The effect of disorder-induced mode coupling on vibrational spectra of glasses

A theoretical approach based on the Green's function method is proposed for investigation of disorder-induced mode coupling in glasses and of its effect on the intensity of the first order Raman and Infrared spectra. It is shown that while the spectral line widths depend on the Green's function of the glass, dG¢, the mode coupling coefficients depend on dG²¢. The numerical results, obtained in self-consistent approximation for dG¢ and in ladder approximation for dG²¢, demonstrate that the structural disorder leads to strong coupling between modes only if they are of the same type and with close frequencies.     

On the determination of double diffraction dissociation cross section at HERA

The excitation of the proton into undetected multiparticle states (double diffraction dissociation) is an important background to single diffractive deep-inelastic processes epMe'p'A⁰,r e'p'J/O,r e'p'X at HERA. We present estimates of the admixture of the double diffraction dissociation events in all diffractive events. We find that in the J/O photoproduction, electroproduction of the A⁰ at large Q² and diffraction dissociation of real and virtual photons into high mass states X the contamination of the double diffraction dissociation can be as large as ~30%, thus affecting substantially the experimental tests of the pomeron exchange in deep inelastic scattering at HERA. We discuss a possibility of tagging the double diffraction dissociation by neutrons observed in the forward neutron calorimeter. We present evaluations of the spectra of neutrons and efficiency of neutron tagging based on the experimental data for diffractive processes in the proton-proton collisions.     

Atomic and electronic structure of the corundum (0001) surface: comparison with surface spectroscopies

The electronic structure and geometry of the Al-terminated corundum (0001) surface were studied using a slab model within the ab-initio Hartree-Fock technique. The distance between the top Al plane and the next O basal plane is found to be considerably reduced on relaxation (by 0.57 Å, i.e. by 68% of the corresponding interlayer distance in the bulk). An interpretation of experimental photoelectron spectra (UPS He I) and metastable impact electron spectra (MIES) is given using the calculated total density of states of the slab and the projections to the atoms, atomic orbitals, and He 1s floating atomic orbital at different positions above the surface. Calculated projected densities of states exhibit a strong dependence on the relaxation of surface atoms. The good agreement of simulated and experimental UPS and MIES spectra supports the correctness of calculated surface relaxation.     

Solutions of the Dirac–Fock Equations for Atoms¶and Molecules

The Dirac-Fock equations are the relativistic analogue of the well-known Hartree-Fock equations. They are used in computational chemistry, and yield results on the inner-shell electrons of heavy atoms that are in very good agreement with experimental data. By a variational method, we prove the existence of infinitely many solutions of the Dirac-Fock equations "without projector", for Coulomb systems of electrons in atoms, ions or molecules, with Z h 124, N h 41, N h Z. Here, Z is the sum of the nuclear charges in the molecule, N is the number of electrons.     

Methods of filtering densities reconstructed from line projections

We present details important for practical applications of a new algorithm for filtering densities reconstructed from their line projections. The method is applied to densities reconstructed via the Cormack method from both model projections and 2D angular correlation of positron annihilation radiation spectra in CeIn₃ and ErGa₃. It is shown that this algorithm filters the experimental noise as well as enhances some details of the reconstructed densities, which then reveals their fine features. We also compare this way of filtering with the effects of imposing symmetry conditions on densities reconstructed for cubic systems. PACS 78.70.Bj; 87.59.Fm; 74.25.Jb     

