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No description available.

Superconducting materials exhibit effects which can be used to improve a wide range of existing technologies, and enable the creation of entirely new ones. The infinite conductivity of superconductors results in high specific power and efficiency, making them an attractive replacement for conventional conductors. By far the most technologically useful material is Yttrium Barium Copper Oxide (YBCO), which superconducts well above the temperature of cheap, readily available liquid nitrogen (77K), which makes cooling it to the required temperature economically viable. Improvement of flux pinning and thus the critical current, <i>J_c</i>, that can be carried in YBCO is crucial for achieving widespread applications of this technologically important material. This thesis presents an investigation into pinning in thin films of YBCO, via inclusions of a non-superconducting stable oxide phase, RE₃TaO₇, where RE = Sm, Gd, Er or Yb. This second phase provides both ideally sized <i>c</i> axis correlated and random pinning sites and results in an order of magnitude improvement of critical current over pure YBCO at 1 Telsa. Additionally, RE₃TaO₇ does not depress the superconducting transition temperature, as has been found with the closest performing additives based on BaWO₃, where W = Zr or Sn. The thin films studied were grown by pulsed laser deposition and characterised with a standard four point measurement technique, transmission electron microscopy, X-ray diffraction, atomic force microscopy and X-ray Photoelectron Spectroscopy. Additionally, the importance of surface pinning in doped films was investigated and the mixing of the second phase with YBCO during the initial layers of film growth was found to have a large effect of the pinning at that interface.

No description available.

Nanomechanical resonators have extremely low masses (~ 10−15 kg) and frequencies from a few megahertz all the way up to the gigahertz range. These properties along with a small damping rate make them very useful or ultrasensitive detection applications, now pushing into the realm of zeptonewtons (10−21 N) and zeptograms (10−21 g). On a more fundamental level, nanomechanical resonators are expected to display quantum mechanical effects when cooled down to millikelvin temperatures. The understanding of dissipation in nanomechanical resonators is important for device applications and to study quantum mechanical effects in such systems. However, despite a range of experiments on semiconducting and metallic devices, dissipation in nanomechanical resonators at low temperatures is not yet well understood. Although mechanical resonators have traditionally been operated in the linear regime, exploiting their nonlinearities can prove advantageous for industrial applications as well as opening up new experimental windows into the fundamental study of the nonlinear dynamics of mesoscopic systems. In this thesis, we present results from low temperature dissipation studies on pure gold and on gold-coated high-stress silicon nitride nanomechanical resonators. A theory, which predicts the existence of tunnelling two-level systems (TLS) in bulk disordered solids at low temperatures, is used as a framework to describe the data. The nonlinear interactions between different flexural modes of a single silicon nitride device, are explored experimentally and theoretically. The resonators were fabricated as doubly-clamped beams using a combination of optical lithography, electron-beam lithography, dry and wet etching techniques. The motion of the resonators was actuated and detected using the magnetomotive scheme. At low temperatures, all the beams had resonant frequencies between 3 and 60 MHz and quality factors in the range 105 − 106. The strong variation observed in dissipation and resonant frequency at the lowest temperatures (below 1 K) indicates the presence of tunnelling TLS in nanomechanical resonators.

Measurements are made on superconducting Niobium on Sapphire and oxidized Silicon microwave coplanar resonators for quantum bit experiments. Device geometry and materials are investigated and quality factors in excess of a million have been observed. The resonant frequency as a function of temperature of a coplanar resonator is characterised in terms of the change in the number density of superconducting electrons. At lower temperatures, the resonant frequency no longer follows this function, and evidence is shown that this is associated with the resonant coupling of the resonant frequency with two level systems in the substrate. At T<2.2 K the resonant frequency scales logarithmically with the temperature, indicating that two level systems distributed in the volume of the Silicon Dioxide affect the electric permittivity. Applying higher input microwave power levels is shown to saturate these two level systems, essentially decoupling them from the CPR resonance. This is observed as an increase in resonant frequency and Q factor. The resonant frequency is also shown to have a high sensitivity to a magnetic field applied perpendicular to the plane of the coplanar resonator, with a quadratic dependence for the fundamental, second and third harmonics. Frequency shift of hundreds of linewidths are obtained. Coplanar resonator are fabricated and measured with current control lines built on chip, and these have shown to produce frequency shifts of tens of Kilohertz.

