Browsing by Subject "Condensed matter physics"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Classical and quantum aspects of non-linear sigma models with a squashed sphere target space
    (2022-07) Schubring, Daniel
    Various aspects of non-linear sigma models with an SU(N) x U(1) symmetric target space are considered. In the case N=2, three-dimensional topological defects are discussed which are relevant for frustrated magnetic systems and which may offer a new perspective on the Skyrme model. An apparent discrepancy between the large N expansion and the weak coupling expansion noted earlier in the literature is reviewed and clarified. A systematic approach to the operator product expansion at sub-leading order in large N is developed and the spinon two-point function is expanded as a trans-series in which all ambiguities in the Borel plane are shown to cancel.
  • Thumbnail Image
    Item
    Disorder effects in the Kitaev spin liquid
    (2024-09) Kao, Wen-Han
    One of the long-standing open questions in condensed matter physics, which has recently garnered significant attention, is how a quantum spin liquid (QSL) state responds to the various forms of structural disorder that are inevitable in real materials. Despite numerous theoretical proposals and candidate materials for QSLs over the past half-century, the existence of this exotic magnetic phase remains contentious due to two primary challenges in experimental identification. First, the absence of long-range magnetic order in QSLs results in a featureless ground state, making its characterization largely dependent on the excitation spectrum and dynamical probes. Second, the unavoidable presence of structural disorder in real materials can significantly affect the spin-liquid phase or even lead to its destruction. This underscores the urgent need for a deeper understanding of the effects of disorder in quantum spin liquids. The Kitaev honeycomb model, discovered in 2006, is an exactly solvable model that hosts quantum spin liquid phases and holds the potential for realization in transition-metal compounds with strong spin-orbit coupling. The spin fractionalization into locally conserved fluxes and itinerant Majorana fermions underpins the model's exact solvability, even in real-space representation. This characteristic makes the Kitaev model an ideal testbed for studying disorder effects in quantum spin liquids, allowing us to address the aforementioned challenges by enabling the calculation of the energy spectrum and dynamical response without relying on translational invariance. In this dissertation, various aspects of disorder within the Kitaev spin liquid model have been explored, including disorder-induced flux binding, localization of Majorana modes, power-law divergence in the density of states, signatures of Majorana zero modes in the local dynamical correlation function, and strong-disorder criticality in the quasi-one-dimensional Kitaev model. These studies provide not only a deeper understanding of the role of disorder in the Kitaev spin liquid, but also propose potentially observable consequences in thermal conductivity, specific heat, and inelastic tunneling conductance.
  • Thumbnail Image
    Item
    Solid foundations: structuring American solid state physics, 1939–1993
    (2013-05) Martin, Joseph Daniel
    When solid state physics formed in 1940s America, it was unusual. It violated the longstanding convention that physics should only be subdivided according to natural classes of research problems or consistent sets of techniques. Instead, solid state incorporated a wide range of concepts and methodological approaches that had only the most superficial similarities. The unifying force behind the field was the explicit professional goal of bringing academic and industrial researchers into closer dialogue. The non-traditional manner in which solid state formed was symptomatic of a sea change in the American physics community as some physicists in the 1940s began thinking about professional and institutional structures as tools with which they could actively define and maintain the scope and mission of physics. This shift had consequences both for solid state, and for American physics as a whole. Solid state was initially defined in terms of 1940s professional challenges, and so was forced to continually reimagine itself as the context changed around it. Eventually, it fractured into subgroups with divergent perspectives about the field’s goals and how best to address them. One of these, condensed matter physics, has typically been understood as a simple renaming of solid state physics. A close examination of the process by which condensed matter emerged, however, indicates that it represented an intentional return to defining a sub-disciplinary on the basis of natural phenomena and investigatory techniques. Condensed matter physics grew from pointed reactions against the segment of solid state that was closely aligned with industry. It crafted an identity that emphasized the intellectual puzzles physical studies of complex systems could address. As broadly conceived fields like solid state physics established themselves and grew, both in population and in influence, physics as a whole became a broader enterprise. Research areas that might otherwise have branched off into engineering or become independent specialties were offered a place in sub-disciplines like solid state physics. Additionally, other elements of the physics community adopted solid state’s mode of discipline formation, making the definition of “physics” more fluid and responsive to contemporary professional pressures. The evolution of solid state physics was guided throughout by a philosophical debate over the nature of fundamental knowledge. The disagreement persisted mostly between solid state physicists, who advocated the stance that fundamental knowledge could be found at any level of complexity, and high energy physicists, who restricted fundamental knowledge to the theories and concepts that governed the smallest constituents of matter and energy. The progress of this debate was driven by professional concerns about funding and intellectual prestige, and the philosophical positions physicists developed helped, in turn, to shape the field’s professional infrastructure.
  • Thumbnail Image
    Item
    Transport properties of superconducting nanostructures.
    (2012-07) Snyder, Stephen David
    This work is concerned with the transport properties of superconducting nanorings at extremely low temperatures in magnetic field. The goal of this work was to experimentally observe a prediction on the crossover from h/2e to h/e period oscillations of transition temperature when the size of the ring becomes small relative to the superconducting coherence length. Impurities in the aluminum nanostructures studied here hinder the direct observation of this crossover. However, the proper direction to take in future experiments on this subject has been evaluated and firmly established. Along the way, an interesting effect has been observed in the form of a high resistance state in superconducting nanorings. This is remarkable because it has a resistance higher than the normal state resistance even though it is superconducting. Therefore, it seems to be phase coherent even though it is resistive. There have been other similar observations in the literature in wires and disks but not rings. It can be explained in terms of nonequilibrium relaxation of quasiparticles near normal-superconductor interfaces that occur naturally in such constricted structures. The relevant physics of this is discussed.