Gleason's Theorem for Rectangular JBW-Triples

A JBW^*-triple B is said to be rectangular if there exists a W^*-algebra A and a pair (p,q) of centrally equivalent elements of the complete orthomodular lattice P(A)\mathcal{P}(A) of projections in A such that B is isomorphic to the JBW^*-triple pAq. Any weak^*-closed injective operator space provides an example of a rectangular JBW^*-triple. The principal order ideal CP(A)_(p,q)\mathcal{C}\mathcal{P}(A)_{(p,q)} of the complete ^*-lattice CP(A)\mathcal{C}\mathcal{P}(A) of centrally equivalent pairs of projections in a W^*-algebra A, generated by (p,q), forms a complete lattice that is order isomorphic to the complete latticeI(B)\mathcal{I}(B) of weak^*-closed inner ideals in B and to the complete lattice S(B)\mathcal{S}(B) of structural projections on B. Although not itself, in general, orthomodular, CP(A)_(p,q)\mathcal{C}\mathcal{P}(A)_{(p,q)} possesses a complementation that allows for definitions of orthogonality, centre, and central orthogonality to be given. A less familiar notion in lattice theory, that is well-known in the theory of Jordan algebras and Jordan triple systems, is that of rigid collinearity of a pair (e₂,f²) and (e₂,f²) of elements of CP(A)_(p,q)\mathcal{C}\mathcal{P}(A)_{(p,q)}. This is defined and characterized in terms of properties of P(A)\mathcal{P}(A). A W^*-algebra A is sometimes thought of as providing a model for a statistical physical system. In this case B, or, equivalently, pAq, may be thought of as providing a model for a fixed sub-system of that represented by A. Therefore, CP(A)_(p,q)\mathcal{C}\mathcal{P}(A)_{(p,q)} may be considered to represent the set consisting of a particular kind of sub-system of that represented by pAq. Central orthogonality and rigid collinearity of pairs of elements of CP(A)_(p,q)\mathcal{C}\mathcal{P}(A)_{(p,q)} may be regarded as representing two different types of disjointness, the former, classical disjointness, and the latter, decoherence, of the two sub-systems. It is therefore natural to consider bounded measures m on CP(A)_(p,q)\mathcal{C}\mathcal{P}(A)_{(p,q)} that are additive on centrally orthogonal and rigidly collinear pairs of elements. Using results of J.D.M. Wright, it is shown that, provided that neither of the two hereditary sub-W^*-algebras pAp and qAq of A has a weak^*-closed ideal of Type I², such measures are precisely those that are the restrictions of bounded sesquilinear functionals {_m on pAp 2 qAq with the property that the action of the centroid Z(B) of B commutes with the adjoint operation. When B is a complex Hilbert space of dimension greater than two, this result reduces to Gleason's Theorem.     

Photoluminescence spectra of MOCVD-grown P-doped GaAS/Alx Ga1-x As MQW

PhotoLuminescence (PL) measurement techniques have been used to investigate on MOCVD grown P-doped GaAs/Al_xGa_1mx As (x=0.3) Multiple Quantum Wells (MQW). The spectra reveal extrinsic luminescence characteristics of e-A⁰ transitions for interface and centre of well acceptors in addition to both bound and free exciton emissions.     

Determining the polarization ellipse parameters of a multimode radiowave signal

A spectral-polarization method is proposed to determine the polarization-ellipse parameters of a radiowave signal in the wave front plane by analysis of three mutually orthogonal projections of the field vector. First, we calculate the complex Doppler spectra for the temporal variations of those projections. Then, these data are used to determine the polarization parameters of the electromagnetic wave for each spectral component. Formulas illustrating the proposed technique are presented, and the results of numerical simulation showing the possibility of practical implementation of the idea are given. Institute of Solar-Terrestrial Physics, Siberian Branch of the Russian Academy of Sciences; Higher Aircraft Engineering Military School of Irkutsk, Irkutsk, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 4, pp. 324–332, April 1999.     

Comparative analysis of the growth of SmBa2Cu3O7-x thin films by off-axis pulsed laser deposition on different substrates

The growth of SmBa₂Cu₃O_7-x superconducting thin films by off-axis pulsed laser deposition on different substrates (SrTiO₃, MgO, LaAlO₃, and YSZ) has been analyzed by means of resistance vs. temperature and X-ray diffraction measurements. The onset and width of the resistive transition depend on the substrate type and are in the ranges (89-80) K and (1-9) K, respectively. X-ray diffraction spectra show only the 00l reflections, from which the lattice parameter c can be estimated. Moreover, the rocking curves of the 005 peaks give an indication of the films' crystallinity and oxygen stoichiometry.     

