Dissertations / Theses on the topic 'Superconducting multilayers'

Magnetic properties of Nb/Ni superconducting (SC) / ferromagnetic (FM) multilayers exhibit interesting properties near and below SC transition. A complex Field (H) – Temperature (T) phase boundary is observed in perpendicular and parallel orientation of ML with respect to DC field. We address the critical need to develop methods to make reliable magnetic measurements on SC thin films and ML, in spite of their extreme shape anisotropy and the strong diamagnetic response of the SC state. Abrupt, highly reproducible “switching” of the SC state magnetization near the normal-state FM coercive fields has been observed in Nb/Ni ML. The SC penetration depth l(Nb) > the SC coherence length xo(Nb) » 40 nm >> the FM layer thickness y(Ni) = 5 nm, abrupt magnetic reversals might be driven by strong supercurrent densities (J x M torques) that have the potential to flow into the Ni layers. Alternatively, sharp magnetization anomalies also can result from strong flux pinning by the periodic layered structure of ML, including “lock-in” of quantized flux lines (FL) parallel to the ML plane. Strong confinement of the supercurrents within ML planes might also lead to various phase transitions of the FL lattice (FLL) composed of one-dimensional chains and other unusual structures. Possible mechanisms for the switching anomalies must be evaluated while considering other experimental properties of Nb(x)/Ni(y) ML: 1) The upper critical magnetic field Hc2(T) exhibits a highly unusual anisotropy where the SC transition temperature Tc (H®0) for DC field H ^ ML plane exceeds the value for H || ML by ~ 0.5 K. 2) Nb/Ni ML samples do not consistently exhibit magnetic signatures for the onset of superconductivity, depending on the details of the sample mounting procedure and the AC or DC method used in SQUID magnetometry experiments. 3) Unusual “wiggles” or oscillations of order 10-30 mK were observed in Hc2(T) in AC SQUID experiments with H || ML and can be even larger (~0.16 K), depending upon the AC drive amplitude ho and frequency f .

PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO
Neste trabalho foram preparadas multi-camadas supercondutor(SC)/ ferromagneto(FM) Nb/Co via pulverização catódica (Magnetron Sputtering). O principal objetivo é estudar o efeito de diferentes espessuras da camada ferromagnética (Co) nas propriedades supercondutoras do Nb. Era esperado que, após tratamentos térmicos, as camadas de Co formassem um plano de nanopartículas magnéticas ordenadas, cujo efeito deve ser muito diferente das nanopartículas aleatoriamente orientadas e camadas magnéticas continuas. As microestruturas foram investigadas por Difração de Raios-X em baixos ângulos (LAXRD), Microscopia de Força Atômica (AFM) e Microscopia Eletrônica de Transmissão (TEM). Propriedades magnéticas e de transporte tem sido estudadas com o Sistema de Medição de Propriedade Físicas (PPMS), da empresa Quantum Design. As medidas magnéticas e de transporte mostram que, com o aumento da espessura das camadas de Co, a temperatura de transição supercondutora (Tc) aumenta significativamente para as amostras como preparadas. Foi relatado na literatura que quando a espessura das camadas magnéticas da ordem de alguns nanômetros, a Tc aumenta e diminui periodicamente com o aumento da espessura das camadas magnéticas. No entanto, nesta pesquisa, a espessura das camadas magnéticas é de dezenas de nanômetros, sendo muito maior do que este alcance e portanto, não pode ser explicado baseando-se no mesmo modelo. Propusemos que a rugosidade da interface entre as camadas de Co e Nb desempenha um papel importante para este comportamento. Os resultados de AFM e XRD mostram que a rugosidade máxima da interface é da ordem de 7 a 10 nm, o que é comparável à espessura de camadas de Co (de 5 a 20 nm). Introduzimos um parâmetro R igual a d, onde R é a rugosidade da interface e d é a espessura da camada magnética, para discutir o efeito da interface sobre as propriedades supercondutoras da nossa amostra. Quando delta maior que 1, a camada magnética pode ser considerada uma forma não-continua e somente quando delta menor que 1, as camadas magnéticas continuas podem ser formadas. Com base em observações de topografia de interfaces na nano-escala , podemos compreender que primeiro a rugosidade aumenta a área da interface, resultando em um efeito de proximidade mais forte, além de aumentar o efeito do campo de dispersão na Tc. Este efeito depende não somente da rugosidade, mas também da espessura da camada magnética. Verificou-se que o parâmetro determina o efeito das camadas magnéticas. As diferentes propriedades magnéticas abaixo da Tc para diferentes amostras também pode ser explicada por este modelo. Após o tratamento térmico, a Tc das amostras diminuiu e as propriedades magnéticas também se tornam piores do que as amostras como preparadas. Os resultados de TEM mostram que as camadas de Co estam interconectadas e depois do recozimento não há indícios de interdifusão entre as camadas Nb e Co. Mais medidas são necessárias para verificar se as camadas magnéticas podem induzir vórtices espontâneos, assim como para explicar a diferença entre as amostras com nanopartículas magnéticas ordenadas comparadas com aquelas orientadas aleatoriamente.
