Rozprawy doktorskie na temat „Niobium superconductor”

Superconductivity and ferromagnetism are two antagonistic physical phenomena which their coexistence in a uniform material can be resolved only under extraordinary conditions. The reason for that is the phonon-mediated attraction energy between electrons which results in the formation of the so-called Cooper pairs, is usually smaller that the exchange (Zeeman) interaction between electrons which tend to align the electron spins. However, non-zero total momentum Cooper pairs can be accomplished even in the presence of an exchange field as surprisingly! predicted first by Fulde and Ferrel [1] and independently by Larkin and Ovchinikov [2] nearly 50 years ago. This coexistence has already been observed experimentally in both bulk samples [3, 4] and in thin films [5-7] which result from a different type of electron-pairing mechanism which electrons with spin pointing in the same direction team up to form Cooper pairs with one unit of spin, resulting in the so-called triplet superconductivity. Apart from this so-called ferromagnetsuperconductors which both superconducting and ferromagnetism order parameters are present in a uniform material, hybrid systems [8] are made form materials with different or even mutually exclusive properties. Therefore the overall property can be strongly affected by the interaction between constituent materials. The present work, concerns such a hybrid system where Nb, a superconducting metal having transition temperature below 9.5K, is placed in contact with a ferromagnetic metal, Gd with bulk Curie temperature of around 290 K in a form of a nanocomposite. The mutual immiscibility of these two elements gives us the opportunity to take advantage of both the superconduction and ferromagnetism properties of the constituents and further study the transport and magnetic behavior of the system and their effects on each other specially on the critical current of the superconductor which is expected to be modified by the proximity of the ferromagnetic metal.

The antiferromagnetic coupling between a rare-earth (RE) and a tran- sition metal (TM) ferromagnet can be exploited to engineer normal state and superconducting functional devices. RE/TM ferromagnetic multi- layers were previously used as spin-mixers to generate spin-triplet su- percurrents. This was possible due to magnetic inhomogeneity present in the devices, however the precise nature of the inhomogeneity was not understood. Here we present a comprehensive study of the Ni/Gd/Ni system using a powerful element-specific measurement technique: x-ray magnetic circular dichroism. In order to analyse the experimental results we present a novel model based on the Stoner-Wohlfarth model, which shows that significant inhomogeneity exists at the Ni/Gd interfaces due to the competition between the exchange energies within the system and the Zeeman energy of the applied magnetic field. The experiment and model together provide a complete overview of the Ni/Gd/Ni system due to the breadth of temperatures and thicknesses studied. The knowledge gained from this work is then applied to designing and test- ing new spin valves based on the intrinsic inhomogeneity at the RE/TM interface, and both Ni/Gd- and Gd/Ho-based devices show reversible magnetic switching behaviour which alters the superconducting critical temperature.

A high resolution pressure gauge has been developed for use in thermodynamic measurements along the lambda line in liquid helium. The gauge was designed to operate at cryogenic temperatures and provide pressure measurements up to 30bar, with an accuracy of 3 × 10¯¹ºbar in a 1Hz bandwidth. Experiments reported here show the gauge to have met these specifications; at least for measurements close to zero pressure, at temperatures close to 4.5K. It is expected that operation at higher pressures, and at temperatures closer to the lambda transition, will result in similar or even improved performance. The gauge consists of a titanium-alloy diaphragm with a superconducting position transducer read-out. Compensation techniques internal to the superconducting circuit were used to eliminate any significant sensitivity to temperature fluctuations and in-line acceleration. For high values of common-mode rejection, thermal compensation revealed a non-linear temperature characteristic which was exploited to provide a further reduction in the temperature sensitivity. Acceleration compensation was achieved up to a common-mode rejection of more than 78dB. Present performance appears to be limited by thermal gradient fluctuations at low frequencies and at higher frequencies by a noise source which appears to originate beyond the superconducting transducer. It is expected that some further improvement may be gained in this higher frequency band simply by trapping a larger persistent current in the superconducting circuit. In the course of development and characterization of the gauge several anomalous effects were discovered and investigated. In response to changes in temperature, the gauge was found to exhibit irreversible behaviour in a variety of ways. These phenomena were fully investigated and found to be complex in nature. A critical state model was employed which was successful in explaining many of the observed effects. Other authors have observed apparently related behaviour in samples of niobium and some have developed similar critical state models which give results generally consistent with those reported here. However, these latter works have not investigated the presence of such effects within superconducting wires; neither have they considered the implications for devices based upon superconducting wire circuits. It appears this anomalous behaviour may be relevant to a broad range of instruments employing superconducting wire circuits similar to that used here. If this is the case, the results presented here have significant consequences for the performance of such devices

