Electronic Transport In Twisted Graphene And Graphene Nanoconstrictions

Graphene,due to its linear dispersive band structure,has unique physical properties.Twisted graphene moiré superlattices and graphene nanostructures can significantly modulate the band structure of graphene,resulting in novel physical phenomena,including correlated insulating states,unconventional superconductivity,quantum anomalous Hall effect,and electron coherent transport.Different twisted heterostructures and different nanostructures will be more convenient to achieve different physical properties.At present,the research on the physical mechanism of unconventional superconducting states and electron strongly correlated states,as well as the application of graphene nanostructures in micro and nano electronics devices,is still ongoing,in which high-quality devices are essential for the detailed study of novel physical phenomena.This thesis mainly introduces the preparation of highquality twisted graphene heterostructures,the study of fractal energy spectrum and electron correlation states,and the modulation of electron transport behaviors in graphene nanoconstrictions.High-quality twisted graphene without interlayer bubbles is prepared by an improved dry transfer technique that is using a hemispherical structure made of a harder epoxy resin and an organic adhesive layer.The hyperfine fractal spectrum of the Hofstadter butterfly is observed in the 0.98° twisted bilayer graphene.This is a self-similar fractal quantum transport phenomenon caused by the matching of the period length of the potential field of the moiré superlattice and the characteristic length of the magnetic field.We observe three types of Dirac points.At the first neutral Dirac point,we observe the quantum Hall effect both with four-fold degeneracy and with degeneracy totally lifted,the latter corresponding to the spin and valley symmetry breaking completely.At the second Dirac point observed at eightelectron filling factor,the quantum states have four-fold degeneracy.At the thirdgeneration Dirac points formed in the cross region of the former two,the magnetic flux passing through the superlattice unit cell under the magnetic field of these points is exactly an integral part of magnetic flux quantum,1/,and the distinguishable fractal structure is still observed at up to = 16.The fine energy spectrum of the system under small magnetic field provides a platform for further investigation of the change of the fractal gaps with the topological quantum number or even the fractal structure in the case of incompatibility between Bloch bands and Landau levels.We prepare 1.18 ° twisted monolayer-bilayer graphene and observe perpendicular displacement field electrically tunable correlated states.In the direction from the bilayer towards the monolayer graphene,the half filling correlated insulating state appears and a metal-insulator transition can occur in perpendicular magnetic field.The insulation gap increases significantly with the increase of the magnetic field,indicating that it has a polarized ground state and the formation of in-plane electron orbitals.However,only incipient correlated insulting sates are formed at one-quarter and three-quarter fillings,where it only exhibits the increase of resistance peaks without the formation of band gaps.In the opposite direction of displacement field,no correlated states under each filling are found.This is due to the low symmetry of the twisted monolayer-bilayer graphene,which causes the spontaneous symmetry breaking of spin and valley under different integer fillings of flat bands and forms topological nontrivial sub-bands and the corresponding diverse polarization ground states,and is also due to electrostatic screening from the addition of the weakly charged third graphene sheet to reduce strength of electron interactions between monolayer and bilayer graphene.We transfer graphene to hexagonal boron nitride by dry transfer technique and prepare graphene nanoconstrictions by feedback controlled electroburning technique.We realize that a crossover of electron transport behaviors can be reversibly controlled by a single back gate,from electron tunneling to Coulomb blockade dominated single electron continuous tunneling and finally to electron-wave interference dominated quantum behavior.This is due to the increasing gate voltage raises the Fermi surface in the graphene nanoconstriction to gradually cross the spatial energy profile in the nanostructure,resulting to reduce the height and width of the barriers in the nanoconstriction,which affects the effective size of the nanoconstriction and transmissivity of electrons and finally realizes a crossover of electron transport behaviors.