Transient InSb spin-flip laser - a measurement of T1

A double pulse spin-flip laser technique has been used to evaluate the InSb conduction electron spin-relaxation time T₁; a value of 60 ± 20 nsec (n = 1.2 × 10¹⁶cm^?3, H = 60 kG, T = 20 K)is obtained. The effects of electron heating by free carrier absorption are measured and an energy relaxation time of 20–40 nsec is obtained. A calculation of T₁ based on ionized impurity scattering in the quantum limit regime is in order of magnitude agreement with the experiment.     

Spin-flip Raman scattering from electrons in localized donor states in n-InSb

We report the first observation of spin-flip Raman scattering from electrons localized in shallow donor states in InSb. For a non-degenerate n-InSb sample (8×10¹³ cm^-3) measurements of the spin-flip Raman gain and the effective g-value as a function of the magnetic field show lineshapes and magnetic field dependences completely different to that of an InSb sample with the electron gas being in a degenerate regime (1.35×10¹⁵ cm^-3). For the 8×10¹³ cm^-3 InSb sample, at magnetic fields greater than 11.5 kG, a splitting of the spin-flip Raman line into two lines is observed which may be an indication that two shallow donor states with different effective g-values are concerned.     

A strong magnetic dipole excitation in 40Ca observed in high-resolution inelastic electron scattering and coherent spin-flip transitions due to ground-state correlations

An ususually strong magnetic dipole transition from the ground state to a state at Ex = 10.319 MeV in ⁴⁰Ca has been observed in high-resolution inelastic electron scattering, contrary to expectations from the pure independent particle shell model. The transition strength, however, can be accounted for by coherent spin-flip transitions due to strong ground-state correlations.     

Microscopic magnetic properties of nonstoichiometric V2O3+x—NMR and inelastic spin-flip neutron scattering measurements

The local magnetic properties of the V sites in the nonstoichiometric V₂O_3+x (0 ? x <0.08) have been examined by nuclear magnetic resonance and inelastic spin-flip neutron scattering techniques. The samples with x = 0.01 and 0.02 show a paramagnetic metal (PM)-antiferromagnetic insulator (AFI) transition. In the AFI phase, two distinct ⁵¹V NMR signals with hyperfine fields H_n = 184.9±0.5 kOe and 71±1 kOe were observed at 1.8 K, which were assigned as due to V³⁺ and V³⁺ sites, respectively. On the other hand, the samples with x = 0.04 and 0.06 were metallic down to 1.4K, and showed a paramagnetic (PM)-antiferromagnetic (AFM) transition at about 10 K. In these samples, a ⁵¹V NMR signal with H_n = 58±2 k0e and one with 〈H_n〉 = 9kOe were observed at 1.8 K, which were assigned as due to V³⁺-like sites and the matrix V sites, respectively. These results are entirely consistent with those obtained from the neutron experiment. We propose that in the metallic phase (0.04 ? x < 0.08) the minority V⁴⁺-like sites are magnetically localized in the delocalized V matrix and may be responsible for the antiferromagnetic long range order below 10 K.     

Spin-flip probability for 7.6 MeV neutrons scattered inelastically (Q = −2.23 MeV) by a sulfur target

The spin-flip probability for 7.6 MeV (ΔE = 0.250 MeV) neutrons scattered inelastically (Q = ?2.23 MeV) from naturally occuring sulfur has been measured at scattering angles between 40° and 160° using a γ-correlated neutron time-of-flight method. The scattering by the natural sulfur target was assumed to be characteristic of ³²S. The results of the measurement were compared with an incoherent sum of statistical compound-nucleus (CN) and direct-interaction (DI) contributions. The DI contributions were obtained using either a DWBA calculation or the coupled-channel (CC) formalism. Neither combination of CN plus DI contributions reproduced the experimental spin-flip probability angular distribution which was peaked near 110° with a maximum value of 0.33 ± 0.08.     

