Structural and optical properties of thermally evaporated thin films

Chalcogenide glass Se₅₅Ge₃₀As₁₅ have amorphous structure in both as-deposited and annealed conditions. The optical properties of the as-deposited and annealed films were studied using spectrophotometric measurements of transmittance, T(λ), and reflectance, R(λ), at normal incidence of light in the wavelength range 200–2500 nm. Neither annealing temperature nor film thickness can influence spectral response on refractive index and absorption index of films. The type of electronic transition responsible for optical properties is indirectly allowed transition with energy gap of 1.94 eV and phonon energy of 40 meV. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple–Didomenico (WD) model. The width of band tails of localized states into the gap (ΔE), the single oscillator energy (E_o), the dispersion energy (E_d), the optical dielectric constant (ε_∞), the lattice dielectric constant (ε_L), the plasma frequency (ω_p) and the free charge carrier concentration (N) were estimated.     

Recurrence relations for polynomial sequences via Riordan matrices

We give recurrence relations for any family of generalized Appell polynomials unifying so some known recurrences for many classical sequences of polynomials. Our main tool to get our goal is the Riordan group. We use the product of Riordan matrices to interpret some relationships between different polynomial families. Moreover using the Hadamard product of series we get a general recurrence relation for the polynomial sequences associated to the so called generalized umbral calculus.     

SAMBADENA Hyperpolarization of 13C-Succinate in an MRI: Singlet-Triplet Mixing Causes Polarization Loss

The signal enhancement provided by the hyperpolarization of nuclear spins of biological molecules is a highly promising technique for diagnostic imaging. To date, most ¹³C-contrast agents had to be polarized in an extra, complex or cost intensive polarizer. Recently, the in situ hyperpolarization of a ¹³C contrast agent to >20 % was demonstrated without a polarizer but within the bore of an MRI system. This approach addresses some of the challenges of MRI with hyperpolarized tracers, i. e. elevated cost, long production times, and loss of polarization during transfer to the detection site. Here, we demonstrate the first hyperpolarization of a biomolecule in aqueous solution in the bore of an MRI at field strength of 7 T within seconds. The ¹³C nucleus of 1-¹³C, 2,3-²H₂-succinate was polarized to 11 % corresponding to a signal enhancement of approximately 18.000. Interesting effects during the process of the hydrogenation reaction which lead to a significant loss of polarization have been observed.     

Singularity formation in general relativistic dynamics of homogeneous scalar fields

Collapsing dynamics of a wide class of self-interacting, self-gravitating homogeneous scalar field models is analyzed. The assumptions made on the potential satisfy some general conditions allowing to show that the generic evolution is divergent in a finite time. Combining results shown here with the ones from [R. Giambó, F. Giannoni, G. Magli, J. Math. Phys. 49 (2008) 042504], dealing with sub-exponential growing potentials, allows us to obtain the same results of singularity formation for more general potentials. Moreover it turns out that these models can be completed to find radiating collapsing star models of the Vaidya type, where blackholes are generically formed.     

Optimal Eigenvalue Estimate for the Dirac–Witten Operator on Bounded Domains with Smooth Boundary

Eigenvalue estimate for the Dirac–Witten operator is given on bounded domains (with smooth boundary) of spacelike spin hypersurfaces satisfying the dominant energy condition, under four natural boundary conditions (MIT, APS, modified APS and chiral conditions). Roughly speaking, any eigenvalue of the Dirac–Witten operator satisfies

Towards physically motivated proofs of the Poincaré and geometrization conjectures

Although the Poincaré and the geometrization conjectures were recently proved by Perelman, the proof relies heavily on properties of the Ricci flow previously investigated in great detail by Hamilton. Physical realization of such a flow can be found, for instance, in the work by Friedan [D. Friedan, Nonlinear models in 2+ε$2+\epsilon$ dimensions, Ann. Phys. 163 (1985) 318–419]. In his work the renormalization group flow for a nonlinear sigma model in 2+ε$2+\epsilon$ dimensions was obtained and studied. For ε=0$\epsilon =0$, by approximating the β$\beta$-function for such a flow by the lowest order terms in the sigma model coupling constant, the equations for Ricci flow are obtained. In view of such an approximation, the existence of this type of flow in Nature is questionable. In this work, we find totally independent justification for the existence of Ricci flows in Nature. This is achieved by developing a new formalism extending the results of two-dimensional conformal field theories (CFT’s) to three and higher dimensions. Equations describing critical dynamics of these CFT’s are examples of the Yamabe and Ricci flows realizable in Nature. Although in the original works by Perelman some physically motivated arguments can be found, their role in his proof remain rather obscure. In this paper, steps are made toward making these arguments more explicit, thus creating an opportunity for developing alternative, more physically motivated, proofs of the Poincaré and geometrization conjectures.     

Aluminium adsorption on Ir(1 1 1) at a quarter monolayer coverage: A first-principles study

We present an ab initio density-functional study for aluminium adsorption on Ir(1 1 1) at high symmetry sites, namely, the fcc-, hcp-hollow, top and bridge sites. In each case, we calculate the atomic geometry, average binding energy, work function, and surface dipole moment at the coverage of 0.25 monolayer. We find the favourable structure to be Al at threefold hcp-hollow site, with a corresponding binding energy of 4.46 eV. We present and compare the electronic properties of the two lowest energy structures, i.e., at the threefold hollow sites and discuss the nature of the Al-Ir bond and binding site preference. In particular, we observe a large hybridization of Al-3s, 3p and Ir-5d states near Fermi level, forming an inter-metallic bonds. This results in a significant electron transfer from the Al atoms to the Ir(1 1 1) substrate, inducing an outward pointing surface dipole moment and a large decrease in the work function of 1.69 eV for Al in the hcp-hollow site. Compared to the fcc-hollow site, adsorption in the hcp-hollow site results in a lower density-of-states at the Fermi level, as well as a greater hybridization in the bonding states.     

Affine conformal vector fields in semi-Riemannian manifolds

This paper is devoted to a systematic presentation of the essential results of research on affine conformal vector fields (ACV) and to exhibit the state of art as it now stands. Of particular interest is the new information on the existence of ACVs in compact orientable semi-Riemannian manifolds, their link with first integrals of the geodesics and the separability structures.     

Variational principle for the Finslerian extension of general relativity

The Lagrangian density for formulating the Finslerian gravitational field equations is constructed by replacing the tangent vectors entering a direction-dependent density by the auxiliary vector field. The Lagrangian derivative is represented in terms of the tensor densities associated with an initial direction-dependent density. A particular case, where the direction-dependent density is chosen in the form of the contraction of the FinslerianK-tensor of curvature multiplied by the Jacobian, is treated in detail.     

Geometry of standard constraints and Weil triviality in supersymmetric gauge theories

By using global geometric constructions on superfibre bundles, we provide a geometric interpretation of the standard constraints in supersymmetric gauge theories together with a proof of Weil triviality that holds for arbitrary superspace topologies.On leave of absence from Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Mostra d'Oltremare Pad. 19, I-80125 Naples, Italy.