Magnetoresistance Of Black Phosphorus Film And Topological-insulator-based Nanostep

In this dissertation,we investigate the magnetoresistance effects in two kind-s of two-dimensional(2D)electronic systems,namely,black phosphorus(BP)films and topological-insulator-based nanostep junction.The energy bands of electrons in BP films possess a high anisotropy and a sizeable band gap which can be tuned by electric fields.We consider the tunneling magnetoresistance of BP films controlled by a rectangular magnetic barrier—a nonuniform magnetic field perpendicular to BP films.We find that the conductance of this system depends strongly on the orientation of the magnetic barrier,which is suppressed maximally when the magnetic barrier is oriented along the armchair direction.Anisotropic magnetoresistance comes from the highly anisotropic energy disper-sion of phosphorene and the magnetic-barrier-induced suppression of available phase space for transmission.The magnetoresistance is enhanced by the reduc-tion of the band gap under the same effective mass components.So far,there is no experimental report on magnetic barriers fabricated on 2D Dirac material-s.For a nanostep junction consisting of two top surfaces and a side surface on a three-dimensional insulator,a uniform magnetic field perpendicular to its side surface acts as a rectangular magnetic barrier for a flat 2D Dirac electron system.For this system we reveal two mechanisms of magnetoresistance.One is that the magnetic field shrinks the phase space for transmission between the two top sur-faces.This is the main mechanism of magnetoresistance when the Fermi energy is close to the Dirac point of the top surface.We also find transmission reso-nances and suppression of Fano factor due to Landau-level-related quasi-bound states when the Fermi energy is near the Dirac point of the side surface.The transmission is blocked when the Fermi energy is between two adjacent quasi-bound state energy levels.This is the second mechanism of magnetoresistance.The critical magnetic field strength needed by closing the conductance channels in this mechanism is considerably lower than that in the first mechanism.