On the mixed nature of the 740 cm band in wurtzite GaN films: A polarized Raman scattering investigation

A detailed study of the polarized Raman scattering of wurtzite GaN films is presented, focusing on the nature of the band centered at 740 cm⁻¹ observed in the X(Z, Z)X configuration. The origin of this band is ascribed to the mixed contribution of the A₁ and E₁ longitudinal phonon modes coupled with the free carrier excitation. The spectral profile of the 740 cm⁻¹ Raman band has been successfully reconstructed through a linear combination of the A₁-E₁ longitudinal phonon plasmon-coupled modes, leading to a free carrier concentration in good agreement with Hall effect measurements.     

Two-dimensional propagators and spatio-temporal correlations for flow in porous media: A comparative study

The two-dimensional displacement joint probability densityPΔ(X,Z) and the two-time probability density W₂(Z₁,Δ₁;Z₂,A₂) for water flowing through several porous systems have been measured by means of pulsed field-gradient nuclear magnetic resonance (PFG-NMR). The simultaneous particle displacementsX and Z perpendicular and parallel to the pressure gradient, respectively, at a given encoding time Δ are obtained from an experiment employing orthogonal magnetic field gradients. Time-correlated propagators which relate the displacement spectra at two consecutive times Δ_l, and Δ₂ with each other were derived by applying rephasing gradients in two steps. Flow through a random packing of glass beads and through natural sandstone is compared to flow through arrays of either oriented or unoriented fibers with different solid volume fractions. The dependence of the dispersion tensor D^* as a function of time is discussed and related to a characteristic length ξ_tT transverse to the flow direction. Within a certain range of Z values, displacements inX and Z are related by a power law <X²(Z)> ∝Z _γ. The spreading exponent γ is found to increase with growing orientational order in the porous system and is largest for fiber bundles being twisted with respect to the mean pressure gradient axis. The evolution of the correlation coefficient px²,z with time gives a measure for the typical correlation length of the system parallel to the flow axis, ξ_‖. Analyzing the shape ofW ₂(Z₁,Δ₁Z₂,Δ₂) allows one to investigate the loss of coherence in flow by an alternative approach. The decay of the two-time correlation coefficient,pZ ₁,Z₂, is sensitive to the change of the z-component of the particle velocity and probes a different lengthscale thanpx ₂z^.     

Numerical evidence for a phase transition in the RP2 spin model in two dimensions

We study the RP²(O(3)/Z₂) spin system in two dimensions using the Monte Carlo technique. The results are indicative of the existence of a phase transition due to vortex condensation, with properties quite similar to that of the X?Y model in two dimensions.     

Ground-state energy and stability of three-particle coulomb systems versus the masses of the particles involved

The ground-state energy of three-particle Coulomb systems (trions) is investigated versus the masses of the particles involved. Variational calculations are performed for 34 asymmetric trions X^±Y^±Z^? consisting of electrons, muons, pions, kaons, nuclei of hydrogen isotopes and their antiparticles, as well as for more than 100 auxiliary three-particle systems involving particles of masses chosen arbitrarily. Wide bases of Laguerre exponential-polynomial functions depending on perimetric particle coordinates are used. Approximate analytic formulas for the ground-state energies of all trions X^±Y^±Z^? with arbitrary particle masses are constructed on the basis of the values found here for the energies of asymmetric trions and the values calculated previously for the energies of symmetric trions X^±X^±Z^?. Particle-mass regions are determined where trions are stable with respect to dissociation. In addition to symmetric trions X^±X^±Z^?, which are stable at any particle masses, asymmetric trions X^±X^±Z^? possess the stability property if the masses of the particles X and Y exceed the mass of the particle Z, where, by Z, we mean, for example, an electron, a muon, a pion, or a kaon. The t⁺d⁺p^? and t⁺d⁺d^? combinations of hydrogen nuclei and antinuclei are also stable with respect to dissociation. The general properties of the ground-state trion energy as a function of the particle masses are discussed.     

