Browsing by Author "Mambakkam, Sivakumar V."
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Item Observation of nonlinear planar Hall effect in magnetic-insulator–topological-insulator heterostructures(Physical Review B, 2022-10-10) Wang, Yang; Mambakkam, Sivakumar V.; Huang, Yue-Xin; Wang, Yong; Ji, Yi; Xiao, Cong; Yang, Shengyuan A.; Law, Stephanie A.; Xiao, John Q.Interfacing topological insulators (TIs) with magnetic insulators (MIs) have been widely used to study the interaction between topological surface states and magnetism. Previous transport studies typically interpret the suppression of weak antilocalization or appearance of the anomalous Hall effect as signatures of the magnetic proximity effect (MPE) imposed to TIs. Here, we report the observation of the nonlinear planar Hall effect (NPHE) in Bi2Se3 films grown on MI thulium and yttrium-iron-garnet (TmIG and YIG) substrates, which is an order of magnitude larger than that in Bi2Se3 grown on nonmagnetic gadolinium-gallium-garnet (GGG) substrate. The nonlinear Hall resistance in TmIG/Bi2Se3 depends linearly on the external magnetic field, while that in YIG/Bi2Se3 exhibits an extra hysteresis loop around zero field. The magnitude of the NPHE is found to scale inversely with carrier density. We speculate that the observed NPHE is related to the MPE-induced exchange gap opening and out-of-plane spin textures in the TI surface states, which may be used as an alternative transport signature of the MPE in MI/TI heterostructures.Item Synthesis and characterization of three-dimensional topological insulator nanostructures(University of Delaware, 2023) Mambakkam, Sivakumar V.Quantum computers offer a promising development in computing technology due to the unique way in which information is stored, known as the quantum bit or qubit. Qubits can take the form of superpositions of discrete electronic energy states in a material. Encoding information in this way can enable certain operations to be performed on a much faster time scale than what could be achieved by a normal computer. What prevents quantum computers from being realized on a wider scale is an issue referred to as decoherence. This is the change in the superposition state of the qubit due to interactions with its surroundings. This can irreversibly change the state, resulting in the loss of information. Currently, the most common method for mitigating this issue involves using cryogenics to cool the system, and surrounding the system with large amounts of cladding and noise-isolation. ☐ One potential route to solving this issue of decoherence is through the use of topological insulators or TIs. TIs are a class of materials that feature linearly dispersed, spin-momentum locked electronic states that are delocalized on the outer surface of the material. Spin-momentum locking in these TI surface states provides the electrons occupying them with a form of scattering resistance, as they require both a change in momentum and a spin-flip before the electron will scatter into a degenerate state. In theory, if a system of energetically separated TI surface states was created, an electron could be supported in a superposition state that would retain the scattering resistance, and thus serve as a robust quantum bit. To explore this possibility, we attempted to use quantum confinement in the topological insulator Bi2Se3 to energetically separate the surface states by creating Bi2Se3 nanoparticles. ☐ In this thesis, we focus on answering two main questions: 1) How can we create Bi2Se3 nanoparticles with controllable dimensions, and 2) Can we observe evidence of energetically discrete TI surface states in these nanoparticles? Over the course of this project, we were able to demonstrate the creation of Bi2Se3 nanoparticles via both top-down and bottom-up approaches and examined the relative advantages and disadvantages of each approach. We then applied various spectroscopic techniques to probe the electronic band structure of these particles. In our study, we were unable to find definitive signatures of the surface states but did obtain valuable insights into the materials and characterization challenges involved in this type of measurement. Lastly, we evaluated the potential for “healing” damage along the sidewalls of etched TI nanostructures via post-fabrication annealing. We found that this method was ineffective and discussed what future work may aid in addressing etch-induced damage in TI nanomaterials.