Elastic and inelastic interactions are studied in tunnel junctions of a superconducting nanoelectromechanical setup and in response to resent experimental superconducting scanning tunneling microscope findings on a paramagnetic molecule. In addition, the electron density of molecular graphene is modeled by a scattering theory approach in very good agreement with experiment. All studies where conducted through the use of model Hamiltonians and a Green function formalism. The nanoelectromechanical system comprise two fixed superconducting leads in-between which a cantilever suspended superconducting island oscillates in an asymmetric fashion with respect to both fixed leads. The Josephson current is found to modulate the island motion which in turn affects the current, such that parameter regions of periodic, quasi periodic and chaotic behavior arise. Our modeled STM setup reproduces the experimentally obtained spin excitations of the paramagnetic molecule and we show a probable cause for the increased uniaxial anisotropy observed when closing the gap distance of tip and substrate. A wider parameter space is also investigated including effects of external magnetic fields, temperature and transverse anisotropy. Molecular graphene turns out to be well described by our adopted scattering theory, producing results that are in good agreement with experiment. Several point like scattering centers are therefore well suited to describe a continuously decaying potential and effects of impurities are easily calculated.

No description available.

Cette thèse porte sur la dynamique quantique dans un dcSQUID inductif. Ce dispositif est une boucle supraconductrice interrompue par deux jonctions Josephson. Sa dynamique est analogue à celle d'une particule massive évoluant dans un potentiel bidimensionnel. Dans la limite quantique, le dcSQUID se comporte comme un atome artificiel à deux degrés de liberté, contrôlé par le courant et le flux de polarisation. Dans la limite où l'inductance de la boucle est petite devant celle des jonctions, celles-ci sont fortement couplées. La dynamique du circuit est alors celle d'un oscillateur anharmonique quantique unidimensionnel. Dans la limite des deux premiers niveaux d'énergie, ce circuit est un qubit de phase. Jusqu'alors la décohérence dans ce circuit était dominée par le bruit en courant. Nous montrons, par des mesures de spectroscopie et d'oscillations cohérentes, que l'effet du bruit en courant s'annule à courant de polarisation nul, permettant une augmentation des temps de cohérence. Dans la limite où l'inductance de la boucle est grande devant celle des jonctions, la dynamique devient bidimensionnelle. Le circuit exhibe alors un spectre d'énergie riche qui peut être décrit comme celui de deux oscillateurs anharmoniques couplés, correspondant aux modes d'oscillations symétrique et antisymétrique des phases des deux jonctions. Nous mettons en évidence ce spectre par des mesures de spectroscopie et nous démontrons la manipulation cohérente des états quantiques de chaque mode. En particulier nous mettons en évidence un couplage non-linéaire entre les deux modes, dans une limite de couplage fort. Ce couplage nous permet alors d'observer des oscillations cohérentes entre les deux modes internes de cet atome artificiel. De plus, dans ce manuscrit, nous présentons une technique innovante de fabrication de jonctions métalliques par évaporations sous angles qui n'a pas recours à un pont de résine suspendu. Finalement nous proposons un modèle simple basé sur les effets de chauffage qui explique pour la première fois une anomalie récurrente observée dans les caractéristiques courant-tension des dcSQUID. / This thesis focuses on the quantum dynamics in inductive dcSQUID. This device is a superconducting loop interrupted by two Josephson junctions. Its dynamics can be described as a massive fictitious particle in a two dimensional potential. A dcSQUID behaves as an artificial atom with two degrees of freedom, controlled by current and flux bias. When the loop inductance is smaller than the Josephson inductance, the junctions are strongly coupled. The device is then described as a one dimensional quantum anharmonic oscillator. In the limit of the two lowest energy levels, a dcSQUID is a phase qubit. Until now decoherence was dominated by the current noise. We show by spectroscopic measurement and coherent oscillations measurement that the effect of the current noise vanishes at zero current bias, enabling longer coherence times. When the loop inductance is larger than the Josephson inductance, the dynamics becomes two dimensional. The device exhibits a rich energy spectrum which can be describe as the one of two coupled anharmonic oscillators, corresponding to symmetric and antisymmetric oscillations modes of the phases across each junctions. We present spectroscopic measurement of this spectrum. We demonstrate the coherent manipulation of the quantum states of each mode. We show evidence of non linear coupling between the modes, in the strong coupling regime. This coupling enables the measurement of coherent oscillations between the internal modes of this artificial atom. In addition we present a novel fabrication technique that allows metallic junction fabrication by angle evaporation without the use of suspended bridge of resist. We propose also a simple model based on heating effects that explain for the first time a frequent anomaly in the IV characteristic of dcSQUID.

