Probing Correlated States in Van der Waals Heterostructures

Abstract

Correlated states can be existing in van der Waals materials in various ways, materialslike NbSe2 can hold both superconducting and charge density wave states intrinsically at low temperatures. Through doping or proximation effects, correlated states can be induced in normal materials like graphene. The advances in the van der Waals heterostructure fabrication techniques that realized high-accuracy rotation angle control between the van der Waals layers. When the two layers are twisted from each other at certain angles, the electronic bands will be flattened, and correlates states will appear when the bands are partially filled. This thesis will focus on the characterization of correlated states and flat bands in three different platforms. The first topic is to probe the proximitized superconductivity and charge density waves in graphene/NbSe2 heterostructure. At a temperature of 4.6 K, we have shown that both superconductivity and charge density waves can be induced in graphene from NbSe2 by proximity effects. By applying a vertical magnetic field, we imaged the Abrikosov vortex lattice and extracted the coherence length for the proximitized superconducting graphene. We further show that the induced correlated states can be completely blocked by adding a monolayer hBN between the graphene and the NbSe2, which demonstrates the importance of the tunnel barrier and surface conditions between the normal metal and superconductor for the proximity effect. The second topic is to characterize the flat bands and correlated states in twisted bilayer graphene at the “magic angle” of 1.1 degrees. This small twist angle leads to a long wavelength moiré unit cell on the order of 13 nm and the appearance of two flat bands. We have found that the wavefunctions at the charge neutrality point show reduced symmetry due to the emergence of a charge-ordered state. The wavefunctions of the lower band are delocalized when the top band is partially filled, meanwhile, the wavefunctions are localized at different parts within the moiré unit cell, which points to the correlated nature of the spin and valley degeneracy broken states. The third topic is to prove the existence of flat bands in twisted bilayer metal dichalcogenides, and characters their electronic properties. We investigated twisted bilayer WSe2 in two rotation angles, 3° and 57.5°. From spectroscopy studies, we have shown the flat bands are near the valence band edge for both configurations. From direct spatial mapping of the wavefunctions at the flat-band energy, we have shown that the shapes of the wavefunctions agree with theoretical calculations.