60 research outputs found
Anisotropy in the dielectric function of BiTe from first principles: From the UV-visible to the infrared range
The dielectric properties of BiTe, a layered compound crystallizing
in a rhombohedral structure, are investigated by means of first-principles
calculations at the random phase approximation level. A special attention is
devoted to the anisotropy in the dielectric function and to the local field
effects that strongly renormalize the optical properties in the UV-visible
range when the electric field is polarized along the stacking axis.
Furthermore, both the Born effective charges for each atom and the zone center
phonon frequencies and eigenvectors needed to describe the dielectric response
in the infrared range are computed. Our theoretical near-normal incidence
reflectivity spectras in both the UV-visible and infrared range are in fairly
good agreement with the experimental spectras, provided that the free carriers
Drude contribution arising from defects is included in the infrared response.
The anisotropic plasmon frequencies entering the Drude model are computed
within the rigid band approximation, suggesting that a measurement of the
reflectivity in the infrared range for both polarizations might allow to infer
not only the type of doping but also the level of doping.Comment: 29 pages, 10 figure
Non-thermal transport of energy driven by photoexcited carriers in switchable solid states of GeTe
Phase change alloys have seen widespread use from rewritable optical discs to
the present day interest in their use in emerging neuromorphic computing
architectures. In spite of this enormous commercial interest, the physics of
carriers in these materials is still not fully understood. Here, we describe
the time and space dependence of the coupling between photoexcited carriers and
the lattice in both the amorphous and crystalline states of one phase change
material, GeTe. We study this using a time-resolved optical technique called
picosecond acoustic method to investigate the \textit{in situ} thermally
assisted amorphous to crystalline phase transformation in GeTe. Our work
reveals a clear evolution of the electron-phonon coupling during the phase
transformation as the spectra of photoexcited acoustic phonons in the amorphous
(-GeTe) and crystalline (-GeTe) phases are different. In particular
and surprisingly, our analysis of the photoinduced acoustic pulse duration in
crystalline GeTe suggests that a part of the energy deposited during the
photoexcitation process takes place over a distance that clearly exceeds that
defined by the pump light skin depth. In the opposite, the lattice
photoexcitation process remains localized within that skin depth in the
amorphous state. We then demonstrate that this is due to supersonic diffusion
of photoexcited electron-hole plasma in the crystalline state. Consequently
these findings prove the existence of a non-thermal transport of energy which
is much faster than lattice heat diffusion
Electron beam charging of insulators: A self-consistent flight-drift model
International audienceElectron beam irradiation and the self-consistent charge transport in bulk insulating samples are described by means of a new flight-drift model and an iterative computer simulation. Ballistic secondary electron and hole transport is followed by electron and hole drifts, their possible recombination and/or trapping in shallow and deep traps. The trap capture cross sections are the Poole-Frenkel-type temperature and field dependent. As a main result the spatial distributions of currents j(x,t), charges, the field F(x,t) and the potential slope V(x,t) are obtained in a self-consistent procedure as well as the time-dependent secondary electron emission rate sigma(t) and the surface potential V0(t) For bulk insulating samples the time-dependent distributions approach the final stationary state with j(x,t)=const=0 and sigma=1. Especially for low electron beam energies E0=4 keV the incorporation of mainly positive charges can be controlled by the potential VG of a vacuum grid in front of the target surface. For high beam energies E0=10, 20, and 30 keV high negative surface potentials V0=−4, −14, and −24 kV are obtained, respectively. Besides open nonconductive samples also positive ion-covered samples and targets with a conducting and grounded layer (metal or carbon) on the surface have been considered as used in environmental scanning electron microscopy and common SEM in order to prevent charging. Indeed, the potential distributions V(x) are considerably small in magnitude and do not affect the incident electron beam neither by retarding field effects in front of the surface nor within the bulk insulating sample. Thus the spatial scattering and excitation distributions are almost not affected
Direct Observation of Propagating Gigahertz Coherent Guided Acoustic Phonons in Free Standing Single Copper Nanowires
We report on gigahertz acoustic phonon waveguiding in free-standing single
copper nanowires studied by femtosecond transient reflectivity measurements.
The results are discussed on the basis of the semianalytical resolution of the
Pochhammer and Chree equation. The spreading of the generated Gaussian wave
packet of two different modes is derived analytically and compared with the
observed oscillations of the sample reflectivity. These experiments provide a
unique way to independently obtain geometrical and material characterization.
This direct observation of coherent guided acoustic phonons in a single
nano-object is also the first step toward nanolateral size acoustic transducer
and comprehensive studies of the thermal properties of nanowires
Phonons in Slow Motion: Dispersion Relations in Ultra-Thin Si Membranes
We report the changes in dispersion relations of hypersonic acoustic phonons
in free-standing silicon membranes as thin as \sim 8 nm. We observe a reduction
of the phase and group velocities of the fundamental flexural mode by more than
one order of magnitude compared to bulk values. The modification of the
dispersion relation in nanostructures has important consequences for noise
control in nano and micro-electromechanical systems (MEMS/NEMS) as well as
opto-mechanical devices.Comment: 5 page
Model d'atenció integrada al pacient crònic: insuficiència cardíaca (IC), malaltia pulmonar obstructiva crònica (MPOC)
Atenció al pacient crònic; Insuficiència cardíaca; Malaltia pulmonar obstructiva crònicaAtención al paciente crónico; Insuficiencia cardíaca; Enfermedad pulmonar obstructiva crónicaChronic patient care; Heart failure; Chronic obstructive pulmonary diseaseAquest document pretén establir un marc d’actuació per a la transformació a mitjà termini de l’atenció que es presta als i les pacients crònics des de l’Institut Català de la Salut (ICS), però tenint en compte que molts dels canvis necessaris només són possibles si s’estableixen els mecanismes de col·laboració adequats entre tots els actors del sistema, específicament, si s’estableixen els incentius per tal de garantir-ne la continuïtat, i si evolucionem cap a un model d’atenció que curi i tingui cura dels i les pacients, i alhora serveixi també per reduir les hospitalitzacions innecessàries. L’abordatge s’ha fet escollint la insuficiència cardíaca (IC) i la malaltia pulmonar obstructiva crònica (MPOC), perquè són dues de les malalties que tenen un major impacte en dependència, discapacitat, qualitat de vida i anys de vida perduts al nostre país, i també, perquè concentren una part molt important del cost de la cronicitat
On the appearance of caustics for plane sound-wave propagation in moving random media
International audienc
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