Dissertations / Theses on the topic 'Superconductivity'
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1
Kouvaris, Christoforos N. "Gapless color superconductivity." Thesis, Massachusetts Institute of Technology, 2005. http://hdl.handle.net/1721.1/32308.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2005.Includes bibliographical references (p. 155-164).
In this thesis, we propose and investigate the "Gapless Color-Flavor Locked" (gCFL) phase, a possible new phase of dense and cold quark matter. At high enough densities, quarks interact with each other and form pairs similarly to electrons in superconductors. This phenomenon in the case of quark matter is called Color Superconductivity. Color superconducting matter must be electrically and color neutral, because otherwise there are huge energy costs, due to the charges. At asymptotically high densities, equal numbers of up, down, and strange quarks make the system neutral, all the quarks pair, and the quark matter is in the Color-Flavor-Locked phase. At intermediate densities however, the strange quark mass changes the number densities of the quarks and this makes the CFL phase unstable. The gCFL phase emerges as a result of the strange quark mass effect and the neutrality conditions. The gCFL phase has gapless modes and non-zero electron density, unlike CFL. These new properties of gCFL have significant astrophysical implications. The interior of neutron stars might have densities at the regime where gCFL dominates. If this is the case, we argue that gCFL will change significantly the cooling of such a star, keeping it hot, even for late times. Also in this thesis we explore the rest of the phase diagram of neutral quark matter at high density as a function of temperature and strange quark mass. We investigate how zero temperature superconducting phases evolve if we heat the system. We derive the phase diagram of dense quark matter using a Nambu-Jona- Lasinio (NJL) model, that might be a good guide for understanding the QCD phase diagram.
by Christoforos N. Kouvaris.
Ph.D.
2
Heron, Dale Robert. "Mathematical models of superconductivity." Thesis, University of Oxford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.296893.
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Barclay, Luke. "Aspects of holographic superconductivity." Thesis, Durham University, 2012. http://etheses.dur.ac.uk/3376/.
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In this thesis we study two different aspects of holographic superconductivity. First we study fully backreacting Gauss-Bonnet (GB) holographic superconductors in 5 bulk spacetime dimensions. We explore the system’s dependence on the scalar mass for both positive and negative GB coupling, α. We find that when the mass approaches the Breitenlohner-Freedman (BF) bound and α→(L^2)/4 the effect of backreaction is to increase the critical temperature, Tc , of the system: the opposite of its effect in the rest of parameter space. We also find that reducing α below zero increases Tc and that the effect of backreaction is diminished. We study the zero temperature limit, proving that this system does not permit regular solutions for a non-trivial, tachyonic scalar field and constrain possible solutions for fields with positive masses. We investigate singular zero temperature solutions in the Einstein limit but find them to be incompatible with the concept of GB gravity being a perturbative expansion of Einstein gravity. We study the conductivity of the system, finding that the inclusion of backreaction hinders the development of poles in the conductivity that are associated with quasi-normal modes approaching the real axis from elsewhere in the complex plane. In the latter part of the thesis we investigate asymptotically anti de-Sitter (adS) and Lifshitz black holes in a bulk gravitational model that has a consistent embed-ding in string theory and that permits an arbitrary dynamical exponent, z ≥ 1. We find numerically that for both types of asymptotic spacetime there exists a two parameter family of black hole solutions. In the adS case these numerical solutions are supported by analytic solutions in the ‘probe’ or non-backreacting limit. Finally, we study the dependence of the black hole’s temperature on these two parameters.
4
Ballestar, Ana. "Superconductivity at Graphite Interfaces." Doctoral thesis, Universitätsbibliothek Leipzig, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-141196.
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The existence of superconductivity in graphite has been under discussion since the 1960s when it was found in intercalated graphitic compounds, such as C8K, C8Rb and C8Cs. However, it was only about 40 years ago when the existence of superconductivity in pure graphite came up. In this work we directly investigate the interfaces highly oriented pyrolytic graphite (HOPG) has in its inner structure, since they play a major role in the electronic properties. The results obtained after studying the electrical transport provide clear evidence on granular superconductivity localized at the interfaces of graphite samples. Zero resistance states, strong current dependence and magnetic field effect on the superconducting phase support this statement. Additionally, an abrupt reduction in the measured voltage at temperatures from 3 to 175 K has been observed. However, the upper value of this transition temperature seems to not have been reached yet. A possible method to enhance it is to increase the carrier density of graphite samples. In order to preserve to quasi-two-dimensional structure of highly oriented pyrolytic graphite, chemical doping has been dismissed in the frame of this work. We used an external electric field to move the Fermi level and, hence, try to trigger superconductivity in multi layer graphene samples. A drop on the resistance at around 17 K has been measured for a large enough electric field applied perpendicular to the graphene planes. This transition is strongly affected by magnetic field and only appeared at low temperatures. As a result of the studies included in this work, it appears clear that graphite has a superconducting phase located at certain interfaces with a very high transition temperature.
5
Ožana, Marek. "Mesoscopic superconductivity : quasiclassical approach." Doctoral thesis, Umeå universitet, Institutionen för fysik, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-91484.
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This Thesis is concerned with the quasiclassical theory of meso-scopic superconductivity. The aim of the Thesis is to introduce the boundary conditions for a quasiclassical Green’s function on partially transparent interfaces in mesoscopic superconducting structures and to analyze the range of applicability of the quasiclassical theory. The linear boundary conditions for Andreev amplitudes, factoring the quasiclassical Green’s function, are presented. The quasiclassical theory on classical trajectories is reviewed and then generalized to include knots with paths intersections. The main focus of the Thesis is on the range of validity of the quasiclassical theory. This goal is achieved by comparison of quasiclassical and exact Green’s functions. The exact Gor’kov Greens function cannot be directly used for the comparison because of its strong microscopic variations on the length-scale of λF. It is the coarse-grain averaged exact Green’s function which is appropriate for the comparison. In most of the typical cases the calculations show very good agreement between both theories. Only for certain special situations, where the classical trajectory contains loops, one encounters discrepancies. The numerical and analytical analysis of the role of the loop-like structures and their influence on discrepancies between both exact and quasiclassical approaches is one of the main results of the Thesis. It is shown that the terms missing in the quasiclassical theory can be attributed to the loops formed by the interfering paths. In typical real samples any imperfection on the scale larger than the Fermi wavelength disconnects the loops and the path is transformed into the tree-like graph. It is concluded that the quasiclassical theory is fully applicable in most of real mesoscopic samples. In the situations where the conventional quasiclassical theory is inapplicable due to contribution of the interfering path, one can use the modification of the quasiclassical technique suggested in the Thesis.
