The technological interest in a material depends very much on its electrical, magnetic, optical and/or mechanical properties. In carbon nanotubes the atoms form a cylindrical structure with a diameter of the order 1 nm, but the nanotubes can be up to several hundred micrometers in length. This makes carbon nanotubes a remarkable model for one-dimensional systems. A lot of efforts have been dedicated to manufacturing carbon nanotubes, which is expected to be the material for the next generation of devices. Despite all the attention that carbon nanotubes have received from the scientific community, only rather limited progress has been made in the theoretical understanding of their physical properties. In this work, we attempt to provide an understanding of the electron-phonon and electron-defect interactions in semiconducting zigzag carbon nanotubes using a tight-binding approach. The electronic energy dispersion relations are calculated by applying the zone-folding technique to the dispersion relations of graphene. A fourth-nearest-neighbour force constant model has been applied to study the vibrational modes in the carbon nanotubes. Both the electron-phonon interaction and the electron-defect interaction are formulated within the tight-binding approximation, and analyzed in terms of their quantum mechanical scattering rates. Apart from the scattering rates, their components in terms of phonon absorption, phonon emission, backscattering and forward scattering have been determined and analyzed. The scattering rates for (5,0), (7,0), (10,0), (13,0) and (25,0) carbon nanotubes at room temperature and at 10K are presented and discussed. The phonon scattering rate is dependent on the lattice temperature in the interval 0-0.17 eV. We find that backscattering and phonon emission are dominant over forward scattering and phonon absorption in most of the energy interval. However, forward scattering and phonon absorption can be comparable to backscattering and phonon emission in limited energy intervals. The phonon modes associated with each peak in the electron-phonon scattering rates have been identified, and the similarities in the phonon scattering rates between different nanotubes are discussed. The dependence of the defect scattering rate on the tube diameter is similar to that of the phonon scattering rate. Both the phonon and the defect scattering rates show strong dependence on the tube diameter (i.e., the scattering rate decreases as a function of the index of the nanotube). It is observed that the backscattering and forward scattering for electrons interacting with defects occur with same frequency at all energies, in sharp contrast to the situation for phonon scattering. It is demonstrated that the differences in the scattering rate between different tubes are mainly due to the differences in their band structures.

The rapid growth in the number of large wind farms has raised serious concerns about their effects on existing radar systems. The large size and rotational movement of the turbine blades can give rise to significant Doppler clutters, which interfere with the detection of moving targets such as aircraft and storms. A previous Air Force study has collected and analyzed the time-varying radar cross section resulting from the blade rotation of a single 1.5 MW turbine. However, multiple interactions taking place in a turbine were not studied in detail. Multiple interactions could play an important role in the propagation of radar signals through wind farms. This thesis sets out to more closely examine the various Doppler features resulting from the scattering due to a single turbine. Backscattered and forward scattered data are measured at Ku-band from various wind turbine models using a motorized turntable in the laboratory. The tested models include a 1:160 scale model turbine, a 3-arm wire model turbine, and a small wind turbine from Bergey Windpower with 2’ blades. The data are processed based on the short-time Fourier transform in order to relate the resulting time-varying Doppler features to various scattering mechanisms. The experimental findings are corroborated by simulations performed using the Numerical Electromagnetics Code (NEC). Furthermore, we propose a post-processing general method to reduce the intensity of the turbine scattered data. This method is applied to filter out simulated Doppler clutter from two different simulation techniques. First, the method is applied to remove the simulated Doppler clutter from the point scatterer model. Next, the algorithm is applied to simulated backscattered data generated using a high-frequency ray tracing code, Ahilo. / text

Le composé SmB6 est un exemple typique de la physique étrange qui peut résulter d'un équilibre de valence. La première configuration du Samarium (Sm2+) correspond à un état isolant non magnétique alors que la seconde (Sm3+) donnerait théoriquement un état magnétique et conducteur. Des mesures de microcalorimétrie sous pression ont permis d'établir le diagramme de phase magnétique de SmB6 : une nouvelle phase magnétique ordonnée à longue distance a été mise en évidence pour des pressions supérieures à 10GPa. D'un autre coté, des mesures de transport réalisées sous pression hydrostatique permettent de situer la transition isolant-métal pour la même pression. Le diagramme de phase sous pression est désormais bien établi et l'observation pour la première fois d'une anomalie magnétique dans les courbes de résistivité au delà de 10GPa permet d'affirmer que la coïncidence des deux phénomènes a bien lieu. Ce changement de comportement du système est discuté dans un nouveau cadre théorique prenant en compte la température Kondo du réseau comme paramètre clef pour la renormalisation de la fonction d'onde vers l'une ou l'autre configuration de valence entière alors que la valence mesurée est toujours nettement intermédiaire. Cette idée générale semble par ailleurs s'appliquer à d'autres composés à valence intermédiaire étudiés (SmS, TmSe). En parallèle, des mesures de résistivité sous contraintes uniaxiales ont été réalisées. Elles mettent en évidence un fort effet d'anisotropie dans le composé SmB6. La comparaison de ces expériences avec les résultats obtenus dans des conditions de pression très hydrostatiques permettent de revisiter le problème de la nature du gap de SmB6.