Three-body model of deuteron breakup and stripping

For a three-body model Hamiltonian, the scattering eigenfunction that corresponds to an incident deuteron is expanded in terms of eigenfunctions of the neutron-proton relative Hamiltonian, as suggested by Johnson and Soper. In this expansion, breakup is represented by an integral over the continuum of neutron-proton scattering states. Only states of zero relative angular momentum are included; the validity and advantages of this approximation are discussed. The continuum is divided into five discrete channels, whose coupling to each other and to the deuteron channel is treated by solving coupled differential equations with appropriate boundary conditions. It is found necessary to use a simple WKB method to take account of the long-range coupling among breakup channels; this method introduces potential matrices W and S that describe local and derivative coupling of the channels. The reaction of breakup on the elastic channel is neglected.The properties of W and S and the breakup wavefunction are examined for the case of 22.9 MeV deuterons incident on a target of mass number A ≈ 40. The Coulomb interaction is ignored, and a local Gaussian shape is used for both the real and imaginary parts of the nucleon-nucleus optical potential.It is found that a rather broad spectrum of n-p continuum states is excited, especially for low center-of-mass angular momentum. This result weakens the justification for the Johnson-Soper adiabatic theory, which emphasizes breakup into states of low relative energy.The breakup part of the wavefunction at zero n-p separation is comparable with the elastic part, but is important only over a surprisingly short range in the center-of-mass coordinate, with the result that breakup cross sections are quite small. Nevertheless, breakup produces major modifications of (d, p) cross sections. These modifications can to some extent be simulated by the Johnson-Soper method. The breakup wavefunctions show several interesting effects in their dependence on angular momentum and radius.     

Aspects of frequency redistribution for non-coherent scattering in a turbulent medium

The scattering of line photons in a turbulent medium is reexamined. We take into account the fact that a photon absorbed in a turbulent element having some velocity will be reemitted by the same element and keeps in that way a certain “memory” of its absorption. Adding the contributions of all turbulent elements at a given point, we construct new redistribution functions describing this correlation. They give the probability that a photon with frequency x in the direction n is absorbed and reemitted at frequency x^′ in the direction n^′. A preliminary discussion of the problem is given in an astrophysical context.     

Orientational disordering in crystals—II: A model for the evaluation of thermodynamic properties of orientational disordering in crystals

A statistical mechanical model for the evaluation of thermodynamic properties of orientational transitions in crystals developed by Aston is generalised by taking into account the existence of more than two allowed molecular orientations (nγ 2) in the rotator crystal phase explicitly. An orientational transition work function is derived in order to generate full thermodynamic functions leading to a formal expression for the configurational energy change in terms of transition enthalpy change and an entropy integral. The value of n for camphor predicted by this equation compares very well with the one computed from another independent method and that reported by previous workers. A general scheme, based on both spectroscopic and thermodynamic data, by which n and phase eigenstates may be computed including the case where configurational and thermal properties are strongly coupled is presented. An analytical expression is obtained for the relative potential energy barrier ν_eff in a single-lattice model which parallels that defined for a double sub-lattice model of positional disorder by Pople and Karasz and their followers. It is shown that the value of ν_eff for a given value of n is not uniquely defined for all substances which may not have common values of other thermodynamic parameters.     

Higher orders of perturbation theory for the Stark effect on an atomic multiplet

The contribution of higher order corrections to the Stark energy is calculated in the anticrossing region of atomic multiplet sublevels. Perturbation theory for close-lying levels is presented that is based on the Schrö dinger integral equation with a completely reduced Green’s function. Analytic formulas are obtained for the splitting of two interacting fine-structure sublevels as a function of the field strength. These formulas take into account fourth-order resonance and nonresonance corrections to both the diagonal and the off-diagonal matrix elements of the dipole moment operator. By the method of the Fues model potential, a numerical analysis of radial matrix elements of the second, third, and fourth orders is carried out that determine a variation in the transition energy between n³P₀ and n³P₂ sublevels of a helium atom for n=2, 3, 4, 5 in a uniform electric field. It is shown that the contribution of the fourth-order corrections in the vicinity of anticrossing of levels for n=2, 3, 4, 5 amounts to 0.1, 5, 10, and 15% of the total variation of energy, respectively. A comparative anal-ysis is carried out with the results of calculations obtained by the method of diagonalization of the energy matrix, which, together with resonance terms, takes into account other states of the discrete spectrum with n≤6.     

Level density for doubly odd deformed nuclei

The level density for doubly odd deformed nuclei near B_n is calculated within the framework of the semi-microscopic approach. The calculated values are found to be in good agreement with the experimental ones. It is shown that for the I ? 1 states it is important to take into account rotational motion.     

