Scattering-matrix method for ballistic electron transport: Theory and an application to quantum antidot arrays

Anisotropic and gate-tunable valley filtering based on 8-Pmmn borophene

We propose a valley filter based on 8-Pmmn borophene which hosts two tilted Dirac cones. It is found that the application of a magnetic-electric barrier provided by a single ferromagnetic gate is sufficient to create valley-polarized current in 8-Pmmn borophene. The valley polarization of output current depends on the barrier orientation. Due to an intrinsic symmetry, it vanishes when the barrier orientation is along the tilted direction of Dirac cones. For the barrier orientation perpendicular to the tilted direction, the valley polarization for a realistic magnetic barrier can approach nearly 100% at proper Fermi energy and gate voltage. The remarkable valley contrast of conductance in this case is attributed to a new transmission resonance. The tilting of Dirac cones is essential for the predicted valley filtering. Our findings are helpful for valleytronic applications of two-dimensional materials with tilted Dirac cones.

Anomalous electron collimation in HgTe quantum wells with inverted band structure

We investigate the electron collimation behavior in HgTe quantum wells (QWs) with a magnetic-electric barrier induced by a ferromagnetic metal stripe. We find that electrons can transmit perfectly through the magnetic-electric barrier at some specific incidence angles. These angles can be controlled by the tuning gate voltage, local magnetic field and Fermi energy of incident electrons in QWs with appropriate barrier length. This collimation feature can be used to construct momentum filters in HgTe QWs and has potential application in nanodevices.

Valley filtering in gapped graphene modulated by an antisymmetric magnetic field and an electric barrier

We investigate valley-dependent electron transport properties of a gapped graphene film modulated by a ferromagnetic metal (FM) stripe with magnetization along the current direction. The antisymmetric stray field of the FM stripe alone does not generate a valley-polarized current due to an intrinsic symmetry. The inclusion of an electric barrier breaks this symmetry. It is shown that highly valley-polarized electron transport can be achieved in this magnetic-electric barrier structure, which results from a valley-dependent phase mechanism. The valley polarization can be tuned by the barrier parameters.

A valley-filtering switch based on strained graphene

We investigate valley-dependent transport through a graphene sheet modulated by both the substrate strain and the fringe field of two parallel ferromagnetic metal (FM) stripes. When the magnetizations of the two FM stripes are switched from the parallel to the antiparallel alignment, the total conductance, valley polarization and valley conductance excess change greatly over a wide range of Fermi energy, which results from the dependence of the valley-related transmission suppression on the polarity configuration of inhomogeneous magnetic fields. Thus the proposed structure exhibits the significant features of a valley-filtering switch and a magnetoresistance device.