Electronic Transport In Twisted Graphene And Graphene Nanostructures

Graphene has drawn wide attention for a long time because of its unique physical properties.Graphene superlattice and nanostructure could significantly modify band structure of graphene,inducing plenty of novel states.This thesis focuses on the investigation of properties of twisted graphene superlattice and graphene nanostructures by low-temperature electric transport measurements,and is depicted as the following two parts:1. electric transport of twisted graphene superlatticeMagic angle??1.1°?twisted bilayer graphene?MA-TBG?exihibits flat bands near the zero Fermi energy.Derived from the flat band,strong electron-electron interactions lead to the emergent correlated insulating states and even unconventional superconductivity.However,the formation of flat band acquires rigorous demand for twist angle,making the controllable modulation of band structure especially significant.MA-TBG doesn't show the ability of electrical tuning flat band structure for its strong interlayer hybridization.Here,using the propertity of band gap opening at charge neutral point under displacement field in AB-stacked bilayer graphene,we first expand the magic-angle system to twisted double bilayer graphene?TDBG?and achieve the electrically tunable flat band.Low-temperature electronic transport results reveal the presence of conduction flat band in our 0.98°?1.33°dual-gated twisted double bilayer graphene fabricated by traditional“tear and stack”technique.Correlated insulating states appear once the flat band is half-filled.Displacement field applied by dual gates can modulate the half-filling band gap which reaches its maximum of 3.2me V at 1.33°device.Enhancement of half-filling insulating state in parallel magnetic field indicates spin polarization of its ground state.Furthermore,we observed linear relationship between resistance and temperature over a wide carrier density,which could be tentatively and qualitatively interpreted by electron-phonon scattering.On the other hand,to disclose the characteristics and origin of Landau levels?LLs? in MA-TBG,we investigated magnetoresistance oscillations in near-magic-angle twisted bilayer graphene.It shows degeneracy transition of 4 fold to 8 fold for LLs from charge neutral point as the Fermi level varies,hence,possibly supporting the presence of C₃ symmetry breaking.Our 1.38°TBG device exihibits clear Hofstadter butterfly features for the first time in twisted graphene system.Closer to magic angle,the 1.26°device shows the same LLs as MA-TBG,which provides novel clues to ascertain the origin of LLs in MA-TBG.2. electric transport of graphene nanostructures based on anisotropic H₂ plasma etchingThe anisotropic H₂ plasma etching on graphene facilitates fabrication of graphene nanostructures such as nanoribbons,quantum dots and quantum point contacts,with zigzag edge.Based on such a technique,we fabricated high-quality single electron transistor and observed clear signals of Coulomb blockade and even excited states.Further,we studied the characterization of zigzag edge state.Once bilayer graphene is gapped under displacement field,the conductance of zigzag-edged bilayer graphene would just come from edge states.We explored the tentative observation of corresponding 4e²/h quantized conductance in devices with short channels.We also attempted to achieve the valley filter effect caused by edge states in zigzag graphene nanoribbons.