6
Chapman, S. J. "Macroscopic models of superconductivity." Thesis, University of Oxford, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.303594.
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After giving a description of the basic physical phenomena to be modelled, we begin by formulating a sharp-interface free-boundary model for the destruction of superconductivity by an applied magnetic field, under isothermal and anisothermal conditions, which takes the form of a vectorial Stefan model similar to the classical scalar Stefan model of solid/liquid phase transitions and identical in certain two-dimensional situations. This model is found sometimes to have instabilities similar to those of the classical Stefan model. We then describe the Ginzburg-Landau theory of superconductivity, in which the sharp interface is `smoothed out' by the introduction of an order parameter, representing the number density of superconducting electrons. By performing a formal asymptotic analysis of this model as various parameters in it tend to zero we find that the leading order solution does indeed satisfy the vectorial Stefan model. However, at the next order we find the emergence of terms analogous to those of `surface tension' and `kinetic undercooling' in the scalar Stefan model. Moreover, the `surface energy' of a normal/superconducting interface is found to take both positive and negative values, defining Type I and Type II superconductors respectively. We discuss the response of superconductors to external influences by considering the nucleation of superconductivity with decreasing magnetic field and with decreasing temperature respectively, and find there to be a pitchfork bifurcation to a superconducting state which is subcritical for Type I superconductors and supercritical for Type II superconductors. We also examine the effects of boundaries on the nucleation field, and describe in more detail the nature of the superconducting solution in Type II superconductors - the so-called `mixed state'. Finally, we present some open questions concerning both the modelling and analysis of superconductors.
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Poenicke, Andreas. "Unconventional Superconductivity near Inhomogeneities." [S.l. : s.n.], 2008. http://digbib.ubka.uni-karlsruhe.de/volltexte/1000007522.
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Plekhanov, Evgueni. "Hubbard U Enhanced Superconductivity." Doctoral thesis, SISSA, 2003. http://hdl.handle.net/20.500.11767/4266.
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We present a. study of the superconducting properties of models containing
the Hubbard rnpulsion term. This strnng on-site repulsion
is considered as a key ingTedient of the high tempera.turn superconductivity.
Though the fact that for the normal, lovv temperature
superconductors, rnpulsion destroys superconducting order, it is argued
in the present thesis, that for the pairing of the d-·wave symmetry
in the strongly correlated electronic systems its effect is to
enhance and may be to cause superconductivity. Various methods
such as Variational Monte Carlo, Gutzwiller Approximation, Time
Dependent Hartree-Fock and Fixed Node Approximation have been
used to investigate t - U - W model, t - U - J - V and pure Hubbard
models.
In this thesis, by considering correlations contribution to the BCS
condensation energy due to the Hubbard U it is shown that the latter
lowers the total energy of a d-wave superconductor in the weak
coupling limit, thus enforcing the stability of such superconductor.
This effect appears to be mainly due to the enhancement of the
spin fluctuations near the nesting vector Q = ( π, π).
It is then studied the crossover from weak to strong coupling
regimes in t - U - J - V model by increasing U. Remarkably in this
model an order of magnitude growth of the superconducting order
parameter is found and explained as being due to the Hubbard repulsion.
vVe fi.nd also, that the pairings, originally induced by spin
or charge fluctuations upon increase of U are differently renormalized,
being the former enhanced, while the latter suppressed.
In the fi.nal part of the thesis the superconductivity in the pure
Hubbard model is carefully studied by means of Variational Monte
Carlo and related numerical methods aimed to improve variational
results. VVe observe the onset of strong coupling superconductivity
at U/t ~ 7 within the Fixed Node Approximation in the systems of
large size and compare our results for small clusters with those of
Lanczos diagonalization. We show that Variational Monte Carlo,
though overestimating the quasiparticle weight (ZvMc > Zexact)
succeeds in reproducing the correct pairing between quasiparticles.
9
Glawe, Henning [Verfasser]. "Descriptors of Superconductivity / Henning Glawe." Berlin : Freie Universität Berlin, 2018. http://d-nb.info/1188239961/34.
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El, Bana Mohammed Sobhy El Sayed. "Superconductivity in two-dimensional crystals." Thesis, University of Bath, 2013. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.589655.
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Since the first isolation of graphene in 2004 interest in superconductivity and the superconducting proximity effect in monolayer or few-layer crystals has grown rapidly. This thesis describes studies of both the proximity effect in single and fewlayer graphene flakes, as well as the superconducting transition in few unit cell chalcogenide flakes. Optical and atomic force microscopy and Raman spectroscopy have been used to characterise the quality and number of molecular layers present in these flakes. Graphene structures with superconducting Al electrodes have been realised by micromechanical cleavage techniques on Si/SiO2 substrates. Devices show good normal state transport characteristics, efficient back-gating of the longitudinal resistivity, and low contact resistances. Several trials have been made to investigate proximity-induced critical currents in devices with junction lengths in the range 250-750 nm. Unfortunately, no sign of proximity supercurrents was observed in any of these devices. Nevertheless the same devices have been used to carefully characterise proximity doping, (due to the deposited electrode), and weak localisation/anti-localisation contributions to the conductivity in them. In addition this work has been extended to investigations of the superconducting transition in few unit-cell dichalcogenide flakes. Four-terminal devices have been realised by micromechanical cleavage from a 2H-NbSe2 single crystal onto Si/SiO2 substrates followed by the deposition of Cr/Au contacts. While very thin NbSe2 flakes do not appear to conduct, slightly thicker flakes are superconducting with an onset ܶ that is only slightly depressed from the bulk value (7.2K). The resistance typically shows a small, sharp, high temperature transition followed by one or more broader transitions, which end in a wide tail to zero resistance at low temperatures. These multiple transitions appear to be related to disorder in the layer stacking rather than lateral inhomogeneity. The behaviour of several flakes has been characterised as a function of temperature, applied field and back-gate voltage. The resistance and transition temperatures are found to depend weakly on the gate voltage. Results have been analysed in terms of available theories for these phenomena.
