Shot Noise in a Spin-Diode Geometry

We apply the master equation technique to calculate shot noise in a system composed of single level quantum dot attached to a normal metal lead and to a ferromagnetic lead (NM-QD-FM). It is known that this system operates as a spin-diode, giving unpolarized currents for forward bias and polarized current for reverse bias. This effect is observed when only one electron can tunnel at a time through the dot, due to the strong intradot Coulomb interaction. We find that the shot noise also presents a signature of this spin-diode effect, with a super-Poissonian shot noise for forward and a sub-Poissonian shot noise for reverse bias voltages. The shot noise thus can provide further experimental evidence of the spin-rectification in the NM-QD-FM geometry.     

Spin Effects on Heat Current Through a Quantum Dot Attached to Ferromagnetic Leads

A heat current originating from electron–phonon coupling in a quantum dot (QD) molecule connected to ferromagnetic leads is studied by the non-equilibrium Green’s function technique. The system is driven out of equilibrium by a temperature gradient (thermal bias) applied across the two terminals of the structure. We find that when the magnetic moments of the two leads are arranged in parallel configuration, the heat current is not sensitive to the leads’ ferromagnetism, whereas in the case of antiparallel configuration, the magnitude of the heat current increases with increasing spin polarization of the leads, with the reduction of the electric current’s intensity. We also find that the ferromagnetism on the leads can amplify the heat rectification effect occurring for some particular dot levels, i.e., the strength of the heat flowing between the QD and the phonon bath can be very small for one direction of the temperature gradient, while it becomes quite large when the corresponding direction of the temperature gradient is reversed.     

Nonequilibrium Magnetic Polarization and Spin Currents Controlled by Gate Voltage in a Ferromagnetic Single-Electron Transistor

Spin-dependent electronic transport in a single-electron transistor with ferromagnetic external electrodes and nonmagnetic central part (island) is analyzed theoretically in the sequential tunneling and cotunneling regimes. Nonequilibrium magnetic polarization of the island due to spin accumulation (spin splitting of the chemical potential) is taken into account. The accumulation takes place when the spin relaxation time on the island is sufficiently long. The crossover from slow to fast spin relaxation limits is also analyzed. Magnetic polarization of the island and spin polarization of the flowing current are examined as a function of the gate and transport voltages.     

Control of Spin-Valley Current in Strain-Engineered Graphene Magnetic Junction

We investigate the spin-valley current in a strain-engineered graphene magnetic system, normal region (N)/strain region(S)/ferromagnetic region (F)/normal region (N) junction. Locally strained region S leads to valley-dependent current, and ferromagnetic region F leads to the spin-dependent current. We find that pure valley current can be created easily by applying the real-vector potential that is equal to the pseudo-vector potential caused by strains in the S-region. In this work, we focus on the spin current in each valley when exchange field is applied in the F-region, and find that the linear control of spin-valley polarization by gate potential is possible. It is also found that when the current is carried only by the carriers in one valley (pure valley current), pure spin-up current can be linearly altered to pure spin-down current by tuning the gate potential, in the case of very large-F-thickness junction. Our work is applicable for devising controllable spin-valley-current electronics circuits.     

Effects of a ferromagnetic metal stripe and a Schottky metal stripe on the electron transport in a nanostructure

In this paper, the spin-dependent electron transport in a two-dimensional electron gas (2DEG) modulated by a ferromagnetic (FM) metal stripe and a Schottky metal (SM) stripe is in detail studied. It is found that the position and the width of the SM stripe as well as the incident energy of electron play an important role on the spin polarization. It is also found that the spin polarization is obviously dependent on the electric-barrier height induced by an applied voltage to the SM stripe and such a device can be used as a voltage-tunable electron-spin filter.     

Disorder enhancement of quasiparticle lifetimes in superconductors

We calculate the quasiparticle lifetime due to Coulomb interactions in a disordered superconductor in perturbation theory with respect to the screened Coulomb interaction. We find a diffusion enhancement of the relaxation rate similar to the normal metal case. The temperature dependence of the lifetime is found to be the same as in a clean superconductor. The relation of these results to experiment and ways to improve the theory are discussed.     

