Quantum pumping in graphene with a perpendicular magnetic field

Spin dependent transport characteristics through normal graphene/ ferromagnetic graphene/ normal graphene junction is investigated. The conduction of this junction is derived by solving Dirac equation for both parallel and anti-parallel spin alignments of electrons. Numerical calculations are performed for the conductance for both spin alignments. Oscillatory behavior of the conductance for the two cases is due to the interplay between the photons of the induced AC-signal with both spin-up and spin-down subbands. These oscillations are due to the modulation of the Fermi energy by the potential of the magnetic insulator and photon-energy. Also, the calculations of spin polarization and giant magneto-resistance show that these parameters could be modified by the barrier height and the angle of incidence of electrons on the corresponding region of the present device. Quantum pumping by induction of external photons could enhance spin transport mechanism through such investigated device. The present results show that the cut-off frequency for both parallel and anti-parallel spin alignments varies strongly in the range of THz to 10 19 Hz. The present investigation could be found for designing very high speed nano-electronic devices and applications in the field of nano-biotechnology, for example, imaging processing. [Mina D. Asham, Walid A. Zein, Adel H. Phillips. Quantum Pumping Driven by an AC-field in Graphene Field Effect Transistor. J Am Sci 2012;8(7):374-381]. (

Applied Physics Letters, 2012

We present a proposal for an adiabatic quantum pump based on a graphene monolayer patterned by electrostatic gates and operated in the low-energy Dirac regime. The setup under investigation works in the presence of inhomogeneous spin-orbit interactions of intrinsic-and Rashba-type and allows to generate spin polarized coherent current. A local spin polarized current is induced by the pumping mechanism assisted by the spin-double refraction phenomenon.

The European Physical Journal B

We study the spin transport phenomena in two-dimensional graphene-like materials with arbitrary tilted Dirac cones. The tilt arises due to next-nearest hopping when the bottom of the conduction band and top of the valence band does not simultaneously coincide at Dirac point. We consider normal-ferromagnetic-normal (N-F-N) junction of the materials and using the generalized scattering approach calculate the spin current. Here, we show that tilting the Dirac cones can strongly change the transport properties by modifying the period of oscillation of the spin current. The spin conductance can be effectively tuned by the tilt with taking advantage of the modified interference condition. A pure spin current reversal also possible with a smooth variation of the tilting. We further study the spin current by the adiabatic precession of a doped ferromagnet on top of the material. It is shown that the spin-mixing conductance and hence the spin current can become zero by turning the tilt of the Dirac cone. Our findings provide an efficient way towards high controllability of spin transport by tuning the tilt of the ferromagnetic junction and can be very useful in the field of spintronics. The model also presents a simplified way to measure the tilt of Dirac cone of those materials.

Physical Review B, 2017

We demonstrate experimentally that a sizable chiral charge pumping can be achieved at room temperature in graphene/Yttrium Iron Garnet (YIG) bilayer systems. The effect, which cannot be attributed to the ordinary spin pumping, reveals itself in the creation of a dc electric field/voltage in graphene as a response to the dynamic magnetic excitations (spin waves) in an adjacent out-of-plane magnetized YIG film. We show that the induced voltage changes its sign when the orientation of the static magnetization is reversed, clearly indicating the broken mirror reflection symmetry about the planes normal to the graphene/YIG interface. The strength of effect shows a non-monotonous dependence on the spin-wave frequency, in agreement with the proposed theoretical model.

2011

We propose a quantum pump mechanism based on the particular properties of graphene, namely chirality and bipolarity. The underlying physics is the excitation of evanescent modes entering a potential barrier from one lead, while those from the other lead do not reach the driving region. This induces a large nonequilibrium current with electrons stemming from a broad range of energies, in contrast to the narrow resonances that govern the corresponding effect in semiconductor heterostructures. Moreover, the pump mechanism in graphene turns out to be robust, with a simple parameter dependence, which is beneficial for applications. Numerical results from a Floquet scattering formalism are complemented with analytical solutions for small to moderate driving.

