Seminar by Luca Galletti
Condensed Matter Seminar Series
Niels Bohr Institute
Topological states of the Dirac Semimetal Cd3As2
In this presentation, I will review the scientific results achieved as an Assistant Project Scientist at UC Santa Barbara, with special focus on transport characterization of epitaxial thin films of the three-dimensional Dirac semimetal Cd3As2. Topological Dirac semimetals offer an extremely versatile platform to engineer their topological states using thin films technologies. While topological Dirac semimetals have been studied extensively in bulk crystals, thin films offer unique opportunities to tune and control their topological states. The electronic state of Dirac semimetals is characterized by band inversion, which determines a ℤ2 topological invariant in the bulk. The electronic bands touch in doubly degenerate bulk Dirac nodes at the Fermi level and disperse linearly around them. Topological surface states, extensively studied in bulk crystals as Fermi arcs, show different features in thin films, and critically depend on the film orientation.
Epitaxial films were grown by molecular beam epitaxy on GaAs, GaSb and CdTe substrates in either (112) or (001) orientations. For sufficiently thin, (112)-oriented Cd3As2 films, the bulk Dirac nodes become gapped by quantum confinement. Magnetotransport studies on gated Hall bars show a quantum Hall effect. A spectroscopic signature of a zero energy Landau level can be detected, at high magnetic field, while an ambipolar transport with a linear energy dispersion of the 2D states is observed at zero field. The nature of the topological states, and their signatures in magnetotransport, change dramatically for (001) thin films. We show that at large negative values of gate bias, the filling factor ν = 1 is abruptly preempted by an insulating state that is accompanied by the collapse of the well-developed quantum Hall effect. I will discuss the data in the context of the topological edge states, which are required by the ℤ2 invariant, in close analogy to 3D topological insulators.