Dilute magnetic semiconductors (DMSs) are attractive. They are candidate materials for applications in novel spintronic devices. Because of the giant Zeemaneect in the paramagnetic state, a magnetic eld can be used to manipulate the spin and charge of carriers in DMSs. One possibility is to exploit the nonhomogeneous magnetic elds due to superconductors. In this thesis, the heterostructures of the planar DMS and superconductors in dierent geometries and superconducting states are investigated to understand the electronic structure of electrons in the DMS. The combination of a superconducting disk in the Meissner state and the planar DMS is studied using both simple and realistic models of the magnetic eld associated with the disk. The giant Zeeman interaction is found to substantially inuence the energies of magnetically conned states in the adjacent DMS. In the simple model eld, the giant Zeeman energy acts as an extra conning potential and results in spin dependent electron states exhibiting dierent spatial distributions, while the more realistic model eld results in conned states exhibiting a variety of mixed spin characters. The hybrid of a superconducting lm in a superconducting vortex state and the DMS is then explored. The concentrated magnetic eld due to an isolated vortex is shown to trap strongly spin polarised electron states. In the case of an Abrikosov lattice of vortices, interactions between vortex-bound states result in a band structure which can be controlled by the magnitude of an external uniform magnetic eld. It is found that the numerical band structures obtained using a basis of Landau states dier from those previously reported, leading to the development of a tight-binding theory to conrm their corrections. Another hybrid investigated is a square superconductor above the DMS. In this case, the arrangement of vortices is distorted by the boundary of the sample, leading to the possibility of multivortex state and/or giant vortex states. It is discovered that the magnetic eld due to the former state induces \molecular" electron states in the DMS, while that due to the latter state induces electron states with increased spatial distribution. Tight-binding theory is again used to describe the observed energy levels and the interactions between electron states induced by the magnetic elds due to separated vortices in the multivortex state.

No description available.

Transition edge sensors are the detectors of choice for a wide range of applications; from dark matter search, neutrino search, to cosmic radiation detection from near infrared to millimeter wavelengths. We are developing transition edge sensors using superconducting iridium thin films and we are proposing their use for future dark matter and neutrino search experiments. Our Ir films are deposited using an radio frequency (RF) magnetron sputtering and photolithographic techniques and measured using an adiabatic refrigerator capable of reaching temperatures of a few tens of mK. This thesis presents a detailed description of superconducting iridium thin films from the fabrication process to the characterization of the film properties at room temperature and low temperature. Alternative options for the bias circuit used to read out the TES signals will be discussed, we are proposing the use of RLC resonant circuits and transformers instead of SQUIDS.

The characteristics of liquid and solid aerosols have been intensively investigated by means of optical diagnostic techniques. Part I describes the characteristics of liquid aerosol formation formed by heat transfer fluids (HTFs) from bulk liquids. Part II investigates the characteristics of convective transport behavior of solid particles in virtual impactor (VI). The objective of part I is to establish correlations which offer predictions on atomized particle size of HTFs which are widely and commonly used in process industries. There are numerous reports stating that mist explosions formed from leakage cause disastrous accidents in process industries. For safety concerns, the characteristics of mist formation should be known in order to prevent HTFs from catching on fire or exploding. The empirical data on formation of mist are collected by the optical measurement technique, the Fraunhofer diffraction. The Buckingham-PI theorem is applied to establish a correlation between empirical data and representative physical properties of HTFs. Final results of correlations are solved by a statistical method of linear regression. The objective of part II is to investigate the characteristics of convective transport behavior in virtual impactor (VI) which is used to sort polydisperse precursor powder in the process industries of superconductor wire. VI is the device to separate polydisperse particles as a function of particle size by using the difference in inertia between different sizes of particles. To optimize VI performance, the characteristics of convective transport should be identified. This objective is achieved by visualization techniques. The applied visualization techniques are Mie-scattering and laser induced fluorescence (LIF). To investigate analytically, a local Stokes number is introduced in order to offer criteria on predicting the efficiency of VI performance and boundary effect on particle separation. The achieved results can enhance performance and eliminate defects by having knowledge of the behavior of solid particles in VI.