Redistribution of electrons between ellipsoids in a magnetic field in the quantum limit range in n-Bi-Sb alloys

The magnetoresistivities ρ₂₂(H) and ρ₃₂(H) and the Hall coefficient R _32.1 for single-crystal samples of the n-Bi_0.93Sb_0.07 semiconducting alloy have been measured at low temperatures in magnetic fields up to H=14 T at H∥C ₂. The samples with three electron concentrations n ₁=1.25 × 10¹⁶ cm^s-3, n ₂=3.5×10¹⁶ cm^s-3, and n ₃=1.6×10¹⁷ cm^s-3 have been studied. The strong anisotropy of the electron spectrum of the alloys has made it possible to observe quantum oscillations of the magnetoresistivity ρ₂₂ (H) at H∥C ₂ for electrons of the secondary ellipsoids with the transition to the quantum limit in high magnetic fields. However, in the same magnetic fields, the quantization condition for electrons of the main ellipsoid is not satisfied. An increase in the energy of electrons of the secondary ellipsoids in the magnetic fields of the quantum limit leads to their migration to the main ellipsoid. After the complete migration, the Fermi energy for the alloy samples with the electron concentrations n ₁, n ₂, and n ₃ increases from 7.0 to 11.3 meV, from 11.0 to 17.1 meV, and from 20.2 to 30.6 meV, respectively. After the migration, the magnetoresistivity for electrons of the main ellipsoid increases with an increase in the magnetic field and the specific features in the behavior of the kinetic coefficients are observed in the vicinity of the magnetic field H=10 T. Therefore, the electronic topological transition from the three-valley electron spectrum to the single-valley electron spectrum occurs in the Bi_0.93Sb_0.07 single crystals for H∥C ₂ at low temperatures in the range of magnetic fields of the quantum limit.     

Korringa relaxation time of magnetic ion system near a two-dimensional electron gas

Spin relaxation of Mn ions in a (Cd,Mn)Te quantum well with quasi-two-dimensional carriers (Q2DEG) is investigated. The mechanism of energy transfer is spin-flip scattering of Mn spin with electrons making transitions between spin subbands accompanied by a change in the Mn spin. A calculation of the spin-flip scattering rate shows that the Mn spin relaxation rate is proportional to the coupling constant squared, the density of states squared, and the electron temperature, the so called Korringa relaxation rate. It was found that for small Mn ion concentration, the relaxation time ≈10−7-10−6s is in a good agreement with experimental results. Moreover, the relaxation rate scales with L⁻², L being the well width, and it can be enhanced over its value in bulk.     

Quantum transitions of relativistic electrons in a superstrong magnetic field

The probability of emission of a hard γ-quantum in relativistic electron transitions to the ground (or near it) level in a magnetic field H ～ H₀ = m²c³/e₀? = 4.41 × 10¹³G is obtained. For the inverse transitions from these levels the cross-section of electron photoexitation is calculated.     

Electric spin resonances in light nuclei

Very recent pion inelastic scattering experiments at LAMPF have revealed the existence of strong spin-flip E1 resonances in the vicinity of the GDR in several light nuclei. We present here the results of shell-model calculations of S = 0 and S = 1 E1 strength distributions which offer a broad theoretical context for the discussion of electric spin excitations. Our results for ¹⁶O and ⁴⁰Ca corroborate the LAMPF data and indicate that a major fraction of the spin-flip strength still lies above the GDR.     

The orientational mechanism of nonlinearity and the self-focusing of He-Ne laser radiation in nematic liquid crystal mesophase (theory and experiment)

A giant optical nonlinearity of self-focusing type in the oriented mesophase of nematic liquid crystals (NLC) due to the director reorientation under the action of a light wave field is predicted. Self-focusing of He-Ne laser radiation with power ～10^?2 W and power density ～50 W/cm² in a planar oriented 60 μm thick NLC layer has been carried out experimentally. The measured value of the nonlinearity effective constant ?₂ = 0.07 cm³/erg corresponds to theoretical predictions, and turns out to be larger than the CS₂ nonlinearity by ? 10⁹ times.     

Theory of the upper critical field in layered superconductors with magnetic intercalates

We propose a model for the layered superconductors with electronically isolated magnetic intercalates in which the electrons propagate freely in the dirty superconducting layers with arbitrarily strong spin-orbit scattering, and spin-flip scatter off the magnetic ions during interlayer tunneling. We calculate the upper critical field H_c2 including demagnetization effects. In a parallel field below the dimensional-crossover temperature T¹, a new type of spin ordering is predicted.     