Nesta dissertação nós revisamos a teoria básica que descreve a junção Josephson bosônica para uma e duas espécies partindo do modelo de Bose-Hubbard. Em seguida explicamos como é possível gerar campos de calibe artificiais em um sistema de átomos neutros, como é o caso do condensado de Bose-Einstein. Finalmente, utilizando os conhecimentos teóricos desenvolvidos anteriormente nós buscamos os estados estacionários de um sistema de pseudospin 1/2 submetido a um campo de calibre não-Abeliano artificial, que torna a dinâmica da junção muito mais complexa e rica. Nós também exploramos um novo desbalanceamento de população que surge no sistema, devido a presença do campo de calibre, com características similares as do Macroscopic Quantum Self-Trapping. / In this dissertation we review the basic theory that describes the bosonic Josephson junction for one and two species using the Bose-Hubbard model. Afterwards, we explain how it is possible to generate artificial gauge fields for neutral atoms, like a Bose-Einstein condensate. Finally, using this theoretical background we search for stationary states of a pseudospin 1/2 system subject to a non-Abelian artificial gauge field which turns the dynamic of the junction much more complex and rich. We also explore a possible new populational imbalance that appears on the system due to the presence of the gauge field, with similar features as the Macroscopic Quantum Self-Trapping.

Le sujet de thèse est une étude théorique des modes normaux d’oscillations plasma dans des chaînes de jonctions Josephson supra-conductrices. Les propriétés de ces modes normaux peuvent être contrôlés en choisissant une modulation spatiale appropriée de paramètres des jonctions le long de la chaîne et/ou un couplage approprié à l'environnement extérieur. Le travail théorique au sein du LPMMC se fait en étroite collaboration avec l'équipe expérimentale"Cohérence Quantique" à l'Institut Néel. Les problèmes spécifiques étudiés dans la thèse sont : modélisation détaillée du couplage des modes normaux à l'environnement pour leur caractérisation dans une expérience de transmission de micro-ondes, dissipation intrinsèque des oscillations du plasma à cause de quasi-particules hors équilibre, l'optimisation de la structure spatiale de la chaîne de jonctions Josephson pour son utilisation en tant qu'une super-inductance. / The subject of thesis is a theorerical study of normal modes of plasma oscillations in superconducting Josephson junction chains. The properties of these normal modes can be controlled by choosing an appropriate spatial modulation of the junction parameters along the chain and/or an appropriate coupling to the external environment. The theoretical work at LPMMC is performed in a close collaboration with the experimental Quantum Coherence group at Néel Institute. The specific problems studied in this thesis are : detailed modeling of the normal mode coupling to the environment for probing them in a microwave transmission experiment, intrinsic dissipation of plasma oscillations due to the presence of non-equilibrium quasi-particles, optimization of the spatial structure of the Josephson junction chain for its use as a super-inductance.