Positive and negative Coulomb drag in vertically integrated one-dimensional quantum wires

Electron interactions in and between wires become increasingly complex and important as circuits are scaled to nanometre sizes, or use reduced-dimensional conductors such as carbon nanotubes, nanowires and gated high-mobility two-dimensional electron systems. This is because the screening of the long-range Coulomb potential of individual carriers is weakened in these systems, which can lead to phenomena such as Coulomb drag, where a current in one wire induces a voltage in a second wire through Coulomb interactions alone. Previous experiments have demonstrated Coulomb electron drag in wires separated by a soft electrostatic barrier of width ?80?nm (ref.?12), which was interpreted as resulting entirely from momentum transfer. Here, we measure both positive and negative drag between adjacent vertical quantum wires that are separated by ～15?nm and have independent contacts, which allows their electron densities to be tuned independently. We map out the drag signal versus the number of electron sub-bands occupied in each wire, and interpret the results both in terms of momentum-transfer and charge-fluctuation induced transport models. For wires of significantly different sub-band occupancies, the positive drag effect can be as large as 25%.     

Graphene nanoring as a tunable source of polarized electrons

We propose a novel spin filter based on a graphene nanoring fabricated above a ferromagnetic strip. The exchange interaction between the magnetic moments of the ions in the ferromagnet and the electron spin splits the electronic states, and gives rise to spin polarization of the conductance and the total electric current. We demonstrate that both the current and its polarization can be controlled by a side-gate voltage. This opens the possibility to use the proposed device as a tunable source of polarized electrons.     

Effect of Ferromagnetic Metal Stripe and Strained Barrier on Electron Transport Characteristics in a Graphene

In this paper, the effect of ferromagnetic metal stripe and the strained barrier on the valley-dependent transport characteristics of electrons is studied in a graphene nanostructure. The numerical results show that a large valley polarization can be obtained in such a graphene, and the valley-dependent transport characteristics can be well controlled by changing the strength of the magnetic field induced by the ferromagnetic metal stripe, the width of the ferromagnetic metal stripe and the position of the strained barrier. Therefore, the valley polarization can be modulated by controlling the ferromagnetic metal stripe and the strained barrier. This work can promote the research and development of the new valleytronic devices, and then meet the practical application needs.

     

Graphene as a tunnel barrier: graphene-based magnetic tunnel junctions

Graphene has been widely studied for its high in-plane charge carrier mobility and long spin diffusion lengths. In contrast, the out-of-plane charge and spin transport behavior of this atomically thin material have not been well addressed. We show here that while graphene exhibits metallic conductivity in-plane, it serves effectively as an insulator for transport perpendicular to the plane. We report fabrication of tunnel junctions using single-layer graphene between two ferromagnetic metal layers in a fully scalable photolithographic process. The transport occurs by quantum tunneling perpendicular to the graphene plane and preserves a net spin polarization of the current from the contact so that the structures exhibit tunneling magnetoresistance to 425 K. These results demonstrate that graphene can function as an effective tunnel barrier for both charge and spin-based devices and enable realization of more complex graphene-based devices for highly functional nanoscale circuits, such as tunnel transistors, nonvolatile magnetic memory, and reprogrammable spin logic.     

DNA Molecule as a Spintronic Device

In this work, we study the spin-dependent electron transport of a segment of DNA chain. We model the system by using the Fishbone model for DNA, which is characterized by a tight binding Hamiltonian. We predict that the spin-dependent transport can be observed in short DNA molecules coupled to metal contacts by applying an external magnetic field. A detailed analysis of spin-dependent current and spin polarization as a function of the bias voltages is carried out.     

Fano and Dicke effects in a double Rashba-ring system

The electronic transport in a system of two quantum rings side-coupled to a quantum wire is studied via a single-band tunneling tight-binding Hamiltonian. We derived analytical expressions for the conductance and spin polarization when the rings are threaded by magnetic fluxes with Rashba spin-orbit interaction. We show that by using the Fano and Dicke effects this system can be used as an efficient spin filter even for small spin-orbit interaction and small values of magnetic fluxes. We compare the spin-dependent polarization of this design and the polarization obtained with one ring side-coupled to a quantum ring. As a main result, we find better spin polarization capabilities as compared to the one-ring design.     