Interaction de l'oxygène aux basses pressions avec la solution solide niobium-oxygène à haute température: II. Désorption d'oxydes

Interations between oxygen under low pressure and a niobium-oxygen solid solution at high temperature (2200 >T > 1800 K) have been studied. Evidence was found of the validity of a model where the rates of oxide desorption (Z_NbOx) are related to superficial coverage θ : Z_NbOx = K_NbOxθ^x. It is shown experimentally that it is possible to derive from this model an expression which relates flux to bulk concentration C : Z_NbOx = K_NbOxC^x From the equivalence of these two expressions it results that: (1) the enthalpy of transfer (ΔH_SV between the surface and the bulk of solid solution, and (2) the bond energy (x) of the superficial oxygen atoms, have the same dependence on superficial coverage θ (i.e. influence of the interactions between adsorbed oxygen atoms on both X and ΔH_SV is the same). So, X and Δ_SV are theoretically in the form: ΔH_SV = ΔH_SV(0) ? αθ and X = x(0) ? αθ. In fact, there is a discrepancy beiween experimental values which is discussed.     

Antiferromagnetic resonance spectrum in LaMnO3: Interrelation of the orbital structure and the magnetic properties

The relation between the orbital ordering and magnetic structure of the crystal LaMnO₃ is investigated. The dependence of the exchange parameters on the angle Φ of the orbital structure is determined. When the isotropic exchange interaction and the single-ion anisotropy, which depends on the angle Φ and the rotational distortions, are introduced into the spin Hamiltonian, a four-sublattice structure (A _X, F _Y, G _Z) is obtained with orientation of the magnetic moments of the sublattices near the long axis of the orthorhombic cell of the crystal in the basal plane of the crystal (A _X ? G _Z, F _Y). The effect of a magnetic field on the magnetic structure and the antiferromagnetic resonance spectrum are investigated taking account of the nonequivalent, anisotropic, orbitally-dependent g tensors. The spin-flop and spin-flip transition fields are calculated.     

Z-production in deep inelastice P-scattering

We discussZ ⁰-production in the processe ^? +P → e ^? +Z ⁰ +X in the context of the standard modelSU (3) _C ×SU (2) _L ×U (1) of the strong and electroweak interactions. Besides results for the total cross section and the differential cross section as a function of the parameters of the outgoing electron, we present results for the energy spectrum and the angular distribution of the producedZ ⁰, for energies available at HERA in the near future. We find that the energy and angular distributions forZ ⁰-production are strongly peaked, which will permit a clear identification of the producedZ ⁰.     

The effect of resonance interactions on the absorption spectra of (SF6)2 dimers in low-temperature matrices: Calculations and experiment

The IR absorption spectra of (SF₆)₂ dimers in Ar and N₂ matrices are investigated at 11 K. As a result of the resonance dipole-dipole interaction, the band of the triply degenerate vibration v ₃ is split into two components v _{X, Y} and v _Z. In comparison with the gaseous state, the splitting Δv = v _{X, Y} ? v _Z in the argon matrix decreases to 18.45 cm^?1, whereas, in the nitrogen matrix, the band v _{X, Y} is split into components v _X and v _Y, with the splitting being equal to δ ≈ 0.9 cm^?1. A model that takes into account the influence of the matrix on the spectra of dimers is developed. The model makes it possible to successively (i) calculate the resonance spectrum of an isolated dimer in terms of the model of local modes taking into account resonance interactions, (ii) determine with the help of the Monte Carlo method the structure of a matrix consisting of 512–1440 Ar (or N₂) atoms and a rigid (SF₆)₂ dimer, and (iii) take into account interactions of local dipole moments of a dimer with particles of the matrix in the approximation of dipole-induced dipole interactions. The model developed satisfactorily describes the experimental results. The calculated frequencies v _Z, v _X, and v _Y of a dimer in the matrix are shifted toward smaller frequencies as compared to the gaseous state, while the resonance splitting decreases virtually by 2 cm^?1. It is shown that, in an argon matrix with a symmetric arrangement of argon atoms nearest to a dimer, the splitting of v _{X, Y} proves to be smaller than 0.05 cm^?1. In a nitrogen matrix, this splitting increases virtually to 0.4 cm^?1.     