Local Hall probe measurements and differential magneto-optical imaging with high spatial resolution have been used to investigate the magnetic state of high temperature superconducting Bi2Sr2CaCu2O8+� (BSCCO) micro-disks and platelet single crystals. The results obtained by magneto-optical imaging demonstrate that the field at which flux quantised vortices enter the disks decays exponentially with increasing temperature and the measured data agree well with analytic models for the thermal excitation of individual pancake vortices over Bean-Livingston surface barriers. Scanning Hall probe microscopy images are used to directly map the magnetic induction profiles of individual micro-disks at different applied fields and the results can be quite successfully fitted to analytic models which assume a continuous distribution of flux in the sample. At low fields, however, the characteristic mesoscopic compression of vortex clusters in increasing magnetic fields has been observed. Even at higher fields, where single vortex resolution is lost, it is still possible to track configurational changes in the vortex patterns, since competing vortex orders impose unmistakable signatures on local magnetisation curves as a function of the applied field. These observations are in excellent agreement with molecular dynamics numerical simulations which lead to a natural definition of the lengthscale for the crossover between discrete and continuum behaviours in this system. In closely related experiments, Hall magnetometry is used to probe the out-of-plane local magnetisation of platelet BSCCO single crystals. The magnetisation is found to depend on the strength and direction of an in-plane magnetic field in the crossing vortex lattices regime. The remanent magnetisation in zero out-of-plane field is found to exhibit a pronounced anisotropy, being largest with the in-plane field parallel to the crystalline a-axis, and smallest when it is parallel to the orthogonal b-axis. This behaviour is attributed to the presence of underlying linear disorder. Finally, spectral analysis of the local magnetisation data is used to estimate a lower cutoff for the characteristic frequency of thermal fluctuations of vortex positions.

Στην παρούσα διπλωματική εργασία διερευνήθηκε η δυνατότητα του ρομποτικού θεοδόλιχου στην ορθή καταγραφή ταλαντώσεων με στόχο την ακρίβεια προσδιορισμού του εύρους. Για το σκοπό αυτό χρησιμοποιήθηκε η συκευή παραγωγής ελεγχόμενων εξαναγκασμενων ταλαντώσεων (μονοβάθμιος ταλαντωτής) και ακολουθήθηκε στατιστική αναλυση δεδομένων. / -

The subject of this thesis is the study of a particular open quantum system consisting of a resonator coupled to a superconducting single electron transistor (SSET). The theoretical model we use is applicable to both mechanical and superconducting stripline resonators leading to a large parameter regime that can be explored. The SSET is tuned to the Josephson quasi-particle resonance, in which the transport occurs via Cooper pairs coherently tunnelling across one junction followed by the incoherent tunnelling of quasi-particles across the other. The SSET can be thought of as an artificial atom since it has a similar energy level structure and transitions to an atom. We investigate to what extent the current and current noise through the SSET can be used to infer the state of the resonator. In order to carry out these investigations we describe the system with a Born-Markov master equation, which we solve numerically. The evolution of the density matrix of the system is described by a Liouvillian superoperator. In order to better understand the results we perform an eigenfunction expansion of the Liouvillian, which is useful in connecting the behaviour of the resonator to the current noise. The mixture of coherent and incoherent processes in the SSET leads to interesting back action effects on the resonator. For weak coupling the SSET acts as an effective thermal bath on the resonator. Depending on the operating point the resonator can be either heated or cooled in comparison to its surroundings. In this regime we can use a set of mean field equations to describe the system and also capture certain aspects of the behaviour with some simple models. For sufficient coupling the SSET can drive the resonator into states of self-sustained oscillations. At the transition between stable and oscillating states of the resonator we also find regions of co-existence between oscillating and fixed point states of the resonator. The current noise provides a way to identify these transitions and the state of the resonator. The system also shows analogies with quantum optical systems such as the micromaser. We calculate the linewidth of the resonator and find deviations from the expected behaviour.

