Thesis (Ph. D.)--University of New South Wales, 2003. / Also available online.

Thesis (Ph. D.)--Virginia Commonwealth University, 2007. / Prepared for: School of Engineering. Bibliography: leaves 170-185. Also available online via the Internet.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2005. / Includes bibliographical references (leaves 155-176). Also available in electronic version.

Thesis (Ph.D.)--Brown University, 2005. / Vita. Thesis advisor: J. M. Xu. Includes bibliographical references (leaves 112-121). Also available online.

No description available.

Compte tenu de considérations environnementales les matériaux multiferroïques sans plomb de type BixTR1-xFeO3 (TR :La, Sm, Dy) constituent une alternative de choix au PbZrxTi1-xO3 (PZT) pour l'utilisation dans des systèmes transducteurs. En effet, il a récemment été observé, dans ces composés, d'importantes propriétés piézoélectriques en relation avec une zone de phase morphotropique (ZPM) qui agirait comme un pont structural entre la phase R3c et la phase Pnma. Cette ZPM présente une variété de structures (adaptive, antiferroélectrique, incommensurable) et de propriétés encore largement incomprises. Notre approche pour mieux comprendre ces systèmes repose sur la synthèse et l'étude de super-réseaux à base de BiFeO3 et LaFeO3 (BFO/LFO). Les principales étapes de ce travail ont donc porté sur l'élaboration par ablation laser et la caractérisation structurale (par diffraction de rayons X, microscopie électronique en transmission et spectroscopie Raman) de super-réseaux de BFO(1-x)Λ/ LFOxΛ et de BFO0,5Λ/LFO0,5Λ. L'étude à température ambiante sur ces super-réseaux a démontré l'existence d’une structure antiferroélectrique de type PbZrO3 dans les couches de BFO. Cette phase anti-polaire présente un domaine de stabilité bien précis puisque une modification structurale vers une phase non polaire de type Pnma a pu être mise en évidence via la modification des proportions relatives dans la période ou du nombre d'interfaces. Les études en température ont par ailleurs démontré une transition de phase structurale de la phase anti-polaire type PbZrO3 vers une phase paraélectrique de type Pnma à haute température. Une modulation importante de la Tc en fonction de l'épaisseur de BFO dans la période montre le rôle important des contraintes. Cependant l'existence de contraintes seules ne peut expliquer cette phase antiferroélectrique. La compatibilité des systèmes de tilt et de rotation des octaèdres d'oxygènes aux interfaces des super-réseaux peut expliquer l'apparition de cette phase / Due to environmental issue the lead free multiferroic system Bi1-xTRxFeO3 (TR :La, Sm, Dy) constitute an alternative to PbZrxTi1-xO3 (PZT) widely used in transducers. Indeed, it was recently observed in these compounds an important piezoelectric properties linked to a morphotropic phase boundary (MPB), which acts like a bridge between R3c and Pnma phases. This MPB shows a variety of structure (adaptive, incommensurate, antiferroelectric) and properties that are not understood yet. Our approach to better understand this system lies on the growth and characterization of superlattices based on BiFeO3 and LaFeO3 (BFO/LFO). The principal steps of this work were the growth by pulsed laser deposition and the structural characterization (X-ray diffraction, transmission electron microscopy, Raman spectroscopy) of BFO(1-x)Λ/LFOxΛ and BFO0,5Λ/LFO0,5Λ superlattices. Room temperature structural investigations reveal the existence of PbZrO3 like anti-polar ordering in BFO layers. The anti-polar order presents specific domain stability and an induced structural modification toward a Pnma state is observed depending on the relative ratio x in the period and the number of interfaces. Temperature-dependent investigations demonstrate a structural phase transition from the PbZrO3 like anti-polar order to the Pnma like paraelectric structure at high temperature. An observed scaling of the Tc with the BFO thickness highlights the role of the strain. However strain only cannot explain the observed antiferroelectric like state and oxygen octahedral tilt compatibility at the interfaces of the superlattices must be taken into account.