11
Raut, Dinesh V. "Aspects of superconductivity and fractionalization." Thesis, Massachusetts Institute of Technology, 2005. http://hdl.handle.net/1721.1/32423.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Physics, 2005.Includes bibliographical references (p. 37).
Since their discovery in mid 80's, a complete theory of high temperature superconductors is yet to take its final shape. Theory of fractionalization attempts to explain the phenomenon by assuming that the electron is split into two particles, chargon and spinon, carrying charge and spin respectively. Although capable of producing the qualitative features of the phase diagram, this theory is not been able to account for a number of experimental observations. A simple mean field model based on fractionalization ideas is proposed in this work which can possibly get around some of the drawbacks of the original fractionalization theory. Chapter one discusses various aspects of superconductivity along with BCS theory and chapter two talks about the motivation behind considering this model along with its basic features.
by Dinesh V. Raut.
S.M.
12
Witt, James David Samuel. "Superconductivity and non-homogeneous magnetism." Thesis, University of Cambridge, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.610601.
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Scaffidi, Thomas. "Unconventional superconductivity in strontium ruthenate." Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:af08bf3f-3934-48f1-89af-a897948172fb.
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In this thesis, a weak coupling formalism is developed to study superconductivity in spin-orbit coupled, multi-orbital systems. This formalism is then applied to Sr2RuO4, one of the few candidates for odd-parity superconductivity. We show that spin-orbit coupling and multi-band effects are crucial to understand the physics of this material. Depending on the interaction parameters, the order parameter can either be chiral or helical. In both cases, the gap is highly anisotropic, and has accidental deep minima along certain directions, in accordance with experiments. Focusing then on the chiral case, we show that the total Chern number is -7 instead of the usually assumed +1. This leads to drastically different predictions for the thermal and charge Hall conductances. In particular, we show that the absence of measurable charge edge currents is not incompatible with a chiral state. Finally, we study the evolution of superconductivity in Sr2RuO4 under ?100? uniaxial strain. We find a good agreement with experiments for our prediction of Tc as a function of strain. Furthermore, we find that (1) the absence of a measurable cusp of Tc at zero strain is not incompatible with a chiral state and that (2) there could be a transition to an even-parity state at larger strain close to a Van Hove singularity. We propose Hc,2/Tc2 c as a measurable quantity to identify this transition.
14
Kozii, Vladyslav. "Exotic superconductivity in quantum materials." Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/127702.
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This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2019
Cataloged from the official PDF of thesis.
Includes bibliographical references (pages 345-342).
The theory of superconductivity developed by Bardeen, Cooper, and Schrieffer has proven to correctly describe a wide class of metals, where the effective attraction between electrons is mediated by phonons. Despite huge success, this theory fails to explain certain types of superconductivity, which includes but not limited to topological superconductivity and superconductivity in systems with low carrier density. We study new exciting properties of these materials and discuss possible microscopic mechanisms for exotic superconductivity. In Part I of this thesis, we explore the properties of two-component superconductors with strong spin-orbit coupling. Our study is motivated by the experiments on a topological superconductor candidate material, Bi2Se3 doped with Cu, Sn, or Nb atoms. Generally, superconductivity in such systems comes in two flavors: nematic, which breaks rotational symmetry of the crystal, and time-reversal breaking chiral.
We study the relative energetics and different features specific to each of these flavors. We find that, in three dimensions, the nematic superconductors generically possess full pairing gap on the Fermi surface, thus representing a solid-state realization of a time-reversal-invariant topological superconductor. On the contrary, chiral superconductors host non-degenerate point nodes on the Fermi surface and represent the superconducting analog of topological Weyl semimetals; the low-energy excitations in these materials are itinerant Majorana fermions. In Part II, we suggest possible microscopic mechanisms for unconventional superconductivity. We show that strong fluctuations of the inversion-breaking order parameter induce instability in an odd-parity superconducting channel, suggesting a route towards topological superconductivity. Using bosonization, we generalize this result to one-dimensional systems.
We apply our findings to study superconductivity in three-dimensional Dirac materials with extremely low density of carriers. Finally, we discuss the mechanism for nematic superconductivity from density wave fluctuations in two-dimensional systems, with possible application to twisted bilayer graphene. The results presented in this thesis are mainly based on Refs. [1, 2, 3, 4, 5, 6, 7].
by Vladyslav Kozii.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Physics
15
Luk, Kwok-hing R. "Quadrupolar impurity suppression of superconductivity /." The Ohio State University, 1992. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487759914761644.
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Farrell, Aaron. "Topological superconductivity without proximity effect." Thesis, McGill University, 2013. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=119741.
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The search for a Majorana Fermion has been an area of intense interest in condensed matter research of late. This elusive particle, predicted to exist in 1937, has been sought after for both fundamental and practical reasons. On the fundamental level, no particle to date has been observed to be a Majorana fermion, meanwhile on the practical level a Majorana fermion, if found, would represent a non-abelian anyon and could thus be used to build a quantum computer. The search for a Majorana Fermion has recently shifted to topological superconductivity. Topological superconductors are categorized by the nontrivial winding of their order parameter phase and for this reason are expected to support Majorana Fermions in their vortex cores. Owing to this, the study of topological superconductors has intensified in recent years. Current proposals for a device that may behave as a topological superconductor are based on semiconductor heterostructures, where the spin-orbit coupled bands of a semiconductor are split by a band gap or Zeeman field and superconductivity is induced by proximity to a conventional superconductor. In this setup, topological superconductivity is obtained in the semiconductor layer and the proposed heterostructures typically include two or three layers of different materials. In this thesis we propose a simplification to these types of devices, suggesting a way in which the superconducting layer can be replaced. Part of our proposal includes a model Hamiltonian for these types of systems. This thesis will also develop several different methods to analyze this model Hamiltonian in various different parameter regimes with the ultimate goal of classifying its topology.Récemment, une région d'intérêt en la recherché de la matière condensée est le recherche pour les "Majorana Fermions". Les physiciens sont fascinés avec cette particule pour des raisons fondamentales et pratiques. Fondamentalement, une particule se comporte comme un Majorana Fermion n'a jamais été trouvée avant. Pratiquement, un Majorana Fermion pourrait être utilisé pour la construction d'un ordinateur quantique. Dans les dernières années, les chercheurs ont commencé à chercher pour des Majorana Fermions dans les supraconducteurs. En particulier, les supraconducteurs topologiques sont crus de supportes les Majorana Fermions dans leur vortex cores et de ce fait des nombreux dispositifs supraconducteurs topologiques ont été proposées. Les propositions récemment sont basées sur les hétérostructures de trois ou deux couches. Dans ces hétérostructures, les bandes d'un semiconducteur avec le couplage de spin-orbit sont séparées par le champ Zeeman d'une couche ferromagnétique (ou un champ appliqué). Après cette, supraconductivité topologique est établie dans la couche de semiconductrice en raison de la proximité d'une couche de supraconducteur ordinaire. Dans cette thèse nous proposons une simplification des dispositifs décrits ci-dessus; nous suggérons un moyen d'enlever la couche de supraconductivité. Nous commençons par proposer un Hamiltonian du cette système et procède à développer des nombreuses méthodes pour analyser cette Hamiltonian avec l'objectif ultime de classifier la topologie de ce système.