Energy Structure of an Individual Mn Acceptor in GaAs : Mn

The energy structure of the Mn acceptor, which is a complex of Mn²⁺ ion plus valence band hole, is investigated in the external magnetic field and under presence of an uniaxial stress has been studied. The spin-flip Raman spectra are studied under resonant excitation of exciton bound to the Mn acceptor. The gfactors of the ground F = 1 and the first excited F = 2 states are determined and selection rules for the optical transitions between the acceptor states are described. The value of the random field (stress or electric field) acting on manganese acceptor and the deformation potential for the exchange interaction constant of the Mn²⁺ + hole complex are obtained. A theoretical model is developed that takes into account the influence of random internal and uniaxial external stress and magnetic field. The proposed model describes well the lines of spin-flip Raman scattering of Mn acceptor.     

The methanation activity of W(110)

The methanation activity of W(110) was measured over a range of reactant partial pressures and temperatures (P_H2 = 1–1000 Torr, P_CO = 0.1–10 Torr, T = 475–820 K). Plotting the results in an Arrhenius fashion yielded a lower apparent activation energy (E_a = 56 kJmol^?1) than previously determined for Ni(100) (E_a = 103 kJmol^?1) with an activity surpassing that of Ni at lower temperatures. The H₂ pressure dependence of the methanation activity was found to be much stronger for W(110) than for Ni(100), the surface becoming increasingly inactive at the lowest H₂ pressures investigated. Auger electron spectroscopy revealed the active catalytic surface to be carbidic in nature.     

Analysis of the spin splitting of maxima of quantum oscillations of the resistivity of n-Bi-Sb semiconductor alloys in a magnetic field parallel to the bisector axis

In samples of semiconductor alloys n-Bi_0.93Sb_0.07 with different electron concentrations (n ₁ = 8 × 10¹⁵ cm^?3, n ₂ = 1.2 × 10¹⁷ cm^?3, and n ₃ = 1.9 × 10¹⁸ cm^?3), dependences of the electrical resistivity on magnetic fields up to 45 T parallel to the current and the bisector axis (H ‖ C ₁ ‖ j) have been measured at temperatures of 1.5, 4.5, and 10 K. The obtained dependences ρ₂₂(H) demonstrate quantum oscillations of the resistivity (Shubnikov-de Haas effect), and, in high magnetic fields, there is a resistivity maximum far away from other maxima. On assumption that this maximum is related to the spin-split Landau level N = 0^? for electrons of the main ellipsoid, the spin-splitting parameters are calculated for electrons of the main ellipsoid: γ₁ = 0.87, γ₂ = 0.8, and γ₃ = 0.73. Using these values, the oscillation maxima can be reliably related to the numbers of split Landau levels for electrons of the main and secondary ellipsoids. The dependences of the resistivity ρ₁₁ and the Hall coefficient R _31.2 on magnetic field have been measured in a transverse magnetic field at H ‖ C ₁ and j ‖ C ₂ on the sample with the electron concentration n ₄ = 1.4 × 10¹⁷ cm^?3. Using similar analysis, the spin-splitting parameter is found to be γ₄ = 0.85, which is close to the value of γ₂ = 0.8 obtained for the sample with close electron concentration (n ₂ = 1.2 × 10¹⁷ cm^?3) during the measurements in a longitudinal magnetic field. The quantum oscillation maxima of Hall coefficient R _31.2 are shifted to the range of high magnetic fields as compared to the quantum oscillation maxima of resistivity ρ₁₁.     

Theory of Raman scattering of light on spin-flip and electronic excitation in Europium chalcogenides

The inelastic scattering of light in magnetic semiconductors from the family of the Europium chalcogenides is discussed in relation with spin-orbit coupling and d-f exchange interaction. It is shown that the Raman processes connected with spin-flip and other electronic excitations can occur in the energy range of 0.1-0.5 eV. In this case and for the typical values of d-f exchange interaction and spin-orbit coupling parameter lying between 0.05 and 0.1 eV, the values of the differentional cross-sections are between 10^-9 - 10^-11 cm²sr^-1 sec. The selection rules for the polarization of the incident and scattered photons as well as the Raman tensors for the four allowed transitions are derived. The possibility of applying spin-flip Raman processes in magnetic semiconductors in the tuning of electromagnetic radiation in Raman lasers is analyzed briefly.     

Ion-electron coincidence study of the thermal He(23S) + H2 Penning reaction

A new Penning-electron-Penning-ion coincidence method is described. It is applied to the study of the thermal reaction of He(2³S) with H₂. The main results reported are separate electron energy spectra that are coincident with the three different ions formed: HeH₂⁺, HeH⁺ and H₂⁺. Based on these results it is shown that the Penning reaction of the He(2³S)/H ₂ system proceeds in two well-separated steps: (i) ionization at distances R (HeH₂) ? 6a₀ in which H₂⁺ (v) is formed in different vibrational states; and (ii) reactive collision of H₂⁺ (v) with He. For the second step the variation of the branching ratios with vibrational quantum numbers v = 0 to v = 10 is derived, and it is shown that these branching ratios may be regarded as relative vibrational-energy-dependent cross-sections for the collision of H₂⁺ (v) with He at an average relative kinetic energy of ～20 meV.     