In this work we prepared Superconductor(SC)/ferromagnet(FM) Nb/Co multi-layers with magnetron-sputtering. The main purpose of this work is to study the effect of different shape of ferromagnetic layers on the superconducting properties of Nb. We expected that after annealing the Co layers can form in-plane ordered magnetic nanoparticles and the effect of ordered magnetic nanoparticles should be very different from randomly oriented nanoparticles and continues magnetic layers. The microstructures have been investigated by means of Low Angle X-ray Diffraction (LAXRD), Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). Magnetic and transport properties have been studied with Physical Property Measurement System (PPMS) from Quantum Design. The magnetic and transport measurements show that with increase of the thickness of Co layers the superconducting transition temperature (Tc) signifficantly increases for the as-prepared samples. It was reported in the literature that when the thickness of the magnetic layers is in the range of several nanometers, Tc increases and decreases periodically with the increase of the thickness of the magnetic layers. In our samples, however, the thickness of the magnetic layers (several tens nanometers) is much larger than that range and therefore, cannot be explained within the same model. We proposed that the roughness of the interface between Co and Nb layers plays an important role for this behavior. The AFM and LAXRD results show that the maxim roughness of the interface is in the range of 7 until 10 nm, which is comparable to the thickness of Co layers (5 until 20 nm). We introduced one parameter R equal d, where R is the roughness of the interface and d is the thickness of the magnetic layer, to discuss the effect of the interface on the superconducting properties of our sample. When delta more 1, the magnetic layer may be in a non-continues form and only when delta less1 continues magnetic layers can be formed. Based upon nano-scale observations of interfaces topography we can understand that the roughness first increases the area of the interface, which gives stronger proximity effect and, second, enhances the effect of the stray eld on Tc. This effect depends not only the roughness but also the thickness of the magnetic layer. It was found out that the parameter determines the effect of the magnetic layers. The different magnetic properties below Tc for different samples can also be explained by this model. After annealing, Tc of the samples decreased and magnetic properties also became worse than the as-prepared samples. The TEM results show that the Co layers is interconnected and after annealing there is no indication of interdiffusion between Nb and Co layers. More measurements are needed to see if the magnetic layers can induce spontaneous vortices and what the difference is between samples with ordered and randomly oriented magnetic nano-particles.