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Yasui, Yuuki. "Superconductivity in Sr2RuO4 micro-rings." Kyoto University, 2019. http://hdl.handle.net/2433/242593.
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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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Zhou, Lubo. "Matrix field theory applications to superconductivity /." College Park, Md. : University of Maryland, 2006. http://hdl.handle.net/1903/4136.
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Thesis (Ph. D.) -- University of Maryland, College Park, 2006.Thesis research directed by: Chemical Physics. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
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Brydon, Philip M. R. "Coexistence of superconductivity and excitonic ferromagnetism." View electronic text, 2002. http://eprints.anu.edu.au/documents/disk0/00/00/07/63/index.html.
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Thesis (BSc. (Hons))--Australian National University, 2002.Available via the Australian National University Library Electronic Pre and Post Print Repository. Title from title screen (viewed Mar. 27, 2003). "A thesis submitted for the degree of Honours in theoretical physics at the Department of Physics and Theoretical Physics, The Faculty of Science, The Australian National University" "Honours thesis submitted November 2002" Includes bibliographical references.
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Yokoyama, Takehito, Seiichiro Onari, and Yukio Tanaka. "Enhanced triplet superconductivity in noncentrosymmetric systems." American Physical Society, 2007. http://hdl.handle.net/2237/11291.
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Peng, Yang [Verfasser]. "Signatures of topological superconductivity / Yang Peng." Berlin : Freie Universität Berlin, 2017. http://d-nb.info/1137867906/34.
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Shomar, Towfic Louis Elias. "Phenomenological realism, superconductivity and quantum mechanics." Thesis, London School of Economics and Political Science (University of London), 1998. http://etheses.lse.ac.uk/2602/.
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The central aim of this thesis is to present a new kind of realism that is driven not from the traditional realism/anti-realism debate but from the practice of physicists. The usual debate focuses on discussions about the truth of theories and how they relate with nature, while the real practices of the scientists are forgotten. The position I shall defend is called "phenomenological realism". The realist doctrine was recently undermined by the argument from pessimistic meta-induction, also known as the argument from scientific revolutions. I argue that phenomenological realism is a new kind of scientific realism that can overcome the problem generated by the pessimistic meta-induction, and which reflects scientific practice. The realist has tried to overcome the pessimistic meta-induction by suggesting various types of theory dichotomy. I claim that the different types of dichotomy normally presented by realists do not overcome the problem, for these dichotomies cut through theory vertically. I argue for a different kind of dichotomy, one that cuts horizontally, between high-level and low-level theoretical representations. I claim that theoretical forms in physics have two distinct types depending on the way they are built. These are theoretical models that are built depending on a top-down approach and phenomenological models that are built depending on a bottom-up approach. I argue that for the most part only phenomenological models are the vehicles of accurate representation. I present two case studies. The first case study is from superconductivity, where I contrast the BCS model of superconductivity with the phenomenological model of Landau and Ginzburg. The other case study is a fresh look at the Bohr-Einstein debate.
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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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Kundu, Joydip 1977. "Two key questions about color superconductivity." Thesis, Massachusetts Institute of Technology, 2004. http://hdl.handle.net/1721.1/29452.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2004.Includes bibliographical references (p. 109-116).
We pose two key questions about color superconductivity: What are the effects of the large strange quark mass, and what are the observable consequences of color superconductivity? Motivated by the first question, we study crystalline color superconductivity. We adapt the Nambu-Gor'kov formalism to study this phase, and go on to examine distinctions between crystalline color superconductivity induced by quark mass differences and by quark chemical potential differences. Turning to the second question, we study neutrino scattering in proto-neutron stars cooling through the critical temperature for color-flavor locked quark matter. We include neutrino interactions with the fermionic excitations, important above and just below the critical temperature, and with the massless collective excitation, important just and well below the critical temperature.
by Joydip Kundu.
Ph.D.
26
Imboden, Matthias. "Diamond nanoelectromechanical resonators: dissipation and superconductivity." Thesis, Boston University, 2012. https://hdl.handle.net/2144/31569.
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Thesis (Ph.D.)--Boston UniversityPLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you.
Nanoelectromechanical systems (NEMS) have become a viable commercial technology and are becoming more and more prevalent in research applications. Through miniaturization, the mechanical response to external sources becomes ever more sensitive. This transduction, coupled to an electrical readout circuit, results in unprecedented sensitivity. This thesis examines dissipation in diamond NEMS resonators in the MHz to GHz range. NCD (Nano-crystalline diamond) has extraordinary properties that make it an intriguing material to study. To begin with, the mechanical hardness allows for a boost in resonance frequency, but beyond that, boron-doped diamond also shows extraordinary electrical behavior. Although scaling benefits speed and sensitivity, dissipation increases dramatically with miniaturization, negating some of the gains in sensitivity. The dissipative mechanisms at play in the MHz range are identified at high temperatures. It is found that extrinsic dissipation mechanisms, mainly circuit and clamping losses, can limit the quality factor (inverse of the dissipation). Furthermore, due to the high surface-to-volume ratio of NEMS, surface defects become significant at the nano-scale. For the first time, quantum dissipation due to assisted phonon tunneling of two level systems is observed in diamond NEMS resonators at millikelvin temperatures. Through scaling, it is shown that the low temperature behavior is universal for a broad range of MHz resonators, including silicon and gallium arsenide, as well as graphene and carbon-nanotubes. Beyond the mechanical response, the superconducting properties of highly boron-doped diamond (BDD) are studied. It is found that the critical temperature of 3.3 K for the thin-film is maintained at the nann-scale. The high critical field, on the order of;) T for thin-films, is strongly suppressed, already at the micro-scale. The zero resistance state is compromised with fields as low as 0.1 T for submicron wide constrictions. It is known that the superconducting state will couple to the strain field. Here, the piezoresistive detection technique is developed for BDD structures in the MHz range at room as well as cryogenic temperatures. This serves as a framework for future studies of strain-superconductivity coupling.
