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DTSTART;TZID=Asia/Kolkata:20260421T100000
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CREATED:20260416T111234Z
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UID:10272-1776765600-1776769200@www.cense.iisc.ac.in
SUMMARY:[Thesis Colloquium] : Probing quantum transport of weak topological insulators\, engineered heterostructures of strange metals with strong topological insulators and 3D Dirac semimetals
DESCRIPTION:Thesis Title: "Probing quantum transport of weak topological insulators\, engineered heterostructures of \nstrange metals with strong topological insulators and 3D Dirac semimetals."\n\nName of the Student: Mr. Nirmal K Sebastian\n\nDegree Registered: Ph.D. Engineering \n\nAdvisor: Prof. Anil Kumar P.S (Physics)  and  Dr. Shivakumara. C (SSCU)\n\nDate: 21st April 2026\, (Tuesday)\, 10:00 AM\n\nVenue:  CeNSE Seminar Hall \n\nAbstract:\n\nTopological materials are known for hosting unusual electronic states that come mainly from strong \nspin-orbit coupling(SOC) and band topology. In this work\, we study three different yet\, closely related \ntopological phases to understand how weak antilocalization (WAL)\, weak localization (WL)\, and quantum interference \nappear in them\, along with the nature of quantum oscillations in surface states with high mobility.\n\n\nIn the first part of this thesis\, we find that thin films of Bi₂Te₃ grown in the R3m phase behave like a weak \ntopological insulator. When the magnetic field is applied perpendicular to the side surface\, we clearly observe \nShubnikov-de Haas quantum oscillations in electrical resistivity at very high magnetic fields. Interestingly\, \nthe quantum oscillations only appear for this geometry\, indicating that the side surface states play the dominant \nrole here and has much larger charge carrier mobility compared to the bulk and the other surfaces. The extracted \noscillatory part after a background subtraction is fitted well with Lifshitz Kosevic equations that validates the \nvery large mobility and associated physical properties of the side surface. This highlights how the side surfaces of \nweak TIs are quite crucial if we want to use them in devices later. Accessing the side surfaces of weak topological \ninsulators is very essential in harnessing their true potential. We also observe WAL feature that fits nicely to the \n2D Hikami-Larkin-Nagaoka (HLN) expression. This further confirms the existence of strong SOC for the side surfaces. \nThis work experimentally realizes the emergence of a weak TI phase in R3m phase of Bi₂Te₃.\n\nIn the next part of this thesis\, we look at thin films of YBa₂Cu₃O₇−δ/Bi₂Se₃ heterostructures\, where YBa₂Cu₃O₇−δ is \nin the strange metal phase. Here\, we see a crossover from WAL to WL with increasing magnetic field and temperature. \nBy fitting the magnetoconductance to a two-component HLN model\, we find competing length scales such as dephasing length\,\nspin-orbit scattering length\, and mean free path. At low temperatures\, the conductance changes logarithmically with \ntemperature\, which suggests that electron-electron interaction effects are also important. This shows that when a correlated \nphase like strange-metal YBCO is placed in close proximity to a topological insulator\, the quantum interference properties \nof the TI surface states can be tuned quite strongly\, and strong correlations can be induced upon proximity of a TI surface \nto a strongly and non-locally entangled electronic species like a strange metal.\n\nFinally\, we study the electrical transport properties of Bi₄Se₃\, which is expected to be a 3D Dirac semimetal. Previous reports \nhave shown it to be a 3-D Dirac semimetal via DFT calculations\, but the electrical transport is still lacking. Here\, we report \nelectrical transport studies of Bi₄Se₃ for the first time. The variation of resistivity with temperature shows a semimetallic nature.\nFurthermore\, upon applying various strengths of magnetic fields\, the Dirac cone doesn’t gap out in bulk\, which is indicative of true \n3-D nature of the Dirac fermions. We also observe a clear WAL signal that is best fit using a 3D version of the HLN model\, \nsupporting that bulk Dirac fermions with strong spin-orbit coupling and a Berry phase of π are present. \nThe structure consists of alternating Bi₂Se₃ layers and Bi bilayers\, and it belongs in the R-3m space group\, \nsimilar to some layered telluride systems. Also\, the variation of the characteristic length scales follow a power \nlaw decaying behaviour\, where the exponent describes dephasing via 3-D electron-electron interactions. All these results \nconfirm that Bi₄Se₃ is a 3-D Dirac semimetal.\n\nTogether\, these results give a more complete picture of how strong SdH oscillations can arise from side surfaces \nof weak TIs\, the elusive nature of competition between WAL and WL for the surface states of strong TIs in proximity \nto a strange metal\, and nature of quantum diffusion and 3-D WAL in 3D Dirac semimetals. Understanding how these \ntopological effects evolve is useful for designing hybrid systems for topological quantum computing and low-dissipation \nelectronic/spintronic applications.
URL:https://www.cense.iisc.ac.in/event/thesis-colloquium-probing-quantum-transport-of-weak-topological-insulators-engineered-heterostructures-of-strange-metals-with-strong-topological-insulators-and-3d-dirac-semimetals/
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