Benjamin Hunt, Ph.D. Assistant Professor, Department of Physics, Carnegie Mellon University

Proximity effects and topological spin currents in van derWaals heterostructures

Abstract

Solids have traditionally been categorized in terms of their electrical conduction: conductors such as aluminum conduct electricity; insulators such as wood and plastics do not; semiconductors such as silicon show an intermediate behavior. In the last decade, this simple picture has been upended. It is now well established that there is a class of solids in which the bulk is insulating, yet the surface of the solid can behave as a very special conductor. These new materials are known as topological insulators (TIs), and the electronic states responsible for the conduction are known as topological surface states. If the TI is atomically thin, the conducting states can be considered one-dimensional and are called topological edge states (TESs). These TESs can have very special properties not generally present in other conductors, such as a strong relationship between the spin direction and the charge current of electrons. Because of these special properties TESs can play host to the remarkable emergent particles known as Majorana fermions, which have been proposed as the building blocks of a potentially transformative technology: the topological quantum computer. However, so far the most useful types of TESs have proven to be the most difficult to create and control experimentally, and thus a new strategy for their realization is needed.

In this proposal, we will investigate a new framework for the creation and manipulation of TESs using the proximity effect in solids: certain properties of one solid can “leak” over a short distance into another in close proximity. Thus, by layering two or more solids, one can design composite materials with novel properties, the whole being greater than the sum of its parts. A new type of artificial layered material, the “van derWaals heterostructure” built from two-dimensional crystals such as graphene and its analogues, shows particular promise for realizing proximity effects because of the atomic flatness and thinness of its constituents as well as the diversity of materials that can be incorporated. We will create unique TESs using the magnetic proximity effect in graphene as well as the superconducting proximity effect in a two-dimensional topological insulator, and demonstrate how these strategies can lead to a scalable platform for the realization of Majorana fermions.

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