2031-01-01
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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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Eguchi, Gaku. "Non-centrosymmetric superconductivity in d-electron compounds." 京都大学 (Kyoto University), 2013. http://hdl.handle.net/2433/175101.
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Pang, Brian SiewHan. "Control of superconductivity in cuprate/manganite heterostructures." Thesis, University of Cambridge, 2004. https://www.repository.cam.ac.uk/handle/1810/34610.
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Research has shown that the spin alignment in an adjacent ferromagnet is capable of suppressing superconductivity. In this project, devices incorporating cuprate/manganite heterostuctres were successfully fabricated to study the effects of spin transport on the high temperature superconductor, YBCO. Deposition of such oxide ferromagnet/superconductor (F/S) multilayers using the 'eclipse' pulsed laser deposition(PLD) technique was also examined. Reproducible multilayers with ultrathin repeats were deposited, which exhibited superconducting and magnetic properties to minimum thicknesses of 3nm for both YBCO and LSMO. Using spin injection, via a ferromagnet, to create a spin imbalance in the superconductor, a suppression of superconducting critical current was observed with increasing injection current. However, the exact cause of this suppression could not be solely attributed to spin-induced nonequilibrium effects, as it proved difficult to eliminate the effects of localized heating, current summation and magnetic field. Interfacial studies of the device junction provided evidence of an alternative current path at the interface. The control of superconductivity was also examined using F/S proximity effects, which improves the understanding of how magnetic and superconducting materials coexist. We observed that oxide F/S samples deposited by high O2 sputtering and 'eclipse' PLD were similar, and that Tc was clearly more suppressed in F/S compared to N(normal metal)/S systems. However, the magnetic moment and exchange coupling, two magnetic properties of significance in ferromagnets, did not, individually, have a major influence on the increased Tc suppression. The Curie temperatures of the multilayers were suppressed with increasing manganite thickness because of structural effects, and also with increasing thickness of the YBCO layer which reduced the coupling between manganite layers. To study the use of the spin-valve effect as a means to control high temperature superconductors, we fabricated an LSMO/YBCO/LC(0.3)MO pseudo spin-valve structure, which is equivalent to a superconductor sandwiched within a spin valve where both parallel and antiparallel configurations of the F layers can be achieved within a single magnetic field sweep. Previous research involving a metallic F/S/F/AF structure, showed that the superconductivity was suppressed when the ferromagnets were in the parallel configuration. From the onset of superconductivity, when the normal metallic behaviour of YBCO switches to superconductivity, a magnetoresistance (MR) peak was observed when the F layers were antiparallel. The MR effect increased with decreasing bias current and temperature, characteristic of a pseudo-spin valve. The result is suggestive of spin transport across the YBCO spacer layer.
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Kaveh-Maryan, Kamran. "Superconductivity near boundaries, a wave-function approach." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2000. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp01/MQ60133.pdf.
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Agarwal, P. "Magnetism and superconductivity in heavy-fermion metals." Thesis, University of Cambridge, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.595373.
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One of the key challenges of contemporary condensed matter physics is to understand how the large variety of states of matter arises in electron systems. While the basic force is in all cases the electrostatic Coulomb interaction, the variety of states of matter spans an impressively broad spectrum, ranging from insulators to superconductors, and including a variety of magnetic phases. In recent years, a series of new, emergent phenomena have added more exotic states to the list of known behaviour in metallic systems, and some of them defy the standard descriptions of 'normal' metals, known as Fermi liquid theory. Examples of these new phenomena include the fractional quantum-Hall effect, high temperature superconductivity and magnetic quantum phase transitions, which are at the focus of this dissertation. Such phase transitions occur when magnetic order is suppressed at low temperatures by means of an external control parameter, such as pressure, doping or applied field. In the vicinity of such a transition, unusual properties are expected due to strong, long range magnetic interactions, which can lead to unconventional normal states and, quite generally, to exotic forms of superconductivity. More specifically, I have studied high purity single crystals of two f-electron metals, namely CeNi2Ge2 and UGe2, both close to their critical lattice densities, and examined the behaviour of their electrical resistivity as a function of temperature and pressure. CeNi2Ge2 at ambient pressure is naturally very close to antiferromagnetic ordering, and for the first time, CeNi2Ge2 is shown to be an unconventional superconductor at ambient pressure at about Tc ≃ 200mK. Furthermore, the normal state of CeNi2Ge2 displays an unconventional power-law of the form Δp T1.2 over 2 decades in temperature. As pressure is applied a slow return to Fermi liquid behaviour is observed at the lowest temperatures, before a second (and as yet unidentified) ordered phase sets in at high pressures. The second system studied in this dissertation is the ferromagnet UGe2, in which magnetism can be suppressed by pressures of about 1.4 GPa. In this material, too, superconductivity is observed at low temperatures over a narrow pressure range, but in the ferromagnetically ordered phase. I present preliminary measurements together with a phase diagram. It seems that UGe2 is the first example of an itinerant ferromagnetic superconductor. Experimental work was carried out using a new adiabatic demagnetization cryostat, specially designed and built as an important part of the research project. The thesis describes the design of the instrument, which allows high sensitivity measurements of the resistivity over a wide range in temperature with high reliability.
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Badr, Mohamed Hosiny. "TUNNELING STUDY OF SUPERCONDUCTIVITY IN MAGNESIUM DIBORIDE." UKnowledge, 2003. http://uknowledge.uky.edu/gradschool_diss/422.