The combination of modern self-assembly techniques with well-established top-down processing methods pioneered in the electronics industry is paving the way for increasingly sophisticated devices in the future[1]. Nanomembranes, made from a variety of materials, can provide the necessary framework for a diverse range of device structures incorporating wrinkling, buckling, folding, and rolling of thin films[2, 3]. Over the past decade, an elegant symbiosis of bottom-up and top-down methods has been developed, allowing the fabrica- tion of hybrid layer systems via the controlled release and rearrangement of inherently strained layers [4]. Self-assembled rolled-up structures[4, 5] have become increasingly at- tractive in a number of fields including micro/nano uidics[6], optics[7](including metama- terial optical fibers[8]), Lab on a Chip applications[9], and micro- and nanoelectronics[10]. The use of such structures for microelectronic applications has been driven by the versatility in contacting geometries and the abundance of material combinations that these devices offer. By allowing devices to expand in the third dimension, certain obstacles that inhibit 2D structuring can be overcome in elegant ways. Similarly, recent progress in nanostructured superconducting electronic structures has been receiving increased attention[11]. The advancement of such devices has been mo- tivated by their use in quantum computation[12], high sensitivity radiation sensors[13], precision voltage standards[14] and superconducting spintronics[15] to name a few. Combining semiconductor with superconductor materials to create new hybrid geometries is advantageous because it adds the functionalities of the semiconductor, including high charge carrier mobilities, gating possibilities, and refined processing technologies. The main focus of the work presented in this thesis is the development of new methods for controlling strain behavior and its applications toward novel semiconduc- tor/superconductor heterostructures based on nanomembranes. More specifically, the goal is to integrate inherently strained semiconductor layer structures with superconducting materials to create innovative electronic devices by the controlled releasing and rearrangement of thin films. By rolling up pre-patterned semiconductor/superconductor layers, device geometries have been realized that are not feasible using any other technique. In this way, superconducting hybrid junctions, or Josephson junctions, have been created and their basic properties investigated. The Josephson effect, and junctions displaying this quantum coherent behavior, have found many essential uses in diverse areas of science and technology. Many research groups around the world are involved in finding new materials and fabrication methods to tune the properties and structure of such Josephson devices further[11]. The inclusion of semi- conductors, for example, allows for a greater control of the charge carrier density within the junction area, thus allowing for "transistor-like" behavior in these superconducting devices. By rolling up the superconductor contacts using a strained semiconductor as scaffolding, the fabrication of hybrid nano-junctions is simplified drastically, removing the need for complicated processing steps such as electron-beam or nano-imprint lithography. Furthermore, the technique allows many nanometer-sized devices to be created in parallel on a single chip which has the advantage that it can be scaled up to full-wafer processing. First, post-growth processing techniques of epitaxial layers are developed in order to extend the control of hybrid device fabrication. Here, three unique concepts for controlling the rolling behavior of strained semiconductor nanomembranes are presented. First an optical method for inhibiting the rolling of the strained layers is described. Next, a selective etching method for destroying the inherent strain within the semiconductor layer is introduced. Finally, a method by which the strain gradient across a trilayer stack is altered in situ during rolling is presented. Next, the fabrication of a hybrid nanomembrane-based superconducting device is presented. Various experimental details of the fabrication process are analyzed, and the electronic properties of the completed device are investigated. The devices created here highlight the fabrication process in which nanometer-sized structures are created using self-assembly techniques and standard microelectronics fabrication methods, presenting a new method to circumvent more complicated processing techniques. References [1] G. M. Whitesides and B. Grzybowski. Self-assembly at all scales. Science 295, 2418{2421 (2002). [2] Y. G. Sun, W. M. Choi, H. Q. Jiang, Y. G. Y. Huang and J. A. Rogers. Controlled buckling of semiconductor nanoribbons for stretchable electronics. Nature Nanotechnology 1, 201{207 (2006). [3] O. G. Schmidt and K. Eberl. Nanotechnology - Thin solid films roll up into nanotubes. Nature 410, 168 (2001). [4] O. G. Schmidt, C. Deneke, Y. Nakamura, R. Zapf-Gottwick, C. Mller and N. Y. Jin-Phillipp. Nanotechnology { Bottom-up meets top-down. Advanced Solid State Physics 42, 231 (2002). [5] V. Ya. Prinz, V. A. Seleznev, A. K. Gutakovsky, A. V. Chehovskiy, V. V. Preobrazhenskii, M. A. Putyato and T. A. Gavrilova. Free-standing and overgrown InGaAs/GaAs nanotubes, nanohelices and their arrays. Physica E 6, 828 (2000). [6] D. J. Thurmer, C. Deneke, Y. F. Mei and O. G. Schmidt. Process integration of microtubes for uidic applications. Applied Physics Letters 89, 223507 (2006). [7] R. Songmuang, A. Rastelli, S. Mendach and O. G. Schmidt. SiOx/Si radial superlattices and microtube optical ring resonators. Applied Physics Letters 90, 091905 (2007). [8] E. J. Smith, Z. W. Liu, Y. F. Mei and O. G. Schmidt. Combined surface plasmon and classical waveguiding through metamaterial fiber design. Nano Letters 10, 1{5 (2010). [9] G. S. Huang, Y. F. Mei, D. J. Thurmer, E. Coric and O. G. Schmidt. Rolled-up transparent microtubes as two-dimensionally confined culture scaffolds of individual yeast cells. Lab on a Chip 9, 263{268 (2009). [10] C. C. B. Bufon, J. D. C. Gonzalez, D. J. Thurmer, D. Grimm, M. Bauer and O. G. Schmidt. Self-assembled ultra-compact energy storage elements based on hybrid nanomembranes. Nano Letters 10, 2506{2510 (2010). [11] G. Katsaros, P. Spathis, M. Stoffel, F. Fournel, M. Mongillo, V. Bouchiat, F. Lefloch, A. Rastelli, O. G. Schmidt and S. De Franceschi. Hybrid superconductor-semiconductor devices made from self-assembled SiGe nanocrystals on silicon. Nature Nanotechnology 5, 458{464 (2010). [12] Y. J. Doh, J. A. van Dam, A. L. Roest, E. P. A. M. Bakkers, L. P. Kouwenhoven and S. De Franceschi. Tunable supercurrent through semiconductor nanowires. Science 309, 272{275 (2005). [13] F. Giazotto, T. T. Heikkila, G. P. Pepe, P. Helisto, A. Luukanen and J. P. Pekola. Ultrasensitive proximity Josephson sensor with kinetic inductance readout. Applied Physics Letters 92, 162507 (2008). [14] S. P. Benz. Superconductor-normal-superconductor junctions for programmable voltage standards. Applied Physics Letters 67, 2714{2716 (1995). [15] Y. C. Tao and J. G. Hu. Superconducting spintronics: Spin-polarized transport in superconducting junctions with ferromagnetic semiconducting contact. Journal of Applied Physics 107, 041101 (2010).

