2,845 research outputs found
S-Matrix Poles Close to Thresholds in Confined Geometries
We have studied the behavior of the S-matrix poles near threshold for quantum
waveguides coupled to a cavity with a defect. We emphasize the occurrence of
both dominant and shadow poles on the various sheets of the energy Riemann
surface, and show that the changes of the total conductivity near threshold as
the cavity's width changes can be explained in terms of dominant to shadow pole
transitions.Comment: 10 pages, 5 figure
The effect of pressure on open-framework silicates: elastic behaviour and crystal-fluid interaction
The elastic behaviour and the structural evolution of microporous materials compressed hydrostatically in a pressure-transmitting fluid are drastically affected by the potential crystal-fluid interaction, with a penetration of new molecules through the zeolitic cavities in response to applied pressure. In this manuscript, the principal mechanisms that govern the P-behaviour of zeolites with and without crystal-fluid interaction are described, on the basis of previous experimental findings and computational modelling studies. When no crystal-fluid interaction occurs, the effects of pressure are mainly accommodated by tilting of (quasi-rigid) tetrahedra around O atoms that behave as hinges. Tilting of tetrahedra is the dominant mechanism at low-mid P-regime, whereas distortion and compression of tetrahedra represent the mechanisms which usually dominate the mid-high P regime. One of the most common deformation mechanisms in zeolitic framework is the increase of channels ellipticity. The deformation mechanisms are dictated by the topological configuration of the tetrahedral framework; however, the compressibility of the cavities is controlled by the nature and bonding configuration of the ionic and molecular content, resulting in different unit-cell volume compressibility in isotypic structures. The experimental results pertaining to compression in "penetrating" fluids, and thus with crystal-fluid interaction, showed that not all the zeolites experience a P-induced intrusion of new monoatomic species or molecules from the P-transmitting fluids. For example, zeolites with well-stuffed channels at room conditions (e.g. natural zeolites) tend to hinder the penetration of new species through the zeolitic cavities. Several variables govern the sorption phenomena at high pressure, among those: the "free diameters" of the framework cavities, the chemical nature and the configuration of the extra-framework population, the partial pressure of the penetrating molecule in the fluid (if mixed with other non-penetrating molecules), the rate of P-increase, the surface/volume ratio of the crystallites under investigations and the temperature at which the experiment is conducted. An overview of the intrusion phenomena of monoatomic species (e.g. He, Ar, Kr), small (e.g. H2O, CO2) and complex molecules, along with the P-induced polymerization phenomena (e.g. C2H2, C2H4, C2H6O, C2H6O2, BNH6, electrolytic MgCl2*21H2O solution) is provided, with a discussion of potential technological and geological implications of these experimental findings
On the nature of spatiotemporal light bullets in bulk Kerr media
We present a detailed experimental investigation, which uncovers the nature
of light bullets generated from self-focusing in a bulk dielectric medium with
Kerr nonlinearity in the anomalous group velocity dispersion regime. By high
dynamic range measurements of three-dimensional intensity profiles, we
demonstrate that the light bullets consist of a sharply localized
high-intensity core, which carries the self-compressed pulse and contains
approximately 25% of the total energy, and a ring-shaped spatiotemporal
periphery. Sub-diffractive propagation along with dispersive broadening of the
light bullets in free space after they exit the nonlinear medium indicate a
strong space-time coupling within the bullet. This finding is confirmed by
measurements of spatiotemporal energy density flux that exhibits the same
features as stationary, polychromatic Bessel beam, thus highlighting the
physical nature of the light bullets
Analogue Gravity and ultrashort laser pulse filamentation
Ultrashort laser pulse filaments in dispersive nonlinear Kerr media induce a
moving refractive index perturbation which modifies the space-time geometry as
seen by co-propagating light rays. We study the analogue geometry induced by
the filament and show that one of the most evident features of filamentation,
namely conical emission, may be precisely reconstructed from the geodesics. We
highlight the existence of favorable conditions for the study of analogue black
hole kinematics and Hawking type radiation.Comment: 4 pages, revised versio
Establishing Multi-User MIMO Communications Automatically Using Retrodirective Arrays
