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Dissertations / Theses on the topic 'High temperature superconductivity'

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Published: 4 June 2021
Last updated: 29 July 2024

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1

Martin, Marianne. "High-Temperature Superconductivity in Doped BaFe2As2." Diss., lmu, 2011. http://nbn-resolving.de/urn:nbn:de:bvb:19-126344.

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2

Macarie, Liliana Sandina. "Correlated electrons and high-temperature superconductivity." Thesis, University of Warwick, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.307992.

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3

Fehrenbacher, Roland. "Theoretical problems posed by high-temperature superconductivity /." Zürich, 1993. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=10256.

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4

Liu, Ru-Shi. "The chemical control of high temperature superconductivity." Thesis, University of Cambridge, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.240144.

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5

Uys, Hermann. "Non-extensive statistics and high temperature superconductivity /." Pretoria : [s.n.], 2001. http://upetd.up.ac.za/thesis/available/etd-11242005-135357/.

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6

Muhammad, Hasnain Syed. "Transient High-Temperature Superconductivity in Palladium Hydride." Thesis, Griffith University, 2016. http://hdl.handle.net/10072/367614.

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Superconductivity in the palladium-hydrogen system has been studied experimentally by measuring the electrical resistivity. Loading a palladium sample to a stoichiometric ratio close to unity resulted in high-temperature superconductivity at ≈ 55 K for PdHx and ≈ 60 K for PdDx. To observe the superconductivity it was necessary to cool the sample quickly from 300 C and measure the resistivity while heating quickly. For this project an important driver was a series of publications reporting the occurrence of superconductivity near room temperature with hydrogen-to-palladium ratio reaching 1. These results are still considered as controversial because they have not been reproduced. One of the other important driving factors was that PdHx formed at about 300 C exhibits different behaviour compared to the hydride formed by passage through the two-phase region at room temperature. This system does not form dislocations if the hydrogen absorption takes place above the thermodynamic critical point, which lies just below 300 C. In the 1970s, a superconducting transition temperature, Tc, of about 9–10 K was reported for pallidum hydride and 11–12 K for palladium deuteride. The new experiments performed in this project revealed that Tc has increased by a factor of about five by preparing the sample at high temperature.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Natural Sciences
Science, Environment, Engineering and Technology
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7

Öktem, Bülent Abukay Doğan. "High temperature superconducting YBa2Cu3O7 thin flims and bolometers/." [s.l.]: [s.n.], 2006. http://library.iyte.edu.tr/tezler/master/fizik/T000555.pdf.

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8

Wang, Qian. "Quantum tunneling, quantum computing, and high temperature superconductivity." Texas A&M University, 2003. http://hdl.handle.net/1969.1/1638.

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In this dissertation, I have studied four theoretical problems in quantum tunneling, quantum computing, and high-temperature superconductivity. I have developed a generally-useful numerical tool for analyzing impurity-induced resonant-state images observed with scanning tunneling microscope (STM) in high temperature superconductors. The integrated tunneling intensities on all predominant sites have been estimated. The results can be used to test the predictions of any tight-binding model calculation. I have numerically simulated two-dimensional time-dependent tunneling of a Gaussian wave packet through a barrier, which contains charged ions. We have found that a negative ion in the barrier directly below the tunneling tip can deflect the tunneling electrons and drastically reduce the probability for them to reach the point in the target plane directly below the tunneling tip. I have studied an infinite family of sure-success quantum algorithms, which are introduced by C.-R. Hu [Phys. Rev. A {\bf 66}, 042301 (2002)], for solving a generalized Grover search problem. Rigorous proofs are found for several conjectures made by Hu and explicit equations are obtained for finding the values of two phase parameters which make the algorithms sure success. Using self-consistent Hartree-Fock theory, I have studied an extended Hubbard model which includes quasi-long-range Coulomb interaction between the holes (characterized by parameter V). I have found that for sufficiently large V/t, doubly-charged-antiphase-island do become energetically favored localized objects in this system for moderate values of U/t, thus supporting a recent conjecture by C.-R. Hu [Int. J. Mod. Phys. B {\bf 17}, 3284 (2003)].
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9

Mackenzie, Andrew Peter. "The role of stoichiometry in high temperature superconductivity." Thesis, University of Cambridge, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.386134.

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10

Roberts, Alison Laura Udal. "Structural investigations of high temperature superconductivity in cuprates." Thesis, University of Oxford, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.306346.

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11

Doluweera, D. G. Sumith Pradeepa. "Effect of Weak Inhomogeneities in High Temperature Superconductivity." University of Cincinnati / OhioLINK, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1227215152.

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12

Huang, Hua. "High Tc superconductors and the contact properties." Thesis, University of Oxford, 1992. http://ora.ox.ac.uk/objects/uuid:283bec01-d830-4ce2-bcf2-de0918dd0588.

