Ph.D. Defense- Zhiming Zhang
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Abstract:
Two-dimensional van der Waals (vdW) materials can exhibit various interesting properties when heterostructures were created in different configurations. Correlated states like superconductivity are particularly interesting in these heterostructure devices, as they can be emergent from previously non-superconducting components like graphene by proximation effects or moiré modulations.
In this dissertation, we probe into the correlated states in three vdW heterostructure systems and image their wavefunctions with scanning probe microscopy and spectroscopy. The first system consists of two layers graphene rotated ~1.1° away from each other, known as the "magic-angle" twisted bilayer graphene. We have found that the symmetry of the wavefunctions is closely related to the spin and valley degeneracy of correlated states at different doping level. The second system consists of two layers of WSe2 rotated away from each other by 3° or 183°, where flat bands were found near their valence band edge, the shapes of the wavefunction at the flat band energy are different for 3° and 183°. Both of these observations are in excellent agreement with theoretical calculations. In the last system, graphene/NbSe2 heterostructure, superconducting gap and Abrikosov vortex lattice were resolved in proximity-induced superconducting graphene. Moreover, with the insertion of a single layer of hBN, we have found that the proximity effects can be blocked.
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