Theoretical Investigation On Quantum Transport Properties In Multilayer Graphene Systems

As the thinnest conductive and elastic material,graphene is expected to play a crucial role in post-Moore era.Besides applications on electronic devices,graphene has shown great potential for nano-electromechanical systems.While interlayer inter-actions play a key role in modifying the electronic structures of layered materials,no attention has been given to their impact on electromechanical properties.Here we report the positive piezoconductive effect observed in suspended bi-and multi-layer graphene.The effect is highly layer number dependent and shows the most pronounced response for tri-layer graphene.The effect,and its dependence on the layer number,can be un-derstood as resulting from the strain-induced competition between interlayer coupling and intralayer transport,as confirmed by the numerical calculations based on the non-equilibrium Green's function method.Our results enrich the understanding of graphene and point to layer number as a powerful tool for tuning the electromechanical properties of graphene for future applications.The existence of inequivalent valleys K and K' in the momentumspace of 2D hexagonal lattices provides a new electronic degree of freedom,the manipulation of which can potentially lead to new types of electronics,analogous to the role played by electron spin.In materials with broken inversion symmetry,such as an electrically gated bilayer graphene(BLG),the momentum-space Berry curvature ? carries opposite sign in the K and K' valleys.A sign reversal of ? along an internal boundary of the sheet gives rise to counterpropagating 1D conducting modes encoded with opposite-valley indices.These metallic states are topologically protected against backscattering in the absence of valley-mixing scattering,and thus can carry current ballistically.In BLG,the reversal of 2 can occur at the domain wall of AB-and BA-stacked domains,or at the line junction of two oppositely gated regions.The latter approach can provide a scalable platform to implement valleytronic operations,such as valley valves and waveguides.Experimental results show that the conductance is around 0.5e2/h rather than quantized.This observation can be explained from our numerical results,which suggest that the scattering between topological confinement states and bound states and the presence of atomic scale disorders that provide inter-valley scattering can effectively reduce the conductance to about 0.5e2/h.We further and out-of-plane magneticeld can strongly suppress these scattering mechanisms and gives rise to nearly quantized conductance.On one hand,the presence of magnetic field makes bound states become Landau levels,which reduces the scattering between zero-line mode and bound states.On the other hand,the wave function distributions of oppositely propagating topological confine-ment states at different valleys are spatially separated,which can strongly suppress the inter-valley scattering.Specically speaking,the conductance can be increased to 3.2 e2/h at 8 T even when the atomic Anderson type disorders are considered.The conduc-tance can almost reach the quantum transport limit 4 e2/h with the Fermi levels closed to band gap edge at B = 9 T?We demonstrate new mechanisms for gate-tunable current partition at topological zero-line intersections in a graphene-based current splitter.In the zero-field limit the control on current routing and partition can be achieved within a range of 0-1 of the total incoming current by tuning the carrier density at tilted inter-sections or by modifying the relative magnitude of the bulk band gaps via gate voltage.We discuss the implications of our findings in the design of topological zero-line net-works where finite orbital magnetic moments are expected when the current partition is asymmetric.We theoretically investigate the localization mechanism of the quantum anomalous Hall effect(QAHE)in the presence of spin-flip disorders.We show that the QAHE stays quantized at weak disorders,then enters a Berry-curvature mediated metallic phase at moderate disorders,and finally goes into the Anderson insulating phase at strong disor-ders.From the phase diagram,we find that at the charge neutrality point although the QAHE is most robust against disorders,the corresponding metallic phase is much easier to be localized into the Anderson insulating phase due to the interchange of Berry curva-tures carried,respectively,by the conduction and valence bands.In the end,we provide a phenomenological picture related to the topological charges to better understand the underlying physical origin of the QAHE Anderson localization.