Probing the valley filtering effect by Andreev reflection in a zigzag graphene nanoribbon with a ballistic point contact

Abstract

Ballistic point contact (BPC) with zigzag edges in graphene is a main candidate of a valley filter, in which the polarization of the valley degree of freedom can be selected by using a local gate voltage. Here, we propose to detect the valley filtering effect by Andreev reflection. Because electrons in the lowest conduction band and the highest valence band of the BPC possess opposite chirality, the inter-band Andreev reflection is strongly suppressed, after multiple scattering and interference. We draw this conclusion by both the scattering matrix analysis and the numerical simulation. The Andreev reflection as a function of the incident energy of electrons and the local gate voltage at the BPC is obtained, by which the parameter region for a perfect valley filter and the direction of valley polarization can be determined. The Andreev reflection exhibits an oscillatory decay with the length of the BPC, indicating a negative correlation to valley polarization.

Giant magnetoresistance and spin-valley polarization of Dirac fermions modulated by magnetic fields

“…[7][8][9][10][11] In the pioneering work by Beenakker, the valley filter and valley valve effect have been proposed in graphene nanoribbons with a zigzag edge, 12) which could be detected by Andreev reflection. 13) The signal of pure valley current has been verified experimentally in bilayer graphene as a result of the broken inversion symmetry induced by the electric field. 14,15) Compared to graphene, silicene has significant advantage in the study of valley pseudospin.…”

The generation and detection of the spin-valley-polarization in semi-Dirac materials

“…Successful fabrication of two-dimensional materials, such as graphene [6] and transition metal dichalcogenides [7][8][9][10][11][12], has facilitated the development of the valleytronics, leading to emerging applications, such as the valley Hall effect [3,[13][14][15][16][17], the valley Zeeman effect [18][19][20][21], and the valley polarization [22,23]. The valley polarization in graphene can be detected by the valley Hall effect [14,17] and the Andreev reflection (AR) [24]. Several approaches to generate the valley polarization in graphene have been proposed, such as introducing a line defect [25], a ballistic point contact with zigzag edges [4], and electric gate pontential in bilayer graphene [26].…”

Direct coupling-induced pseudoparity nonconservation scattering: bipolar spin diode and unipolar spin entanglement pairing

“…Besides the spin and charge degrees of freedom, as is known, the electron in these two-dimensional materials with the honeycomb lattice structure has linear low energy dispersion around two valleys at the K and K points of the Brillouin zone, being degenerate. The two valleys, characterized by a pseudo-spin degree of freedom, are related to each other by time-reversal symmetry like the spin degree of freedom, so a superconducting Cooper pair should be composed of electrons from the two opposite valley bands, which is also the crucial mechanism in valley-based superconducting spintronics [10,17,[43][44][45][46].…”