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Methods of processing large grained textured superconductor have been successfully developed, based on a melt texturing process. Large grained textured superconductor with grain size over 10mm along the growth direction and Jc over 3600A/cm2 (77K, 0.5 Tesla) has been produced in both one - zone and two - zone furnaces with good reproducibility. Two kinds of design of reactive metal contacts have been proposed and investigated, aiming to make low resistivity contacts with strong mechanical strength. Three possible reactive contact metals have been tested for contact making, and the microstructures at the interfaces have been studied to find the relations between contact resistivity and contact processing conditions. Titanium/noble metal multilayer contacts is a promising type of contact technique for low resistivity and strong mechanical bonds. Gold and silver contacts give resistivities among the best reported results in the literature, and they turned out to be extremely stable in time, could withstand repeated thermal cycling from room temperature to 10K and yield very reproducible R-T curves. The electrochemical titration method has been used to increase the oxygen stoichiometry of bulk textured YBa2Cu3O7-x samples. The electrochemical titration method can further oxidize melt textured thick film YBa2Cu3O7-x samples in which it may be difficult to further improve oxygen content by conventional annealing. The solid state electrochemical cell has been used to study the thermodynamic properties of the Y-Ba-Cu-O system at high oxygen pressure by measuring the oxygen activity versus time continuously immediately after the electrochemical titration. A series of computer models have been set up according to the microstructure of the contact interface to simulate the complicated contact resistivity behaviours. The nature of, and geometry of, the reaction products at the contact interfaces may be revealed by the temperature dependence of the contact resistance. This information combined with direct observations on the structure and chemistry of the contacts provided a fuller understanding of conduction mechanism at the contact interface.
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13

Romans, Edward John. "Interfaces between normal metals and high temperature superconductors." Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.389892.

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14

Kale, K. S. "High temperature superconducting microwave devices." Thesis, University of Oxford, 1996. http://ora.ox.ac.uk/objects/uuid:85481181-0e9f-41e4-8f26-71cb51fe7ca8.

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This thesis describes investigations into the application of high temperature superconductoRs(HTS) for microwave devices. The ultimate aim of this research was to produce HTS components that would demonstrate the commercial potential of their use in spacecraft such as satellites or in base stations for terrestrial cellular communication systems. During the course of this research the surface resistance (Rs) of HTS thick and thin films deposited onto different substrates has been measured by a variety of techniques. These have been compared and contrasted with each other. There has been a particular emphasis upon the use of dielectric resonators as characterization tools and this thesis describes their use to measure the Rsof films and the loss tangents of the materials used in them. Consideration has been given to losses in the walls of the enclosures that surround dielectric resonators which is an area that has been neglected in previous work. Many of the films characterized had Rs values lower than normal conductoRsat the same temperature and frequency including a thick film which had an Rsof 2.7mΩ±8.0% at 77K and scaled to 10 GHz which represents the second lowest result achieved in a thick film to date. Using such high quality films a number of devices have been made including microstrip resonatoRsoperating at 1.7-8.4 GHz, 5.58 GHz, 23 GHz and 24.75 GHz dielectric resonatoRsand filteRswith centre frequencies from 5.58-13 GHz. Such devices have shown performance enhancements of up to 700% over their normal conductor equivalents. Benefits have been delivered in light, low volume packages which is of particular advantage to the satellite communications industry. Finally, there has been a demonstration, for the first time, of a tunable HTS thin film filter.
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15

Xia, Xiangjun. "Theoretical investigation of high-Tc cuprate superconductors using t-J model : transport properties /." View abstract or full-text, 2005. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202005%20XIA.

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16

Fang, Yue. "Interfacial studies of high temperature superconducting Bi₂Sr₂Can-₁Cun 0₂n+₄ and Ag." Diss., Georgia Institute of Technology, 1995. http://hdl.handle.net/1853/17890.

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17

俞大風 and Tai-fung Yu. "An analysis of electrical transport and magnetic susceptibility properties of YBa2Cu3O7-[delta] and La2-xSrxCuO4 high Tcsuperconductors." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1997. http://hub.hku.hk/bib/B31238014.

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18

Yu, Tai-fung. "An analysis of electrical transport and magnetic susceptibility properties of YBa2Cu3O7-[delta] and La2-xSrxCuO4 high Tc superconductors /." Hong Kong : University of Hong Kong, 1997. http://sunzi.lib.hku.hk/hkuto/record.jsp?B19036607.

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19

TOSCHI, ALESSANDRO. "Intermediate-to-strong coupling superconducivity: a SMFT study of the attractive hubbard model." Doctoral thesis, La Sapienza, 2005. http://hdl.handle.net/11573/917146.

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20

Lidmar, Jack. "Phase transitions in high-temperature superconductors." Doctoral thesis, Stockholm : Tekniska högsk, 1998. http://www.lib.kth.se/abs99/lidm0111.pdf.

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21

Bloom, Scott Harris. "Superconducting and normal compounds : some high field/high pressure effects /." Thesis, Connect to Dissertations & Theses @ Tufts University, 1989.