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Although the pairing mechanism in MgB2 is thought to be phonon mediated, there are still many experimental results that lack appropriate explanation. For example, there is no consensus about the magnitude of the energy gap, its temperature dependence, and whether it has only one-gap or not. Many techniques have been used to investigate this, like Raman spectroscopy, farinfrared transmission, specific heat, high-resolution photoemission and tunneling. Most tunneling data on MgB2 are obtained from mechanical junctions. Measurements of energy gap by these junctions have many disadvantages like the instability to temperature and field changes. On the other hand, sandwich-like planar junctions offer a stable and reliable measurement for temperature dependence of the energy gap, where any variation in the tunneling spectra can be interpreted as a direct result from the sample under study. To the best of our knowledge, we report the first energy gap temperature- and magnetic field-dependence of MgB2/Pb planar junctions. Study of the temperature-dependence shows that the small gap value (reported by many groups and explained as a result of surface degradation) is a real bulk property of MgB2. Moreover, our data is in favor of the two-gap model rather than the onegap, multi-gap, or single anisotropic gap models. The study of magnetic field effect on the junctions gave an estimation of the upper critical field of about 5.6 T. The dependence of energy gap on the field has been studied as well. Our junctions show stability against temperature changes, but "collapsed" when the magnetic field (applied normal to the junction barrier) is higher than 3.2 T. The irreversible structural change switched the tunnling mechanism from quisiparticle tunneling into Josephson tunneling. Josephson I-V curves at different temperatures have been studied and the characteristic voltages are calculated. The estimated MgB2 energy gap from supercurrent tunneling in weak link junctions agrees very well with that from quasiparticle tunneling. Reported properties on polycrystalline, single crystal and thin film MgB2 samples are widely varied, depending on the details of preparation procedure. MgB2 single crystals are synthesized mainly by heat treatment at high temperature and pressure. Single crystals prepared by this way have the disadvantages of Mg deficiency and shape irregularity. On the other hand, improving the coupling of grain boundaries in polycrystalline MgB2 (has the lowest normal state resistivity in comparison to many other practical superconductors) will be of practical interest. Consequently, we have been motivated to look for a new heat treatment to prepare high quality polycrystalline and single crystal MgB2 in the same process. The importance of our new method is its simplicity in preparing single crystals (neither high pressure cells nor very high sintering temperatures are required to prepare single crystals) and the quality of the obtained single crystal and polycrystalline MgB2. This method gives high quality and dense polycrystalline MgB2 with very low normal state resistivity (σ(40 ) = 0.28 cm). Single crystals have an average diagonal of 50 m and 10 m thickness with a unique shape that resembles the hexagonal crystal structure. Furthermore, preparing both forms in same process gives a great opportunity to study inconsistencies in their properties. On the other hand, magnesium diboride thin films have also been prepared by magnetron sputtering under new preparation conditions. The prepared thin films have a transition temperature of about 35.2 K and they are promising in fabricating tunnel junctions.
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Longmore, A. "Neutron scattering and praseodymium suppression of superconductivity." Thesis, University of Oxford, 1995. http://ora.ox.ac.uk/objects/uuid:46caa612-2ff0-40ae-8cdc-898a9bb9c639.
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PrBa2Cu3O6+x is anomalous among the compounds which can be made by substituting different rare-earth ions for yttrium (Y) in YBa2Cu3O6+x:it is the only compound which has the same structure as YBa2Cu3O6+x, and yet does not superconduct. This unusual property makes it an important system to study, since the differences between the two compounds could produce theoretical insights into the mechanisms of cuprate, or high-temperature, superconductivity. This thesis describes neutron scattering investigations of the magnetic properties of PrBa2Cu3O6+x. I have investigated the magnetic ordering in well-characterised, single-crystal samples, both with and without Al-contamination. In the data analysis, proper account is taken of the sample mosaicity and the different intrinsic peak shapes encountered. Some of the most striking new characteristics reported here are that (i), there is a clear interaction between the Pr ions and the Cu-O2 planes, (ii), the ordered Pr moments tilt away from the c-axis (as suggested previously by Mossbauer spectroscopy), and (iii), there is two-dimensional ordering in the Al-containing crystals. I show how observed trends in the magnetic ordering, as O- and Al-contents vary, may be understood in terms of charge redistribution involving the hybridisation of the Pr ions. I have also examined the crystal field transitions of the praseodymium ion. Because of the difficulties previously encountered in measuring these in PrBa2Cu3O6+x, I have used instead the compounds PrO2 and Y0.3Pr0.7Ba2Cu4O8, which contain praseodymium in a chemically similar environment. Y0.3Pr0.7Ba2Cu4O8 shows the same broadening effects as PrBa2Cu3O6+x; PrO2 shows signs of mixed-valency, with temperature-dependent lifetime effects. The application of symmetry considerations and the point-charge model to the crystal field measurements is considered in some detail.
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Cowan, Greig Alan. "Single-colour and single-flavour colour superconductivity." Thesis, University of Glasgow, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.412941.
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Wooldridge, Jennifer. "Superconductivity and magnetism in sodium cobaltate oxyhydrate." Thesis, University of Warwick, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.443624.
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Powell, Benjamin James. "On the interplay of superconductivity and magnetism." Thesis, University of Bristol, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.251063.
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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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McCulloch, Neil. "A search for superconductivity in conjugated polymers." Thesis, Heriot-Watt University, 2010. http://hdl.handle.net/10399/2324.
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The aim of this thesis is to search for a superconducting phase transition in a conjugated polymer with implanted magnetic atoms. To carry out this investigation we have performed a modified worldline quantum Monte Carlo (QMC) algorithm concentrating on the polymer cis-polyacetylene at finite temperatures. The Hamiltonian used is that of the extended Su-Schrieffer-Heeger discrete tight-binding electron-lattice model which incorporates on-site and nearest-neighbour electron-electron Hubbard terms, Brazovskii-Kirova symmetry breaking, and Kondo impurities. We have found that through changing the conduction charge doping level real space-charge pairs, bipolarons, can be stabilised in the presence of magnetic impurities within certain parameter regimes. This has been established through direct observation of the calculated lattice order parameter/staggered lattice distortion and net charge distributions. Thus we have demonstrated that real space charge pairs and magnetic atoms can coexist. In order to search for a second-order phase transition, indicative of possible superconducting behaviour, we have calculated the impurity magnetic staggered and uniform susceptibilities. This was done at discrete temperatures in the 60K – 300K range in the presence of stable hole bipolarons. The results show, at higher temperatures, an enhancement of the staggered over the uniform susceptibility for next-nearest-neighbour impurity separation. This is indicative of anti-ferromagnetic fluctuations arising from indirect exchange. In contrast, at lower temperatures the staggered and uniform susceptibilities converge consistent with the Kondo effect and destruction of the indirect exchange mechanism. When the impurity separation is increased to six lattice sites both susceptibilities are qualitatively the same at all temperatures as the impurities fluctuate independently. Thus the indirect exchange mechanism is short-ranged and the impurities should be relatively close to each other to facilitate anti-ferromagnetic fluctuations in the presence of bound hole pairs. The susceptibility measurements showed no signature of a phase transition within the aforementioned temperature range. The investigation is however inconclusive as lower temperatures proved computationally prohibitive.