Las cintas superconductoras de alta temperatura (HTS) han emergido como materiales prometedores para sus uso en el campo de l'energía puesto que permiten reducir a mitad el tamaño de los equipos de energía eléctrica respecto a los convencionales, reducir las pérdidas de energía, aumentar la eficacia en la generación, la transmisión y la distribución de la misma, y así la reducir el impacto ambiental.Sin embargo, diversamente de los conductores típicos, los materiales superconductores basados en óxido son frágiles, se dañado fácilmente y son así difíciles de procesar. Hasta ahora ha sido posible producir longitudes de un kilómetro de cables HTS de primera generación, para el uso en el trasporte de corriente eléctrica. Las cintas superconductoras de YBCO, por ejemplo, pueden soportar altas densidades de corrientes críticas y por esto representan un candidato prometedor en el trasporte de electricidad.Uno del los substratos disponible para suportar el superconductor es un acero policristalino con una película epitaxial de YSZ en cima, YSZ(IBAD)/Stainless. El segundo que hemos utilizado es el NiO(SOE)/Ni(Rabit), una cinta de nickel previamente texturada por laminación y sucesivamente oxidata de manera controlada. Numerosas técnicas están disponible para la deposición de YBCO epitaxiale, nosotros elegimos el proceso más barato y industrialmente interesante: la técnica sol-gel. Para evitar la interacción entre el YBCO superconductor y el substrato epitaxial, evitando así la reducción de la corriente que el superconductor puede trasportar, es importante interponer un material inerte que transfiera su epitaxia al YBCO; este clase de películas se llaman capas tampón. El objetivo principal de esta tesis ha sido optimizar el crecimiento de las capas tampón por técnica química y finalmente estudiar la deposición del YBCO por TFA sobres esas muestras optimizadas. Las capas de cerámica que hemos estudiados han sido: CeO2, BaZrO3, CaZrO3, SrZrO3, SrTiO3, BaCeO3 , y depositadas por el método químico: metal 2-4-&#61538;-diketone disuelto en ácido acético, o los metales isopropoxidos disuelto en metanol. Por depositar las soluciones precursoras hemos utilizado la técnica de spin coating. Controlando los diversos parámetros, velocidad, aceleración y la concentración de la solución obtuvimos películas homogéneas con diverso grosor. La fase de la cristalización se alcanza en un horno donde se controla l'atmósfera, la temperatura y la velocidad de calefacción. Durante esta investigación hemos adquirido un conocimiento total del acrecimiento de las películas delgada de MOD-CeO2. De una combinación de las análisis de TEM, de XRD y de RHEED observamos que el mecanismo de crecimiento tiene un comportamiento anómalo comparado con el otro materiales crecidos con la misma técnica.En este proceso de síntesis de la ceria, la nucleation homogénea de hecho esta favorita debido al bajo valor de Tnuc./Tmel ceria (Tnuc./Tmel=0.21). Solamente los granos nucleados sobre el substrato resultan texturados. La dependencia del tamaño de grano con temperatura sigue una relación de tipo Arrhenius, características de un crecimiento 3D del grano. Los análisis de EELS revelaron una fracción significativa de C residual que adorna los límites de grano, es probable que el crecimiento del límite de grano se quede bloqueando debido estas impurezas.Un proceso del recocido en aire a posteriores, ha demostrado la posibilidad de crecer las películas de CeO2 totalmente epitaxiales. Los análisis de EELS de tales muestras tratadas en oxígeno demuestran claramente que los límites de grano quedan limpios de las impurezas de C, desbloqueando así el crecimiento del grano. Después de un proceso largo de optimización de los parámetros de síntesis, podemos ahora controlar exactamente el crecimiento epitaxiale de la ceria. Se ha verificado que el óxido del cerio se puede crecer en YSZ(IBAD)/SS con solamente la orientación (00l). Para preservar la cinta del metal contra la oxidación, el proceso optimizado se ha adaptado a la deposición sobre substrato de acero inoxidable reduciendo la temperatura de síntesis a 900ºC. Hemos optimizado también la preparación de SrTiO3(STO) y BaZrO3(BZO) sobre MgO y YSZ mono-cristales y en seguida también sobre YSZ(IBAD)/SS y NiO(SOE)/Ni.La arquitectura más prometedora resultó ser STO/BZO/NiO(SOE)/Ni. Por ultimo depositamos YBCO por método TFA (Trifluoracéticetatos) sobre las capas tampones optimizadas. Una muestra de TFA-YBCO sobre CeO2/YSZ(IBAD)/SS preparada en aire a 900ºC en 8 h ha dato como resultado una densidad corriente crítica, Jc de 7 MA/cm2 a 5K, y 6·105A/cm2 a 77K. Estos valores están cerca de objectivo de un millón A/cm2 a 77K. Los experimentos sobre las capas tampón de BZO y de STO han demostrado la posibilidad de usar este sistema doble sobre NiO(SOE)/Ni como plantilla alternativa para el crecimiento de YBCO. Alcanzado una