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Thesis (Ph.D.)--Tufts University, 1989.
Submitted to the Dept. of Physics. Includes bibliographical references (leaves 192-204). Access restricted to members of the Tufts University community. Also available via the World Wide Web;
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22

Gillett, Jack. "High-temperature superconductivity in a family of iron pnictide materials." Thesis, University of Cambridge, 2011. https://www.repository.cam.ac.uk/handle/1810/241040.

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The work in this thesis falls roughly into three parts, which I characterise loosely as a developmental stage, an exploratory stage, and an attempt to contribute to understanding of the field. In the developmental stage, I have worked to design a variety of methods to create high-quality samples of various Iron Pnictide superconductors, to dope them with various chemicals and to characterise the resulting crystalline samples. I discuss in depth the signature of good quality crystals and the various experiments that they have been used in by myself and my collaborators. These processes are ongoing and will hopefully continue to contribute to my research group's capabilities. My exploratory work involves a detailed survey of one particular family, Sr(Fe1-xCox)2As2, as the level of Cobalt is varied, and the mapping of the phase diagram for the system. I have also made a comparison to the better-measured Barium analogue, and discuss the reasons for the differences in character between the two, most notably the lack of a splitting of the structural and magnetic transitions in the first species. I also discuss the effect of pressure, which can lead to superconductivity in lightly doped samples for very modest pressures; and annealing, which increases transition temperatures within samples, on a limited quantity of crystals. Finally, I attempt to contribute to the understanding of the field via a series of Resonant Ultrasound Spectroscopic experiments conducted by a collaborator on my crystals and analysed by me. I see distinct first-order transitions in the parent compounds, characterisable above the high-T structural transition within a Ginzburg-Landau pseudoproper ferroelastic scheme for a transition coupling weakly to strain but driven by another order parameter. My observations allow several statements about the symmetry of the order parameter and are suggestive of a non-magnetically driven structural transition. In the case of doped samples a much richer behavior is seen, with a broad transition and simultaneous relaxation of all elastic peaks and a broad temperature range of significant dispersion. The effect of the softening is seen far above TN and lends strong support to the family of models predicting such high-T fluctuations.
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23

Koralek, Jacob D. "Laser based angle-resolved photoemission spectroscopy and high-temperature superconductivity." Diss., Connect to online resource, 2006. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3219043.

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24

Durrell, John H. "Critical current anisotropy in high temperature superconductors." Thesis, University of Cambridge, 2001. https://www.repository.cam.ac.uk/handle/1810/34606.

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After nearly 15 years of research effort, High Temperature Superconductors (HTS) are finding a wide range of practical applications. A clear understanding of the factors controlling the current carrying capacity of these materials is a prerequisite to their successful technological development. The critical current density (Jc) in HTS is directly dependent on the structure and pinning of the Flux Line Lattice (FLL) in these materials. This thesis presents an investigation of the Jc anisotropy in HTS. The use of thin films grown on off c-axis (vicinal) substrates allowed the effect of current directions outside the cuprate planes to be studied. With this experimental geometry Berghuis, et al. (Phys. Rev. Lett. 79,12, pg. 2332) observed a striking flux channelling effect in vicinal YBa2Cu3O7-δ (YBCO) films. By confirming, and extending, this observation, it is demonstrated that this is an intrinsic effect. The results obtained, appear to fit well with the predictions of a field angle dependent cross-over from a three dimensional rectilinear FLL to a kinked lattice of strings and pancakes. The pinning force density for movement of strings inside the cuprate planes is considerably less than that on vortex pancake elements. When the FLL is entirely string-like this reduced pinning leads to the observed channelling minima. It is observed that anti-phase boundaries enhance the Jc in vicinal YBCO films by strongly pinning vortex strings. The effect on the FLL structure cross-over of increasing anisotropy has been elucidated using de-oxygenated vicinal YBCO films. Intriguingly, the counter intuitive prediction that the range of applied field angle for which the kinked lattice is fully developed reduces with increasing anisotropy, appears to be confirmed. Although vortex channelling cannot be observed in c-axis YBCO films, the pinning force density for vortex string channelling has been extracted by observing string dragging. By studying the effect of rotating the applied field at a constant angle to the cuprate planes, it is possible to observe the cross-over into the string pancake regime in c-axis films. In the 3D region, the observed behaviour is well explained by the anisotropic Ginzburg-Landau model. Measurements were also made on thin films of the much more anisotropic Bi 2Sr2CaCu2O8+x material, grown on vicinal substrates. The absence of any flux channelling effect and clear adherence to the expected Kes-Law behaviour in the observed Jc characteristics does not provide evidence for the existence of the predicted 'crossing lattice' in Bi 2Sr2CaCu2O8+x.
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25

Eames, Sara Jennifer. "Novel light absorbing species for a wavelength-selective high-temperature superconducting bolometer /." Full text (PDF) from UMI/Dissertation Abstracts International, 2000. http://wwwlib.umi.com/cr/utexas/fullcit?p3004256.

