February 8, 2022
Visualizing Topological and Correlated Electronic States at the Atomic Scale in 2D Devices
Reduced screening and enhanced Coulomb interactions cause electron correlations to play a significant role in determining the electronic properties of 2D materials, but these same properties can make them difficult to image at the atomic scale. I will discuss how we have explored topological and correlated electron phenomena in single-layer and few-layer 2D systems by incorporating them into gate-tunable devices that can be integrated with a scanning tunneling microscope (STM). This allows carrier concentration to be continuously tuned, thus providing an important experimental knob for exploring correlated states in 2D systems. I will first discuss our STM study of correlated insulating states in gate-tunable twisted monolayer-bilayer graphene (tMBLG), where we observe charge gaps at ½- and ¾-filling of the tMBLG conduction flat band, consistent with the emergence of symmetry-breaking insulating phases. Application of a magnetic field confirms that the ¾-filling insulating state corresponds to a C=±2 Chern insulating state, where the sign of the Chern number can be controlled using the device back gate. I will next discuss our study of an even lower-dimensional system: the 1D Tomonaga-Luttinger liquid (TLL) found in mirror twin boundaries (MTB) of single-layer 1H-MoSe2. Incorporation of single-layer 1H-MoSe2 into 2D devices that can be integrated with STM spectroscopy allows us to visualize TLL behavior in MTBs for different electron densities, thus enabling precise characterization of electron-electron interaction effects. Our measurements unambiguously identify collective density-wave excitations in 1H-MoSe2 MTBs that have two distinct velocities, in agreement with spin-charge separation predicted by finite-length TLL theory.