Quantum Pumping Research Articles

Generation of large spin currents in graphene using adiabatic quantum pumping

We study adiabatic quantum pumping of Dirac fermions in a monolayer graphene in the large pumping amplitude regime in order to study how to generate a large spin polarized current. Spin current is generated by applying two periodic oscillating gate voltages to a monolayer graphene with exchange splitting induced by ferromagnetic proximity. We find that pumped charge and spin currents are sensitive functions of the Fermi energy and pure spin current and spin current with different degrees of polarization and large magnitudes are obtained in our scheme. We also study the effects of parameters of the system, such as the barrier separation and the exchange splitting, on the pumped currents. The spin current generated by this method can be as high as 80-100 nA. This shows the method can be used in the study of spin physics in graphene and the realization of graphene spintronic devices.

Rashba spin-orbit-interaction-based quantum pump in graphene

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 currents. A local spin polarized current is induced by the pumping mechanism assisted by the spin-double refraction phenomenon.

Scattering by an oscillating barrier: Quantum, classical, and semiclassical comparison

We present a detailed study of scattering by an amplitude-modulated potential barrier using three distinct physical frameworks: quantum, classical, and semiclassical. Classical physics gives bounds on the energy and momentum of the scattered particle, while also providing the foundation for semiclassical theory. We use the semiclassical approach to selectively add quantum-mechanical effects such as interference and diffraction. We find good agreement between the quantum and semiclassical momentum distributions. Our methods and results can be used to understand quantum and classical aspects of transport mechanisms involving time-varying potentials, such as quantum pumping.

Quantum pump effect induced by a linearly polarized microwave in a two-dimensional electron gas

A quantum pump effect is predicted in an ideal homogeneous two-dimensional electron gas (2DEG) that is normally irradiated by linearly polarized microwaves (MW). Without considering effects from spin–orbital coupling or the magnetic field, it is found that a polarized MW can continuously pump electrons from the longitudinal to the transverse direction, or from the transverse to the longitudinal direction, in the central irradiated region. The large pump current is obtained for both the low frequency limit and the high frequency case. Its magnitude depends on sample properties such as the size of the radiated region, the power and frequency of the MW, etc. Through the calculated results, the pump current should be attributed to the dominant photon-assisted tunneling processes as well as the asymmetry of the electron density of states with respect to the Fermi energy.

Coherent superconducting quantum pump

We demonstrate nonadiabatic charge pumping utilizing a sequence of coherent oscillations between a superconducting island and two reservoirs. The pumping rate for each elementary cycle is limited by the coupling between the island and the reservoirs given by the Josephson energy. Our experimental and theoretical studies show that relaxation can be employed to reset the pump in order to avoid accumulation of errors due to nonideal control pulses. Thus our results demonstrate the effects of nonadiabatic quantum pumping and dissipation.

Current-induced forces in mesoscopic systems: A scattering-matrix approach.

Nanoelectromechanical systems are characterized by an intimate connection between electronic and mechanical degrees of freedom. Due to the nanoscopic scale, current flowing through the system noticeably impacts upons the vibrational dynamics of the device, complementing the effect of the vibrational modes on the electronic dynamics. We employ the scattering-matrix approach to quantum transport in order to develop a unified theory of nanoelectromechanical systems out of equilibrium. For a slow mechanical mode the current can be obtained from the Landauer–Büttiker formula in the strictly adiabatic limit. The leading correction to the adiabatic limit reduces to Brouwer’s formula for the current of a quantum pump in the absence of a bias voltage. The principal results of the present paper are the scattering-matrix expressions for the current-induced forces acting on the mechanical degrees of freedom. These forces control the Langevin dynamics of the mechanical modes. Specifically, we derive expressions for the (typically nonconservative) mean force, for the (possibly negative) damping force, an effective “Lorentz” force that exists even for time-reversal-invariant systems, and the fluctuating Langevin force originating from Nyquist and shot noise of the current flow. We apply our general formalism to several simple models that illustrate the peculiar nature of the current-induced forces. Specifically, we find that in out-of-equilibrium situations the current-induced forces can destabilize the mechanical vibrations and cause limit-cycle dynamics.

