Browsing by Subject "transition metal dichalcogenides"
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Item Explorations of constructs for unconventional and topological superconductivities(2022-09) Heischmidt, BrettIn recent years, topology has risen as a prominent topic of study within the physics community. At its core, topology is simply a classification system, where all objects within a particular class (or more formally, space) hold a common property. Physicists tend to find topology interesting for a few reasons. First, the classification system can be extremely neat (clean), as when an integral over a physical space comes out as an integer multiple of some constant. Second, interesting physical manifestations can arise when a system lives in one topological class compared to another. Third, other physical manifestations can arise when crossing between topological classes. This thesis work centers itself around various topologies. The central topology is that related to the phenomenon of Majorana Zero Modes (MZMs), which are superconducting excitations at the split between particles and holes (i.e., zero energy). The topological classes relevant here are arrangements of certain systems that give rise to the MZM. There is a secondary topology associated with MZMs tied to their use in so-called "topological quantum computing." In this type of quantum computing, excitations are moved around one another in such a way that they remember where they have been by accumulation of a particular phase. Due to the physical process and its inherent memory of its path, this process has been dubbed "braiding." Aligned with previous language, the topological classes here, then, are the braids. This work studies two systems within the above motivations, NbSe$_2$ and magnet-semiconductor interfaces. NbSe$_2$ is predicted to be a nodal topological superconductor, wherein classes within the topology are defined on the nodes in the Bogoliubov-de Gennes (BdG) spectrum. (By convention, no nodes is trivial, and presence of nodes gives "nontrivial" classes.) Further, MZMs are predicted to arise when the nodes are present. Another platform for realizing MZMs is a combination of a semiconducting nanowire, s-wave superconductor, and magnetic element. Realizing unambiguous signatures of MZMs has been particularly tricky, however, leading to substantial efforts to understand the interactions of the three elements. The magnet-semiconductor interface studies fit within this context. Chapter 1 introduces some concepts motivating this work. The first concept presented is topology in quantum mechanical systems followed by its tie to superconductivity. The next concepts that are presented are tied to unconventional superconductivity and are central to its use in quantum computing. Chapter 2 presents an experimental analysis of NbSe$_2$. After outlining some history and motivation, device and measurement specifics are described. A main result of two-fold anisotropy in magnetoresistant properties of the superconducting state is presented followed by multiple efforts to rule out trivial causes. With these ruled out, an interpretation is presented describing a competition of superconducting instabilities. Chapter 3 addresses quantum spin transport in InSb nanowires. InAs and InSb nanowires are introduced for their role in experimentally showing MZMs. Experimental work on VLS InSb is sketched, although the focus here is a brief description of simulations relevant to the experimental picture. Progress toward exploring other platforms for this work is then presented. Chapter 4 moves into a computational study of Heusler / III-V semiconductor interfaces, with the motivation of studying the semiconductor-magnet interface. Grounding concepts are presented, followed by computational details for two interfaces, Ti$_2$MnIn-InSb and Ni$_2$MnIn-InAs. Results are finally discussed. Chapter 5 summarizes the work.Item Interplay of Symmetry and Topology in 2D Non-Centrosymmetric Superconductors Illustrated in 1H Transition Metal Dichalcogenides(2020-08) Shaffer, DanielThe subject of this dissertation is at the intersection of two major fields of condensed matter physics: unconventional superconductivity (SC) and topological phases of matter. Both conventional and unconventional superconductors exhibit similar qualitative behavior: they pass currents with zero resistance and expel magnetic fields, both effects due to formation of a Cooper pair condensate. Broadly, an unconventional superconductor is simply one that is not described by the textbook Bardeen-Cooper-Schrieffer theory. There are at least three things that can make a superconductor unconventional: the pairing mechanism, the symmetry of the Cooper pair, and topology. In many unconventional superconductors the paring mechanism is thought to arise due to the 2D nature of the material that can exhibit strong quantum fluctuations. Unconventional pairing can lead to spin-triplet Cooper pairs with a non-zero orbital momentum, or even a non-zero total momentum, in which case they can realize the so-called pair density wave (PDW). Since magnetic fields align spins that can only form spin-triplet states, one possibility for realizing unconventional superconductors is to look for superconductors that survive in large magnetic fields. This is well-known to occur in systems with strong spin-orbit coupling (SOC) that is also well-known to lead to possible topological phases and topological superconductors in particular, which exhibit Majorana edge modes that may one day be useful for building a quantum computer. All of these elements come together in a family of monolayer materials with strong SOC known as the 1H transition metal dichalcogenides (TMDs) that have recently been found to be superconducting. The thesis of this dissertation is that they can indeed host interesting unconventional and topological SC phases. To show this, in Chapter 2 we analyze the possible symmetry breaking instabilities using the parquet renormalization group that has been successfully used in other unconventional superconductors. We find that Coulomb interactions can lead to unconventional SC and PDW. In Chapter 3, we explain what makes phases in general topological, and how the topology is restricted by their symmetry. In Chapter 4, we combine the results of the two previous chapter to study 1H-TMDs in a mean-field theory, and find unconventional topological phases. In Chapter 5 we study the PDW phase in more detail, and in Chapter 6 we conclude by looking at recent experimental data that indeed suggests that 1H-NbSe\(_2\) may be an unconventional superconductor, but perhaps not the one we anticipated in theory.