Relativistic calculations of ground states of single-electron diatomic molecular ions

We have performed relativistic calculations of ground-state energies for a series of single-electron homonuclear dimers A ₂ ^(2Z?1)+ with nucleus charge Z = 1, 2, 10, 20, 30, 40, 50, 60, 70, 80, 90, 92, and 100 and internuclear distances R = 2/Z. The work involves the Born-Oppenheimer approximation and the single-electron two-center Dirac Hamiltonian, which describes the interaction between an electron and two immovable point charges. Analysis of the convergence process and comparison with data presented in other works for H ₂ ⁺ and Th ₂ ¹⁷⁹⁺ dimers shows that the relative error of the obtained results is on the order of 10^?11–10^?12. High-accuracy values of ground-state energies for some dimers other than Z = 1 and 90 have been obtained in this work for the first time.     

Chemisorption of H2 on W(100): Absolute sticking probability,coverage, and electron stimulated desorption

Measurements of both the absolute sticking probability near normal incidence and the coverage of H₂ adsorbed on W(100) at ~ 300K have been made using a precision gas dosing system; a known fraction of the molecules entering the vacuum chamber struck the sample crystal before reaching a mass spectrometer detector. The initial sticking probability S₀ for H₂/W(100) is 0.51 ± 0.03; the hydrogen coverage extrapolated to S = 0 is 2.0 × 10¹⁵ atoms cm^?2. The initial sticking probability S₀ for D₂/W(100) is 0.57 ± 0.03; the isotope effect for sticking probability is smaller than previously reported. Electron stimulated desorption (ESD) studies reveal that the low coverage β₂ hydrogen state on W(100) yields H⁺ ions upon bombardment by 100 eV electrons; the ion desorption cross section is ~ 1.8 × 10^?23 cm². The H⁺ ion cross section at saturation hydrogen coverage when the β₁ state is fully populated is ? 10^?25 cm². An isotope effect in electron stimulated desorption of H⁺ and D⁺ has been found. The H⁺ ion yield is ? 100 × greater than the D⁺ ion yield, in agreement with theory.     

Spin-flip probability in the inelastic scattering of 16.9 MeV neutrons from 12C

The neutron spin-flip probability S(θ) for inelastic scattering of unpolarized 16.9 MeV neutrons to the 4.44 MeV state of ¹²C has been determined by measuring the absolute directional correlation between the scattered neutrons and the subsequent E2 deexcitation γ- radiation emitted perpendicular to the scattering plane. Time-of-flight techniques with carbon recoil detection in a plastic scintillator were used to separate elastically and inelastically scattered neutrons. The neutron spin-flip data were found to be in close agreement with 20.0 MeV proton spin-flip results. Comparison of the measurement is made with the predictions of a microscopic antisymmetrized distorted wave calculation in which the direct reaction mechanism is supplemented by a two step resonance contribution. This theoretical analysis reveals the presence of a strong quadrupole resonance at 20.5 MeV excitation in ¹²C. The results are also influenced to a lesser extent by the E1 giant dipole resonance.     

Optical properties of electron irradiation induced defects in InP

The optical properties of deep hole traps H₄ and H₅ in p type and of the deep electron trap E₁₁ in n type InP, introduced by electron irradiation, have been studied using deep level optical spectroscopy. Comparison of the optical threshold with the thermal activation energy of H₅ level shows that it is highly relaxed with a Frank-Condon shift d_Fc = 0.45 eV. The electron level E₁₁ is weakly relaxed and its optical cross section σ ⁰ is well accounted for by transitions to the Γ_6c minimum. The optical absorption σ_p⁰ associated to level H₄ shows two successive onsets at hν = 0.5 and hν = 1.2 eV which can be attributed to hole transitions to the Γ_7–8 and to the L_4–5 valence band extrema, respectively. The deduced Frank-Condon shift, d_Fc = 0.23 eV, agrees with the measured difference of 40 meV between its apparent activation energy E_a and its thermal activation energy E_T.