Study of Rashba Spin-orbit Coupling in Oxide Heterostructures

Abstract

Spintronic devices hold the promise of being resilient, energy-efficient, and non-volatile compared to conventional electronic devices. One of the fundamental aims of spintronics is to develop efficient methods for controlling electrons’ spin degrees of freedom. Interestingly, the Rashba spin-orbit (RSO) effect, which arises due to the interplay of intrinsic spin-orbit coupling and broken structural inversion symmetry offers exactly what is required for an efficient spintronics device. This is because the structural symmetry can be broken by an electric field that can be applied either externally or can be generated internally. Moreover, in recent years, a special branch of spintronics, named spin-orbitronics, has been garnering a lot of attention from researchers for its potential to revolutionize the landscape of electronic devices In this respect, transition metal-based oxide heterostructures have emerged as the front-runner for the implementation of these ideas for the following reasons: the possibility of stronger intrinsic spin-orbit coupling, higher spin-flip length, and ease of generating an internal electric field that breaks the structural inversion symmetry. However, the most well-known transition metal-based oxide heterostructures, which show a prominent RSO effect, are ill-suited for device applications due to the emergence of magnetism at the interface. In this thesis, we employ functional theory (DFT) based first-principles calculations to study a polar-nonpolar sandwich heterostructure made of LaAlO3 (LAO)/SrTiO3 (STO) and the polar-polar heterostructure made of LaAlO3/KTaO3 (KTO) and demonstrated that these structures are nonmagnetic while hosting the RSO effect prominently. Surprisingly, we have found a strong inter-orbital coupling between t2g and eg orbital, which contributes to the RSO effect, in the LAO2.5/STO5.5/LAO2.5 heterostructure. Moreover, we find that the strength of the cubic Rashba splitting in this system surpasses the linear Rashba splitting. On the other hand, we have found evidence of both cubic and linear Rashba interactions in the LAO/KTO heterostructure. It is noteworthy that the linear RSO coupling strength found in this system is the highest one for the non-magnetic oxide heterostructure. Interestingly, a potential photocurrent transition path has been discovered in this heterostructure, making it an excellent platform to study the circularly polarized photogalvanic effect.