Orientador: Oscar Ferreira de Lima
Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataglin
Made available in DSpace on 2018-09-24T18:20:24Z (GMT). No. of bitstreams: 1 Oliveira_TarcisoMesquitade_M.pdf: 3236428 bytes, checksum: f8799c6f4d4ac1a196ca499b0ad67efb (MD5) Previous issue date: 2005
Resumo: Nesta tese estudamos possíveis origens do Segundo Pico da Magnetização (SPM) em supercondutores de alta e baixa temperatura crítica, usando amostras monocristalinas de Bi2Sr2CaCu2O8+d e de Nb. Para estudar o SPM fizemos medidas de curvas de magnetização M x H e suscetibilidade AC (vs. h e T). Através da aplicação de uma lei de escala, para suscetibilidade AC, obtivemos o expoente de creep na região do SMP. Em amostras de Nb verificamos os efeitos sobre as curvas M x H de não homogeneidades na amostra: deslocamentos de planos e oxigênio intersticial. Em amostras de alta pureza (bulk e monocristalina) observamos que as curvas M x H não apresentam o SPM nem o Efeito Pico, próximo a Hc2.Ao adicionar não homogeneidades nas amostras, através de dopagem com oxigênio e deformação por elongação, observamos o aparecimento do Efeito Pico, de instabilidades termomagnéticas e de anisotropia do campo Hc2. Em amostras de Bi2Sr2CaCu2O8+d observamos que o SPM está associado com a componente do campo aplicado paralela à direção c da rede cristalina, ou seja, que o SPM está relacionado com vórtices de Abrikosov e que vórtices Josephson parecem não influenciar no SPM. Ao clivarmos a amostra observamos que a intensidade do SPM se reduz e diminui a janela de temperatura onde ele é visto. Obtivemos o expoente de creep e observamos que na região do SPM o arrasto dos vórtices diminui em campos que antecedem o HSPM , mas na região de HSPM o arrasto aumenta rapidamente e depois volta a cessar. Interpretamos a variação brusca no expoente de creep como uma mudança de fase no sistema de vórtices, que passa de um estado de quase-rede para um estado emaranhado de vórtices
Abstract: In this thesis we have studied possible origins for the Second Magnetization Peak (SMP) in high and low critical temperature superconductors, using samples of Bi2Sr2CaCu2O8+d and Nb single crystals. To study the SMP we did measurements of magnetization curves (M x H) and AC susceptibility (vs. h and T). We obtained the creep exponent in the SMP region, using a scaling law for the AC susceptibility data. In the Nb samples we have verified the effects on M x H curves due to inhomogeneities like dislocations and interstitial oxygen. In high pure samples (bulk and single crystals) we have observed that the M x H curves do not present either SMP or Peak Effect, near H2c . By adding inhomogeneities in the samples, like doping them with oxygen or deforming by elongation, we have observed the appearance of Peak Effect, thermomagnetic instabilities, and H2c anisotropy. In Bi2Sr2CaCu2O8+d samples we have observed that the SMP is associated with the magnetic field component parallel to the crystal c direction of crystal net. This means that the SMP is related to Abrikosov¿s vortices and that Josephson¿s vortices seem not to influence the SMP. After cleaving the sample we observed that the SMP intensity was reduced and the temperature window were it occurs was decreased. We have obtained the creep exponent and have observed that in the SMP region the vortices creep diminish for fields below HSPM , but around HSPM the creep increases abruptly and after the SPM it decreases again. We have interpreted the abrupt creep exponent variation as a phase transition in the vortex system, that pass from a quasi-lattice state to an entangled vortex state
Mestrado
Física
Mestre em Física

Thin-film niobium mechanically controlled break junctions and resistively shunted niobium mechanically-controlled break junctions were developed and successfully microfabricated. Using these devices, high-stability atomic size contacts were routinely produced and investigated both in the normal and superconducting states. Investigations of the two-level conductance fluctuations in the smallest contacts allowed the calculation of their specific atomic structure. Embedding resistive shunts close to the superconducting atomic-sized junctions affected the coherence of the electronic transport. Finally, point contact spectroscopy measurements provide evidence of the interaction of conduction electrons with the mechanical degrees of freedom of the atomic-size niobium contacts.

