Prof. Emanuel Tutuc, Department of Electrical & Computer Engineering, UT-Austin

"Magnetotransport and Tunneling in Rotationally Controlled van der Waals Heterostructures"

Coffee and cookies will be served at 3:45pm in RLM 4.102

Abstract: Starting with the isolation of graphene in 2004, a wide range of atomic, two-dimensional (2D) layers of bulk crystals have emerged as a new class of materials, with electronic properties ranging from metals, to semi-metals, and to insulators. This large palette offers a seemingly endless range of possibilities to combine 2D materials in designer van der Waals (vdW) heterostructures, otherwise not be available through other techniques, and to stabilize novel quantum states as a result of strong electron interaction and confinement. We discuss here recent techniques that allow the realization of vdW heterostructures with unprecedented control of the layers rotational alignment and their interfaces, and illustrate the applicability of these techniques to designer moiré patterns and resonant tunneling heterostructures. Two examples of novel quantum states that may be stabilized in such vdW heterostructures will be discussed. We first review magnetotransport studies in transition metal dichalcogenide (WSe2, MoSe2) atomic layers, which reveal clear quantum Hall states, and allow the extraction of fundamental electronic properties, such as carrier effective mass at band extrema, Landau level degeneracy, and Zeeman splitting. The carrier density dependence on gate bias reveals negative electronic compressibility, and the quantum Hall state sequence dependence on carrier density reveals an interaction-enhanced Landau level Zeeman splitting consistent with the large (0.5-1me) carrier effective mass measured in these materials, which suggest a ferromagnetic state can be stabilized in the low density limit. We also discuss the realization of rotationally aligned double bilayer graphene separated by a WSe2 tunnel barrier. Thanks to translation invariance the interlayer current-voltage characteristics show resonant tunneling when the band extrema of the two layers are energetically aligned. Surprisingly, the tunneling conductance is strongly enhanced at reduced temperatures when the two layers are populated with electrons and holes with equal densities. The strongly enhanced tunneling at overall neutrality departs markedly from single-particle model calculations that otherwise match the measured tunneling current-voltage characteristics well, and suggests the emergence of a many-body state with condensed interbilayer excitons when electrons and holes of equal densities populate the two layers.