densidad de corriente crítica de Jc(5K) = 5·105A/cm2 con la mejor muestra de YBCO/STO/BZO/NiO(SOE)/Ni. / High-temperature superconducting (HTS) tapes have emerged as promising materials for superconducting power applications since they make possible electric power equipment that is half the size of conventional alternatives, with half energy losses, increasing the efficiency in the generation, transmission and distribution of the electric energy, and thus reducing the impact of power delivery on the environment. However, unlike typical conductors, oxide based superconductor materials are brittle and easily damaged and thus they are difficult to process and handle, specially forming large and flexible wires.Up to now it has been possible to produce kilometre lengths of the first generation of HTS wires for use in electrical transmission cables. YBCO coated conductors can support high critical current densities and is a promising candidate. One of the substrate available is a polycrystalline metal substrate with an epitaxial YSZ film on it, the ion-beam assisted deposition YSZ(IBAD)/Stainless Steel. The second is the textured NiO(SOE)/Ni(Rabit).Numerous methods are available for epitaxial deposition of YBCO, including vacuum techniques but we choose the cheaper non vacuum sol-gel processes.To avoid the interaction between the superconductor it is important to interpose a inert material that can transfer the epitaxy from the substrate to the YBCO, these kind of films are called buffer layers, avoiding than the reduction of the current that the superconductor can support. The main aim of this thesis was optimising the buffer layer growth by chemical technique and finally studying the deposition of TFA-YBCO on those optimised templates.The ceramic buffer layers studied:CeO2 , BaZrO3 , CaZrO3, SrZrO3, SrTiO3,BaCeO3The Sol-Gel system used was the Metal &#61538;-diketone dissolved in Acid Acetic, or Metal isopropoxide dissolved in methanol.The deposition step was performed by spin coating. Controlling the different parameters of rate and acceleration of the spinner and the precursor solution concentration we obtained homogenous films with different thickness. The crystallisation step is achieved in a furnace in a controlled atmosphere, temperature and heating rate.Important knowledge on the MOD-CeO2 thin film growth has been acquired during this research. From a combination of TEM, XRD and RHEED analyses it was observed that its growth mechanism exhibits an anomalous behaviour compared with other CSD derived films. The homogeneous nucleation in fact is favoured in this MOD process due to the low Tnuc./Tmel value for ceria film (Tnuc./Tmel=0.21). Only grains nucleated on the substrate are textured as observed in XTEM images. The grain size dependence with temperature follows an Arrhenius relation: <r>2=&#61537;otexp(-Q/kT), characteristics of 3D undergoing thermally activated grain growth . EELS analyses revealed a significant fraction of residual C decorating the grain boundaries, that very likely acts as a growth by blocking grain boundary motion. A process of post annealing or direct growth in static air, have demonstrated the possibility of growing completely epitaxial CeO2 films. EELS analyses of such samples clearly demonstrates that the oxygen clean up grain boundaries from C impurities thus unblocking grain growth. After a long process of synthesis parameter optimisation, we are now able to control exactly the epitaxial growth of ceria growth. It has been verified that Cerium oxide can be grown on YSZ(IBAD)/SS with only the (00l) orientation. The optimised process has been adapted to stainless steel substrate reducing the synthesis temperature at 900ºC in order to preserve the metal tape against oxidation. We observed an interesting phenomenon of in plane texture improvement of the ceria film with respect to the underlying YSZ(IBAD)/SS substrate, from &#61508;&#61542;YSZ = 8.3º and to &#61508;&#61542;CeO2 = 7.5º.The solution preparation and the deposition conditions for STO and BZO on MgO and YSZ have been also optimised. After several experiments of buffer deposition on YSZ(IBAD)/SS and NiO(SOE)/Ni technical metal substrates the most promising architecture resulted to be the STO/BZO/NiO(SOE)/Ni. We have grown YBCO by the TFA(Trifluor Acetic Acid) method on the optimized buffer layers. A sample of TFA-YBCO on a CeO2/YSZ(IBAD)/SS template prepared in air at 900ºC for 8 h has shown a critical current density, Jc has a value of 7 MA/cm2 at 5K, and 6·105A/cm2 at 77K. These values are near the target of one million A/cm2 at 77K. The experiments on BZO and STO buffer layers have demonstrated the possibility of using the double buffer on NiO(SOE)/Ni as an alternative template for YBCO deposition. A critical current density of Jc(5K)= 5·105A/cm2 has been achieved for the best sample of YBCO/STO/BZO/NiO(SOE)/Ni.