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26

Delap, Martin Richard. "Thermal conductivity studies of YBa₂Cu₃O₇₋δ." Thesis, Durham University, 1990. http://etheses.dur.ac.uk/9301/.

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Apparatus to measure the thermal conductivity of YBa(_2)Cu(_3)O(_7-δ) at temperatures between 20K and 120K has been designed and constructed. The thermal conductivity is measured using a longitudinal steady state heat flow technique. Thermal conductivity measurements have been performed upon a sample of YBa(_2)Cu(_3)O(_7-δ) which has been subjected to a series of heat treatments in order to remove oxygen from the material. The measurements show conclusively that the thermal conductivity of YBa(_2)Cu(_3)O(_7-δ) is very strongly influenced by the oxygen content of the material. A reduction of the oxygen content of the material results in a substantial lowering of the thermal conductivity. To explain this result, a quantitative model has been constructed; the model demonstrates that consideration of the changes in phonon interactions alone cannot account for the differences in the behaviour of the thermal conductivity of YBa(_2)Cu(_3)O(_6) and YBa(_2)Cu(_3)O(_7). In addition; the model, shows that there must be a significant carrier contribution to the thermal conductivity in both the normal and superconducting states. A physical process has been proposed which provides the required large carrier contribution below T(_c). Further studies have been performed on a series of samples of YBa(_2)Cu(_3)O(_7-δ) which were sintered at slightly different temperatures. Qualitative analysis of the physical properties, of these samples has been performed.
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27

Giblin, S. P. "Preparation and magnetic properties of mercury deficient Hg←1←-←xBa←2CuO←4←#delta#." Thesis, University of Bristol, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.389091.

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28

Baghdadi, Mehdi. "Design and fabrication of the HTS synchronous motor using 2G-HTS stacked tapes." Thesis, University of Cambridge, 2016. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.709482.

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29

Richardson, Kurt Antony. "The manufacture of high temperature superconducting tapes and films." Thesis, University of Southampton, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.242413.

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30

Yang, Kaiyu. "Theoretical study of high transition temperature superconducting Cu-oxide." Click to view the E-thesis via HKUTO, 2006. http://sunzi.lib.hku.hk/hkuto/record/B37683147.

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31

Yang, Kaiyu, and 楊開宇. "Theoretical study of high transition temperature superconducting Cu-oxide." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2006. http://hub.hku.hk/bib/B37683147.

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32

Connolly, Malcolm. "Magnetometry of high temperature superconducting micro-disks and single crystals." Thesis, University of Bath, 2008. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.492292.

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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.
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33

Ainslie, Mark Douglas. "Transport AC loss in high temperature superconducting coils." Thesis, University of Cambridge, 2012. https://www.repository.cam.ac.uk/handle/1810/244077.

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In this dissertation, the problem of calculating and measuring AC losses in superconducting coils is addressed, with a particular focus on the transport AC loss of coils for electric machines. In order to model the superconducting coil's electromagnetic properties and calculate the AC loss, an existing two dimensional (2D) finite element model that implements a set of equations known as the H formulation, which directly solves the magnetic field components in 2D, is extended to model a superconducting coil, where the cross-section of the coil is modelled as a 2D stack of superconducting coated conductors. The model is also modified to allow the nclusion of a magnetic substrate, which is present in some commercially available HTS wire. The analysis raises a number of interesting points regarding the use of superconductors with magnetic substrates. In particular, the presence of a magnetic substrate affects the penetration of the magnetic flux front within the coil and increases the magnetic flux density within the penetrated region, both of which can increase the AC loss significantly. In order to investigate these findings further, a comprehensive analysis on stacks of tapes with weak and strong magnetic substrates is carried out, using a symmetric model that requires only one quarter of the cross-section to be modelled. In order to validate the modelling results, an extensive experimental setup is designed and built to measure the transport AC loss of a superconducting coil using an electrical method based on inductive compensation by means of a variable mutual inductance. Measurements are carried out on the superconducting racetrack coil and it is found that the experimental results agree with the modelling results for low current, but some phase drift occurs for higher current, which affects the accuracy of the measurement. In order to overcome this problem, a number of improvements are made to the initial setup to improve the lock-in amplifier's phase setting and other aspects of the measurement technique. New measurements are carried out on a single, circular pancake coil and the discrepancies between the experimental and modelling results are described in terms of the assumptions made in the model and aspects of the coil that cannot be modelled. Using the original measured properties of the superconducting tape, there is an order of magnitude difference between the experiment and model. The properties of the superconductor can degrade during the winding and cooling processes, and a critical current measurement of the coil showed that the tape critical current reduced from nearly 300 A, down to around 100 A. Applying this finding to the model, the experimental and modelling results show good agreement, and the difference in the slope of the AC loss curve can be described in terms of the B-dependent critical current dependency Jc(B) used in the model. Finally, methods used to mitigate AC loss in superconducting wires and coils are summarised, and the use of weak and strong magnetic materials as a flux diverter is investigated as a technique to reduce AC loss in superconducting coils. This technique can achieve a significant reduction in AC loss and does not require modification to the conductor itself, which can be detrimental to the superconductor's properties.
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34

Di, Bucchianico Maria Elena. "Modelling high temperature superconductivity : a philosophical inquiry in theory, experiment and dissent." Thesis, London School of Economics and Political Science (University of London), 2009. http://etheses.lse.ac.uk/2755/.

