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Spintronic transistor: discovery and characterization of topological insulators

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Author(s):
Carlos Augusto Mera Acosta
Total Authors: 1
Document type: Doctoral Thesis
Press: São Paulo.
Institution: Universidade de São Paulo (USP). Instituto de Física (IF/SBI)
Defense date:
Examining board members:
Adalberto Fazzio; Gustavo Martini Dalpian; Gennady Gusev; Tome Mauro Schmidt; Luis Gregorio Godoy de Vasconcellos Dias da Silva
Advisor: Adalberto Fazzio
Abstract

The main goal of spintronics is to understand the mechanisms to efficiently control both spin configurations and spin currents, aiming the use of the spin degree of freedom as the basic element of digital devices, e.g., the \"spintronic transistor\" in which the ON and OFF are defined by the spin electron orientation. Many of the most promising proposed mechanisms are based on spin current generation in Rashba and/or topological semiconductors mainly mediated by the spin-orbit coupling and electric fields. However, despite topological insulators (TIs) are predicted to feature boundary (surface/edge for three/two dimensional systems) states protected by a given symmetry against disorder, the proposed TI candidates are extremely sensitive to fabrications processes, impurities, and temperature effects; indeed, it is difficult to observe the current known phenomena or even to experimentally achieve the spin transport regime governed by the topologically protected boundary states. In this thesis, based on first-principle calculation, tight-binding models and topological invariant calculations we propose possible solutions for these problems, not only systematically predicting new topological insulator candidates with suitable conditions to achieve the boundary states transport regime, but also suggesting novel phenomena allowing for the spin current control. Specifically, we have i) explored the honeycomb-lattice family proposing a new kind of band inversion; ii) used machine learning to systematically predict new two-dimensional TIs; iii) proposed that instead of focused on finding TIs exhibiting large band gaps, the bulk states can be intrinsically protected by the time-reversal symmetry; iv) found that an external electric field breaking the mirror symmetry in dual topological insulators can be used to control the spin polarization, leading to a non-dynamic spin-polarization generation and allowing the construction of a spintronic transistor; and v) studied the influence of the bulk states in the surface electronic transport. To address this issues we have also implemented the topological invariants: Chern Number Cn and the Z2 invariant within the SIESTA, VASP and AIMS codes which are used to performed first-principles calculations, and we made a model for the electronic transport considering spin-orbit coupling. We believe that our work advances the understanding of the properties of TIs, the external field effects in these systems, and their potential for device applications. We also believe that our proposal, i.e., the spin-polarization controlled by the mirror symmetry breaking, could open a new research area in TIs. (AU)

FAPESP's process: 14/12357-3 - Tansistor based on spintronics: topological characterization and ballistic transport in topological insulators
Grantee:Carlos Augusto Mera Acosta
Support Opportunities: Scholarships in Brazil - Doctorate