Anti de Sitter Quantum Field Theory and a New Class of Hypergeometric Identities

We use Anti-de Sitter quantum field theory to prove a new class of identities between hypergeometric functions related to the Källén-Lehmann representation of products of two Anti-de Sitter two-point functions. A rich mathematical structure emerges. We apply our results to study the decay of unstable Anti-de Sitter particles. The total amplitude is in this case finite and Anti-de Sitter invariant.     

Orbital-decomposed electronic and magnetic properties of the double perovskite Sr2FeReO6

The detailed orbital-decomposed electronic structures and magnetic properties of the double perovskite Sr₂FeReO₆ have been studied using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation (GGA). Both occupied and unoccupied s and three p states of Fe³⁺ ion are located far away from the Fermi level, while all up-spin states and most down-spin states are completely filled for the s and three p states of Re⁵⁺ ion. The octahedral crystal field of the oxygen atoms around transition-metal (TM) sites splits the five-fold degenerate d states of the free TM atoms into triply degenerate t_2g states with smaller bonding-antibonding splitting and doubly degenerate e_g states with larger bonding-antibonding splitting. The Fe³⁺ and Re⁵⁺ ions are in the states (3d⁵, S=5/2) and (5d², S=1) with magnetic moments 3.70 and −0.86μ_B, respectively and thus antiferromagnetic coupling via oxygen between them. There are no direct interactions between two nearest Fe-Fe or Re-Re pairs, whereas along each Fe-O-Re-O-Fe or Re-O-Fe-O-Re chains, the hybridizations between Fe 3d and 4s, O 2s and 2p, as well as Re 5p, 5d and 6s orbitals are fairly significant.     

Ultrabroadband conical emission generated from the ultraviolet up to the far-infrared during the optical filamentation in air

Ultraviolet and infrared conical emissions were observed during the filamentation in air of powerful femtosecond laser pulses produced by a portable terawatt laser system. The broadband spectrum was measured from 200 nm up to 14 microm and covered the complete optical transmission window of the atmosphere. The angularly resolved spectrum showed some X-wave structure across the frequency range analyzed. However, we demonstrated that the strong conical emission observed in the mid- and far-infrared is mainly owing to the four-wave mixing between the pump pulse and its blueshifted conical emission.     

Modeling the sound transmission between rooms coupled through partition walls by using a diffusion model

In this paper, a modification of the diffusion model for room acoustics is proposed to account for sound transmission between two rooms, a source room and an adjacent room, which are coupled through a partition wall. A system of two diffusion equations, one for each room, together with a set of two boundary conditions, one for the partition wall and one for the other walls of a room, is obtained and numerically solved. The modified diffusion model is validated by numerical comparisons with the statistical theory for several coupled-room configurations by varying the coupling area surface, the absorption coefficient of each room, and the volume of the adjacent room. An experimental comparison is also carried out for two coupled classrooms. The modified diffusion model results agree very well with both the statistical theory and the experimental data. The diffusion model can then be used as an alternative to the statistical theory, especially when the statistical theory is not applicable, that is, when the reverberant sound field is not diffuse. Moreover, the diffusion model allows the prediction of the spatial distribution of sound energy within each coupled room, while the statistical theory gives only one sound level for each room.     

Variety of available coordination sites for extra-framework iron in LTA and MFI zeolites

Variations in coordination states of extra-framework iron are studied in low iron content ferrisilicates (Si/⁵⁷Fe ≈ 200) during various in situ treatments. In Fe-LTA complete Fe³⁺ ? Fe^2?+? reversibility is observed. In Fe-MFI extra-framework iron can be stabilized in Fe^2?+? state in spite of ambient oxidizing conditions (N₂O, 620 K). Further, in Fe-MFI simultaneous stabilization of Fe^2?+? and Fe^3?+? may take place providing centres for redox catalytic processes.     

Reciprocal space constraints create real-space anomalies in doped carbon nanotubes

When graphite is doped with electrons, carbon-carbon bonds lengthen and Raman-active phonons soften as antibonding states fill. However, in semiconducting carbon nanotubes, one Raman-active G-band mode increases in frequency at low doping levels. We show how phase constraints on the conduction-band wave function expose a latent bonding character in the conduction band of certain nanotubes. In these tubes, filling the lowest conduction band shortens the axial bonds even as it lengthens the circumferential bonds. The A{1}{LO} phonon, which preferentially stretches the axial bonds, then hardens even as the other phonons soften. Quantum confinement eliminates the angular averaging taken for granted in higher-dimensional systems and develops a new class of states, neither bonding nor antibonding, whose character depends on the angular orientation of the bonds in question.     

Enhanced quantum well infrared photodetector focal plane arrays for space applications

A 30 months European Space Agency project started in March 2008, whose overall purpose is to expand and assess the performance of broadband (11–15 μm) quantum detectors for spectro-imaging applications: Dispersive Spectrometers and Fourier Transform Spectrometers. We present here the technical requirements, the development approach chosen as well as preliminary experimental results. Our approach is fully compatible with the final array format (1024 × 256, pitch 50–60 μm). We expect the requested uniformity, operability and SNR levels to be achieved at temperatures close to the goal values. The performance level will be demonstrated on 256 × 256, 50 μm pitch arrays. Also, operability and uniformity issues will be addressed on large mechanical 1024 × 256 hybrid arrays.     

Dynamic response of converse magnetoelectric effect in a PMN-PT-based multiferroic heterostructure

A multiferroic heterostructure, consisting of a 25 μm thick Metglas® ribbon affixed to a lead magnesium niobate–lead titanate (PMN-PT) crystal, was systemically studied to investigate the time response of converse magnetoelectric coupling under the application of electric fields at low frequencies (0.05<f<10 Hz). This multiferroic heterostructure exhibits a considerably strong converse magnetoelectric effect, CME=?80%, where CME=[M(E)?M(0)]/M(0), and a converse ME coupling constant, A=22.5 Oe-cm/kV, at frequencies below 1 Hz and near saturation electric polarization. A switching time (t _s), representing the response time of the CME coupling, is measured to be 0.6 seconds for this heterostructure under the application of instantaneous electric fields. The switching time results in significant influences on the magnetoelectric effect especially at frequencies higher than 2 Hz. The dynamic response of CME coupling is predominantly determined by ferroelectric relaxation within the PMN-PT crystal, as opposed to the magnetic relaxation of the Metglas® ribbon. A model was used to describe the dynamic behavior of CME coupling in disordered systems such as PMN-PT.     

Nearest-neighbor configuration in (GaIn)(NAs) probed by x-ray absorption spectroscopy

Ga(1-x)In(x)N(y)As(1-y) is a promising material system for the fabrication of inexpensive "last-mile" optoelectronic components. However, details of its atomic arrangement and the relationship to observed optical properties is not fully known. Particularly, a blueshift of emission wavelength is observed after annealing. In this work, we use x-ray absorption fine structure to study the chemical environment around N atoms in the material before and after annealing. We find that as-grown molecular beam epitaxy material consists of a nearly random distribution of atoms, while postannealed material shows segregation of In toward N. Ab initio simulations show that this short-range ordering creates a more thermodynamically stable alloy and is responsible for blueshifting the emission.