Dirty Dirac Materials: Magnetotransport and impurity physics
Main list: Cond Mat Seminars
In contrast to usual metals, where quasiparticles obey a Schrödinger-like Hamiltonian, a large number of materials such as graphene, topological insulators, and even helium-3 have low-energy excitations well described by a massless Dirac Hamiltonian. These "Dirac materials" often have unique responses to impurities and symmetry breaking perturbations - such as a magnetic field -, for example, electrons on the surface of a topological insulator are protected from backscattering due to spin-momentum locking. In this talk I will discuss three phenomena in 2D and 3D Dirac materials both theoretically and in comparison to experiments:(1) I will show how spin-momentum locking can be probed using quasiparticle interference (QPI). A comparison of theory and experiment enables a detailed characterisation of a topological insulator surface.(2) Protection from backscattering can be lifted by a magnetic field parallel to the topological insulator surface. I will demonstrate that such a field retains the Dirac physics of the surface but enables backscattering predominantly in the direction parallel to the magnetic field. The resulting anisotropy of magnetoresistance is a sensitive probe of this loss of topological protection. We compare our theory with transport measurements on bulk-insulating Bi2−xSbxTe3, where we find very good agreement, in particular as a function of gate voltage.(3) Crystal symmetries often mean that 3D Dirac materials contain multiple Dirac nodes within their Brillouin zone. In a large magnetic field the Dirac quasiparticles form effective 1D channels parallel to the magnetic field, meaning only the dispersion parallel to the field is retained. I will argue that a positive magnetoresistance occurs when the magnetic field is orientated in a direction perpendicular to a vector connecting a pair of Dirac nodes i.e. when the two nodes are located at the same location in momentum space projected parallel to the field. This transport mechanism allows one to "spectroscopically" map the bulk electronic structure of a 3D Dirac or Weyl semi-metal with multiple nodes. Our theory will be compared to experiments on the Dirac semi-metal Pb1-xSnxSe.Further details on (2) can be found in Nat. Comms. vol. 8, 1340 (2017). Manuscripts for (1) & (3) are in preparation.
University of Cologne
May 1, 2019
From: 13h00 To: 14h00