Detailed, accurate measurements of critical current density and resistivity to determine the upper critical field have been made on a technological NbsAl conductor in magnetic fields up to 15 T, temperatures from 4.2 K up to the critical temperature and in the strain range from -1.8% to 0.7%. The uncertainty in temperature above 4.2 K was equivalent to ± 100 mK with a stability during the measurements of < 5 mK up to a limiting current of 80 A and a typical noise level of 1 µ Vm(^-1).When B(_c2){T,ε) is defined at 5%pn, 50%pn or 95%/%pn, an empirical relation is found where and an approximate relation, holds. The Jε data were parameterised using F(_p) = J(_E)B = A(ε)[Bc(_2)](^n)b(^p)(1-b)(^9) where b = B/B(_c2)(T,ε). When B(_c2)(T,ε) is constrained to be the value at 50%pn or 95%pn, the scaling law for F(_p) breaks down such that p and q are strong functions of temperature and q is also a strong function of strain. However, when B(_c2)(T,ε) is defined at 5%pn, there is good scaling where p and q are constants independent of temperature and strain. F(_p) can also be approximated by a Kramer form where the Ginzburg-Landau constant is γ is the electronic density of states and is interpreted as the average B(_c2) for the bulk where percolative current flow occurs. The critical current density of Hot Isostatic Pressed (HIP'ed) and unHIP'ed Nb(_3)Sn Modified Jelly Roll wires has also been measured at 4.2 K. The critical current and upper critical field were decreased for the HIP'ed sample. The reduced upper critical field of the HIP'ed wire was found to be less sensitive to strain than the unHIP'ed wire. The exponent of B(_c2) in the flux pinning scaling law increased from 0.86 to 2.14 as a result of the HIP processing.

Ce travail est consacre a l'etude des variations structurales et des proprietes physiques en fonction de x dans les systemes supraconducteurs li#1#-#xti#2o#4 et li#1#-#xnbo#2 (0x<1). Nous avons travaille entierement sur des monocristaux afin de pouvoir controler l'extraction chimique des quantites precises de li#+ et de favoriser une bonne homogeneite des echantillons. Nos resultats montrent que le depart de li#+ dans li#1#-#xti#2o#4, a temperature ambiante, produit, a partir de x. 15, un deplacement des cations de titane sur les sites octaedriques vides 16c de la structure spinelle. Nous avons montre que en operant l'extraction du li#+ a 255k et en conservant les echantillons a 77 k, on peut eviter le mouvement des atomes de titane. Dans ce cas le reseau spinelle reste ordonnee et la t#c du systeme augmente de 11. 2 a 13. 2k lorsque x croit de 0 a 0. 25. Par contre, la t#c diminue lorsque le degre de desordre du titane augmente, jusqu'a la disparition complete de la supraconductivite dans les echantillons tres desordonnes. Dans les deux cas, la t#c varie comme la valence du titane. Notre etude du systeme li#1#-#xnbo#2 a montre que la diminution de x: i) impose l'apparition de la supraconductivite dans ce systeme pour x 0. 25 et ii) produit une surstructure qui implique le dedoublement de la maille hexagonale suivante c et qui est liee a un ordre d'occupations de sites de li#+. Il en resulte un abaissement de la symetrie qui, d'hexagonale (p6#3/mmc) dans le linbo#2 devient trigonale p3ml dans le li#0#. #7#5nbo#2. Ces variations structurales impliquent des variations de la valence du niobium. L'apparition de la supraconductivite est liee a c une augmentation de 0. 15 u. V. De la valence moyenne du niobium. Celui-ci, qui est de 3#+ u. V dans le semi-metal linbo#2 se dismute en 3. 7#+ u. V et 2. 6#+ u. V dans le supraconducteur li#0#. #7#5nbo#2