Metalloradical EPR Signals from the YZ·S-State Intermediates in Photosystem II

The redox-active tyrosine residue (Y_Z) plays a crucial role in the mechanism of the water oxidation. Metalloradical electron paramagnetic resonance (EPR) signals reflecting the light-induced Y_Z· in magnetic interaction with the CaMn₄-cluster in the particular S-state, Y_Z·S_X intermediates, have been found in intact photosystem II. These so-called split EPR signals are induced by illumination at cryogenic temperatures and provide means to both study the otherwise transient Y_Z· and to probe the S-states with EPR spectroscopy. The illumination used for signal induction grouped the observed split EPR signals in two categories: (i) Y_Z in the lower S-states was oxidized by P680⁺ formed via charge separation, while (ii) Y_Z in the higher S-states was oxidized by an excited, highly oxidizing Mn species. Applied mechanistic studies of the Y_Z·S_X intermediates in the different S-states are reviewed and compared to investigations in photosystem II at physiological temperature. Addition of methanol induced S-state characteristic changes in the split signals’ formation which reflect changes in the magnetic coupling within the CaMn₄-cluster due to methanol binding. The pH titration of the split EPR signals, on the other hand, could probe the proton-coupled electron transfer properties of the Y_Z oxidation. The apparent pK _as found for decreased split signal induction were interpreted in the fate of the phenol proton.     

Production of D*+(2010) mesons by high energy neutrinos from the Tevatron

We discuss some issues arising in the search for CP violation in the decays Z → τ⁺ τ^? and Z → bb?X. The form-factor and the effective Lagrangian approach to parametrize CP-violating effects are compared. We emphasize the interest to study both real and imaginary parts of CP-violating form factors like the weak dipole moment form factor ${d_?u^Z}={(m_Z^2)}$ of the τ-lepton. We propose an “optimal” way to search for CP violation when Z → bb?X events have to be selected on a statistical basis from the total number of Z hadronic decays.     

Total cross section forZ 0 production in electron proton collisions

We present an exact calculation of the total cross section π_tot of theZ ⁰ production ine ^?1+p→e ^?+Z ⁰+X processes. We find a total cross section π_tot?1.0·10^?36cm² for an energy in the center of mass of 300 GeV and aZ ⁰ mass of 90GeV, which is quite a measurable result in somee ^?1 proton collider experiments, like HERA or SPS/LEP.     

Reaction of molecular hydrogen with the 100 face of MoO3: I. Kinetics in the low pressure range (10−8–10−6 Torr) in the presence of platinum particles

Three successive processes are observed when the 100 face of a MoO₃ single crystal covered with a specific density of platinum particles of known diameter is exposed to molecular hydrogen in a pressure (P) domain between 10^?8 and 10^?6 Torr. During the first step, called the activation step, the rate process is a function of the surface area of the Pt particles and it is slightly thermally activated. During this activation step, H₂ dissociatively chemisorbed on the platinum surface and the hydrogen atoms are inserted into the MoO₃ surface layers. The inserted atom is partially ionized, the electrons given to the lattice reducing Mo⁶⁺ into Mo⁵⁺ and Mo⁴⁺. The rate of hydrogen insertion (V_a) in the activation step increases about linearly with time. The activation step is followed by a stationary step where V is independent of the extent of the Pt surface, V = 0.7Z(1?X), where Z is the number of collisions between H₂ and the MoO₃ surface and X the electronically reduced fraction of the surface area. $\text{X =}\text{a}\text{P}\text{(1 + a}\text{P}\text{)}$, where a is a function of the temperature only. In the stationary step, the platinum particles do not play any role and molecular hydrogen is dissociatively chemisorbed on the MoO₃ surface. The stationary step is followed by a deceleration step. The slowing down of the reaction rate is due to the formation of fractures within the crystal. During all these three steps, the surface is self-cleaning, since protons diffuse into the bulk. The final product of the insertion process would be H_1.6MoO₃, the properties of which have been studied in detail elsewhere. The model which is summarized above is very different from that which would result from the classical views on the so-called hydrogen spillover process since the Pt surface producing the H atoms spilling over the surface would operate only during the activation step.     

