration of major impurity N ≈ 3 × 10¹⁶ cm^?3 and with a varied, but very small, compensation K was measured as a function of the electric field E in various magnetic fields H-σ(H, E). It was found that, at K < 10^?3 and in moderate E, where these samples are characterized by a negative nonohmicity (dσ(0, E)/dE < 0), the ratio σ(H, E)/σ(0, E) > 1 (negative magnetoresistance). With increasing E, these inequalities are simultaneously reversed (positive nonohmicity and positive magnetoresistance). It is suggested that both negative and positive nonohmicities are due to electron transitions in electric fields from impurity ground states to states in the Mott-Hubbard gap.     

Effects of the overlap between wave functions of impurity centers on the activation energy of hopping conduction

The hopping conductivity σ₃ has been studied in samples of slightly counterdoped crystalline Si: B with a boron concentration of 2×10¹⁶ cm^?3<N<10¹⁷ cm^?3 and a compensation of 10^?4≤K≤10^?2. It is found that at K≤10^?3 the activation energy ε₃ is not lower (as it must be according to classical notions at finite K) but larger than the value ε_N=e ² N ^1/3/κ, where e is the electronic charge and κ is the dielectric constant. With decreasing N, the energy ε₃ drops slower and, with decreasing K, grows faster than follows from the standard theory. At K≤10^?4, ε₃ is higher than ε _N by a factor of 1.5–2. The result is explained by the effect of the overlap between wave functions of neighboring impurity centers on the structure of the impurity band.