Noise in 2d topological edges and spin Hall systems

William Marsh Rice University, Houston TX

Investigators

Abstract

Non-Technical Abstract: Electrons carry both electrical charge and, much like a top, angular momentum called "spin", and while ordinary electronic devices typically only use the charge, there is increasing interest in taking advantage of the spin for information storage and processing. In some materials with particular structures, a current of charge in one direction can drive a current of spin in a transverse direction (the "spin Hall effect"), and a net imbalance of spin can build up at the material's surface. In other materials (two-dimensional "topological insulators"), charge and spin are predicted to flow only around the perimeter of the material. In both systems, because charge and spin come in discrete amounts and are coupled together, a lot can be learned about how they move by measuring not just the charge current, but fluctuations in the charge current called "shot noise". This project measures the shot noise at radio frequencies produced by current flow along the edges of 2d topological insulators, to determine the nature of that edge transport and the scattering mechanisms that limit its flow. This information is revealed by the geometry, temperature, and magnetic field dependence of the noise. This project applies the same measurement approach to look for shot noise predicted to result when the spin Hall effect builds up spin polarization at a material surface. This is one avenue to determine the amount of spin build-up without the need for complicated device structures involving magnetic materials. This project provides research training and professional development for two graduate students, as well as research opportunities for undergraduates. Results are disseminated through publications, presentations at conferences and universities, and through the principal investigator's blog. The principal investigator is working with K12 teachers and undergraduates to explain this and related research at level appropriate for a general audience, in writings distributed in the blog and in cooperation with the Houston Chronicle. The principal investigator is also developing resources for science writers and journalists, to aid in explaining condensed matter and nanoscale physics concepts to the public. Technical Abstract: There is a growing appreciation that the interplay between spin and orbital degrees of freedom of electrons provides opportunities to manipulate spin and create previously unprecedented electronic systems. The intellectual merit of this effort focuses on understanding two physical systems. In particular materials, band structure predicts the formation of a two-dimensional "topological insulator" state in the bulk, with topologically protected, 1d helical edge states girding the perimeter. Understanding these edge states is essential for realizing their benefits in dissipationless transport and computing. Electron-electron interactions, magnetic impurities, and disorder are expected to affect these edge states in ways testable through measurements of nonequilibrium noise in the edge state conduction. In addition, the related spin Hall effect is a viable means of generating nonequilibrium spin accumulation in metals, with tremendous potential for information storage and processing. Charge noise is predicted to be a way to characterize that spin accumulation without the need for ferromagnetic materials. This project measures nonequilibrium noise and charge transport to examine edge states at 2d TI interfaces and SHE-driven spin accumulation in metal nanostructures. Specific scientific questions include: Are edge states at semiconductor heterointerfaces trivial or topological? What limits ballistic transport in such edge states? To what extent does spin accumulation from the spin Hall effect produce charge current noise? These science questions have relevance for technological applications, including exotic computational architectures and magnetic information storage and processing. Beyond technologies, the broader impacts of this project involve the professional training of graduate students and undergraduates as the next generation of technological workforce, and public outreach including K12 education. The principal investigator works with Rice K12 programs and an ongoing Rice REU program focusing on Houston-area community college students; and public outreach based on his decade-established blog. Specific products include a primer/guide to condensed matter/nano physics for science journalists; "tip-sheets" to science journalists regarding new developments in the field; and science writing contributions for the Houston Chronicle.