Pure spin current generation in monolayer graphene by quantum pumping

We study a method to generate pure spin current in monolayer graphene over a wide range of Fermi energy by adiabatic quantum pumping. The device consists of three gate electrodes and two ferromagnetic strips, which induce a spin-splitting in the graphene through the proximity effect. A pure spin current is generated by applying two periodic oscillating gate voltages. We find that the pumped pure spin current is a sensitive oscillatory function of the Fermi energy. Large spin currents can be found at Fermi energies where there are Fabry–Perot resonances in the barriers. Furthermore, we analyze the effects of the parameters of the system on the pumped currents. Our predicted pumped spin current can be of the order of 100 nA which is measurable using the current technology. The proposed method is useful in the realization of graphene spintronic devices.

Charge pumping in monolayer graphene driven by a series of time-periodic potentials

We applied the Floquet scattering-matrix formalism to studying the electronic transport properties in a mesoscopic Dirac system. Using the method, we investigate theoretically quantum pumping driven by a series of time-periodic potentials in graphene monolayer both in the adiabatic and non-adiabatic regimes. Our numerical results demonstrate that adding harmonic modulated potentials can break the time reversal symmetry when no voltage bias is applied to the graphene monolayer. Thus, when the system is pumped with proper dynamic parameters, these scatterers can produce a nonzero dc pumped current. We also find that the transmission is anisotropic as the incident angle is changed.

Electron and spin transport in adiabatic quantum pump based on armchair graphene nanoribbons

The phenomenon of adiabatic quantum pump in double-barrier structures based on armchair graphene nanoribbons has been theoretically analyzed within the framework of the tight binding approximation. An analytical expression for a bilinear response is obtained that is valid at small Fermi energies. Using the effect of proximity to a ferromagnetic dielectric, it is possible to obtain both electron and pure spin currents. Dependences of the generated electron and spin currents on the Fermi energy have been numerically calculated. The validity of the adiabatic approximation is assessed based on the dwell time for electron travelling through the system.

Pumped shot noise in adiabatically modulated graphene-based double-barrier structures

Quantum pumping processes are accompanied by considerable quantum noise. Based on the scattering approach, we investigated the pumped shot noise properties in adiabatically modulated graphene-based double-barrier structures. It is found that compared with the Poisson processes, the pumped shot noise is dramatically enhanced where the dc pumped current changes flow direction, which demonstrates the effect of the Klein paradox.

Single-parameter pumping in graphene

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.

Electrically controlled pumping of spin currents in topological insulators

Pure spin currents are shown to be generated by an electrically controlled quantum pump applied at the edges of a topological insulator. The electric rather than the more conventional magnetic control offers several advantages and avoids, in particular, the necessity of delicate control of magnetization dynamics over tiny regions. The pump is implemented by pinching the sample at two quantum point contacts and phase modulating two external gate voltages between them. The spin current is generated for the full range of parameters. On the other hand, pumping via amplitude modulation of the interboundary couplings generates both charge and spin currents, with a pure charge current appearing only for special values of the parameters for which the Aharonov-Bohm flux takes integer values. Our setup can therefore serve to fingerprint the helical nature of the edges states with the zeros of the pumped spin and charge currents occurring at distinct universal locations where the Fabry-P\'erot or the Aharonov-Bohm phases take integer values.

Electron and spin transport in adiabatic quantum pumps based on graphene nanoribbons

The adiabatic quantum pump effect in double-barrier structures based on graphene nanoribbons has been analyzed numerically. The charge transmitted through the system in one cycle of variation of pumping potentials (barrier heights) has been calculated. This charge is shown to be quantized under certain conditions. The possibility of the generation of not only electron currents but also spin and purely spin ones is considered. In devices based on zigzag nanoribbons, the spin current emerges when taking into account their magnetic structure and applying a transverse electric field breaking the symmetry between the up and down spins. In devices based on armchair nanoribbons without any magnetic structure, this symmetry breaks when using a ferromagnetic insulator (deposited, for example, on the region between gates) through the proximity effect.

Spin-polarized quantum pumping in bilayer graphene

We study adiabatic quantum pumping in bilayer graphene where two-barrierpotentials are weakly modulated as pumping parameters. Comparing the resultswith those for a normal quantum pump of non-chiral quasiparticles, we find thatthe chirality of quasiparticles in bilayer graphene heavily affects the pumpedcurrent through chiral tunnelling. When an exchange splitting induced by theproximity of a ferromagnetic insulator is introduced, the pumped current becomesspin-polarized. It is interesting that an almost 100% polarized charge currentand a pure spin current with vanishing charge current can all be achieved undersuitable conditions. The experimental feasibility and the interlayer asymmetric effectin bilayer graphene caused by the gate and the ferromagnet structures are alsodiscussed. The results are useful for spintronics applications based on graphene.