Since the discovery of high superconducting transition temperature in layered copper oxides, many researchers have searched for similar behavior in other layered metal oxides, such as cobalt and nickel. The sodium cobalt oxyhydrate is the first cobalt-oxide layered superconductor. We present the studies of low-temperature specific heat C(T, H) in NaxCoO2¡EyH2O (x ~ 0.35, y ~ 1.3). At H = 0, a very sharp anomaly was observed at T ~ 4.7 K indicating the existence of bulk superconductivity. There exists an £\T2 term in C(T, H=0) in the superconducting state manifesting the line nodal superconducting order parameter. The feature at the superconducting transition is rather sharp, becoming broad and strongly suppressed in an applied magnetic field. The transition temperature also changed in an applied magnetic field. Thus an abrupt change of slope in H vs. Tc curve was observed. Possible scenarios such as the multiple phase transitions in the mixed state are discussed.

The characteristics of liquid and solid aerosols have been intensively investigated by means of optical diagnostic techniques. Part I describes the characteristics of liquid aerosol formation formed by heat transfer fluids (HTFs) from bulk liquids. Part II investigates the characteristics of convective transport behavior of solid particles in virtual impactor (VI). The objective of part I is to establish correlations which offer predictions on atomized particle size of HTFs which are widely and commonly used in process industries. There are numerous reports stating that mist explosions formed from leakage cause disastrous accidents in process industries. For safety concerns, the characteristics of mist formation should be known in order to prevent HTFs from catching on fire or exploding. The empirical data on formation of mist are collected by the optical measurement technique, the Fraunhofer diffraction. The Buckingham-PI theorem is applied to establish a correlation between empirical data and representative physical properties of HTFs. Final results of correlations are solved by a statistical method of linear regression. The objective of part II is to investigate the characteristics of convective transport behavior in virtual impactor (VI) which is used to sort polydisperse precursor powder in the process industries of superconductor wire. VI is the device to separate polydisperse particles as a function of particle size by using the difference in inertia between different sizes of particles. To optimize VI performance, the characteristics of convective transport should be identified. This objective is achieved by visualization techniques. The applied visualization techniques are Mie-scattering and laser induced fluorescence (LIF). To investigate analytically, a local Stokes number is introduced in order to offer criteria on predicting the efficiency of VI performance and boundary effect on particle separation. The achieved results can enhance performance and eliminate defects by having knowledge of the behavior of solid particles in VI.