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This thesis tells the story of the Balkanization of the theory community in High Temperature Superconductivity (HTS) and of the many roles experimental evidence has been playing in the battles there. In the twenty-five years that followed the discovery of HTS, the Condensed Matter Physics (CMP) community has experienced extreme difficulty in trying to reach a consensus on a 'final' theory. I will explore some of the reasons for such dissent, starting from testimonies that I collected through personal interviews with HTS physicists. I will focus on the way experiments actively contribute to the formulation of theories. I claim that there is a tension between the different methods and aims of two scientific traditions as they implement the contribution from experiments. This tension will be illustrated through the discussion of several episodes from the history of Superconductivity and CMP research. In particular the paradigmatic quarrels between two of the major players in the history of superconductivity, physicists PW Anderson and B Matthias, will be presented to explore the meeting of theoretical and experimental driving forces and their impact on the evaluation of theories and research programmes. I will also argue that the ambiguity in the theories of evidence employed by the warring camps in HTS allows each of them to claim empirical adequacy for itself and deny it to the opponents, and I shall raise questions about whether the standards of evidence employed there are consistently applied and grounded.
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35

Minor, Bill. "Density matrix renormalization group study of the enhanced hole-hopping model of high temperature superconductivity /." *McMaster only, 1996.

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36

Mesoraca, Salvatore. "Growth of spinel oxide thin films for high efficiency room temperature spin filtering." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/275286.

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In this thesis, the potential of all-spinel oxide tunnel junctions in the field of spintronics has been investigated. In particular, the suitability of the metallic and superconducting LiTi2O4 as non-magnetic electrode in an almost defect-free CoFe2O4-based spin-filter tunnel junction has been explored. For this purpose, high-quality spinel LiTi2O4 and CoFe2O4 thin films have been deposited by pulsed laser deposition on MgAl2O4 substrates. Both films were extensively characterised in terms of structural, surface, magnetic and transport properties. The LiTi2O4 showed metallic and superconducting properties, and the CoFe2O4 had insulating and ferromagnetic properties. A careful tuning of the different growth conditions of these oxides followed in order to grow CoFe2O4/LiTi2O4 bilayers in which LiTi2O4 maintains its metallic and superconducting properties and CoFe2O4 its insulating ferromagnetic characteristics. Transport measurements at low temperature have been carried out to explore details of the tunnelling in symmetric tunnel junctions of the form LiTi2O4/CoFe2O4/LiTi2O4. The measured current–voltage characteristics of these junctions revealed clear Josephson junction behaviour due to superconductivity of the LiTi2O4 electrodes. This conclusive evidence of the tunnel nature of these junctions proves that LiTi2O4 can be used as bottom electrode in all-spinel oxide tunnel junctions.
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37

Chandler, Simon John. "Electron tunnelling study of high-temperature superconductors." Thesis, University of Cambridge, 1994. https://www.repository.cam.ac.uk/handle/1810/270328.

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This dissertation describes work carried out between June 1987 and October 1991, in the Low Temperature Physics Group at the Cavendish Laboratory, Cambridge. The aim of this work was to use electron tunnelling spectroscopy to measure the density of excitation states of the recently discovered high-temperature superconductors. Tunnelling is the most sensitive method for measuring a superconductor’s energy gap, and historically has provided important evidence for the microscopic mechanism of superconductivity in conventional metals. It was hoped that electron tunnelling would prove equally successful in revealing the mechanism of superconductivity in these new materials. Preliminary experiments showed that a thick, degraded surface layer prevented preparation of high-quality tunnel junctions by conventional evaporation techniques. For this reason, apparatus for the formation and fine control of low-temperature point-contact junctions was constructed, together with new measurement electronics and a computer-controlled data-acquisition system. To test this apparatus, point-contact junctions were formed on conventional superconductors. By increasing pressure of the tip on the sample the junction could be moved from the tunnelling to the metallic regime. Point-contact measurements were then made on a number of ceramic, single-crystal and thin-film high-temperature superconducting materials; some not previously investigated by tunnelling. Although ‘gap-like’ structure was occasionally observed, anomalous features (e.g., voltage-dependent background, broadening, large zero-bias conductance) were always present and usually dominated the tunnelling characteristics. These complicate estimation of the energy gap and preclude measurement of more subtle properties such as gap anisotropy or the effective phonon spectrum, α2F. The origins of these non-ideal features have been much debated in the literature and are reviewed in this dissertation. In the case of thin films deposited by laser ablation the tunnelling characteristics were dominated by single-electron tunnelling effects (Coulomb gap and staircase structure). The results suggest that the surface region consists of numerous, isolated normal metal particles.
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38

Price, Stephen [Verfasser]. "Interplay between magnetism and superconductivity in iron based high temperature superconductors / Stephen Price." Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2013. http://d-nb.info/1051419441/34.