Adiabatic quantum pumping in normal-metal–insulator–superconductor junctions in a monolayer of graphene

We investigate adiabatic quantum pumping through a normal-metal--``insulator''--superconductor (NIS) junction in a monolayer of graphene. The pumped current is generated by periodic modulation of two gate voltages, applied to the insulating and superconducting regions, respectively. In the bilinear response regime and in the limit of a thin high insulating barrier, we find that the presence of the superconductor enhances the pumped current per mode by a factor of 4 at resonance. Compared to the pumped current in an analogous semiconductor NIS junction, the resonances have a $\ensuremath{\pi}/2$ phase difference. We also predict experimentally distinguishable differences between the pumped current and the tunneling conductance in graphene NIS junctions.

Crossover from Adiabatic to Antiadiabatic Quantum Pumping with Dissipation

Quantum pumping, in its different forms, is attracting attention from different fields, from fundamental quantum mechanics, to nanotechnology, to superconductivity. We investigate the crossover of quantum pumping from the adiabatic to the antiadiabatic regime in the presence of dissipation, and find general and explicit analytical expressions for the pumped current in a minimal model describing a system with the topology of a ring forced by a periodic modulation of frequency ω. The solution allows following in a transparent way the evolution of pumped dc current from much smaller to much larger ω values than the other relevant energy scale, the energy splitting introduced by the modulation. We find and characterize a temperature-dependent optimal value of the frequency for which the pumped current is maximal.

Multiharmonic fields driven adiabatic quantum pumps in nanowire structures

A dc current can be pumped through a nanostructure by two out-of-phase ac driving fields. In this approach, we investigate the pumped current properties driven by multiharmonic ac fields in nanowire structures. Considering the first five harmonics, the adiabatically pumping properties driven by approximate square, sawtooth, and triangle-shape ac waves are discussed. It is found that the pumped current varies with the phase difference in a square and sawtooth shape, respectively, under corresponding driven fields. Sinusoidal relation governs in the pump driven by a triangle-shape ac field.

A Josephson quantum electron pump

Mesoscopic charge pumping, a transport mechanism that relies on the explicit time-dependence of some properties of a nanoscale conductor, was envisaged theoretically a few decades ago1,2,3,4. So far, nanoscale pumps have been realized only in systems exhibiting strong Coulombic effects5,6,7,8,9,10,11,12, whereas evidence for pumping in the absence of Coulomb blockade has been elusive. A pioneering experiment by Switkes et al. 13 evidenced the difficulty of modulating in time the properties of an open mesoscopic conductor at cryogenic temperatures without generating undesired bias voltages due to stray capacitances14,15. One possible solution to this problem is to use the a.c. Josephson effect to induce periodically time-dependent Andreev reflection amplitudes in a hybrid normal–superconducting system16. Here we report the experimental detection of charge flow in an unbiased InAs nanowire embedded in a superconducting quantum interference device (SQUID). In this system, quantum pumping may occur via the cyclic modulation of the phase of the order parameter of different superconducting electrodes. The symmetry of the current with respect to the enclosed magnetic flux17,18 and bias SQUID current is a discriminating signature of pumping. Currents exceeding 20 pA are measured at 250 mK, and exhibit symmetries compatible with quantum pumping.

Spin polarized quantum pump effect in zigzag graphene nanoribbons

The spin polarized adiabatic quantum pump effect in zigzag graphene nanoribbons has been numerically analyzed. Since the ground state of such a ribbon is antiferromagnetic (the opposite spin electrons are located on the opposite edges of the ribbon), the spin currents can be generated in this system with the help of the quantum pump effect when symmetry between the opposite spin states is broken. Two methods of this breaking by means of defects at the ribbon edge and the transverse electric field have been proposed. It has been shown that the generation of not only the electron and spin currents, but also the purely spin current is possible in both cases.

Gate driven adiabatic quantum pumping in graphene

We propose a new type of quantum pump made out of graphene, adiabatically driven by oscillating voltages applied to two back gates. From a practical point of view, graphene-based quantum pumps present advantages as compared to normal pumps, like enhanced robustness against thermal effects and a wider adiabatic range in driving frequency. From a fundamental point of view, apart from conventional pumping through propagating modes, graphene pumps can tap into evanescent modes, which penetrate deeply into the device as a consequence of chirality. At the Dirac point the evanescent modes dominate pumping and give rise to a universal response under weak driving for short and wide pumps, even though the charge per unit cycle is not quantized.