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CAPES
O estudo em sistemas de multicamadas supercondutoras alcan cou grande interesse na pesquisa de novos materiais, pois al em de apresentarem ampla aplicabilidade tecnol ogica, oferecem fascinantes possibilidades de observar novos fen^omenos na supercondutividade. Esta ultima caracter stica e a de maior motiva c~ao para esta tese, uma vez que pouco ainda se tem reportado a respeito da supercondutividade em multicamadas constitu das por supercondutor(S)/supercondutor(S0). Neste trabalho, estudamos tr^es novos sistemas nanoestruturados formados por supercondutores elementares, de baixa temperatura cr tica, tais como ni obio (Nb), chumbo (Pb) e estanho (Sn), a saber, a tricamada Nb(5)/Pb(500)/Nb(50) e as multicamadas Nb(100)/[Sn(50)/Nb(50)]7 e Cr(10)/Nb(100)/[Sn(50)/Nb(50)]7, onde o termo entre par^enteses indica a espessura da referida camada em nan^ometros e os colchetes indicam que a estrutura Nb/Sn e repetida 7 vezes. Para compara c~ao, tamb em s~ao analisados lmes de refer^encia de Nb, Pb e Sn. Os lmes e as multicamadas foram crescidos a temperatura ambiente por deposi c~ao via sputtering, utilizando fontes DC e RF. As amostras foram caracterizadas por difra c~ao de raios X de baixo ^angulo, microscopia eletr^onica de varredura e por microscopia de for ca at^omica. As propriedades supercondutoras foram estudadas atrav es de medidas magn eticas e de transporte el etrico. Magnetiza c~ao e resistividade foram medidas como fun c~oes da temperatura e do campo magn etico aplicado perpendicular e paralelamente as camadas. Os tr^es sistemas apresentaram transi c~oes supercondutoras abruptas, tanto na magnetiza c~ao quanto na resistividade em fun c~ao da temperatura, com os valores de TC obtidos de 7,2 K para o Nb/Pb/Nb, 5,2 K para o Nb/[Sn/Nb]7 e 3,7 K para o Cr/Nb/[Sn/Nb]7. As propriedades supercondutoras dos lmes de Pb, Nb e Sn, tais como TC, 0Hc2 e comprimentos caracter sticos apresentaram coer^encia com os valores reportados na literatura, con rmando a boa qualidade das camadas. Flux jumps s~ao observados nos loops de histerese em todos os sistemas, os quais s~ao atribu dos a instabilidades termomagn eticas. A amostra Nb/Pb/Nb apresentou uma curvatura positiva na depend^encia de 0Hc2(T), indicando que a supercondutividade ocorre preferencialmente na camada de Pb para T > T e preferencialmente na camada de Nb para T < T , de acordo com a teoria de Takahashi-Tachiki. Os comprimentos caracter sticos s~ao calculados, classi cando as multicamadas como supercondutores do tipo II. Veri camos grande in u^encia do material magn etico Cr na supercondutividade da multicamada, causando a diminui c~ao da temperatura cr tica do sistema e modi cando o comportamento dos campos cr ticos inferior e superior. A depend^encia de 0Hc1(T), em todos os sistemas, revelou um comportamento n~ao convencional, que e atribu do a uma manifesta c~ao de sistemas multicomponentes com componentes espacialmente separadas.
The study of superconducting multilayers systems has achieved much interest in the research of new materials. Besides their wide technological applicability, these structures o er fascinating possibilities to observe new phenomena in superconductivity. The latter characteristic is the fundamental motivation for this thesis, since a little has been reported about multilayers constituted of superconductor(S)/superconductor(S0). In this work, we have studied three new nanostructured systems formed by low critical temperature conventional superconductors, such as niobium (Nb), lead (Pb) and tin (Sn): the trilayer Nb(5)/Pb(500)/Nb(50) and the multilayers Nb(100)/[Sn(50)/Nb(50)]7 and Cr(10)/Nb(100)/[Sn(50)/Nb(50)]7. The term in parenthesis indicates the thickness of the layer in nanometers and the brackets that Nb/Sn structure is repeated 7 times. For comparison, the reference lms of Nb, Pb and Sn were also analyzed. The thin lms and multilayers were growth at room temperature via sputtering deposition, by using DC and RF sources. The samples were characterized for small angle X ray di raction, scanning electron microscopy (SEM), and atomic force microscopy (AFM). The superconducting properties were investigated through magnetic and electric transport measurements. The magnetization and resistivity were obtained as functions of temperature and magnetic eld applied both perpendicularly and parallel to the layers. All the multilayers systems have showed sharp superconducting transitions in the dependence of magnetization and resistivity with the temperature. The multilayers critical temperatures were found to be 7.2 K for Nb/Pb/Nb, 5.2 K for Nb/[Sn/Nb]7 and 3.7 K for Cr/Nb/[Sn/Nb]7. The superconducting properties of reference lms Pb, Nb and Sn, such as critical temperature, upper critical eld and characteristic lengths were consistent with the literature values, con rming the good quality of the samples. In the three systems ux jumps were observed, which are attributed to thermomagnetic instabilities. The Ginzburg-Landau parameter is estimated, classifying the multilayers as type II superconductors. In the case of Nb/Pb/Nb sample, it presented an upward curvature in the 0Hc2(T) diagrams, which is a signature of superconductivity nucleation in the each layer, in accordance with Takahashi-Tachiki theory for multilayered systems. We found out a noticeable in uence of the magnetic material Cr on the multilayer superconducting properties, reducing the critical temperature of the system and modifying the lower and upper critical elds behavior. The dependence of 0Hc1(T), of all the multilayers, revealed a non-conventional behavior feature, which is consistent with a multicomponent behavior with spatially separated components.