Les gènes situés sur la molécule d'ADN au coeur de nos cellules nonseulement portent une information héréditaire, mais de plusparticipent dynamiquement au fonctionnement de la cellule ensynthétisant plus ou moins activement des protéines. Ces dernièressont susceptibles de nombreuses interactions non linéaires avecd'autres acteurs, menant à la formation de réseaux biochimiques quipeuvent présenter des comportements dynamiques divers et complexes. Enparticulier, la régulation de l'activité des gènes (leur taux detranscription) par des protéines spécifiques peut aboutir à laformation de boucle de rétroaction. Ces boucles tissent des réseauxgénétiques où un ensemble de gènes régulent réciproquement leursexpressions.Des travaux expérimentaux récents ont montré que la régulation del'activité du gène n'est pas toujours instantanée. Nous avons alorsétudié l'influence d'une dynamique transcriptionnelle intrinsèque dansun circuit simple constitué d'un gène réprimé par sa propre protéine.Nous obtenons un critère analytique pour l'apparition desoscillations, qui nous permet de montrer que ces dernières sontfavorisées lorsque le temps de réponse du gène prend une certainevaleur. L'échelle de temps ainsi repérée est pertinente à la fois dansune description déterministe et dans une description stochastique.Ce sont également des réseaux génétiques qui sont à l'origine decertains rythmes biologiques, induisant les oscillations del'expression de protéines clés. Ces réseaux sont alors des horlogesendogènes qui permettent à de nombreux êtres vivants d'anticiper lesmodifications cycliques de l'environnement. Parmi celles-ci, l'horlogecircadienne permet à l'organisme de s'adapter au cycle diurnal enétant entraîné par l'alternance du jour et de la nuit. À partir de données expérimentales, nous avons modélisé l'horlogecircadienne de l'algue unicellulaire Ostreococcus tauri. Lemodèle est basé sur une boucle de rétroaction transcriptionnellenégative impliquant deux gènes se régulant mutuellement. L'accordentre le modèle et les données expérimentales est excellent et met enévidence une absence de signature du couplage dans les données lorsquel'horloge est à l'heure, ce qui révèle une propriété de robustesse aux fluctuations d'éclairement. / Genes located on the DNA macromolecule inside our cells do not onlycarry hereditary information. They also contribute dynamically tobiological functions by synthesizing proteins at a variable rate.Proteins are subject to many nonlinear interactions with other actors,forming vast biochemical networks which may display a number ofcomplex behaviors. In particular, regulation of gene activity (i.e.,of their transcription rate) by specific proteins creates feedbackloops. These loops form genetic networks where a set of genes regulatetheir expressions reciprocally. Recent experimental studies have shown that gene regulation is notalways instantaneous. We have thus studied the influence of anintrinsic transcriptional dynamics in the simple circuit where a geneis repressed by its own protein. We have obtained an analyticalcriterion for the appearance of oscillations, which allows us to showthat oscillations are favored when gene response time is close to acharacteristic value. The time scale thus identified is relevant bothin a deterministic and a stochastic description. Gene regulatory networks are also at work in some biological rhythms,inducing oscillations in the concentrations of some key proteins.These networks then serve as endogeneous clocks, which allow manyliving organisms to anticipate periodic changes in the environment. Inparticular, the circadian clock is used by organisms to adapt to thediurnal cycle by being entrained by the day/night cycle. Using experimental data, we have constructed a mathematical model ofthe circadian clock of the unicellular alga Ostreococcus tauri.This model is based on a transcriptional negative feedback loop, whichinvolves two genes regulating each other. Agreement between numericalsimulations and experimental data is excellent and unveils the factthat there is no signature of coupling in data when the clock is ontime. This reveals a strong robustness to daylight fluctuations.