1,395 research outputs found
Physically founded phonon dispersions of few-layer materials, and the case of borophene
An increasing number of theoretical calculations on few-layer materials have
been reporting a non-zero sound velocity for all three acoustic phonon modes.
In contrast with these reports, here we show that the lowest phonon dispersion
branch of atomistically described few-layer materials should be quadratic, and
this can have dramatic consequencies on calculated properties, such as the
thermal conductivity. By reformulating the interatomic force constants (IFC) in
terms of internal coordinates, we find that a delicate balance between the IFCs
is responsible for this quadraticity. This balance is hard to obtain in
ab-initio calculations even if all the symmetries are numerically enforced a
posteriori, but it arises naturally in our approach. We demonstrate the
phenomenon in the case of borophene, where a very subtle correction to the
ab-initio IFCs yields the physically correct quadratic dispersion, while
leaving the rest of the spectrum virtually unmodified. Such quadraticity
nevertheless has a major effect on the computed lattice thermal conductivity,
which in the case of borophene changes by more than a factor 2, and reverses
its anisotropy, when the subtle IFC correction is put in place
Spectra: Architecture in Transit
Journeys are not always one-way streets that architects travel towards a distant or dreamed-of reality. It is also possible for those distant, seemingly static places and times to physically move towards viewers. During this passage, architecture is often filtered, taking on the category of an ideal or an icon. These secondary realities become something perhaps even more real and true than what is tangible. It may even make the first type of journey no longer necessary in the normal sense of the word. ----------------- Los viajes no son siempre un mecanismo unidireccional en los que el arquitecto se desplaza hacia una realidad lejana o soñada. También es posible que aquellos lugares y momentos, estáticos, se desplacen “físicamente” hacia el espectador. A menudo, en ese tránsito, la arquitectura se filtra e idealiza, asumiendo la categoría de ideal o icono. Esas realidades se convierten en algo quizá más real y cierto que lo tangible, haciendo incluso innecesario el viaje en el sentido común del término
Clear detection of dusty torus signatures in a Weak-Line Radio Galaxy: the case of PKS 0043-42
We report the clearest detection to date of dusty torus signatures in a
Weak-Line Radio Galaxy (WLRG). The deep Spitzer InfraRed Spectrograph (IRS)
rest-frame mid-infrared (MIR) spectrum of the WLRG PKS 0043-42 (z=0.116) shows
a clear spectral turnover at wavelengths longer than ~20 micron suggestive of
warm dust, as well as a 9.7 micron silicate absorption feature. In addition,
the hard X-ray results, based on Chandra data, strongly support a picture in
which PKS 0043-42 has a torus and accretion disc more typical of Strong-Line
Radio Galaxies (SLRGs). The MIR and X-ray spectra are markedly different from
those of other WLRGs at similar redshifts, and here we show that the former can
be successfully fitted with clumpy torus models with parameters characteristic
of Type-2 AGN tori: close to edge-on (i=74 deg) and relatively broad (torus
angular width=60 deg), with an outer radius of 2 pc, hydrogen column density
~1.6x10^(23) cm^(-2), and AGN bolometric luminosity ~1.6x10^(44) erg s^(-1).
The presence of a compact torus in PKS 0043-42 provides evidence that this WLRG
is fuelled by cold, rather than hot, gas accretion. We suggest that WLRGs are a
diverse population, and PKS 0043-42 may represent a type of radio galaxy in
which the AGN activity has been recently re-triggered as a consequence of
intermittent gas supply, or in which the covering factor of the Narrow-Line
Region (NLR) clouds is relatively low.Comment: 7 pages, 6 figures, 1 table. Accepted by MNRA
Vibrational inelastic scattering effects in molecular electronics
We describe how to treat the interaction of travelling electrons with
localised vibrational modes in nanojunctions. We present a multichannel
scattering technique which can be applied to calculate the transport properties
for realistic systems, and show how it is related to other methods that are
useful in particular cases. We apply our technique to describe recent
experiments on the conductance through molecular junctions.Comment: LaTeX, 12 pages, 3 figure
El concurso del Colegio Mayor 'Nuestra Señora de Luján' de 1964. Política, viajes y encuentros, dibujos
En septiembre de 1963 el Ministerio de Cultura y Educación de la República Argentina, con el patrocinio de la Sociedad Central de Arquitectos convocaba un concurso de anteproyectos para la construcción en la Ciudad Universitaria de Madrid de un Colegio Mayor para alojar a becarios e investigadores argentinos. El texto desvela las motivaciones políticas que existieron en el trasfondo de la convocatoria y traza los viajes, encuentros y dibujos que el matrimonio de arquitectos ganadores, Horacio Baliero y Carmen Córdova, hicieron por España y cómo estos viajes, reflejados en los dibujos de Baliero, propiciaron modificaciones del proyecto original
¿abstracción y arquitectura = esencia ≠ forma?
El texto recoge una reflexión de los autores para el Campus Ultzama Internacional 2011 LLegadas/Partidas, donde se debatió acerca de la idea de abstracción en la arquitectura
Surface roughness and thermal conductivity of semiconductor nanowires: going below the Casimir limit
By explicitly considering surface roughness at the atomic level, we
quantitatively show that the thermal conductivity of Si nanowires can be lower
than Casimir's classical limit. However, this violation only occurs for deep
surface degradation. For shallow surface roughness, the Casimir formula is
shown to yield a good approximation to the phonon mean free paths and
conductivity, even for nanowire diameters as thin as 2.22 nm. Our exact
treatment of roughness scattering is in stark contrast with a previously
proposed perturbative approach, which is found to overpredict scattering rates
by an order of magnitude. The obtained results suggest that a complete
theoretical understanding of some previously published experimental results is
still lacking.Comment: 11 pages, 4 figure
Triple-Band Concurrent Reconfigurable Matching Network
Reconfigurable Matching Networks (RMN) have found a wide range of applications, such as antenna impedance matching (Antenna Tuning Units -ATU-), the design of reconfigurable power amplifiers, applications in Magnetic Resonance Imaging (MRI), adjustable low noise amplifier design, etc. In this paper, we propose the experimental design and verification of a reconfigurable impedance synthesis network that can simultaneously work in three different bands and is completely independent so that the impedance variations in a frequency band are approximately transparent to the rest. The variable elements used in this paper are varactors. To verify its operation, it is applied to a process of matching a laser modulator in three different frequency bands for C-RAN (Cloud Radio Access Networks) applications. Experimental results demonstrate, as expected, that losses may depend on the state in which they are driven. Consequently, a state that can guarantee a good match could also imply greater losses, leading to a certain trade-off. The application of genetic algorithms in this context points out that it may be convenient to optimize the insertion losses of the complete chain instead of the return losses
Heat conductance is strongly anisotropic for pristine silicon nanowires
We compute atomistically the heat conductance for ultra-thin pristine silicon
nanowires (SiNWs) with diameters ranging from 1 to 5 nm. The room temperature
thermal conductance is found to be highly anisotropic: wires oriented along the
direction have 50-75% larger conductance than wires oriented along the
and directions. We show that the anisotropies can be qualitatively
understood and reproduced from the bulk phonon band structure. Ab initio
density functional theory (DFT) is used to study the thinnest wires, but
becomes computationally prohibitive for larger diameters, where we instead use
the Tersoff empirical potential model (TEP). For the smallest wires, the
thermal conductances obtained from DFT- and TEP calculations agree within 10%.
The presented results could be relevant for future phonon-engineering of
nanowire devices.Comment: 7 pages, 5 figure
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