In part I, a numerical study of the mixed states in a mesoscopic type-II superconducting cylinder is described. Steady-state configurations and transient behavior of the magnetic vortices for various values of the applied magnetic field H are presented. Transitions between different multi-vortex states as H is changed is demonstrated by abrupt changes in vortex configurations and jumps in the B vs H plot. An efficient scheme to determine the equilibrium vortex configuration in a mesoscopic system at any given applied field, not limited to the symmetry of the system, is devised and demonstrated. In part II, a superconducting thin film is subject to a non-uniform magnetic field from a vertical magnetic dipole, consisting of two magnetic monopoles of opposite charges. For a film with constant thickness and with no pins, it has been found that the film carries two pairs of vortex-antivortex in the steady state in the imposed flux range of 2.15 < (Phi)+ < 2.90 (in units of flux quantum) and no vortex at all for (Phi)+ <= 2.15. Transitions from a superconducting state with 3 pairs of vortex-antivortex to one with 2 pairs, where a pair of vortex-antivortex annihilates, have been observed in the pseudo-time sequence. With a perturbation with antidots (holes), vortexantivortex pair has been created for lower magnetic fluxes down to (Phi)+ = 1.3. In the sample of size 16(Xi) x 16(Xi), the attraction force between the vortex and antivortex always dominates over the pinning force, so that they eventually come out of pins, move toward each other, and annihilate each other. The annihilation rate, measured with time taken for the annihilation, is reduced noticeably by the increase of the distance between pins, or the increase in the pin size. A simulation of the magnetic vortex pinning in the sample of size 32(Xi) x 32(Xi) suggests we are likely to achieve pinning of the vortex-antivortex pair with the sample size around this and vortex-antivortex separation of 22(Xi). Using this sample as a template, the maximum density of pinned vortices achievable is calculates to be about 7.6 x 10^14 vortices/m2 for (Xi) =~ 1.6A°.

In this thesis I describe work on developing theoretical and numerical models of supercon- ducting thin-film microwave resonators. Superconducting resonators are used in a variety of applications, one of which is as kinetic inductance detectors (KIDs). KIDs are ultra-low noise, highly sensitive, multiplexable detectors, with uses in a wide variety of fields including astrophysics, medical imaging, and particle physics. Resonators are also crucial for supercon- ducting qubit readout, superconducting mixers and parametric amplifiers, and as multiplexers for other devices. The results described in this thesis apply to all thin film resonators, but are primarily described in the context of KIDs. I develop models of how constant absorbed power affects superconducting thin films, driving them out of thermal equilibrium with the bath. Using the Chang & Scalapino equations, I calculate numerically the steady-state quasiparticle and phonon distributions for various sources of power (sub-gap and above-gap photons and phonons, single frequency and broadband). Many new results emerge, a few of which are: the quasiparticle heating effects of microwave power, explaining the experimentally measured saturation of resonator performance; the frequency dependence of the quasiparticle generation efficiency in the sub-mm wavelength range; and the importance of phonon trapping in thin films at low temperatures. I then use these nonequilibrium results in higher-level device models. Starting from a generic framework for resonators, I present and implement a model of a KID with variable thermal isolation. I calculate the effects of quasiparticle heating on both the large-signal and small-signal behaviour, highlighting the effects of electrothermal feedback caused by readout power. Electrothermal feedback is shown to be able to increase or decrease the magnitude and bandwidth of the responsivity and noise, depending on the operating point. Finally, I propose an experimental measurement of quasiparticle heating effects in a new device – a four-port ring resonator. In such a device, unlike in the standard two-port resonator, the device can be heated and read out independently. I develop detailed models for the thermal and electrical behaviour of these devices, and suggest a scheme by which the quasiparticle heating effects of microwave power, predicted in this thesis, can be measured.

Ce travail de thèse a pour objet l'étude de la famille des supraconducteurs à base de fer, composés découverts en 2008 et présentant un diagramme de phase très riche dans lequel se trouve de la supraconductivité, du magnétisme, ainsi que d'autres ordres électroniques tel que la nématicité électronique. Ceci offre la possibilité de fortes interactions et/ou compétitions entre ces divers types d'ordres électroniques, et l'étude de ces composés vise à mieux comprendre la nature et le lien entre ces différents ordres. Ceci devrait permettre de mieux comprendre de façon générale les comportements collectifs des électrons corrélés dans les solides cristallins. Les supraconducteurs à base de fer sont en général des métaux et présentent un caractère multi-orbital,ce qui donne des degrés de liberté supplémentaires aux électrons. Dans cette thèse,nous avons étudié certains de ces matériaux par photoémission