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39

Huang, Dennis. "Nanoscale Investigations of High-Temperature Superconductivity in a Single Atomic Layer of Iron Selenide." Thesis, Harvard University, 2016. http://nrs.harvard.edu/urn-3:HUL.InstRepos:33493535.

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The potential of interface engineering to enhance electronic properties is exemplified in a single atomic layer of FeSe grown on SrTiO$_3$, which exhibits an order-of-magnitude increase in its superconducting transition temperature ($T_c$ up to 110 K) compared to bulk ($T_c$ = 8 K). Since this discovery in 2012, efforts to reproduce, understand, and extend this finding continue to draw both excitement and scrutiny. In this thesis, we report the use of a combined molecular beam epitaxy (MBE) and scanning tunneling microscopy (STM) system to grow and image films of superconducting FeSe/SrTiO$_3$. In particular, we investigate and harness atomic defects in as-grown films to derive microscopic insights in two directions. First, we image quasiparticle interference (QPI) patterns generated around defects in order to reconstruct the electronic structure of the unperturbed system, and uncover pieces of the puzzle of high-$T_c$ superconductivity in a monolayer of FeSe. Second, we characterize the atomic structure of defects using density functional theory (DFT), with possible implications on film quality and nanostructuring.
Physics
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40

Straub, Andreas. "Flux Trapping in Superconducting Pellets." PDXScholar, 1992. https://pdxscholar.library.pdx.edu/open_access_etds/4535.

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This research concerns the effects on samples of nominal composition Bil.8Pb0.2Sr2Ca2cu30y which were exposed to hot, dense argon in a ballistic compressor. The investigations were concentrated on two specimens which were exposed to hot, dense argon at about 1800 K (peak pressure 330 atm) and 1500 K (peak pressure 230 atm), respectively. Sample Bi #1 showed a completely melted surface structure after triple exposure in the ballistic compressor at 1800 K while the surface of sample Bi #7 was just partly melted after double exposure at 1500 K. Changes in flux trapping capability and qualitative Meissner effect were investigated in addition to the properties described by Duan, et al. ( 17, 18, 3 2] , who reported changes in critical temperature, crystal structure, surface morphology and composition after exposure of samples to hot, dense argon. After triple exposure in the ballistic compressor at a temperature of approximately 1800 K, sample Bi #1 showed an enhanced Meissner effect on the exposed side compared to the unexposed side of the pellet, while no difference in Meissner effect was found between the exposed and the unexposed side of sample Bi #7. EDS analysis showed that both samples are inhomogeneous in chemical surface composition. Oxygen loss due to exposure to hot, dense argon could not be demonstrated. X-ray analysis indicated that the melted surface layer of sample Bi # 1 after triple exposure to hot, dense argon contains smaller crystals than before exposure in the ballistic compressor. Tc measurements gave varying results which are explainable by the chemical inhomogeneity of the specimens. An increase in the amount of trapped flux due to exposure of the samples to hot, dense argon could not be demonstrated
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41

Elkaseh, Akram Abdulsalam. "Fabrication of high-temperature superconducting nanobridges using atomic force microscopy." Thesis, Link to the online version, 2006. http://hdl.handle.net/10019/1019.

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42

Babkevich, Peter. "Quantum materials explored by neutron scattering." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:5f6ef05d-e846-47e1-b59f-864ea4fa2f3f.

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This thesis describes neutron scattering experiments on strongly correlated systems exhibiting a range of emergent phenomena: antiferromagnetism, charge order, superconductivity and multiferroicity. I have examined the La_{2}CoO_{4} compound which is a Mott insulator and orders antiferromagnetically near room temperature. The La_{2}CoO_{4} sample was studied using spherical neutron polarimetry and I present magnetic structure models to describe the two antiferromagnetic phases of the compound. Furthermore, the magnetic fluctuations have been investigated using neutron time-of-flight technique. This has allowed us to extract the dominant exchange interactions in the system. More interestingly, the work on La_{2}CoO_{4} presented in this thesis provides a basis for the experimental evidence of an hourglass dispersion in La_{5/3}Sr_{1/3}CoO_{4}, previously only observed in the copper oxide based superconductors. This dispersion has been understood in terms of a stripe ordered magnetic phase and was found to be well described by a linear spin-wave model. Neutron scattering experiments were also carried out on the new iron-based high-temperature superconductors, FeSe_{x}Te_{1−x}. A range of compositions were studied, including both antiferromagnetically ordered and superconducting. Below the superconducting phase transition temperature, a spin resonance mode was found centred on the antiferromagnetic wavevector. This is an important feature shared by many unconventional superconductors. The spin resonance intensity was found to reflect the order parameter of the superconducting state. Polarised inelastic neutron scattering experiments have revealed a small anisotropy between the in-plane and out-of-plane magnetic fluctuations at the resonance. This anisotropy cannot be readily explained by the usual anisotropic terms in the Hamiltonian. This could be evidence of new physics in the FeSe_{x}Te_{1−x} superconductors. Finally, I have studied CuO – a high-temperature multiferroic. Analysis of polarised neutron diffraction experiments shows that the magnetic domain population can be varied using an externally applied electric field. This unambiguously demonstrates coupling between the magnetic and ferroelectric degrees of freedom. Using representation analysis I derive the incommensurate magnetic structure in the multiferroic phase. The origin of the magnetoelectric coupling is consistent with models based on the inverse Dzyaloshinskii-Moriya interaction.
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43