Photoelectron (He(I), He(II) and X-ray) spectroscopy of γ-pyrone and its related sulphur derivatives: valence and core ionization energies and shake-up satellites

The photoelectron He(I) and He(II) spectra of 4H-pyran-4-one (1), 4H-thiopyran-4-one (2), 4H-pyran-4-thione (3) and 4H-thiopyran-4-thione (4) are reported. The assignments are based on experimental evidence, taking into account the results of theoretical calculations. The outermost orbital sequences proposed (X = O, S) are n_X (σ), 3b₁ (π), 1a₂(π) for 1, 2 and 4; and 3b₁(π), n_X(σ) and 1a₂(π) for 3. The shifts of the core-ionization energy values for 1–4 are ascribed to a drift of π-charge from the intracyclic heteroatom towards the carbonyl or thiocarbonyl group. Low-energy shake-up satellites (up to 25% with respect to the main line) are observed in the various energy regions of the XPS spectra. They are qualitatively reproduced by CNDO/2 calculations. The most important satellites derive from the transition in the ion corresponding to the 3b₁ (π) → 4b₁(π*) transition in the UV-visible spectra of the neutral molecules. Charge rearrangements accompanying this transition lead to charge depletion of the core-ionized atom.     

Cyclotron masses in n-GaAs/Ga 1−xAl xAs heterojunctions

Electron cyclotron resonance is studied in single layer GaAs/Ga _1−xAl _xAs heterostructures. The cyclotron mass is measured over a wide range of the two-dimensional electron density N _s and the magnetic field strength B to enable detailed comparison with self-consistent theoretical results that take into account the effects of band nonparabolicity. The calculations are performed using an effective 2 × 2 subband Hamiltonian, which is derived from a five-level k·p-model by fourth order perturbation theory and includes remote band contributions. Close agreement between experimental and theoretical cyclotron masses is achieved and the importance of band nonparabolicity in these systems is demonstrated.     

Spatial-dispersion induced birefringence in cubic crystals: CuBr and CuI

A natural birefringence in single crystals of copper halides is observed and measured in the exciton part of the spectrum. This intrinsic birefringence is due to the spatial dispersion of the dielectric constant ?(ω, q). Its spectral dependence and strong enhancement near the energy gap can be explained only, if we take account of the strong excitonic contribution in these compounds. This effect is related to the valence band anisotropy (warning) and thus allows a direct determination of the Luttinger band parameters | γ₂ - γ₃ | for these compounds.     

Decay times for second-chance fission of 239U studied by crystal blocking

Neutron-induced fission of ²³⁸U has been studied by the crystal-blocking technique for neutron energies just below and above the threshold for second-chance fission. In agreement with earlier measurements, in this energy range the lifetime for first-chance fission is found to be too short to have an observable effect on the blocking dips. Above the threshold, however, an appreciable filling-in of the dips is observed. The results are analyzed in terms of a two-component lifetime distribution and then indicate an average lifetime of a few fsec for second-chance fission at a neutron energy of E_n ≈ 7.2 MeV, in agreement with results from a simple calculation. It is shown that in this analysis it is important to take into account the anisotropy of the fission-fragment distribution and, in particular, the difference between the angular distributions for first- and second-chance fission.     

Low-frequency behavior of optical spatial dispersion effects

A theoretical explanation is given for the frequency independence of the nonreciprocal birefringence of light, which was recently observed in the semiconductors Cd_1?x Mn_xTe, Zn_1?x Mn_xTe, and GaAs in the frequency range below the frequency corresponding to the interband absorption edge. It is shown that the symmetry of the effect becomes higher at such frequencies if the light-induced excitation energy ?ω_n(k) only slightly depends on the photon momentum k. In this case, the nonreciprocal birefringence is completely determined by the second-rank magnetoelectric tensor. It is shown that the nonreciprocal birefringence of light can be observed in magnetic media with a tensor order parameter.     

On the influence of shell structure and matrix element fluctuations on the pairing correlation in nuclei

The uniform model for the nuclear pairing-force problem is extended to take into account the effect of fluctuations in nucleon orbital level densityρ and in pairing matrix elementsG _vv′. Simple formulas for the average dependence of level density on energy are presented, and alternative ways of estimating effects of shell bunching on the pairing correlation are derived. It is argued also that the pairing constantG might be systematically overestimated in the usual BCS numerical calculations; a semiconstant pairing force with diagonal elements larger than off-diagonal is thus suggested.     

Vacancies in binary alloys: III. The energy of vacancy formation

A general expression for the energy of formation of a single vacancyE _F in substitutional binary alloys having two types of lattice sites is derived, which takes into account the interaction of neighbouring atoms. It is shown, that near the Curie temperature Θ the formation energy exhibits an anomaly.     

Drastic influence of plasma quasi-static electric fields on satellites of helium forbidden lines

The emission spectrum has been calculated of helium- and alkali-like quantum systems in plasma with quasi-monochromatic electric field E₀ cos ωt. It was demonstrated that taking into account even relatively weak quasi-static plasma electric fields S could drastically alter the known Baranger-Mozer results, concerning the intensities of the forbidden line satellites. The way for the correction of Baranger-Mozer-type diagnostics of the field E₀ cos ωt is pointed out. It was also found that intensive satellites of a new type appear at the distance ±ω from the allowed line.     

