Spin-polarized quantum pumping in bilayer graphene

Electrically controllable sudden reversals in spin and valley polarization in silicene

We study the spin and valley dependent transport in a silicene superlattice under the influence of a magnetic exchange field, a perpendicular electric field and a voltage potential. It is found that a gate-voltage-controllable fully spin and valley polarized current can be obtained in the proposed device, and the spin and valley polarizations are sensitive oscillatory functions of the voltage potential. In properly designed superlattice structure, the spin and valley polarizations can be reversed from −100% to 100% by a slight change in the external voltage potential. The energy dispersion relations of the superlattice structure are also investigated, which helps us to understand the effects of the superlattice structure. The switching of the spin direction and the valley of the tunneling electrons by a gate voltage enables new possibilities for spin or valley control in silicene-based spintronics and valleytronics.

Equilibrium spin current in graphene with Rashba spin-orbit coupling

The existence of a background spin current under thermodynamic equilibrium is an interesting phenomenon in the two-dimensional electron gas with Rashba spin-orbit coupling (RSOC). Here we study the equilibrium spin current (ESC) in graphene with RSOC. For an infinite graphene with uniform RSOC, we found that the ESC is proportional to λ² with λ the Rashba strength and mainly comes from the energy window [−λ, λ] near Dirac points. In the regime of energy far away from Dirac points, the λ³ dependence as that in a normal two-dimensional electron gas is recovered. In a system with a normal graphene strip inserted between two Rashba graphene sheets, we found that the ESC can penetrate through the normal graphene layer (perpendicular to the interface). This unique effect can be understood by considering the spin-filtered scattering from the normal region to the RSOC region. The finding of the ESC through the normal region without RSOC advances the understanding of ESC and provides a new way to generate a pure spin current in graphene. For an experimentally accessible strength of Rashba spin-orbit coupling, the ESC remains over room temperature.

Gigahertz quantized charge pumping in graphene quantum dots

Single-electron pumps are set to revolutionize electrical metrology by enabling the ampere to be redefined in terms of the elementary charge of an electron. Pumps based on lithographically fixed tunnel barriers in mesoscopic metallic systems and normal/superconducting hybrid turnstiles can reach very small error rates, but only at megahertz pumping speeds that correspond to small currents of the order of picoamperes. Tunable barrier pumps in semiconductor structures are operated at gigahertz frequencies, but the theoretical treatment of the error rate is more complex and only approximate predictions are available. Here, we present a monolithic, fixed-barrier single-electron pump made entirely from graphene that performs at frequencies up to several gigahertz. Combined with the record-high accuracy of the quantum Hall effect and proximity-induced Josephson junctions, quantized-current generation brings an all-graphene closure of the quantum metrological triangle within reach. Envisaged applications for graphene charge pumps outside quantum metrology include single-photon generation via electron-hole recombination in electrostatically doped bilayer graphene reservoirs, single Dirac fermion emission in relativistic electron quantum optics and read-out of spin-based graphene qubits in quantum information processing.