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Optoelectronic study of topological insulators and topological insulator - magnetic insulator heterostructures
Restricted (Penn State Only)
Doctor of Philosophy
Date of Defense:
June 16, 2017
Nitin Samarth, Dissertation Advisor
Nitin Samarth, Committee Chair
Jun Zhu, Committee Member
Chaoxing Liu, Committee Member
Venkatraman Gopalan, Outside Member
Topological insulators have attracted much contemporary interest due to the gapless spin-textured surface states which reside within the bulk band gap. The spin-momentum locking in these helical surface states provides a unique opportunity for "topological spintronics" devices that function at technologically relevant temperatures (300 K and above). Optoelectronic methods have been adopted to control electron spin and charge currents in 3D TIs. In particular, experiments have shown that circularly polarized light induces a directional helicity-dependent photocurrent (HDPC) in 3D TIs. The ready observation of this phenomenon at 300 K promises interesting opportunities for developing opto-spintronic devices wherein electron currents might be steered optically. Progress toward such technological applications is however impeded by a lack of understanding about the physics underlying this phenomenon. Thus, a comprehensive study of the helicity-dependent photocurrent in 3D TIs as a function of the incidence angle of the optical excitation, its wavelength and the gate-tuned chemical potential is described in the first part of the dissertation (Chapter 3). I unambiguously identify the circular photo-galvanic effect as the dominant mechanism for the helicity-dependent photocurrent. Additionally, a theoretical analysis and numerical calculation of the photocurrent relates the directional nature of the photocurrent to asymmetric optical transitions between the topological surface states and bulk bands. Though the topologically protected surface states are interesting by its own, the emerging field of ’topological spintronics’ relies on interfacing the helical Dirac surface states of TIs with magnetism. Heterostructures that combine TIs with magnetic insulators (MIs) are particularly relevant within this context. The magnetic proximity effect and the spin pumping at the interface are of great importance in the research field of topological spintronics. Thus, the second topic of the dissertation (Chapter 4) is to explore the spin pumping by light illumination on the heterostructure. The discovery of a spin-dependent photocurrent (PC) is a signature of the spin pumping, which maps out the magnetization at the interface, as confirmed by a direct comparison with magneto-optical Kerr effect measurements. Further insight into this phenomenon is gained by studying the spin-dependent PC as a function of the chemical potential of the TI film, as well as by examining its variation with the temperature and the wavelength of the optical excitation. The optoelectronic study in the heterostructure of TI-MI not only identifies the spin pumping, but also provides a new method for magnetization measurements.
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