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Saravanamuttu, Nagulan Sriharan. "Coexistence and competition of superconductivity and magnetism." Thesis, University of Cambridge, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.615972.
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Wu, Beilun. "Unconventional superconductivity in the ferromagnetic superconductor UCoGe." Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAY010/document.
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Cette thèse discute essentiellement sur le champ critique supérieur du supraconducteur ferromagnétique UCoGe. La conductivité thermique et d'autres méthodes expérimentales ont été utilisées pour confirmer les nombreux comportements particuliers de Hc2 dans UCoGe, précédemment observés dans des études de résistivité. Ces caractéristiques, y compris une anisotropie forte et des courbures anormales, ne peuvent pas être interprétées en termes de théories classiques pour Hc2. Au lieu de cela, un phénomène spécifique aux supraconducteurs ferromagnétiques - la dépendance en champ de l'interaction d'appariement doit être considéré. Nous montrons que cet effet peut être analysé de façon cohérente avec des propriétés de la phase normales et peut être aussi comparé quantitativement avec une théorie existante. Ceci conduit à une clarification nette pour le cas de H//c dans UCoGe, et explique en même temps le comportement différent de Hc2 dans UCoGe et URhGe. Ces résultats soutiennent fortement l'origine magnétique de la supraconductivité dans ces systèmes. Pour H//b, nous montrons que certaines observations expérimentales convergentes suggèrent un possible changement d'état supraconducteur induit par le champ magnétique transversal dans UCoGe. Indépendamment du reste de l'étude, le dernier chapitre présente quelques résultats expérimentaux sur la phase normale de UCoGe et sur l'autre système de fermions lourds UBe13This thesis mainly discuss the upper critical field of the ferromagnetic superconductor UCoGe.Thermal conductivity and other experimental methods have been used to confirm the numerous particularbehaviors of Hc2 in UCoGe, previously observed in resistivity studies. These features, including the stronganisotropy and the anomalous curvatures, cannot be interpreted in terms of classical theories for Hc2.Instead, a phenomenon specific to the ferromagnetic superconductors - the field dependence of the pairinginteraction, needs to be considered. We show that this effect can be consistently analyzed with normalphase properties, and is quantitatively compared with existing theory. This leads to a net clarificationfor the case of H//c in UCoGe, and at the same timeexplains the different behavior of Hc2 in UCoGe and URhGe. These resultsstrongly support the magnetic origin of superconductivity in these systems. For H//b, we showconvergent experimental observations that suggest a possible change of the superconducting state inducedby the transverse magnetic field in UCoGe. Independent from the rest of the study, the last chapter presents someexperimental results on the normal phase of UCoGe and on the other heavy-fermion system UBe13
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Daido, Akito. "Novel topological superconductivity and bulk-boundary correspondence." Kyoto University, 2020. http://hdl.handle.net/2433/253072.
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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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Gomes, Niladri, and Niladri Gomes. "Superconductivity in Strongly Correlated Quarter Filled Systems." Diss., The University of Arizona, 2017. http://hdl.handle.net/10150/625678.
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The objective of this thesis is to reach theoretical understanding of the unusual relationship between charge-ordering and superconductivity in correlated-electron systems. The competition between these broken symmetries and magnetism in the cuprate high temperature superconductors has been extensively discussed, but exists also in many other correlated-electron superconductors, including quasi-two-dimensional organic charge-transfer solids. It has been suggested that the same attractive interaction is responsible for both charge-order and superconductivity. We propose that the specific interaction is the
tendency in correlated-electron systems to form spin-singlet bonds, which is strongly enhanced at the commensurate carrier density p of ½ a charge carrier per site, characteristic of all superconducting charge-transfer solids. To probe superconductivity driven by electron correlations, a necessary condition is that electron-electron interactions enhance superconducting pair-pair correlations, relative to the non-interacting limit. We have performed state of the art numerical calculations on the two-dimensional Hubbard model on different triangular lattices, as well as other lattices corresponding to K-BEDT-TTF based organic charge transfer solids, for the complete range of carrier densities per site p (0 ≤ p ≤ 1). We have shown that pair-pair correlation for each cluster is enhanced by electron-electron interaction only for p ≃ 0.5, far away from the density range thought to be important for superconductivity. Although initial focus is on charge-transfer solids, the results of the research will impact the field of correlated electrons as a whole. We believe our calculations will provide fundamental and fresh insight to the theory of superconductivity in strongly correlated systems.
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Kittaka, Shunichiro. "Superconductivity of Sr2RuO4 and its eutectic systems." 京都大学 (Kyoto University), 2010. http://hdl.handle.net/2433/120634.
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Tada, Yasuhiro. "Theoretical Study on Superconductivity in Noncentrosymmetric Systems." 京都大学 (Kyoto University), 2011. http://hdl.handle.net/2433/142356.
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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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Carlström, Johan. "Multicomponent superconductivity : Vortex matter and phase transitions." Doctoral thesis, KTH, Statistisk fysik, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-136279.