Quantum superlattice with a narrow energy band is an artificial semiconductor structure demonstrating both nonlinear and active high-frequency electromagnetic properties. These types of superlattices are used as key elements in various miniature electronic devices including frequency multipliers and quantum cascade lasers. Interaction between terahertz radiation and magnetic field in semiconductor superlattices has been the subject of growing research interest, both theoretical and experimental. In this thesis, we study the nonlinear dynamics of electrons in minibands of the semiconductor superlattices subjected to a terahertz electric field and a magnetic field. Electron transport in a semiconductor superlattice with an electric field and a tilted magnetic field has been studied using semiclassical equations. In particular, we consider how dynamics of electron in superlattices evolve with changing the strength and the tilt of a magnetic field. In order to investigate the influence of a tilted magnetic field on electron transport, we calculate the drift velocity for different values of the magnetic field. Studies have shown that the resonance of Bloch oscillations and cyclotron oscillations produces additional peaks in drift velocity. We also found out that appearance of these resonances can promote amplification of a small ac signal applied to the superlattice. In the presence of the electromagnetic field, the superlattice is expected to demonstrate the Hall effect, which however should have a number of very specific features due to an excitation of Bloch oscillations and a significant electric anisotropy. Here, we theoretically study the Hall effect in a semiconductor superlattice both for the steady electron transport and for the transient response. We studied the coherent Hall effect in an extraordinary configuration where the electric field is applied in the transverse direction of the superlattice growth direction. By mapping the momentum dynamics to the pendulum equivalent, we distinguished the two regimes of the oscillations from the viewpoint of the effective potentials. We discuss the experimental manifestation of the Hall effect in a realistic superlattice. We also made the numerical simulations of the polarized THz field and the time-resolved internal electro-optic sampling (TEOS) signals where we found the unusual shaped waveforms of the THz signals.

<p> Thi thesis focuses on modelling electron dynamics in biased semiconductor superlattices, in the time domain. The numerical framework created on the basis of recently developed transparent boundary conditions proved to be an efficient and stable means to carry out simulations. A number of phenomena were modelled, with particular attention given to interminiband Rabi oscillations (RO); a quantitative description of the latter was given. We also report an observation of RO across three minibands in the high field regime. The detailed resolution of wavepacket dynamics allowed for conclusions to be drawn regarding the physical basis of RO. Due to similarity of the investigated system with other areas of physics, the approach developed could be further applied to study non-linear transport phenomena in cold atom traps and photonics. </p> / Thesis / Master of Science (MSc)

Acoustic modes guided by thin-film metal superlattices have been investigated using Brillouin spectroscopy. Samples were grown on both single-crystal sapphire and fused silica substrates by alternately sputtering two different metals to yield a total thickness in the range 0.3 - 0.5 μm. Structural and chemical characterization of the polycrystalline films was performed using x-ray diffraction. Rutherford backscattering and optical interferometry. Thermally excited acoustic waves in the metal film create a surface ripple which weakly interacts with light incident from a single mode argon laser. A tandem Fabry-Perot consisting of two synchronized 3-pass cavities is used to measure the frequency shift of light which is inelastically scattered from acoustic waves. The contrast ratio of this interferometer exceeds 10¹⁰ and provides sufficient stray light rejection to detect the surface Rayleigh wave and as many as 13 higher order acoustic modes. The elastic stiffness constants of the anisotropic superlattices were estimated by fitting the measured acoustic mode velocities to a parameterized acoustic model. A comparison is made between these elastic constants and those predicted from the properties of the separate bulk constituents. The dependence of bilayer wavelength on the elastic properties of both Cu/Nb and Mo/Ta superlattices over the range of roughly 10 to 200 Å was determined. The unexpected softening of Cu/Nb superlattices within a range of bilayer wavelengths near 20 Å which was reported previously is qualitatively similar to the measurements reported here. It is shown that the elastic stiffness coefficient with the largest variation is c₄₄. The stiffness variations determined for the Mo/Ta samples are much smaller than for Cu/Nb. It is suggested that this is due to either structural differences (Cu/Nb is fcc-bcc and Mo/Ta is bcc-bcc) or the smaller interfacial lattice mismatch for Mo/Ta. Interfacial strain is found to be strongly correlated with the stiffness variations of the Mo/Ta samples. However, the underlying cause of these variations in stiffness remains anomalous. This dissertation also reports the first observations of Love waves and Stoneley waves by Brillouin scattering. The purely transverse Love waves guided by Cu/Nb films were detected by elasto-optic scattering from the evanescent acoustic strain in the sapphire substrate. The stiffness coefficient c₁₂ of the hexagonally symmetric metal film cannot be determined by the other guided acoustic waves which ripple the surface. Molybdenum in contact with fused silica is predicted to support a Stoneley wave which is guided by the interface. The lowest order Sezawa made guided by a molybdenum film was found to evolve to the Stoneley wave as the film becomes thicker. These measurements together with measurements of the surface Rayleigh wave show that the stiffness of the sputtered metal films is quite homogeneous and independent of film thickness.

No description available.