Dans la premiere partie nous decrivons la mise au point d'une experience de mesures de transport a basses temperatures. Nous avons construit un dispositif experimental qui permet de piloter la rotation de l'echantillon a basse temperature sous fort champ magnetique a partir de l'exterieur du cryostat. La deuxieme partie de la these est consacree a l'etude systematique des siliciures isostructuraux et isoelectroniques vsi#2, nbsi#2 et tasi#2. Les echantillons sont monocristallins. Les trois composes sont metalliques et leur resistivite a haute temperature est anisotrope. Des mesures de magnetoresistance nous ont permis d'explorer la surface de fermi de nbsi#2. Des mesures de chaleur specifique a tres basse temperature revelent que la densite d'etats electronique au niveau de fermi est deux fois plus elevee pour vsi#2 que pour les deux autres siliciures. Les temperatures de debye des trois composes varient en fonction de m##1#/#2 (m: masse molaire) indiquant que les forces interatomiques sont identiques. Nbsi#2 et tasi#2 sont supraconducteurs en dessous respectivement 130 mk et 353 mk. Des mesures de susceptibilite montrent que vsi#2 est paramagnetique, nbsi#2 et tasi#2 sont diamagnetiques. Les spectres de phonons obtenus par des mesures de spectroscopie de pointes sont en bon accord avec les temperatures de debye obtenues par les autres methodes. Des etudes sur les proprietes de transport des siliciures semiconducteurs crsi#2, resi#1#,#7#5 et ru#2si#3 sont decrites dans le troisieme chapitre. La preparation de monocristaux de ces materiaux est plus difficile que celle des siliciures metalliques. La resistivite des trois composes est anisotrope. Ru#2si#3 et resi#1#,#7#5 se comportent comme des semiconducteurs dopes en regime extrinseque. Crsi#2 montre plutot le comportement d'un mauvais metal. La magnetoresistance de ru#2si#3 a basse temperature peut s'interpreter dans un modele de localisation faible ou les interactions electron-electron sont dominantes

碩士
國立清華大學
物理系
104
We implement a tunable vertical graphene-superconductor tunnel junction and aim to investigate the tunneling behaviors between Dirac fermions and Cooper pairs. Our device consists with two serial coupled Niobium (Nb)-Graphene junctions, which are coated with an ionic gel as a top-gate electrode for controlling the Fermi-level (EF) of grapheme at room temperature. When EF is tuned away from the charge-neutral point (CNP or called the Dirac point) of grapheme and the temperature is lower to the critical temperature of Nb (~ 9 K), we observe a clear dip structure of differential conductance in the vicinity of zero-bias source-drain voltage (VSD), and peak features as VSD approaches the superconducting gap, which indicate clear behaviors of a superconducting Nb-graphene junction. The general features aforementioned evolve with EF ; intriguingly the differential conductance becomes fluctuating as EF is tuned closed CNP. Our data analysis suggests the observed tunneling behaviors are consistent with the theoretical predictions based on the approaches of Bogoliubov-de-Genn equation when EF is away from CNP. Near CNP, the tunneling behaviors are dominated by the combinations of the effects of interfacial disorders and specular Andreev reflection.

碩士
國立清華大學
物理系
104
We investigate the electric properties through graphene-Nb superconductor junction and aim to study Andreev reflection (AR) as a function of Fermi-level EF of graphene. The Nb thin film is deposited on quartz with superconducting temperature Tc close to 9 K. The graphene is prepared by chemical vapor deposition (CVD) and transfer on top of Nb-pattern, in which two series coupled junctions are defined. In particular, we carefully treat Nb surface so to ensure clean interface is achieved. To control EF of graphene, we employ ion-gel on graphene as a gate electrode. We first characterize the resistance of graphene with the gate voltage Vg, determine EF respected to the charge neutral point (CNP) and then cool down the sample below Tc,. We find as EF is tuned away CNP, the differential conductivity dI/dV v.s. V shows a suppressed dip structures, which width is comparable with the expected superconducting gap of Nb. Distinctly, as EF is in the vicinity of CNP, the downward feature of dI/dV becomes more pronounced. Data analysis suggest the retro-AR is enhanced near CNP due to smaller the critical angle of Andreev reflection. Our results provide the first comprehensive studies of electric properties of graphene-supercondutor interface and open a new route to further explore the tunneling processes of AR for Dirac fermions.