Journal of Physics D: Applied Physics, 2015

We investigate pure spin pumping in graphene by imposing a ferromagnet (F) with rotating magnetization on top of it. Using the generalized scattering approach for adiabatic spin pumping, we obtain the spin current pumped through magnetic graphene to a neighboring normal (N) region. The spin current can be easily controlled by gate voltages and under certain conditions, becomes sufficiently large to be measurable in current experimental setups. In fact it reaches a maximum value when one of the spins are completely filtered due to the vanishing density of states of the corresponding spin species in the ferromagnetic part. Considering an N|F|N structure with a finite ferromagnetic region, it is found that in contrast to the metallic ferromagnetic materials the transverse spin coherence length can be comparable to the length of F denoted by L. Subsequently, due to the quantum interferences, the spin current becomes oscillatory function of JL/ vF in which J is the spin splitting inside F. Finally we show, originated from the controllability of pumped spin into two different normal sides, a profound spin battery effect can be seen in the hybrid N|F|N device.

We investigate pure spin pumping in graphene by imposing a ferromagnet (F) with rotating magnetization on top of it. Using the generalized scattering approach for adiabatic spin pumping, we obtain the spin current pumped through magnetic graphene to the normal (N) region. This spin current which can be easily controlled by gate voltages, reaches sufficiently large values measurable in current experimental setups. The spin current reaches its maximum when one of the spins are completely filtered because of its vanishing density of states in the ferromagnetic part. In order to study the effect of ferromagnetic part length on pumped spin current, the N|F|N structure is considered. It is found that in contrast to the metallic ferromagnetic materials the transverse spin coherence length can be comparable to the length of F. Subsequently, due to the quantum interferences inside middle F region, the spin current becomes oscillatory function of JL/ vF in which J is the spin splitting and L is the length of F. Finally controllability of pumped spin into two different normal sides in N|F|N hybrid device gives rise to the spin battery effect.

Nano letters, 2015

With the discovery and first characterization of graphene, its potential for spintronic applications was recognized immediately. Since then, an active field of research has developed trying to overcome the practical hurdles. One of the most severe challenges is to find appropriate interfaces between graphene and ferromagnetic layers, which are granting efficient injection of spin-polarized electrons. Here, we show that graphene grown under appropriate conditions on Co(0001) demonstrates perfect structural properties and simultaneously exhibits highly spin-polarized charge carriers. The latter was conclusively proven by observation of a single-spin Dirac cone near the Fermi level. This was accomplished experimentally using spin- and angle-resolved photoelectron spectroscopy, and theoretically with density functional calculations. Our results demonstrate that the graphene/Co(0001) system represents an interesting candidate for applications in devices using the spin degree of freedom.

2010 14th International Workshop on Computational Electronics, 2010

An effective approach of quantum transport of Dirac carriers in mono-and bi-layer graphene structures and devices is presented. Initially based on the Green's function formalism to treat the Dirac Hamiltonian of massless particles in twodimensional mono-layer graphene, the model has been extended to to small bandgap materials and to bi-layer graphene with massive carriers. It is applied to investigate some transport problems as the minimum conductivity, the tunneling properties the spin-polarized transport through single-barrier structures, and the operation of graphene field-effect transistors.

arXiv (Cornell University), 2016

Quantum spin Hall effect was first predicted in graphene. However, the weak spin orbit interaction in graphene meant that the search for quantum spin Hall effect in graphene never fructified. In this work we show how to generate the quantum spin-valley Hall effect in graphene via quantum pumping by adiabatically modulating a magnetic impurity and an electrostatic potential in a monolayer of strained graphene. We see that not only exclusive spin polarized currents can be pumped in the two valleys in exactly opposite directions but one can have pure spin currents flowing in opposite directions in the two valleys, we call this novel phenomena the quantum spin-valley Hall effect. This means that the twin effects of quantum valley Hall and quantum spin Hall can both be probed simultaneously in the proposed device.