Spin and topological effects in 2D quantum material devices

Abstract

Topological quantum materials, such as topological insulators, Weyl semimetals and magnetic topological insulators, have electronic band structures with a nontrivial topology and protected surface or edge states that cannot be destroyed by disorder or small perturbations. These band structures contain characteristic spin textures where the electron's spin is locked to its momentum direction, thereby generating a spontaneous spin polarisation in response to an electric field. Additionally, the electronic states host fundamental quantum geometric features, which can give rise to nonlinear transport phenomena.In this dissertation, spin and topological effects are explored in van der Waals quantum materials through spin and electronic transport in microscopic devices. First, spin textures in the ferromagnets cobalt and Fe5GeTe2 are sensed electrically through pure spin-polarised transport in lateral graphene spin-valve devices. Next, similar graphene heterostructure devices are employed to observe charge-spin interconversion in the topological bands of the topological insulator Bi1.5Sb0.5Te1.7Se1.3 and the Weyl semimetal TaIrTe4. Finally, nonlinear electronic transport effects are utilised as a sensitive probe of the magnetic phases and the quantum geometry of the magnetic topological insulator MnBi2Te4. Together, these findings demonstrate the exciting physics of spin and topology in van der Waals topological quantum materials, which can lead towards new fundamental insights as well as novel spintronic and quantum devices for next-generation technologies.