Communications in the mmWave and THz bands will be a key technological pillar for next-generation wireless networks. However, the increase in frequency results in an increase in path loss, which must be compensated for by using large antenna arrays. This introduces challenging issues due to power consumption, signalling overhead for channel estimation, hardware complexity, and slow beamforming and beam alignment schemes, which are in contrast with the requirements of next-generation wireless networks. In this paper, we propose the adoption of a retro-directive antenna array (RAA) at the user equipment (UE) side, where the signal sent by the base station (BS) is reflected towards the source after being conjugated and phase-modulated according to the UE data. By making use of modified Power Methods for the computation of the eigenvectors of the resulting round-trip channel, it is shown that, in single and multi-user multiple-input multiple-output (MIMO) scenarios, ultra-low complexity UEs can establish parallel communication links automatically with the BS in a very short time. This is done in a blind way, also by tracking fast channel variations while communicating, without the need for ADC chains at the UE as well as without explicit channel estimation and time-consuming beamforming and beam alignment schemes
A parallel perturbed biharmonic solver
AbstractAn algorithm for the solution of the finite difference approximation of the perturbed biharmonic problem is proposed which consists of both direct and iterative stages. The optimality of this algorithm is proved under the opportune hypothesis on the perturbation
A novel universal primer pair for prokaryotes with improved performances for anammox containing communities
Understanding the social norms of cooling in Chinese offices: Predominance, professionalism, and peer respect
Challenging the international spread of highly energy-consuming air-conditioning technologies, this research explores how cooling demand is supported by the local emergence of relevant social norms. This paper presents the results of an interview study focused on the cooling norms currently found inside corporate offices in two large cities in southern China. Building on existing, mostly quantitative, studies of office cooling in areas of China with hot summers, and with a focus on how social norms shape everyday actions, the accounts of 17 financial sector workers are examined through a thematic analysis. Three themes are discussed: how air conditioning was perceived to dominate, how expectations concerning professional self-image were negotiated, and how respect for colleagues at different levels influenced personal cooling in the workplace. Together they point to the potential of novel strategies for promoting more sustainable office cooling in China and suggest the value of further research on dynamic cooling norms
Energy Dependence of Breakup Cross Sections of Halo Nucleus 8B and Effective Interactions
We study the energy dependence of the cross sections for nucleon removal of
8B projectiles. It is shown that the Glauber model calculations with
nucleon-nucleon t-matrix reproduce well the energy dependence of the breakup
cross sections of 8B. A DWBA model for the breakup cross section is also
proposed and results are compared with those of the Glauber model. We show that
to obtain an agreement between the DWBA calculations, the Glauber formalism,
and the experimental data, it is necessary to modify the energy behavior of the
effective interaction. In particular, the breakup potential has a quite
different energy dependence than the strong absorption potential.Comment: 13 pages, 4 figure
Poly(Alkylene 2,5-thiophenedicarboxylate) polyesters: A new class of bio-based high-performance polymers for sustainable packaging
In the present study, 100% bio-based polyesters of 2,5-thiophenedicarboxylic acid were synthesized via two-stage melt polycondensation using glycols containing 3 to 6 methylene groups. The so-prepared samples were characterised from the molecular point of view and processed into free-standing thin films. Afterward, both the purified powders and the films were subjected to structural and thermal characterisation. In the case of thin films, mechanical response and barrier properties to O2 and CO2 were also evaluated. From the results obtained, it emerged that the length of glycolic sub-units is an effective tool to modulate the chain mobility and, in turn, the kind and amount of ordered phases developed in the samples. In addition to the usual amorphous and 3D crystalline phases, in all the samples investigated it was possible to evidence a further phase characterised by a lower degree of order (mesophase) than the crystalline one, whose amount is strictly related to the glycol sub-unit length. The relative fraction of all these phases is responsible for the different mechanical and barrier performances. Last, but not least, a comparison between thiophene-based homopolymers and their furan-based homologues was carried out
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