Transport Study On Graphene Moiré Superlattice

The emergence of van der Waals and superlattice structures enables us unprecedented abilities to create and modify materials and band structures.Two-dimensional moirésuperlattice have attracted great attention from physicists for their high tunability,leading to a series of novel physical phenomena that includes superconductivity,correlated insulator states,anomalous quantum Hall effects,and etc.This paper focuses mainly on graphene moirésuperlattice materials,and uses low-temperature electrical transport measurement methods to study its physical properties.The specific research topics are as follows:1.Scattering between minivalleys in twisted double bilayer graphene.We systematically studied the quantum oscillation behavior in weakly-coupled twisted double bilayer graphene.Magnetotransport measurements revealed two distinct types of quantum oscillations:Shubnikov-de Haas oscillations(Sd HO)due to Landau quantization and magneto-intersubband oscillations(MISO)due to the electronic scattering between minivalleys.High-quality quantum oscillation data allowed us to calculate the interlayer capacitance(C_m)using a capacitance model and therefor to quantitatively analyze the strength of interlayer coupling in the system.For valence band,it gives a constant C_m of～6.3 u F/cm~2,regardless of carrier density,suggesting a well-defined decoupled system;for conduction band,it shows a strong carrier density dependence,and the C_m changes from 19 u F/cm~2 at low density to～7 u F/cm~2 at high density,indicative of a stronger coupling compared to the case in the valence band.Furthermore,our data showed that MISO could be periodically modulated by a displacement field,and after the disappearance of Sd HO,the period of MISO with respect to magnetic field doubled.Temperature-dependent MISO suggested that electron-electron interactions between minivalleys played a crucial role in scattering.Our study demonstrated that twisted double bilayer graphene,as a highly tunable moirésystem,provided an ideal platform for studying interactions between Landau levels,moiréband structures,and electronic scattering behavior.2.Chirality-dependent topological states in twisted double bilayer graphene.We have experimentally detected that although the AB-AB and AB-BA configurations of twisted double bilayer graphene have similar band structures,they exhibit completely different Hofstadter butterfly spectra under magnetic field due to their different band topological properties.In the AB-AB configuration of twisted double bilayer graphene,the orbital magnetic moments in the k~+and k~-valleys are both zero,and the valley degeneracy is preserved under a perpendicular magnetic field.Only the Landau levels with a degeneracy of 4 originating from charge-neutral point and superlattice bandgaps appear in its Hofstadter butterfly spectrum.In the AB-BA configuration of twisted double bilayer graphene,the k~+and k~-valleys have opposite and nonzero orbital magnetic moments,and the valley degeneracy is broken under a perpendicular magnetic field.The degeneracy of the Landau levels originating from superlattice bandgaps changes from 4 to 2,and Landau levels with a degeneracy of 2also appear at the filling factors of±1/2.Our work provides the first experimental verification of the topologically nontrivial properties of the band structure of the AB-BA configuration of twisted double bilayer graphene.3.Epitaxial growth of trilayer graphene/hexagonal boron nitride moirésuperlattice.We have successfully achieved the epitaxial growth of trilayer graphene/hexagonal boron nitride moirésuperlattice.Using atomic force microscopy,we observed that the grown trilayer graphene/hexagonal boron nitride superlattice has a 15 nm moiréperiod,consistent with the lattice mismatch between graphene and hexagonal boron nitride.Electrical transport results show that our epitaxially grown trilayer graphene exhibits the characteristics of an ABC stacking configuration.More importantly,we observed additional resistance peaks at the hole filling factor of 1/2,indicating the strong electron-electron interaction in our epitaxially grown trilayer graphene/hexagonal boron nitride moirésuperlattice.Our work demonstrates that epitaxial growth can provide a simple method for preparing stable and reproducible two-dimensional strongly correlated electronic materials.