dc.contributor.author Hsu, Yi-Ting dc.date.accessioned 2018-04-26T14:18:01Z dc.date.available 2018-04-26T14:18:01Z dc.date.issued 2017-08-30 dc.identifier.other Hsu_cornellgrad_0058F_10554 dc.identifier.other http://dissertations.umi.com/cornellgrad:10554 dc.identifier.other bibid: 10361649 dc.identifier.uri https://hdl.handle.net/1813/56972 dc.description.abstract The experimental discovery and subsequent theoretical understanding of the integer quantum Hall effect, the first known topological phase, has started a revolutionary breakthrough in understanding states of matter since its discovery four decades ago. Topological phases are predicted to have many generic signatures resulting from their underlying topological nature, such as quantized Hall transport, robust boundary states, and possible fractional excitations. The intriguing nature of these signatures and their potential applications in quantum computation has intensely fueled the efforts of the physics community to materialize topological phases. Among various topological phases initially predicted on theoretical grounds, chiral topological superconductors and time-reversal symmetric topological insulators (TI) in three dimension (3D) are two promising candidates for experimental realization and application. The family of materials, $\rm{Bi_2X_3}$ (X=Se, Te), has been predicted and shown experimentally to be time-reversal symmetric 3D TIs through the observation of robust Dirac surface states with Rashba-type spin-winding. Due to their robust surface states with spin-windings, these 3D TIs are expected to be promising materials for producing large spin-transfer torques which are advantageous for spintronics application. As for topological superconductors, despite the exotic excitations that have been extensively proposed as qubits for topological quantum computing, materials hosting topological superconductivity are rare to date and the leading candidate in two dimensions (2D), $\rm{Sr_2RuO_4}$, has a low transition temperature ($T_c$). The goal of my phd study is to push forward the current status of realization of topological phases by materializing higher $T_c$ topological superconductors and investigating the stability of Dirac surface states in 3D TIs. In the first part of this thesis, I will discuss our double-pronged objective for topological superconductors: to propose how to enhance the $T_c$ of the existing leading candidate $\rm{Sr_2RuO_4}$ and to propose new material candidates for topological superconductors. First, by carrying out perturbative renormalization group (RG) analysis, we predicted that straining the ruthenate films will maximize the $T_c$ for triplet pairing channel when the Fermi surface is close to van Hove singularities without tuning on to the singularity. Then with a similar RG approach and a self-consistent calculation for the gap equations, we investigated the repulsion-mediated intrinsic and proximity-induced superconductivity in a family of lightly hole-doped noncentrosymmetric semiconductors, monolayer transition metal dichalcogenides (TMDs). We found that thanks to the spin-valley locking in lightly hole-doped TMDs, two distinct topological pairing states are favored for the intrinsically superconducting case: an interpocket paired state with Chern number 2 and an intrapocket paired state with finite pair momentum. Moreover, nematic odd-parity pairing with a possibly high $T_c$ can be induced when proximitized by a cuprate. A confirmation of our predictions will open up possibilities for manipulating unconventional and topological superconductivity at a higher temperature on the device-friendly platform of strained ruthenate films and monolayer TMDs. In the second part, I will discuss our studies on the stability of the Dirac surface states in 3D TIs in the presence of bulk states and in TI-ferromagnetic metal heterostructures. We constructed simple microscopic models with Fano-type couplings between localized and extended states for each situation. Then with ab initio calculations we investigated the fate of the Dirac surface states in terms of the spectrum, the spatial profile and the spin-texture. Based on our results, we proposed explanations for existing experimental spectroscopic and spin-torque results. dc.language.iso en_US dc.subject superconductivity dc.subject Physics dc.subject Topological Insulator dc.subject Condensed matter theory dc.subject Ruthenates dc.subject Topological phases dc.subject Transition metal dichalcogenides dc.title Topological phases in the real world dc.type dissertation or thesis thesis.degree.discipline Physics thesis.degree.grantor Cornell University thesis.degree.level Doctor of Philosophy thesis.degree.name Ph. D., Physics dc.contributor.chair Kim, Eun-Ah dc.contributor.committeeMember Mueller, Erich dc.contributor.committeeMember Ralph, Daniel C. dcterms.license https://hdl.handle.net/1813/59810 dc.identifier.doi https://doi.org/10.7298/X4MG7MN9
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