résolue en angle (ARPES),technique permettant de sonder leur structure électronique et d'estimer la force et la nature des corrélations électroniques dans chaque orbitale. Nous nous sommes intéressés à des phases non supraconductrices afin d'avoir une vision plus globale des supraconducteurs au fer. La première partie de cette thèse se concentre sur le composé FeSe. L'intérêt pour ce composé vient de l'émergence d'un nouveau type d'ordre électronique : la nématicité, où les électrons brisent spontanément la symétrie de rotation, ce qui rend les propriétés électroniques anisotropes.Grâce à l'ARPES, nous avons pu suivre l'évolution des différentes orbitales dxz, dyz et dxy,présentes au niveau de Fermi, à travers la transition structurale pour caractériser l'anisotropie de la structure électronique. Pour interpréter ces évolutions, nous avons collaboré au niveau théorique avec le groupe de Lara Benfatto de l'université de Rome. Nous avons élaboré un modèle où les fluctuations de spins, bien présentes dans FeSe même s'il n'y a pas d'ordre magnétique, sont capables de modifier la surface de Fermi. Ceci rend compte des modifications que nous observons par ARPES dans la phase nématique si on suppose que ces fluctuations deviennent anisotropes. La deuxième partie de cette thèse se penche sur les pnictures de la forme AM ₂ As ₂ (A = Ba, Sr, Ca, M = Fe, Co). Cette famille présente plusieurs cas où la nature du magnétisme est différente de celui que l'on trouve à proximité de la phase supraconductrice. Dans les composés à base de fer, il peut se produire une transition vers une phase "collapsée" qui a pour effet de supprimer les moments magnétiques locaux portés par les fers. Nous avons cherché à comprendre comment les propriétés électroniques sont affectées par cette transition. Dans les composés à base de cobalt, des fluctuations ferromagnétiques apparaissent dans les plans CoAs, et CaCo ₂ As ₂ s'ordonne magnétiquement à 72K. C'est un magnétisme plutôt itinérant, assez différent de celui des phases au fer, et nous avons voulu observer cette transition et l'évolution des corrélations dans cette limite. Durant cette étude nous avons été amené à nous poser des questions sur la façon dont l'ARPES, qui est une technique de surface, peut visualiser l'évolution des structures électroniques, notamment la tridimensionnalité, entre les différents composés où la distance entre les plans CoAs varie fortement suivant la nature de A. / This thesis focuses on the study of the family of iron-based superconductors,compounds discovered in 2008 and displaying a rich phase diagram where there is superconductivity, magnetism and also other electronic orders such as electronic nematicity. This offers the possibility for strong interactions and/or competitions between those various electronic orders,and the study of these compounds aim for a better understanding of the nature and the link between these orders. This should allow generally a better understanding of the collective behavior of correlated electrons in crystalline solids. These compounds are generally metals and havea multi-orbital nature, giving extra degrees of freedom to the electrons. During this PhD, westudied some of these materials using Angle Resolved Photoelectron Spectroscopy (ARPES),a technique allowing to probe their electronic structure and estimate the nature and strength of electronic correlations in each orbital. We focused on phases that are not superconducting,in order to have a more global view of iron-based superconductors. The first part of this PhD focuses on FeSe. The interest for this compound comes from the emergence of a new sort of electronic order : the nematicity, where the electrons break spontaneously the rotational symmetry, making the electronic properties anisotropic. Thanks to ARPES, we were able to follow the evolution of the various orbitals dxz, dyz and dxy, existing at the Fermi level, through the structural transition in order to characterize the electronic structure anisotropy. To interpret these evolutions, we collaborated with the theory group of Lara Benfatto from the Roma uni-versity. We elaborated a model where spin fluctuations, which exist in FeSe although there isno magnetic order, are able to modify the Fermi surface. This reflects the modifications that we observe by ARPES in the nematic phase if we suppose these fluctuations become anisotropic.The second part of this PhD focuses on the pnictides with the form AM ₂ As ₂ (A = Ba, Sr, Ca,M = Fe;Co). This family displays several cases where the magnetism is different from that found near the superconducting phases. In the iron-based compounds, there is a transition towards a collapsed phase where the magnetism is suppressed. We have tried to understand how the electronic properties are affected by this transition. In the cobalt-based compounds,ferromagnetic fluctuations appear within the CoAs planes, and CaCo ₂ As ₂ orders at 72K. This magnetism is itinerant, quite different from that found in the iron-based compounds, and we wanted to observe this transition and the evolution of the correlations in this limit. During this study we have come to question ourselves about the way ARPES, which is a surface technique,can visualize the evolution of the electronic structure, notably its three dimensionality, among the different compounds where the distance between CoAs layers strongly changes depending on the nature of A.