Charge-Spin Conversions in Helical Metals and Chiral Materials

Florida State University, Tallahassee FL

Investigators

Abstract

Non-technical Abstract: Conventional semiconductor microelectronics utilize only the charge of electrons for logic operations. It has been demonstrated that harnessing the spin of electrons can not only lead to enhanced performance such as dramatic reduction in power consumption but also produce fundamentally new functionalities such as nonvolatility and reconfigurable logics. An essential ingredient of spin-based semiconductor electronics is the creation of electron populations and currents with well-defined spin orientations, i.e., spin polarization, in a nonmagnetic semiconductor. This project aims to generate, control, and detect polarized spins in semiconductors through pure electrical means, and in some cases without even using any magnetic material. One way is by driving an electric current through the surface of a topological insulator, where the electron velocity and spin are always locked in orthogonal directions; the other is by pushing electrons through a chiral medium, which can be a monolayer of chiral organic molecules or inorganic materials with inherent chiral crystal structures. Practically, the research may lead to new paradigms of spintronics devices free of magnetic materials. At the fundamental level, they offer ideal platforms for studying microscopic mechanisms of charge-spin conversions due to structural chirality and electronic interactions. The project also presents an invaluable combination of basic science research and technology development, an effective venue for preparing graduate students for careers in academia and industry. Technical Abstract: This project is aimed at the generation, control, and detection of polarized spins in semiconductors through pure electrical means, and in some cases without using any magnetic material. The project has two distinct research thrusts, which share a common objective of addressing the manifestations and microscopic mechanisms of charge-spin conversions due to structural chirality and electronic interactions. The two lines of research specifically target demonstrating and understanding electrical generation of spin currents from charge motion in two different systems: i) 3D strong topological insulators whose surface states exhibit helical spin textures in momentum space and, ii) materials with intrinsic structural chirality in real space. In the first system, the research introduces photocurrent injection and correlated characterizations of the topological surface state, which offers high probability of deciphering the microscopic mechanism behind the current-induced spin polarization and determining its device application potentials. The study on the spin filtering effect of chiral materials is designed to produce fundamental insights on spin-dependent transport in the chiral media. The results from these physical chiral structures have broad implications in pertinent studies of emergent spin-helical states in topological materials. Practically, the research may lead to conceptually new methods of spin injection and detection, forming the basis for spintronics device platforms free of magnetic materials. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.