A Level 1 Large-Deviation Principle for the Autocovariances of Uniquely Ergodic Transformations with Additive Noise

A large-deviation principle (LDP) at level 1 for random means of the type $$M_n \equiv \frac{1}{n}\sum\limits_{j = 0}^{n - 1} {Z_j Z_{j + 1} ,{\text{ }}n = 1,2,...}$$ is established. The random process {Z _n} _n≥0 is given by Z _n = Φ(X _n) + ξ _n , n = 0, 1, 2,..., where {X _n} _n≥0 and {ξ _n} _n≥0 are independent random sequences: the former is a stationary process defined by X _n = T ⁿ(X ₀), X ₀ is uniformly distributed on the circle S ¹, T: S ¹ → S ¹ is a continuous, uniquely ergodic transformation preserving the Lebesgue measure on S ¹, and {ξ_n} _n≥0 is a random sequence of independent and identically distributed random variables on S ¹; Φ is a continuous real function. The LDP at level 1 for the means M _n is obtained by using the level 2 LDP for the Markov process {V _n = (X _n, ξ _n , ξ _n+1)} _n≥0 and the contraction principle. For establishing this level 2 LDP, one can consider a more general setting: T: [0, 1) → [0, 1) is a measure-preserving Lebesgue measure, $\Phi :\left[ {0,\left. 1 \right)} \right. \to \mathbb{R}$ is a real measurable function, and ξ _n are independent and identically distributed random variables on $\mathbb{R}$ (for instance, they could have a Gaussian distribution with mean zero and variance σ²). The analogous result for the case of autocovariance of order k is also true.     

Measurements ofK-shell ionization with132Xe and208Pb projectiles in the energy range of 1.4–5.9 MeV/u

Projectile and targetK-shell ionization cross sections induced by 3.6-, 4.7-, and 5.9 MeV/u¹³²Xe ions and 1.4-, 3.6-, 4.7-, and 5.9 MeV/u²⁰⁸Pb ions from the UNILAC in thin solid targets between C and U are measured. The cross sections are discussed in terms of the molecular model of innershell vacancy production in heavy ion-atom collisions. The sharing of 2p _1/2σ vacancies between theK shells of the two collision partners in these very heavy ion-atom collisions is found to deviate from the Meyerhof-Demkov formula forR≦10^?2. The measured ionization cross sections are compared with theoretical calculations for 1sσ and 2p _1/2σ excitation cross sections. AZ _UA=Z₁+Z ₂ dependence is found independent ofZ ₁/Z ₂. Outer-shell vacancy configurations measured in these close encounters are reported.     

