Parametric excitation of helicon and acoustic waves in piezoelectric semiconductor-plasma

The parametric excitation of a helicon and an acoustic wave in a piezoelectric semiconductor-plasma in the presence of a strong magnetic field has been investigated using the coupled mode theory. The expressions for the threshold value of the electric field required for the onset of instability and for the growth rate well above the threshold have been obtained. It is observed that an acoustic wave of higher frequency and higher phase velocity than that of the pump wave cannot be excited. The analysis has been applied to the case of n type InSb sample where the threshold value of the electric field is found to be of the order 5.2 × 10³ Vm^?1 and the growth rate at an electric field 5.2 × 10⁴ Vm^?1 is of the order of 8.7 × 10¹⁰ sec^?1.     

Evaporation of millimeter water drops in the field of IR laser radiation

The evaporation of large (∼10⁻³ m across) water drops in the field of IR (10.6 μm) laser radiation of power density (4–12)×10⁵ W/m² is considered. Qualitative analysis shows that the process is described well with an optical cavity model. A mechanism of water evaporation is found to be associated with surface electromagnetic waves, which break hydrogen bonds in the high-frequency (≈3×10¹³ s⁻¹) field of the laser. The theory is in good agreement with experimental data. Original Russian Text ? V.I. Trigub, 2006, published in Zhurnal Tekhnicheskoĭ Fiziki, 2006, Vol. 76, No. 4, pp. 120–123.     

237Np and 57Fe Mössbauer study of NpFeGa5

⁵⁷Fe and ²³⁷Np Mössbauer ōmeasurements have been performed for NpFeGa₅, which is one of the so-called neptunium 1-1-5 compounds. The ⁵⁷Fe Mössbauer spectra below T _N = 118 K show the magnetically ordered state. The magnitude of the hyperfine magnetic field at the ⁵⁷Fe nucleus is determined to be 1.98 ± 0.05 T at 10 K. From the ²³⁷Np Mössbauer spectrum at 10 K, the hyperfine magnetic field at the ²³⁷Np nucleus is 203 T and the hyperfine coupling constant is determined to be 237 T/μ_B using the Np atomic magnetic moment of 0.86 μ_B determined by the neutron diffraction study.     

Mössbauer studies of europium and ytterbium hydrides

Mössbauer absorption spectra were obtained for the 21·6 ke V transition of ¹⁵¹Eu in EuH₂ at various temperatures and for the 84·3 keV transition of ¹⁷⁰Yb in YbH₂ at 4·1°K. The isomer shift of EuH₂ relative to Eu³⁺: Sm₂O₃ is ? 12·1 ± 0·3 mm. sec^?1, and the magnetic hyperfine field equals ? 305 ± 5 kOe at saturation. The Curie temperature is found to be 16·2 ± 0·05°K, and the critical parameters of the transition are D = 1·17 ± 0·02 and β = 0·35 ± 0·01. The magnetic field is perpendicular to the principal axis of the electric field gradient and the values of the quadrupole hyperfine interaction e²qQ₀(3 cos² θ ? 1)/8 is ? 28 ± 4 Mc . sec^?1. A large increase of the resonance area (21%) occurs at the transitio to the ferromagnetic state. The isomer shift of YbH₂ relative to Yb: TmAl₂ is ?0·11 ± 0·01 mm . sec^?1. The value of the quadrupole coupling constant e²qQ_c/4is ? 91·5 ± 2 Mc . sec^?1 and the asymmetry parameter of the electric field gradient equals 0·89 ± 0·05. The data for EuH₂ and YbH₂ is shown to be consistent with the hydridic model for the rare earth hydrides.     

Initial slope of the critical magnetic field curve of yttrium thorium sesquicarbide

The initial slope of the critical magnetic field curve of yttrium thorium sesquicarbide was observed to be 2.4±0.2 × 10⁴ Oe/K. Assuming no paramagnetic limitation, as was the case for lanthanum sesquicarbide, the zero temperature critical-magnetic field is estimated to be 280 ± 24 kOe.     

Temperature dependence of the work function of hafnium islands on tungsten

Two-dimensional islands of hafnium near {100{ faces of tungsten and their terraces were examined by field electron microscopy. The two-dimensional sublimation energy of the islands was found to be 6.1±0.5 eV. The field electron emission from the islands grows with temperature, the effect being considerably greater than theoretically expected. The negative temperature coefficient of ϕ, α_T, for the islands was estimated at (4–6)×10⁻⁴ eV grad⁻¹ or more, whereas for pure tungsten α_T was found to be 2×10⁻⁵ eV grad⁻¹.     

