Quantum transport and many-body effects in encapsulated graphene

Unlike any other material, graphene is all surface. This means that it is strongly affected by its surroundings, including the substrate it lays upon. In the past few years, it was understood that silicon oxide (SiO2), the most popular substrate material for graphene, limits the performance of graphene devices and obscures interesting physics. Hexagonal boron nitride (hBN) has emerged as “the perfect” substrate that results in graphene devices of astonishing electronic quality. In this Thesis, we explore some peculiar nonlocal effects that are found in such new devices due to quantum transport and electron-electron interactions in the coherent as well as the diffusive regime. Chapters 1-4 are devoted to the introduction of some basic concepts and to the main experimental facts that motivated our work. Since encapsulation in hBN crystals makes graphene practically insusceptible to the environment, electrons can travel micrometer distances without scattering. In Chapter 5, in the framework of the Landauer-Büttiker scattering theory, we have developed a scaling procedure for numerical simulations of tight-binding nonlocal transport in realistic graphene devices. We have tested our method against experimental data on transverse magnetic focusing (TMF). This comparison enables a clear physical interpretation of all the observed features of the TMF signal, including its oscillating sign. Moreover, in graphene/hBN superlattices and in bilayer graphene in a perpendicular electric field, which have broken inversion symmetry, topological currents originating from graphene’s two valleys flow in opposite directions and combine to produce long-range charge neutral flow. This effect translates into a nonlocal voltage at zero magnetic fields in a narrow energy range near Dirac points at distances as large as several micrometers. However, the behavior of the observed long-range nonlocality as a function of temperature, band gap, and carrier concentration remained to be understood. In Chapter 6 and 7, we have showen, using a diffusive theory, that this behavior can be explained with bulk topological transport and Coulomb drag between the electrons belonging to the two different valleys, in good agreement with experimental findings. Finally, in Chapter 8, we have studied the crossed Andreev reflection in a three-terminal hybrid graphene/superconductor system in the quantum Hall regime and its effect on the nonlocal resistances.