Effect of oligomers on the growth of amorphous silicon films in a PECVD reactor

A plasma-chemical model of processes in a PECVD reactor is constructed that is an extension of the earlier model and takes into account the formation of oligomers Si_nH_m (n ≤ 5). The corresponding scheme of chemical reactions is developed, and simulation of film growth is carried out. It is found that Si₂H₅ and Si₃H₇ components strongly influence the film growth. It is of interest to obtain more reliable experimental data dramatizing these effects.     

The scattering effects of the redistribution function RIII(x′, n′; x, n) on spectral line formation

The effects of frequency and angular redistribution on line formation are examined in static isothermal atmospheres for scattering described by the redistribution function R_III(x′, n′; x, n). For an optically thin atmosphere, it is found that the emergent line intensities obtained using R_III are essentially the same as R_I and R_II. In the case of a semi-infinite atmosphere, the emergent line intensities obtained using R_III, whether angle-averaged or angle-dependent, did not differ substantially from the complete redistribution results.     

Limits on the mass of a superstring Z′ from the CERN pp̄ collider

We illustrate how recent CERN pp̄ collider data can be used to bound the mass of a possible Z′ in superstring-inspired models. We take into account the fact that the Z′ couplings to quarks and leptons are smaller than those of the conventional Z, and the possibility that the Z′ may be able to decay into exotic particles, thereby suppressing the Z′→ℓ⁺ℓ⁻ branching ratio. In the minimal rank-five model we find m_Z′ > 167GeV if the Z′ can only decay into conventional quarks and leptons and m_Z′ > 118 GeV for the least favourable spectrum of three E₆27 generations of particles and sparticles which is allowed by other experiments. When combined with a global analysis of neutral current data, the latter conservative limit can be increased to m_Z′ > 156 GeV. We derive analogous limits for two other superstring-inspired models. We show how these limits can be improved using future FNAL Tevatron and SLC data.     

Pseudogap behavior in the emery model for the electron-doped superconductor Nd2 − x Ce x CuO4: Multiband LDA + DMFT + Σk approach

We propose a generalization of the LDA + DMFT + Σ _k approach to the multiband case, in which correlated and uncorrelated states are present in the model simultaneously. Using the multiband version of the LDA + DMFT + Σ _k approach, we calculate the density of states and spectral functions for the Emery model in a wide energy interval around the Fermi level. We also obtain the Fermi surfaces for the electron-doped high-temperature superconductor Nd_{2 ? x} Ce _x CuO₄ in the pseudogap phase. The self-energy part Σ _k introduced additionally to take into account pseudogap fluctuations describes the nonlocal interaction of correlated electrons with collective Heisenberg short-range spin fluctuations. To solve the effective impurity model, the numerical renorm-group (NRG) method is used for the DMFT equations. Good qualitative agreement of the Fermi surfaces calculated using the LDA + DMFT + Σ _k approach and experimental angle-resolved photoemission spectroscopic data is attained. The stability of the dielectric solution with charge transfer in the Emery model with correction for double counting is analyzed in the Appendix.     

Study of the equilibrium surface composition of Co1−yFe2+yO4—I: Cation composition with stoichiometric oxygen content

The equilibrium composition at the surface of the mixed spinel Co_1?yFe_2+yO₄ is calculated as a function of the composition parameter y and of the degree of inversion. The ionic standard chemical potentials include the Born-Mayer repulsive energy and the vibrational contribution. The calculated results turn out to be weakly dependent on the bulk inversion, and indicate a pronounced tendency of iron to segregate at the surface. These results are compared and found to be in agreement with some experimental data obtained by AES. In a subsequent paper we will take into account the gas-phase and the charge-transfer effects.     

Inversion and inversion-rotation spectrum of 14NH3 in the ν2 excited state

Using the vibration-inversion-rotation Hamiltonian for ammonia [V. ?pirko, J. M. R. Stone, and D. Papou?ek, J. Mol. Spectrosc.60, 159–178 (1976)], a modified theory is worked out for the Δk = ±3n interactions between the inversion-rotation energy levels of NH₃ which takes into account the large amplitude inversion motion. Eighty frequencies of inversion and inversion-rotation transitions and two perturbation-allowed transitions in the ν₂ state of ¹⁴NH₃ are measured in the far-infrared region around 1 THz (≈33 cm^?1), mostly with the microwave accuracy, by the radiofrequency spectrometer with acoustic detector. By a least-squares fit of these data and the data of the infrared-microwave two-photon and infrared heterodyne measurements of the ν₂ band, a set of molecular constants for the ν₂ state of ¹⁴NH₃ is obtained which reproduces the submillimeterwave data with the accuracy of the experiment.