Nesta dissertação nós revisamos a teoria básica que descreve a junção Josephson bosônica para uma e duas espécies partindo do modelo de Bose-Hubbard. Em seguida explicamos como é possível gerar campos de calibe artificiais em um sistema de átomos neutros, como é o caso do condensado de Bose-Einstein. Finalmente, utilizando os conhecimentos teóricos desenvolvidos anteriormente nós buscamos os estados estacionários de um sistema de pseudospin 1/2 submetido a um campo de calibre não-Abeliano artificial, que torna a dinâmica da junção muito mais complexa e rica. Nós também exploramos um novo desbalanceamento de população que surge no sistema, devido a presença do campo de calibre, com características similares as do Macroscopic Quantum Self-Trapping. / In this dissertation we review the basic theory that describes the bosonic Josephson junction for one and two species using the Bose-Hubbard model. Afterwards, we explain how it is possible to generate artificial gauge fields for neutral atoms, like a Bose-Einstein condensate. Finally, using this theoretical background we search for stationary states of a pseudospin 1/2 system subject to a non-Abelian artificial gauge field which turns the dynamic of the junction much more complex and rich. We also explore a possible new populational imbalance that appears on the system due to the presence of the gauge field, with similar features as the Macroscopic Quantum Self-Trapping.

Au cours de ma thèse, je me suis intéressé à deux aspects de l'interaction charge-rayonnement dans des jonctions tunnel. Premièrement j'ai étudié la dynamique des transferts de charge à travers une jonction tunnel normale lorsque celle-ci est couplée à un environnement dont l'impédance est comparable au quantum d'impédance (RK= h/e^2). Nous avons montré que les uctuations de courant portent des signatures de processus à un, deux et trois photons. Deuxièmement j'ai caractérisé le rayonnement émis par une jonction Josephson lorsque celle-ci est couplée à deux résonateurs de fréquences différentes, nu_1 et nu_2, et soumise à une tension 2eV = h nu_1+h nu_2. Nous avons montré que les photons sont émis pas paires, ce qui constitue un rayonnement non-classique violant l'inégalité de Cauchy-Schwarz.Nos résultats montrent que le blocage de Coulomb dynamique constitue une ressource pour la production de micro-ondes quantiques. / In this thesis work, I have focused on two aspects of the charge-light interaction for tunnel junctions. On one hand, I have investigated the influence of the coupling to electromagnetic radiation on the dynamics of charge transfer through a normal tunnel junction when the impedance of its environment gets comparable to the resistance quantum (RK=h/e^2). We showed that the current fluctuations bear signature of the processes where one, two or three photons are emitted.On the other hand, we used a Josephson junction, embedded in a circuit made of two resonators at different frequencies, nu_1 and nu_2, and biased at the voltage 2eV=h nu_1+h nu_2. We showed that the Josephson junction emits photon pairs corresponding to a non classical state of radiation which violates the Cauchy-Schwarz inequality.Our results extend the toolbox for performing quantum optics experiments in the microwave domain.

This PhD work involves the utilization of the femtosecond laser technique to fabricate the novel S-shaped Josephson Junctions on the high - T_C superconducting YBCO thin films. Initially, it was envisaged as the title states to fabricate nano sized Josephson Junctions using this femtosecond laser technique. However in this PhD work, micron sized (1 – 2 µm) and near nano sized (500 – 800 nm) Josephson Junctions where achieved. / University of South Africa / Electrical and Mining Engineering

We address the excitation of quantum breathers in small nonlinear networks of two and three degrees of freedom, in order to study their properties. The invariance under permutation of two sites of these networks substitutes the translation invariance that is present in nonlinear lattices, where (classical) discrete breathers are time periodic space localized solutions of the underlying classical equations of motion. We do a systematic analysis of the spectrum and eigenstates of such small systems, characterizing quantum breather states by their tunnelling rate (energy splitting), site correlations, fluctuations of the number of quanta, and entanglement. We observe how these properties are reflected in the time evolution of initially localized excitations. Quantum breathers manifest as pairs of nearly degenerate eigenstates that show strong site correlation of quanta, and are characterized by a strong excitation of quanta on one site of the network which perform slow coherent tunnelling motion from one site to another. They enhance the fluctuations of quanta, and are the least entangled states among the group of eigenstates in the same range of the energy spectrum. We use our analysis methods to consider the excitation of quantum breathers in a cell of two coupled Josephson junctions, and study their properties as compared with those in the previous cases. We describe how quantum breathers could be experimentally observed by employing the already developed techniques for quantum information processing with Josephson junctions.