Experimental investigation on field evaporation of singly and doubly charged rhodium

Field ion mass spectrometry and field ion appearance spectroscopy are used to analyze field evaporation processes of singly and doubly charged rhodium ions. Field strengths were ranged between 17 and 41 V/nm at tip temperatures between 600 and 100 K. The appearance energies of doubly charged species were found to increase with increasing field strengths from 29.5 ± 0.4 to 32.0 ± 0.2 eV, those of the singly charged species from 11.2 ± 0.1 to 12.4 ± 0.1 eV. Activation energies as determined from measured temperature dependences of the ion counting rates are equal for both kinds of ionic species and decrease from 1.5 to 0.05 eV with increasing field strengths. The observed field dependences cannot be predicted from the image hump model of field evaporation. Evaporation of Rh¹⁺is better understood using the charge exchange model. Generation of Rh²⁺ is described by the postionization of Rh¹⁺ within the frame of a simple potential energy model from which the average interaction energy of rhodium surface atoms with their nearest neighbours may be derived.     

Charge state of ions in liquid metal field ion sources

The post-ionization model of field evaporation is shown to be consistent with observations of singly and doubly charged ions in liquid metal field ion sources. The model can be used to estimate the field strength at the apex of the Taylor cone which is found to be 1.9–2.0 V/Å for a Ga source. Experiments to test the post-ionization model and to determine the apex field strength more accurately are suggested. A possible method of obtaining ～μA currents of highly charged ions, e.g. Zr⁴⁺, Ta⁴⁺, Ga³⁺, As³⁺, is proposed.     

Magnetic field effect on the H2CS fluorescence from the first excited singlet state Ã1A2

Fluorescence intensity in thioformaldehide vapours (H₂CS), excited to the ÃA₂ different vibronic levels of the ùA₂ ? [Xtilde]¹A₁ transition, were measured as a function of an external magnetic field. On excitation to these levels, dynamics in zero and non-zero field may be described in the small-molecule limit, with fluorescence exhibiting an almost exponential decay. A magnetic field changes the integrated intensity and decay lifetime of the thioformal-dehide fluorescence induced from different vibronic levels of the ù A₂ state. We found that the magnetic field effect grows at lower gas pressures. The measured field dependences of the magnetic field effect can be fitted using field-saturated functions. The measured data were explained by the indirect mechanism theory (nuclear-spin and electron-spin decoupling mechnism).     

Field penetration and band bending near semiconductor surfaces in high electric fields

The existing theory of band bending in the surface space charge region of semiconductors is adapted to problems in field emission, field ionization and field evaporation. The surface field in the space charge layers of semiconductors appropriate for these phenomena ranges from ~10^?2 V/Å to a few 10^?1 V/Å, similar to those encountered in many interface phenomena involving semiconductors. We found that the surface potential resulting from band bending may amount to a few eV. The field penetrates ~10 Å into the semiconductor surface for intrinsic cases, and ~200Å for an n-type semiconductor in a positive field, or for a p-type semiconductor in a negative field. Both the surface potential and the field penetration will affect significantly the electronic properties of the near surface layers. In particular, the photon adsorption edge will be shifted toward the red by the field penetration effect.     

RX J0720.4-3125 as a possible example of magnetic field decay in neutron stars

We consider the possible evolution of the rotation period and magnetic field of the X-ray source RX J0720.4-3125, assuming that this source is an isolated neutron star accreting from the interstellar medium. The magnetic field of the source is estimated to be 10⁶–10⁹ G (the most probable value is about 2·10⁸ G), and it is difficult to explain the observed rotational period 8.38 s without invoking the hypothesis of magnetic field decay. For calculations we used the model of ohmic dissipation of the field in the core of the neutron star. Estimates for the accretion rate (10^?14–10^?16 M_⊙/yr), velocity of the source through the interstellar medium (10–50 km/s), and neutron star age (2·10⁹–10¹⁰ yrs) are obtained.     

Electron-positron pair production by an electron in a magnetic field near the process threshold

The electron-positron pair production by an electron in a strong magnetic field near the process threshold is considered. The process is shown to be more probable if the spin of the initial electron is oriented along the field. In this case, the probability of the process is 10¹³ s⁻¹ when the magnetic field strength is H = 4 × 10¹² G.     

NMR spin echoes and quadrupole effects in nickel single crystals

NMR spin echo measurements of naturally abundant⁶¹Ni have been performed with Néel-type nickel single crystals at 4.2 K. The NMR excitation condition could properly be chosen in order to get signals either from nuclei situated within magnetic domains or within domain walls. In both cases a quadrupole splitting of the NMR line could be observed. By applying an external magnetic field, the direction of the domain magnetization could be varied with respect to the fcc crystal lattice. From the variation of the quadrupole splitting with the crystallographic direction, the tensor of the electric field gradient (EFG) has been derived. The quadrupole splittings and the corresponding field gradients in the principal axis system of the EFG are: Δv_Q kHz, Δv_Q kHz, Δv_Q kHz, V_<111>=(6.6±0.5)·10¹⁸ V/m², V_<110>=4.6±0.5)·10¹⁸ V/m², V_<112>=(2.0±0.5)·10¹⁸ V/m².     