Eğilmez, Mehmet Özyüzer Lütfi. "Microstructural, electrical and mechanical properties of MgB2/Mg metal matrix composites/." [s.l.]: [s.n.], 2004. http://library.iyte.edu.tr/tezler/master/malzemebilimivemuh/T000467.pdf.

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44

Minor, William Roberts. "Density Matrix Renormalization Group study of the enhanced hole-hopping model of high temperature superconductivity." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp02/NQ30159.pdf.

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45

Barnes, Gary James. "Computational modelling for type-II superconductivity and the investigation of high temperature superconducting electrical machines." Thesis, University of Oxford, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.365887.

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46

Wright, Jack Daniel. "Magnetism and superconductivity in iron pnictides and iron chalcogenides." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:c1f7563a-275f-415f-993e-98313b517296.

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This thesis presents a study on several series of unconventional Fe-based superconductors; namely, derivatives of NaFeAs and LiFeAs, as well as molecular-intercalated FeSe. Primarily using muon spin rotation (SR), but also x-ray diffraction (XRD) and magnetic susceptibility measurements, the nature of both magnetic and superconducting phases within these systems is studied. Particular attention is focussed on how these states compete or coexist with one another. The aforementioned experimental techniques are first used to explore the phase diagram of NaFe1xCoxAs. This phase diagram includes regions of long-range antiferromagnetism and short-range order, that both coexist with superconductivity. Magnetism is gradually destroyed, primarily through a diminishment of the size of the ordered moment, as superconductivity is enhanced by Co substitution. This interplay is explored in detail. By contrast, superconductivity in LiFeAs cannot be enhanced by transition metal substitution, suggesting that it is intrinsically optimally-doped. I investigate this conclusion by studying the evolution of the penetration depth in superconducting compositions of LiFe1xCoxAs and LiFe1xNixAs, and comparing these data to those from other electron-doped systems. I also study an unusual and emergent magnetic phase in Li1yFe1+yAs. This work suggests that LiFeAs supports a superconducting phase that resembles those in other Fe-pnictides, but is uniquely close to an additional magnetic instability. I then move on to the study of a recently discovered series, based on FeSe intercalated with ammonia and various metals. I study both the penetration depth and the intrinsic magnetic phases in these systems using SR and compare them with other compounds based on FeSe. I find that these intercalated systems support spacially separated regions of dynamic magnetism and superconductivity and I discuss how much these phases depend on the precise chemical details of the intercalated layer. Finally, I return to the experimental study of NaFe1xCoxAs, extending the range of techniques employed by using high-field magnetometry and high-pressure SR. These studies reveal new features of this system that were not accessible using low-field and ambient-pressure methods. In particular, I show that the magnetic moment size in NaFeAs unexpectedly increases with pressure, suggesting that the electronic structure of this compound may be unique amongst known Fe-based superconductors.
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47

Hetel, Iulian Nicolae. "Quantum Critical Behavior In The Superfluid Density Of High-Temperature Superconducting Thin Films." The Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=osu1204918571.

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48

Zeljkovic, Ilija. "Visualizing the Interplay of Structural and Electronic Disorders in High-Temperature Superconductors Using Scanning Tunneling Microscopy." Thesis, Harvard University, 2013. http://dissertations.umi.com/gsas.harvard:10922.