La réduction du cout de construction et d’exploitation des futurs accélérateurs d particules, a grande et petite échelles, dépend du développement de nouveaux matériaux pour les surfaces actives des structures supraconductrices en radiofréquence (SRF). Les propriétés SRF sont essentiellement un phénomène de surface vu que la profondeur de pénétration (profondeur de pénétration de London, λ) des micro-ondes (RF) est typiquement de l’ordre de 20 à 400 nm en fonction du matériau. Lorsque les procédés de préparation de surface sont optimises, la limite fondamentale du champ RF que les surfaces SRF peuvent supporter est le champ RF maximum, Hc₁, au-delà duquel le flux magnétique commence à pénétrer la surface du supraconducteur. Le matériau le plus utilise pour des applications SRF est le niobium (Nb) massif, avec un champ Hc₁ de l’ordre de 170 mT, qui permet d’atteindre un champ accélérateur de moins de 50 MV/m. Les meilleures perspectives d’amélioration des performances des cavités SRF sont liées à des matériaux et méthodes de production produisant la surface SRF critique de façon contrôlée. Dans cette optique, deux avenues sont explorées pour utiliser des couches minces pour augmenter les performances des structures SRF au-delà du Nb massif, en monocouche ou en structures multicouches Supraconducteur-Isolant-Supraconducteur (SIS) : La première approche est d’utiliser une couche de Nb déposée sur du cuivre (Nb/Cu) à la place du Nb massif. La technologie Nb/Cu a démontré, au cours des années, être une alternative viable pour les cavités SRF. Toutefois, les techniques de dépôt communément utilisées, principalement la pulvérisation magnétron, n’ont jusqu’à présent pas permis de produire des surfaces SRF adaptées aux performances requises. Le récent développement de techniques de dépôt par condensation énergétiques, produisant des flux d’ions énergétiques de façon contrôlée (telles que des sources d’ions ECR sous ultravide) ouvrent la voie au développement de films SRF de grand qualité. La corrélation entre les conditions de croissance, l’énergie des ions incidents, la structure et les performances RF des films produits est étudiée. Des films Nb avec des propriétés proches du Nb massif sont ainsi produits. La deuxième approche est basée sur un concept qui propose qu’une structure multicouche SIS déposée sur une surface de Nb peut atteindre des performances supérieures à celles du Nb massif. Bien que les matériaux supraconducteurs à haute Tc aient un champ Hc₁ inférieur à celui du Nb, des couches minces de tels matériaux d’une épaisseur (d) inférieure à la profondeur de pénétration voient une augmentation de leur champ parallèle Hc₁ résultant au retardement de la pénétration du flux magnétique. Cette surcouche peut ainsi permettre l’écrantage magnétique de la surface de Nb qui est donc maintenue dans l’état de Meissner à des champs RF bien plus importants que pour le Nb massif. La croissance et performance de structures multicouches SIS basées sur des films de NbTiN, pour le supraconducteur, et de l’AlN, pour le diélectrique, sont étudiées. Les résultats de cette étude montrent la faisabilité de cette approche et le potentiel qui en découle pour l’amélioration des performances SRF au-delà du Nb massif
The minimization of cost and energy consumption of future particle accelerators, both large and small, depends upon the development of new materials for the active surfaces of superconducting RF (SRF) accelerating structures. SRF properties are inherently a surface phenomenon as the RF only penetrates the London penetration depth λ, typically between 20 and 400 nm depending on the material. When other technological processes are optimized, the fundamental limit to the maximum supportable RF field amplitude is understood to be the field at which the magnetic flux first penetrates into the surface, Hc₁. Niobium, the material most exploited for SRF accelerator applications, has Hc₁~170 mT, which yields a maximum accelerating gradient of less than 50 MV/m. The greatest potential for dramatic new performance capabilities lies with methods and materials which deliberately produce the sub-micron-thick critical surface layer in a controlled way. In this context, two avenues are pursued for the use of SRF thin films as single layer superconductor or multilayer