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The topic of this thesis is vortex-physics in multi component Ginzburg- Landau models. These models describe a newly discovered class of super- conductors with multiple superconducting gaps, and possess many properties that set them apart from single component models. The work presented here relies on large scale computer simulations using various numerical techniques, but also on some analytical methods. In Paper I, Type-1.5 Superconducting State from an Intrinsic Proximity Effect in Two-Band Superconductors, we show that in multiband supercon- ductors, even an extremely small interband proximity effect can lead to a qualitative change in the interaction potential between superconducting vor- tices, by producing long-range intervortex attraction. This type of vortex interaction results in an unusual response to low magnetic fields, leading to phase separation into domains of two-component Meissner states and vortex droplets. In paper II, Type-1.5 superconductivity in two-band systems, we discuss the influence of Josephson coupling and show that non-monotonic intervortex interaction can also arise in two-band superconductors where one of the bands is proximity induced by Josephson interband coupling. In paper III, Type-1.5 superconductivity in multiband systems: Effects of interband couplings, we investigate the appearance of Type-1.5 superconduc- tivity in the case with two active bands and substantial inter-band couplings such as intrinsic Josephson coupling, mixed gradient coupling, and density- density interactions. We show that in the presence of these interactions, the system supports type-1.5 superconductivity with fundamental length scales being associated with the mass of the gauge field and two masses of normal modes represented by linear combinations of the density fields. In paper IV, Semi-Meissner state and nonpairwise intervortex interactions in type-1.5 superconductors, we demonstrate the existence of nonpairwise in- tervortex forces in multicomponent and layered superconducting systems. We also consider the properties of vortex clusters in a semi-Meissner state of type- 1.5 two-component superconductors. We show that under certain conditions nonpairwise forces can contribute to the formation of complex vortex states in type-1.5 regimes. In paper V, Length scales, collective modes, and type-1.5 regimes in three- band superconductors, we consider systems where frustration in phase dif- ferences occur due to competing Josephson inter-band coupling terms. We show that gradients of densities and phase differences can be inextricably intertwined in vortex excitations in three-band models. This can lead to long-range attractive intervortex interactions and the appearance of type-1.5 regimes even when the intercomponent Josephson coupling is large. We also show that field-induced vortices can lead to a change of broken symmetry from U (1) to U (1) ⇥ Z2 in the system. In the type-1.5 regime, it results in a semi-Meissner state where the system has a macroscopic phase separation in domainswithbrokenU(1)andU(1)⇥Z2 symmetries. In paper VI, Topological Solitons in Three-Band Superconductors with Broken Time Reversal Symmetry, we show that three-band superconductors with broken time reversal symmetry allow magnetic flux-carrying stable topo- logical solitons. They can be induced by fluctuations or quenching the system through a phase transition. It can provide an experimental signature of the time reversal symmetry breakdown. In paper VII, Type-1.5 superconductivity in multiband systems: Magnetic response, broken symmetries and microscopic theory – A brief overview, we give an overview of vortex physics and magnetic response in multi component Ginzburg-Landau theory. We also examine Type-1.5 superconductivity in the context of microscopic theory. In paper VIII, Chiral CP2 skyrmions in three-band superconductors, we show that under certain conditions, three-component superconductors (and, in particular, three-band systems) allow stable topological defects different from vortices. We demonstrate the existence of these excitations, charac- terised by a CP2 topological invariant, in models for three-component super- conductors with broken time-reversal symmetry. We term these topological defects “chiral GL(3) skyrmions,” where “chiral” refers to the fact that due to broken time-reversal symmetry, these defects come in inequivalent left- and right-handed versions. In certain cases, these objects are energetically cheaper than vortices and should be induced by an applied magnetic field. In other situations, these skyrmions are metastable states, which can be produced by a quench. Observation of these defects can signal broken time-reversal sym- metry in three-band superconductors or in Josephson-coupled bilayers of s± and s-wave superconductors. In paper IX, Phase transition in multi-component superconductors, we ex- amine the thermodynamics of frustrated multi-components superconductors and show that their highly complex energy landscape can give rise new types of phase transitions not present in single component superconductors.QC 20131205
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Whitley, William George. "Charge density waves and superconductivity in U6Fe." Thesis, University of Edinburgh, 2016. http://hdl.handle.net/1842/22031.
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U6Fe has the highest superconducting transition temperature TSC ~ 4 K out of all of the U-based compounds. Unusually, the Pauli limit (1:84TSC = 7:36 T) is less than the observed critical field for both the a and c axes in this tetragonal material. Neither Pauli or usual BCS orbital limit is apparently respected. In order to explain why superconductivity exceeds the Pauli limit, it must be considered that either the superconducting state is unaffected by paramagnetic effects, or there is a large amount of spin-orbit scattering. Superconductivity is in the dirty limit for typical samples of U6Fe, which means that the latter cannot be precluded. Another unusual property of the superconducting state of U6Fe is that TSC has a positive dependence on the applied pressure P, for P < 4 kbar. This combined with other subtle signals in various measurements have led to the suggestion that a Charge Density Wave (CDW) state may exist in U6Fe below 110 K. The CDW state is typically favoured by materials with low-dimensional structural features such as chains of atoms. Such materials, if superconductors, are also candidates to exhibit the sought-after Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, an unusual state in which the superconducting order parameter is modulated in real space. The FFLO is expected to be suppressed except in clean, Pauli limited materials. Therefore if U6Fe can be produced at high enough quality to bring the superconducting state into the clean limit, it would be a potential candidate for an FFLO state. Part of this project discusses apparatus and techniques applied with the goal of producing such quality samples of U6Fe. We have succeeded in the application of the Solid State Electrotransport (SSE) method to purifying samples, and have been able to replicate the highest Residual Resistivity Ratios (RRRs) achieved (~9, compared to 4 for typical samples), but for single crystals instead of the polycrystals produced in the past. In parallel with the progress made towards higher quality samples of U6Fe, a new X-ray scanner has been developed for grain mapping of samples. This has found application in the course of our synthesis studies. The best quality samples have been studied by X-ray diffraction on the XMaS beamline at the ESRF in Grenoble, France. Below TCDW ~ 10 K, satellites at (δH; δK; 0) = (±0:11;±0:11; 0) were observed that confirm a CDW state, albeit at much lower temperatures than anticipated. By examination of systematic satellite absences we have determined that the displacement vector → u is perpendicular to the modulation direction in k-space. Additionally it has been found that the symmetry of the lattice below TCDW is reduced from that of the room temperature I4=mcm structure. The appearance of additional Bragg peaks below ~110 K during these experiments were later cast into doubt by multiple scattering. We have, however, detected a signal in the form of a jump at ~110 K in specific heat measurements of our samples. These measurements also show a kink near to TCDW. We have additionally extended the investigation of the effect of pressure on the superconducting state. The maximum of TSC is confirmed in our samples, and the subsequent suppression of TSC and Hc2 is investigated up to 8 GPa. We have analysed our Hc2(T) curves at different pressures under a simple two-band model that fits the observed trends well and suggest that at the highest pressures U6Fe is approaching even more unusually enhanced Hc2 values.
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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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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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