QDev seminar by Jonathan Eroms

Jonathan Eroms

University of Regensburg

Modifying graphene: Artificial crystals and spin injection from 2D magnets

Electronic transport in the 2D material graphene can be controlled by various means. In this talk, I will present transport signatures of Dirac electrons in a one or two-dimensional periodic lattice - an artificial crystal - and of electron spins injected from a ferromagnetic van-der-Waals material. In the former case, a 2D periodic superlattice leads to the fractal energy spectrum first calculated by Hofstadter in 1976. When a magnetic field is applied, the translational crystal symmetry and the phase shift introduced by the vector potential have to be satisfied at the same time, which is only possible for a rational number of flux quanta through a lattice unit cell. The spectrum can be detected in magnetotransport, manifesting in Landau fans emanating from superlattice Dirac points, a non-monotonic sequence of quantum Hall resistances, and, at elevated temperatures, by band conductivity oscillations.

For 1D superlattices, band conductivity oscillations also appear due to the commensurability of the cyclotron radius and the lattice period. In addition, unique to graphene, superlattice-induced Dirac points and anisotropic transport can be observed, and the degeneracy of quantum Hall plateaus is expected to be modified due to the appearance of zero-energy Dirac points.

Regarding spin transport, graphene is ideally suited for preserving spin polarization over several micrometers length due to its intrinsically long spin lifetime and high mobility. Contact to other van-der-Waals materials with vastly different properties, can modify its intrinsic properties, such as spin-orbit coupling. As an example for a device consisting exclusively of van-der-Waals materials, we have realized a lateral spin valve device using the 2D ferromagnet Fe3GeTe2 as an injector/detector, hexagonal boron nitride as a tunnel barrier, and graphene as the spin transport channel.