Superconductor-Insulator-Superconductor structures: Niobium on copper (Nb/Cu) technology for superconducting cavities has proven over the years to be a viable alternative to bulk niobium. However the deposition techniques used for cavities, mainly magnetron sputtering, have not yielded, so far, SRF surfaces suitable for high field performance. High quality films can be grown using methods of energetic condensation, such as Electron Cyclotron Resonance (ECR) Nb ion source in UHV which produce higher flux of ions with controllable incident angle and kinetic energy. The relationship between growth conditions, film microstructure and RF performance is studied. Nb films with unprecedented “bulk-like” properties are produced. The second approach is based on the proposition that a Superconductor/Insulator/Superconductor (S-I-S) multilayer film structure deposited on an Nb surface can achieve performance in excess of that of bulk Nb. Although, many higher-Tc superconducting compounds have Hc₁ lower than niobium, thin films of such compounds with a thickness (d) less than the penetration depth can exhibit an increase of the parallel Hc₁ thus delaying vortex entry. This overlayer provides magnetic screening of the underlying Nb which can then remain in the Meissner state at fields much higher than in bulk Nb. A proof of concept is developed based on NbTiN and AlN thin films. The growth of NbTiN and AlN films is studied and NbTiN-based multilayer structures deposited on Nb surfaces are characterized. The results from this work provide insight for the pursuit of major reductions in both capital and operating costs associated with future particle accelerators across the spectrum from low footprint compact machines to energy frontier facilities

Nous avons concu et realise un systeme de mesure d'impedance de surface z#s de films minces conducteurs ou supraconducteurs par couplage electromagnetique de ces films a un oscillateur a diode tunnel. La mesure de la resistance de films metalliques (or et aluminium) par cette technique inductive (sans contact) nous a permis, en comparant aux mesures directes (4 fils), de valider cette methode. Nous avons ensuite etudie la reponse electromagnetique de films supraconducteurs d'yba#2cu#3o#7 et de multicouches yba#2cu#3o#7/la#2##xsr#xcuo#4. Cette etude a mis en evidence le caractere granulaire de ces materiaux reflete par des valeurs trop elevees de la profondeur de penetration () et la presence d'une resistance residuelle dans la phase supraconductrice. Le comportement de en fonction de la temperature indique que le couplage intergranulaire serait de type sis dans le cas de films et sns dans le cas des multicouches etudiees. Cette difference devrait avoir son origine dans les procedes et conditions de fabrication de ces echantillons

碩士
國立高雄師範大學
物理學系
92
Abstract Single Nb60Ti40 films,(Co/ Nb60Ti40) bilayers, (Co/ Nb60Ti40/Co) trilayers and multilayers were grown onto Si (100) substrate. The superconducting transition temperature,Tc, of single Nb60Ti40 films were observed to decrease monotonically with decreasing superconducting layer thickness. With Co layers added on top or bottom, Tc shifted to lower temperature than single Nb60Ti40 film. It shows that Co will break superconductivity order parameter and reduce Tc. Tc versus trilay thickness was fitted by model, from which we got a critical thickness of 27nm. If Nb60Ti40 layer is thinner, no superconductivity transition could be found. When the behavior of superconducting upper critical field (Hc2 ) crossovers from three-dimension to two-dimension, its temperate dependence changes from linear to non-linear, and angular dependence changes from bell shape to cusp shape. Our data showed these crossover behaviors around 140nm. In three-dimension regime, Hc2⊥ and HC2// crossoverd as temperature cool down. This behavior can be explained by electron energy state in different crystal direction. We measured resistance of multilayers verse both bilayer number and NbTi thickness with C.P.P(Current perpendicular to plan) geometry. A series resistance model was used for quantitative analysis of interface reisitance. We got 2 times unit area resistance value (2AR) of 3.8±0.2fΩm2 and 18.6±0.6fΩm2 for normal and superconducting Co/Nb60Ti40 interface

Thesis (Ph. D.)--University of Wisconsin--Madison, 1996.