The ac transport and giant magnetoimpedance for sol–gel La0.67Ca0.33MnO3

In the present work, the temperature dependences of ac transport and magnetoimpedance for sol–gel La_0.67Ca_0.33MnO₃ manganite are investigated at various frequencies. The ac resistance R with absence of field shows a peak around the metal–insulator transition temperature T_MI for the dc case. A low magnetic field can shift the metal–insulator transition temperature more easily at high frequency than at low frequency. The valley phenomenon is observed in the temperature dependence of reactance X at 2 and 4 MHz, showing the effects of phase separation or grain boundaries. At 30 MHz, the peak of X appears instead of the valley, since the contribution of insulator capacitance term drops greatly and the inductance of metal dominates the X term with an increase of ac frequency in the sample. At 2 and 4 MHz, La_0.67Ca_0.33MnO₃ sol–gel manganite under H=1 kOe shows negative ac magnetoresistance ΔR/R₀, magnetoreactance ΔX/X₀ and magnetoimpedance ΔZ/Z₀ below 275 K, but positive ac ΔR/R₀, ΔX/X₀ and ΔZ/Z₀ near room temperature much higher than the Curie temperature. The peak temperature of R/R₀ moves to low temperature with an increase of the ac frequency from 2 to 4 MHz. At f=30 MHz, the absolute value of negative ac ΔR/R₀ increases monotonically with a decrease of temperature for 175 K<T<275 K, whereas the ΔX/X₀ is positive below 245 K but negative above 245 K. The ΔZ/Z₀ under H=1 kOe is positive below 238 K but negative above 238 K. Such a positive ΔZ/Z₀ is due to an increase of permeability induced by field. With an increase of field, the impedance Z experiences a peak, which is connected with the effect of transverse magnetic anisotropy.     

Dynamics of liquid ammonia from cold neutron scattering

The scattering of 4.1 Å neutrons by liquid ammonia has been measured at 218 K in the angular range of 30–90° using the Trombay rotating crystal spectrometer. The experimental data, after correcting for multiple scattering, have been compared with model calculations, and it is shown that it is possible to get detailed information about the rotational correlation function on the basis of neutron experiments alone. The model assumes Langevin diffusion for translational motions. Rotational motions are described by means of an orientational correlation function which has a gaseous behaviour for times up toτ ₀ and then changes over to a diffusive character with a rotational diffusion constantD _r. Within the framework of the model the correlation function can be described withD _r=0.28×10¹³sec^?1 andτ ₀=0.57×10^?18sec. Corrections for multiple scattering and their dependence on model parameters are discussed.     

Reexamination of the vacuum ultraviolet emission spectrum of CO in the 950–1200 Å region

New spectrograms of CO below 1200 Å reveal that emission bands terminating on the high energy $\text{E}{}_0{}^1\text{Σ}{}^{\mathrm{+}}$ and ¹Π states plus most of the previously reported unidentified emission bands of CO actually originate from molecular nitrogen. Four new emission band systems in CO, tentatively identified as V¹Π-X¹Σ⁺, W¹Π-X¹Σ⁺, Y¹Σ⁺-X¹Σ⁺, and Z¹Σ⁺-X¹Σ⁺, have been observed. The corresponding T_v0′s are 98917, 102804, 99963, and 105724 cm^?1, respectively.     

Die Eigenschwingungen des Metall-Hydratkomplexes in BeSO4·4H2O und BeSO4·4D2O

Infrared reflection spectra of single crystals of BeSO₄·4H₂O and BeSO₄·4D₂O have been obtained in polarized light at 300°K and at 14°K in the region between 4000 cm^?1 and 300 cm^?1. By a Kronig-Kramers analysis, the frequencies of the infrared active transitions have been calculated. These transitions are attributed to internal vibrations of the water molecules and sulfate ions and, in the region between 1000 cm^?1 and 300 cm^?1, especially to internal and external vibrations of the tetrahedral Be⁺⁺·4aqu-complexes. The vibrational modes of these complexes consist of a superposition of translational and librational modes of the water molecules and translational modes of the central Be⁺⁺-ion. The vibrational frequencies and normal modes of this complex have been calculated in a central-force model, and force-constants have been determined by fitting the calculated frequencies to the observed spectra. The calculations have shown that the modes, which comprise mainly translational motions of the water molecules, are strongly coupled with librational motions of the water molecules. On the other hand, there exist pure librational modes with practically no admixture of translational motions. The optimum sets of force constants for the BeSO₄·4H₂O crystal and the BeSO₄·4D₂O crystal differ in a manner which can be understood under the assumption that the dimensions of the Be(D₂O)₄ complex are about 0.1 Å larger than those of the Be(H₂O)₄ complex. Some arguments supporting this conclusion will be discussed.