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The discovery of high-\(T_c\) superconductivity in 1986 generated tremendous excitement. However, despite over 25 years of intense research efforts, many properties of these complex materials are still poorly understood. For example, the cuprate phase diagram is dominated by a mysterious "pseudogap" state, a depletion in the Fermi level density of states which persists above the superconducting critical temperature \(T_c\). Furthermore, these materials are typically electronically inhomogeneous at the atomic scale, but to what extent the intrinsic chemical or structural disorder is responsible for electronic inhomogeneity, and whether the inhomogeneity is relevant to pseudogap or superconductivity, are unresolved questions. In this thesis, I will describe scanning tunneling microscopy experiments which probe the interplay of structural, chemical and electronic disorder in high-\(T_c\) superconductors. First, I will present the imaging of a picoscale orthorhombic structural distortion in Bi-based cuprates. Based on insensitivity of this structural distortion to temperature, magnetic field, and doping level we conclude that it is an omnipresent background not related to the pseudogap state. I will also present the discovery of three types of oxygen disorder in the high-\(T_c\) superconductor \(Bi_2Sr_2CaCu_2O_{8+x}\) two different interstitials as well as vacancies at the apical oxygen site. We find a strong correlation between the positions of these defects and the nanoscale inhomogeneity in the pseudogap phase, which highlights the importance of chemical disorder in these compounds. Furthermore, I will show the determination of the exact intra-unit-cell positions of these dopants and the effect of different types of intrinsic strain on their placement. I will also describe the identification of chemical disorder in another cuprate \(Y_{1−x}Ca_xBa_2Cu_3O_{7−x}\), and the first observation of electronic inhomogeneity of the spectral gap in this material. Finally, I will present definitive identification of the cleavage surfaces in \(Pr_xCa_{1−x}Fe_2As_2\), and imaging of Pr dopants which exhibit lack of clustering, thus ruling out Pr inhomogeneity as the likely source of the high-\(T_c\) volume fraction. To achieve the aforementioned results, we employ novel analytical and experimental tools such as an average supercell algorithm, high-bias dI/dV dopant mapping, and local barrier height mapping.
Physics
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49

Freamat, Mario Vadim. "NORMAL AND SPIN POLARIZED TRANSPORT IN HIGH-TEMPERATURE SUPERCONDUCTOR TUNNELING JUNCTIONS." UKnowledge, 2004. http://uknowledge.uky.edu/gradschool_diss/426.

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One of the challenges facing condensed matter physics nowadays is to understand the electronic structure of high temperature superconductors. This dissertation compiles our contribution to the experimental information concerning this subject. Tunneling conductance spectroscopy a technique capable of probing the electronic density of states in hybrid structures was used to study the current and spin transport properties across junctions between metallic counterelectrodes and Bi2Sr2CaCu2O8- (BSCCO) crystals. Since in these structures the transport is mediated by transmission channels depending on superconductive characteristics, the energy resolved density of states is a signature of the mechanism of superconductivity. For instance, one can observe the superconductive energy gap and the behavior of subgap bound states due to phase sensitive Andreev reflections at the junction interface. In particular, tunneling spectroscopy makes possible the observation of the LOFF state characterized by the coexistence of superconductivity and magnetism. Cuprates like BSCCO are highly anisotropic materials and their superconductivity is almost two dimensional, being confined in the CuO2 planes. Therefore, our junctions combine monocrystals of underdoped samples of BSCCO with various thin film counterelectrodes normal metal (Ag), conventional superconductor (Pb) and ferromagnetic metal (Fe) deposited perpendicular onto the cuprate ab-plane (CuO2 plane). We performed measurements on Ag/BSCCO junctions for two current injection directions into the same crystal. We observed that, near the 110 crystal surface, the conductance spectra show a high zero bias peak (ZBCP) which is a manifestation of zero energy Andreev bound states due to an anisotropic superconductive order parameter. Near the 100 surface, the ZBCP is largely suppressed. This is consistent with a predominantly 2 2 x y d - -wave pairing symmetry. In some cases, the ZBCP splits or decreases in amplitude at low temperatures. This is consistent with the existence of a subdominant s-wave (or xy d ) resulting in a mixed d is + state which breaks time reversal symmetry (BTRS). Since we observe this phenomenon in the underdoped case, we do not confirm the possibility of a quantum critical point close to the optimal doping. Our Pb/BSCCO spectra contradict the theory explaining the BTRS by proximity effect. The Fe/BSCCO junctions measure the effect of spin polarization. We explain the recorded 4-peak asymmetric structure by the combined effect of a spin independent BTRS state and a spin filtering exchange energy in the barrier responsible for a large ZBCP splitting. The LOFF state was observed in the proximity region induced on the ferromagnetic side of multilayered-Fe/Ag/BSCCO structures. As expected for the LOFF order parameter, the spectra develops coherent damped oscillations with the Fe layer thickness probing different regions. The magnitude and sign of the oscillation depends on the energy. The conductances at energy zero or equal to the superconductive gap are modulated in antiphase proving that the order parameters takes successively positive and negative values. Changing the junction orientation with 4 p , results in an opposite behavior for the same distance. The maximal amplitudes in one direction is replaced by minima, showing that, besides space, the LOFF state modulation depends on the phase of the high temperature order parameter inducing the proximity
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50

Kornecki, Michael. "Single fluxoid thermal smearing and the second peak in YBa₂Cu₃O₇ /." free to MU campus, to others for purchase, 2003. http://wwwlib.umi.com/cr/mo/fullcit?p3115563.

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