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This thesis was motivated by the suggestion that selectively implanting YBa₂Cu₃0₇ (YBCO) films with a highly reactive ion, such as Si, could pattern without destroying or removing material. If true, this would greatly simplify conventional methods of patterning multilayer structures. This led to systematic studies on the use of ion implantation to pattern YBCO thin films and multilayers. The samples used were obtained by developing a method referred to as scanning pulsed laser deposition. This technique resulted in the reproducible growth of highly crystalline YBCO thin films with high transition temperatures and critical currents. A study of Si implantation was first done to elucidate the exact mechanism by which it rendered YBCO non-superconducting. Measurements of films implanted at various energies and doses revealed that implantation at required doses severely damaged the films crystalline structure, destroying superconductivity. The growth of high quality multilayers require that the underlying patterned film retain its as-grown crystalline quality. A damaged film can regain that quality (to some extent) with high temperature annealing. Measurements of annealed implanted films revealed that crystalline damage at levels >10 displacements per atom (dpa) could not be completely removed at accessible annealing temperatures. However, when the implantation damage was kept below 1-2 dpa, an annealing temperature of ~ 900°C was successful in recovering most of the original structure. Unfortunately, at these temperatures, the Si-implanted film phase separated, forming islands of a Si-mixed material in a sea of YBCO , and thus regained its original high transition temperature. To address the original claims that Si implantation would not destroy the film, a film was rendered non-superconducting with the appearance that it maintained its crystallinity by implanting only near the films surface. In this case, the film was effectively passivated and a low temperature anneal resulted in oxygen leaving the underlying YBCO structure to the more energetically favored Si0₂ states in the implanted layer. Replenishment of oxygen from the atmosphere was hindered due to the passivating layer capping the film. Patterning films by implanting Si was thus deemed to be unsuitable for multilayer structures. A new technique of substitutional ion implantation patterning was developed using Mg ions. Mg substitutes for the Cu in the Cu-0 planes, drastically reducing transition temperature with very low concentrations. The low required concentrations allowed the use of low implant doses. This, coupled with the relatively low mass of the Mg ion, reduced the implantation damage to levels easily removed with high temperature annealing. As well, the high temperature anneal can incorporate Mg into the YBCO matrix, forming the compound, YBa₂(Cu₁[sub -x]Mg[sub x])₃0₇. The process of implanting Mg into YBCO , followed by a high temperature anneal resulted in the formation of a highly crystalline, non-superconducting material at 77K. Mg implantation was used to successfully pattern films with a resolution of 10 μm. Bilayers with a top YBCO layer and a bottom YBa₂(Cu₁[sub -x]Mg[sub x])₃0₇ were fabricated. Resistivity and x-ray measurements reveal the high quality of both layers.

碩士
國立臺灣海洋大學
光電科學研究所
97
The purpose of this thesis is to study the superconducting flux pinning effect in the multilayered thin films of superconductive Nb layers sandwiched between ferromagnetic Fe, Co, and Ni layers. The samples were grown by sputter deposition with structure (M/S)6/M, where M is the ferromagnetic layer and S is the superconductive Nb layer. The superconducting flux states in samples were created by applying fixed magnetic fields. The excitation of the creep of magnetic flux, which manifested itself as electrical resistance, was accomplished by raising the temperature. The pinning energy of the magnetic flux could be deduced by analyzing the resistance versus temperature curves. The pinning energy was studied with various superconducting layer thickness, applied field orientation, and 'M' materials. Analyses of the dimensional effect of the superconductor were achieved by first determining the three-dimension to two-dimension crossover from the upper critical field, Hc2, as a function of temperature. Our results showed two-dimensional multilayers have larger flux pinning energy. We tried to change different magnetic materials for testing whether these material's characters affected the flux pinning mechanism, maybe the result was no obvious dependence was observed.

