Study On Ballistic Transport Related Phenomena In Graphene Nanoscale Structures

Rich and interesting physical phenomena arise with ballistic transport in graphenenanoscale structures. Among them, negative diferential resistance (NDR) efect, Goos-Ha¨nchen (GH) shift, tunneling group delay, and cyclotron resonances between Landaulevels have extensive applications in electronic devices. However, a basic NDR model,double barrier resonant tunneling diode has not been touched in graphene; how to obtaingiant GH shift larger than the transverse width of an electron beam is to be settled;the widely adopted definition of two dimensional group delay is not complete, how tomeasure the collective group delay at arbitrary Fermi energy is to be resolved; and howto modulate cyclotron resonances by electrostatic method is not clear. To solve theseproblems, in this thesis we theoretically study these phenomena in several graphenebased nanoscale structures.We investigate the NDR in graphene double barrier resonant tunneling diodes bycalculating the I-V characteristic in a rotated pseudospin space. We demonstrate that,the lowest NDR operation window is almost structural parameters-free and can be nearlysolely controlled by the back gate. We indicate that, this remarkable phenomenon stemsfrom the ambipolar transport in graphene and may be applied in operation window-dominated NDR devices. We have also found that, the competition between hole-to-electron transport, Klein tunneling, and resonant tunneling is the physical mechanismfor such a NDR structure. We also show that, appropriate structural parameters arenecessary for the NDR feature, and a tunable band gap can enhance exponentially thepeak-to-valley current ratio.We indicate that the transverse electron beam width plays a critical role in theirspatial splitter. We report giant GH shifts with magnitudes up to the order of transverseelectron beam width and rather small full-widths-at-half-maximum for electron beamstunneling through graphene double barrier structures, which we attribute to the quasi-bound states in the structure. We indicate that these features may be utilized to designvalley and spin beam splitters with wide tunability and high energy resolution. We alsofind that, an induced energy gap can increase the tunability and resolution of the splitters.We reveal the intrinsic contribution of GH shift in the two dimensional group delay.We suggest that, for almost arbitrary Fermi energy, the collective group delay and it inherent GH component can be probed by an induced conductance diference in spinprecession experiments under weak magnetic fields. We also indicate that, it is a nonzeroself-interference delay that relates the group delay and dwell time in graphene.We apply a circle top gate to modulate the Landau levels and cyclotron resonancesin graphene. We find that, the top gate induced potential can induce switchs betweenextended Landau-type bound states and localized quantum dot-type ones. As a result,the cyclotron resonance frequency can be tuned almost linearly by the top gate withinspecific ranges. This phenomenon can be applied as a near linearly controllable photonfrequency splitter.