Andreev reflection (AR) is the underlying phenomena that determines the quasiparticle dynamics at the junction of a superconductor with any non-superconducting material, which in turn determines the transport properties of the junction. The tunability of Fermi energy in the two-dimensional (2D) Dirac semimetal graphene by using an electrostatic gate opened up possibility to realize some new and intriguingly different kind of AR. All of my thesis work is on understanding different kind of AR at graphene – superconductor interfaces, either by conductance measurements or by combining this with shot noise measurements. In conventional Normal metal - superconductor junctions the AR is always retro type. Interplay of superconductivity and relativistic dynamics gives rise to specular type AR (SAR) at graphene - superconductor junction. We carried out the transport measurements in van der Waals junctions of graphene and the quasi 2D niobium diselenide (NbSe_2) superconductor. We investigate the AR near the Dirac point by measuring the differential conductance as a function of Fermi energy and bias energy, which reveal the transition from retro to a non-retro type AR dominated transport near the Dirac point. However, the observation of SAR was restricted due to the large Fermi energy broadening in the graphene. The physics of AR is predicted to alter dramatically in the quantum Hall (QH) regime, where electron transport occurs primarily through the chiral edge states, which themselves are topologically robust manifestations of the Landau levels in the interior of the sample. In another interesting work, we observe signature of AR at the junction of QH edge state in graphene with the NbSe_2 superconductor. Our principal finding is the observation of an anomalous finite temperature conductance peak located precisely at the charge neutrality point, providing a definitive evidence for inter-Landau-level Andreev reflection in a QH system. Further, we carried out shot noise measurements in an edge contacted BLG - Niobium superconductor junction at zero magnetic field as well as QH regime. At the Dirac point we observe Fano factor ~1/3 above the superconducting gap and transition to an enhanced Fano factor ~0.5 below the superconducting gap. By changing the carrier density, we find a continuous reduction of Fano factor for both types of carriers; however, the enhancement of Fano factor within the superconducting gap by a factor of ~1.5 is always preserved. The enhancement of shot noise is also observed in the QH regime, where the current is carried by the edge state, below the critical magnetic field and within the superconducting gap. These observations clearly demonstrate the enhanced charge transport at the BLG - superconductor interface. In a most recent effort, we tried to improve the understanding of AR at QH - superconductor junction by using a high B_C superconductor Molybdenum Rhenium (MoRe) as the superconducting contact to hBN encapsulated graphene. We observe the conductance at the QH plateaus to be very close to the value expected for graphene QH with normal contacts. By measuring shot noise, we observe Fano factor close to half when the bias energy is less than the superconducting gap. The observation of close to half Poissonian value of shot noise together with the observed conductance in QH plateaus uniquely demonstrate the Andreev edge state (AES) at the QH - superconductor interface and also shed light on the underlying phase mixing mechanism.

Measurement of the copper to superconductor ratio (Cu/SC) in composite superconducting wires is necessary for process control, quality assurance, and characterization of the final wire properties. A comparison has been made of three techniques for measuring the Cu/SC ratio on a set of commercial wires produced for the Superconducting SuperCollider (SSC) Laboratory. The simplest and most straightforward technique, chemical etching, was found to display the best reproducibility, while the electrical resistivity technique shows the most variation and sensitivity to measurement errors, as well as being the most difficult to perform. The image analysis technique is fast and fairly reproducible, and is capable of providing much more information on the wire parameters than either of the other techniques.
Graduation date: 1993

Mechanical properties of Nb-Ti composite superconducting wires were tested at room temperature. The results were analysed using simple composite theory, the rule of mixtures. The objective is to predict the mechanical properties of Nb-Ti superconducting composite wires as a function of volume ratio and geometry of the components, the composite wire size and the effect of heat treatment at final drawing wire sizes. To understand the mechanical behaviors of the Nb-Ti composite, mechanical testing of the individual composite components, Nb-Ti filament and copper matrix, was performed, and the geometry of the composite was also studied. The results indicate that for the monofilamentary composite simple composite theory with two components, Nb-Ti filament and copper matrix, can be used as the prediction of the UTS of the composite. For the multifilamentary composite three components make up the composites; a high strength Nb-Ti fiber, a low strength, high ductility bulk copper matrix and a mid-strength (between the Nb-Ti fiber's and bulk copper matrix's) interfilamentary copper matrix. After heavy cold work the UTS of Nb-Ti filaments and bulk copper matrix in the composite saturate, while the UTS of the interfilamentary copper increases as the interfilamentary spacing decreases. The UTS of the interfilamentary copper matrix as a linear function of the reciprocal of interfilamentary spacing is found. The controlling parameters in the manufacturing which determine the mechanical properties of Nb-Ti composite superconducting wires include superconductor to composite ratio, UTS of the Nb-Ti filament and copper matrix, wire final drawing size, and geometry of the composite such as size and number of the filaments, interfilamentary spacing, volume fraction of fringe and core bulk copper in multifilamentary composites.
Graduation date: 1991