Anisotropy in the dielectric function of Bi2_2Te3_3 from first principles: From the UV-visible to the infrared range

The dielectric properties of Bi$_2$Te$_3$, 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

Coherent acoustic phonons generated by ultrashort terahertz pulses in nanofilms of metals and topological insulators

We report the generation of coherent acoustic phonons in materials with terahertz ultrashort pulses. This is demonstrated in metals and topological insulators by exciting an acoustic eigenmode in nanometric-sized thin films. The efficiency of the coupling is quadratic in the terahertz electric field strength within the range of investigation. Owing to a quantitative comparison between terahertz and near-infrared ultrashort pulse excitations, we show that the process of acoustic phonon generation by terahertz radiation is mainly driven by thermoelastic stress. While for the near-infrared light excitation the lattice temperature increase comes from a rapid energy transfer from the hot carriers to the phonon bath during carrier intraband relaxation, the thermoelastic stress induced by the terahertz electric field is linked to the scattering of the accelerated electrons leading to an ultrafast Joule effect

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 ($a$-GeTe) and crystalline ($\alpha$-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

Mechanical behaviour of metal-ceramic bonds

The mechanical behaviour of ceramics bonded with metal is analyzed from the mechanical point of view (interfacial fracture mechanics, applied and residual stress field) and from the physico-chemical point of view taking into account the embrittlement induced by interfacial reactions or diffusion of metal into ceramic and the damages due to the change in interfacial microstructure (segregation, interfacial phases or films, microcracks, ...) during bonding or life. Indeed the behaviour of metal-ceramic bonds is inevitably dependent on the synthesis of all those parameters.Le comportement mécanique des liaisons céramique-métal est analysé du point de vue mécanique (mécanique de la rupture interfaciale, champs des contraintes appliquées et résiduelles) et, du point de vue physicochimique, en tenant compte des effets de fragilisation induits par les réactions interfaciales ou la diffusion du métal dans la céramique et des endommagements dus aux modifications de la microstructure interfaciale (segrégation, phases et films interfaciaux, microfissures ... ) intervenant lors de l'assemblage ou du service. En effet le comportement des assemblages métal-céramique dépend obligatoirement de la synthèse de tous ces paramètres

On the appearance of caustics for plane sound-wave propagation in moving random media

International audienc