Magnetic field effects on the hydrogen abstraction reaction of triplet 4-methoxybenzophenone with thiophenol as studied by a picosecond laser flash photolysis technique

The hydrogen abstraction reaction of triplet 4-methoxybenzophenone with thiophenol at 265 K has been studied with a newly developed picosecond laser flash photolysis apparatus under magnetic fields of 0–1.7 T. The decay rate constant of the radical pair generated was found to increase from 3.42 × 10⁹ s^?1 to 4.15 × 10⁹ s^?1 with increasing the magnetic field from 0 to 1.7 T. The observed magnetic field effects can be explained by the Δg mechanism. Using the simple kinetics model with the Δg mechanism, the rate constant of the escape process from the pair (k _esc) and two rate constants for the T-S spin conversion process (k _T-S) at 0 and 1.7 T were found to be 1.97 × 10⁹ s^?1, 1.45 × 10⁹ s^?1, and 2.12 × 10⁹ s^?1, respectively. From the magnetic field dependence on k _T-S, the difference in the g values of the 4-methoxybenzophenone ketyl and phenylthiyl radicals was estimated to be 0.0087.     

Influence of paired light pulses on the rothstein effect

The influence of paired light pulses with a duration of 1.5 × 10^?6 s and a repetition period from 1.5 × 10^?6 s to 7.2 × 10^?6 s on the Rothstein effect is investigated for the RETINA X-ray film. It is shown that depending on the synchronization of the second light pulse with a single electric field pulse in a photolayer with field strengths of 6 × 10⁶ and 1.5 × 10⁷ V/m, the effect of variation of the integrated optical density of images changes towards higher values as compared to the density of the images formed only by a pair of light pulses. In this case, no negative “field effect” was observed in the given film.     

Influence of the axial magnetic field on the current instability and on the collective acceleration of ions in plasma diodes

It is shown that, while suppressing transverse electron motion, the axial magnetic field with an induction of up to 6.8 × 10⁻² T in the gap of a plasma diode has no significant effect on the current instability and on the acceleration of ions at electron beam currents of ≤40 A. The increase in both the critical current and the period of current oscillations is related to an increase in the plasma density after applying the magnetic field. The maximum energy of the accelerated magnesium ions decreases by ≈25% at an induction of 1.7 × 10⁻² T and does not depend on the magnetic field in the range (1.7–6.8) × 10⁻² T.     

BEEM-π calculations for a series of pyridines and their 14N N.Q.R.

The ⁵⁷Fe Mössbauer spectrum of a powdered sample of phthalocyanine-iron(II) in an applied magnetic field of 3·0 teslas has been measured as a function of temperature in the range 4·2 K to 100 K. Measurements have also been made at 4·2 K with 6·0 teslas applied, and on a single crystal specimen at 4·2 K with 3·0 teslas applied. Independent computer fits to the three measurements taken at 4·2 K were found to be consistent with one another, and showed that detailed information concerning magnetic anisotropy can be obtained even from powdered samples of paramagnets by Mössbauer spectroscopy. Although the asymmetry parameter in the electric field gradient tensor was found to be small, there was a significant departure from tetragonal symmetry in the magnetic properties of the molecule. The magnetic hyperfine field at the ⁵⁷Fe nucleus was found to be positive in all directions, indicating that all three electronic g values are significantly greater than 2·0.     

Speed limitation of Ge far-infrared photoconductive detectors

The speed of Ge submillimetre detectors is investigated with the aid of a pulsed n-InSb Landau emitter. A rise-time of 2.2 nsec with a responsivity of about 10 V/W is achieved for a sample containing 2 × 10¹⁵ donors and 1.2 × 10¹⁵ acceptors per cm³ at an applied electric field of 32 V/cm. The risetime can be further shortened to 0.5 nsec by reducing the bias electric field. From the observation that the capture cross-section does not decrease appreciably with increasing impurity concentration up to 2 × 10¹⁵ impurities per cm³ we conclude that no speed limitation occurs for Ge submillimetre detectors down to rise-times of about 10⁻¹⁰sec.     

Spectroscopic investigation of some rare earth iron garnets

Optical absorption spectra of Dysprosiumirongarnet (DyIG), Holmiumirongarnet (HoIG) and Erbiumirongarnet (ErlG) have been investigated in the near infrared region at temperatures of 4.2°K and 62°K. Exchange field splittings of the crystal field levels have been observed. — For the three garnets the excited levels of the groundterm up to 110 cm^?1 could be determined. The exchange fields acting on the rare earth ions have been deduced as (305±50)·10³ Oe for DyIG and (284±30)·10³ Oe for ErIG.     

The anharmonic force field and equilibrium structure of methane

The anharmonic force field of methane has been refined to fit spectroscopic data from the isotopic species ¹²CH₄, ¹³CH₄, ¹²CH₄, ¹²CH₃D, ¹²CHD₃ and ¹²CH₂D₂. Six of the thirteen cubic force constants have been determined experimentally, the remaining cubic constants being fixed at values derived from ab initio calculations. The quartic force field is very crude, in that only f_rrrr has been refined. It is concluded however that the cubic and quartic force fields, even though they are subject to limitations, provide a considerable improvement in the experimental determination of the r _e structure and the quadratic force field. The equilibrium bond length is found to be r _e(CH) = 1·085₈ ± 0·001 Å.