Simple determination of Na2 scattering lengths using observed bound levels at the ground state asymptote

A simple model is developed to determine a scattering length from the experimental values of the energy of the last, least bound, vibrational levels of the ground state, either ¹S_g⁺^1\Sigma _g^{+} or ³S_u⁺^3\Sigma _u^{+},of an alkali dimer. It is based on an extrapolation of the positions of the few outermost nodes of the bound vibrational wave functions towards the dissociation threshold. It uses the asymptotic part of the molecular potential only. The method is applied to recently measured levels at the asymptote (3s+3s) of Na₂. We obtain precise values for the two scattering lengths respectively associated to the dissociation limits F₁=F₂=1,F=2 (55.1±\pm 1.6 a₀) and F₁=F₂=1,F=0 (50.0±\pm 1.6 a₀); the first value corresponds to the a_1,-1 scattering length usually considered in cold collisions. The extrapolation procedure is also applied to Li₂, using existing experimental data.     

Collisional broadening and shift of Doppler-free two-photon thallium lines perturbed by rare-gas atoms

The extended Omont-Ueda-Kaulakys treatment of collisional effects on quasi-Rydberg states, in which the perturbation of the lower state is taken into account is applied to thallium-rare gas systems. The pressure broadening and shift coefficients of two-photon transitions in thallium involving the 6P_1/2 -n P_1/2,3/2 (n = 9-14) states are calculated and compared with experimental data obtained by Hermann et al. [Eur. Phys. J. D 1, 129 (1998)].     

Angular distributions of light projectile fragments in deep inelastic Pb + Em interactions at 160 A GeV

The nuclear emulsion was exposed at CERN by the lead projectile at 160 A GeV. The angles between any pair of fragments with Z = 2-4 have been measured in the emulsion plane for the events which did not contain heavy fragments. The constant characterizing the normal angle (J) distribution of the fragment momentum projection onto the emulsion plane with respect to initial projectile momentum p₀ is found to be C_J = (0.37 - 0.02) mrad. Corresponding value C₀ = (121 - 6) MeV/c of nucleon momentum distribution in the lead nucleus coincides with that expected from Fermi momentum distribution for this nucleus. The peak in the pair-angle distribution of double-charged fragments, ⁸Be M 2!, is presented for the region of small angles (<0.1 mrad). The fraction of !-particles coming from the decay of the ground state ⁸Be is found to be (13 - 2)% of their whole number.     

Optical tweezers of programmable shape with transverse scattering forces

We propose a non-holographic method to create line traps of arbitrary shape in the sample plane. Setting the phase gradient along theses lines gives control over the transverse forces acting on the confined particles. Phase structures, displayed on a spatial light modulator, are optically processed by a spiral phase filter and imaged onto the object plane of a microscope objective. The resulting bright line structures can be used to trap microparticles. Additionally, they exert transverse scattering forces, which can be exploited for inducing orbital motions or for creating “attracting” or “repelling” points, respectively. We give theoretical and experimental evidence that these scattering forces are proportional to the curvature of the line tweezers.     

Chemical states of oxygen segregated intergranular fracture surfaces of molybdenum

The Auger and electron energy loss spectra (EELS) of a grain boundary fracture plane of bicrystal molybdenum (32 wt.ppm oxygen) are compared with the spectra of pure and oxidized molybdenum. The Auger spectrum of the fracture surface contains molybdenum and oxygen peaks, and the Mo M_4,5NN line coincides with that of the pure metal. The interfacial Auger transition peak is observed on the low energy side of the Mo N_2,3VV Auger peak. Both AES and EELS spectra of the fracture plane are different from those of the oxidized molydenum. These results show that the segregated oxygen is bound to the grain boundary fracture plane as if it were adsorbed.     

