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Direct visualization of the states originating from electron-electron interactions is of great importance for engineering the surface and interfacial properties of graphene-based quantum materials. For instance, the rotational symmetry breaking or nematic phase inferred from spectroscopic imaging has confirmed the existence of correlated states in a wide range of moiré materials. Here, we study the atomic-scale spatial distributions and symmetry of wave functions in gate-tunable twisted double bilayer graphene by employing scanning tunneling microscopy/spectroscopy and continuum model calculations. A series of spectroscopic imaging analyses are used to identify dominant symmetry breaking of the emergent states. Interestingly, in non-integer hole fillings, a completely new localized electronic state with rotational symmetry breaking is observed on the left side of the valence flat band. The degree of anisotropy is found to increase from the conduction flat band through the valence flat band to the new state. Our results provide an essential microscopic insight into the flat band and its adjacent state for a full understanding of their electric field response in twisted graphene systems.

The performance of electronic circuits is becoming limited by on-chip digital information transmission. Graphene plasmons with ultra-high confinement and low damping rates offer an effective solution to this problem as they allow for the implementation of optical interconnects. However, direct contact with the semiconductor always deteriorates the plasmonic properties due to large damping of the plasmon in the semiconductor. Here, we studied graphene plasmons in heterostructures of graphene and monolayer MoS 2 which represents a promising semiconductor for next-generation electronic devices. The graphene plasmons in the heterostructures were systematically evaluated in terms of their resonance frequency, intensity, lifetime and electrical tunability and were found to keep their main characteristics. Experimental results exhibited both a redshift in the plasmon frequencies and a decrease in the resonance intensity for both graphene/MoS 2 and MoS 2 /graphene devices when compared with graphene devices at the same gate bias. These results could be attributed to changes in the dielectric constant and effective doping of graphene. Furthermore, the conductivity saturation on the electron branch in the back-gated graphene/MoS 2 device cancelled the electron plasmons. These findings demonstrate that electrically tunable graphene plasmons can be realized in contact with semiconducting MoS 2 . Our results provide a platform for the investigation of the integration of semiconductor-based electronic and optoelectronic devices with plasmonic devices through van der Waals heterostructures.