碩士
國立臺灣師範大學
光電科技研究所
97
The thesis consists of six chapters. Chapter 1 is to give a brief review of basic properties of a superconductor. Chapter 2 introduces the theoretical methods to be used in the calculation of the effective surface impedance. In chapter 3, the effective surface impedance of a high-temperature superconductor thin film on a semiconductor plasma substrate is calculated. Two possible configurations are considered. The first one is a superconducting film deposited on a semi-infinite semiconductor substrate. It is seen there exists a critical film thickness for superconductor such that a minimum effective surface resistance is attained. The effective surface resistance is strongly dependent on the high-frequency permittivity of semiconductor plasma. The second will be limited to the more practical case, that is, the semiconductor substrate is of finite thickness. The investigation of substrate resonance in the effective surface resistance shows some fundamental distinctions when a semiconductor plasma substrate is introduced. In chapter 4, the structure is a superconducting film deposited on a ferromagnetic substrate. We investigate the effective surface impedance as a function of the different thicknesses of superconductor and ferromagnetic as well. In chapter 5, we investigate the effective surface resistance and surface reactance in the case of normal incidence in a superconductor-semiconductor superlattice. We find there is a critical thickness for superconductor film such that a minimum loss is attained. We next study the effective surface impedance under the oblique incidence. The calculated angle-dependent effective surface impedance indicates that it does not rely on the number of periods when the period is greater than two and, in the meantime, the surface resistance is nearly unchanged as a function of the angle of incidence. The conclusion is given in Chapter 6.

碩士
國立虎尾科技大學
光電與材料科技研究所
92
There are two parts in this thesis. In the first one, we use the transfer-matrix method to analyze the optical properties for the one dimensional periodic superconduction/dielectric photonic structures. Based on the dispersion relation, ω and κ ,for the Bloch wave it is found the existence of the bandgap. In our analysis the two-fluid model is used to describe the electrodynamics of superconductors. In the second part we shall use the Abeles theory to analyze the influence of buffer layers on microwave propagation in a superconducting film layered structure. For numerical analyses of microwave properties, MATHCAD software will be used. YBa2Cu3O7-film on Sapphire with buffer layers SrTiO3 and CeO2,and on Silicon with buffer layers GaF2 and YSZ will be considered.

碩士
國立交通大學
電子物理學系
82
We have investigated some of the fundamental parameters involed in preparing YBCO/(I)/YBCO/NdGaO3 multilayer super- conducting thin film by using KrF Excimer pulsed laser. For single layer YBCO directly deposited onto NdGaO3 single crystal substrate, high quality film were obtained; with Tco =92K, Jc(77K)>8*10MA/ cm2. In particular, due to the optimiza- tion of deposition condition and the near perfect lattice matching between YBCO and NdGaO3, the surface topography of the obtained film, as revealed by atomic force microscopy (AFM), was extremely smooth (with roughness smaller than 5nm over 3 micrometer scanned length). With the results accompli- shed, we have also proceeded to deposited PBCO, as well as NdGaO3 as the insulating layer. It was found that the choice of these intermediate layer of the aimed multilayer structure, has a profound effect on the deposition of the subsequent superconducting layer. The detail characterizations of the microstructure analyses, and transport property measurements were performed to delineate the interplays between the growth- structure-properties.

碩士
國立虎尾科技大學
光電與材料科技研究所
93
Microwave communication has become the main stream in modern communication technology. Microwave devices such as waveguides or transmission lines interconnecting modular and components have dominant effects on the transmission quality. A tunable microwave waveguide can be achieved by applying the static magnetic field. In first part of this thesis we use the electromagnetic theory to analyze the propagation characteristics in the multilayer guided structures consisting of magnetic and dielectric thin films. We shall study the frequency- and field-dependent attenuation and phase constants. In addition, the interaction between the spin wave and microwave will also be investigated. In the second part, we shall employ the London equation and two-fluid model for the superconductor to analyze the propagation properties in a multilayer structure waveguide made of a superconductor (YBCO) and magnetic (YIG) films. The numerical results will be analyzed and discussed further. The results will also be compared with the experimental results reported in the literature.