The W-boson mass and precision tests of the Standard Model

We have examined the electroweak radiative corrections in the LEP precision data in view of the new measurements of M_W and m_t as well as the recent progress in the higher order radiative corrections. From the minimal L²-fit to the experimental Z-decay parameters (with the aid of a modified ZFITTER program), we predict that M_W=80.29(4)(2)rGeV where the first error is due to the uncertainty in the fitted m_t for a fixed m_H and the second error comes from the m_H in the range 60􊖸rGeV, which is to be compared with the current world average M_W=80.23(18)rGeV. The current world average value of M_W and the 1994 LEP data definitely favor nonvanishing electroweak radiative corrections and are consistent with a heavy m_t as measured by the recent CDF report but with a heavy Higgs scalar of about 400rGeV within the context of the minimal standard model. The sensitivity of and the errors in the best fit solutions due to the uncertainties in the gluonic coupling !_s(M_Z) and !(M_Z) are also studied carefully. In addition we discuss how the future precision measurements of M_W can provide a decisive test for the standard model with radiative corrections and give a profound implication for the measurement of t-quark and Higgs masses.     

The glass transition of thin polymer films: some questions, and a possible answer

A simple and predictive model is put forward explaining the experimentally observed substantial shift of the glass transition temperature, T_g, of sufficiently thin polymer films. It focuses on the limit of small molecular weight, where geometrical `finite size' effects on the chain conformation can be ruled out. The model is based on the idea that the polymer freezes due to memory effects in the viscoelastic eigenmodes of the film, which are affected by the proximity of the boundaries. The elastic modulus of the polymer at the glass transition turns out to be the only fitting parameter. Quantitative agreement is obtained with our experimental results on short chain polystyrene (M_W = 2 kg/mol), as well as with earlier results obtained with larger molecules. Furthermore, the model naturally accounts for the weak dependence of the shift of T_g upon the molecular weight. It explains why supported films must be thinner than free standing ones to yield the same shift, and why the latter depends upon the chemical properties of the substrate. Generalizations for arbitrary experimental geometries are straightforward.     

Spectral line shapes of collision-induced light scattering (CILS) and collision-induced absorption (CIA) using isotropic intermolecular potential for H2–Ar

Quantum mechanical line shapes of collision-induced light scattering at room temperature (295 K) and collision-induced absorption at T = 195 K are computed for gaseous mixtures of molecular hydrogen and argon using theoretical values for pair-polarisability trace and anisotropy and induced dipole moments as input. Comparison with other theoretical spectra of isotropic and anisotropic light scattering and measured spectra of absorption shows satisfactory agreement, for which the uncertainty in measurement of its spectral moments is seen to be large. Ab initio models of the trace and anisotropy polarisability which reproduce the recent spectra of scattering are given. Empirical model of the dipole moment which reproduce the experimental spectra and the first three spectral moments more closely than the fundamental theory are also given. Good agreement between computed and/or experimental line shapes of both absorption and scattering is obtained when the potential model which is constructed from the transport and thermo-physical properties is used.     

用极化晶体谱仪探测X射线极化度

为了研究等离子体各向异性及确定高温等离子体中电子束的存在,研制了X射线极化晶体谱仪,探讨了极化光谱学的理论,推演了X射线极化度的计算方法.谱仪能够在两个相互垂直的方向分别对X射线进行探测,垂直方向晶体分析器使用云母球面晶体,水平方向为PET平面晶体,摄谱元件采用成像板.在激光聚变研究中心2×10 J激光装置上进行了摄谱实验.实验结果表明,PET晶体分光效果理想,获得了铝的类He共振线(w)、磁四级M2跃迁x线、互组合跃迁y线以及禁戒谱线z线,适合于研究X射线极化光谱学;云母晶体得到铝的类He共振线,其伴线光谱不明显.分析了谱仪垂直方向信号微弱的原因,提出了改进极化晶体谱仪的方案.     

A scaling for the c-axis resistivity of La1.86Sr0.14CuO4 as a function of temperature, field and field orientation

The c-axis resistivity, A_c(T,rH,r/), of La_1.86 Sr_0.14CuO₄ is experimentally studied as a function of temperature T, magnetic field H and angle / between H and ab-plane. It is argued that the experimental findings cannot be accounted for by previously considered mechanisms. By contrast, they can be explained by a phenomenological model, which is developed by replacing H in the dissipation model for HÁIÁc with the reduced field h=H(sin²/+cos²//%²)^1/2, where % is the anisotropic parameter in magnetic fields. Based on this phenomenological model, it is shown that all the measured A_c(T,rH,r/) curves could consistently map onto a single curve.