Unexpected Density Dependence of Mobility of Electron Bubbles Trapped Below the Free Surface of Normal 3He

We have performed mobility measurements of electron bubbles trapped below the free surface of normal ³He at temperatures between 5 and 100 mK. We find that the mobility increases with increasing temperature, and that the increase at high temperatures is more significant for higher densities of electron bubbles. The observed mobility has no dependence on depth of the electron bubbles, indicating that the surface excitations have no contributions to the mobility. The observed increase of mobility as a function of density suggests that the Coulomb interaction between electron bubbles has some role in the momentum transfer from the electron bubble to ³He quasiparticles.     

Modeling of a ferromagnetic two-dimensional electron gas device

We present a realistic modeling of ballistic electron transport in a hybrid ferromagnetic (FM) two-dimensional electron gas (2DEG) device, consisting of an FM gate on an AlGaAs-GaAs or AlSb-InAs high electron mobility transistor (HEMT) heterostructure. The carriers within the 2DEG are spin-polarized by a combination of magnetic and electrostatic barriers. The magnetic barriers are supplied by a composite FM gate, consisting of two domains made of magnetically hard and soft materials. This gate arrangement breaks the antisymmetry of the fringe B field, and results in a finite spin polarization of the 2DEG current. The B field strength is calculated by considering the pole strength at the gate surfaces and domain boundary, and is significantly weaker than normally assumed. We obtain parameters such as the electrostatic barrier height, Fermi level, and carrier concentration within the 2DEG by a finite-element Poisson calculation, which is self-consistent with the Fermi-Dirac distribution. We calculate the transmission probability and conductance through the 2DEG from these parameter values, assuming a single particle effective mass Hamiltonian and purely ballistic transport. We show that the spin polarization ratio P/sub G/ is extremely sensitive to the gate bias and HEMT doping concentration. However, the maximum P/sub G/ is extremely low for AlGaAs-GaAs (0.003%) and even for AlSb-InAs (0.12%) devices, despite a large Lande g factor. These values are many orders of magnitude smaller than previous predictions of close to 100% polarization, obtained by using simpler models.     

Spin injection in spin FETs using a step-doping profile

We investigate the effect of a step-doping profile on the spin injection from a ferromagnetic metal contact into a semiconductor quantum well in spin field-effect transistors using a Monte Carlo model. The considered scheme uses a heavily doped layer at the metal-semiconductor interface to vary the Schottky barrier shape and enhance the tunneling current. It is found that spin flux (spin current density) is enhanced proportionally to the total current, and the variation of current spin polarization does not exceed 20%.     

Hole Spin Polarization in Metallic Ferromagnetic GaMnAs Multilayers and Superlattices: Lateral and Bloch Miniband Transport

It is shown that spin-polarized currents occur in metallic and ferromagnetic Ga_1–x Mn _x As/GaAs multilayered structures, as a result of the magnetic interaction between holes and the Mn ions. The magnetic layers act as potential barriers for holes with spins aligned parallel to the layer magnetization, and as potential wells for the inverse spin polarization. In the case of currents in-plane, holes with spin parallel and antiparallel to this magnetization move in different regions. By choosing properly the magnetic and the nonmagnetic layers widths, a spin-polarized transport with a difference of an order of magnitude on the mobilities for each spin polarization is predicted to occur. Spin-polarized minibands are also shown to occur in a superlattice based on the same structure. We calculated the dependence of the spin polarization with the superlattice parameters, and we discuss how this polarization affects the Bloch miniband transport in such ferromagnetic superlattice.     

Fano-Rashba effect in quantum dots

We consider the electronic transport through a Rashba quantum dot coupled to ferromagnetic leads. We show that the interference of localized electron states with resonant electron states leads to the appearance of the Fano-Rashba effect. This effect occurs due to the interference of bound levels of spin-polarized electrons with the continuum of electronic states with an opposite spin polarization. We investigate this Fano-Rashba effect as a function of the applied magnetic field and Rashba spin-orbit coupling.     

Spin current,spin accumulation and spin Hall effect

Abstract

Nonlocal spin transport in nanostructured devices with ferromagnetic injector (F1) and detector (F2) electrodes connected to a normal conductor (N) is studied. We reveal how the spin transport depends on interface resistance, electrode resistance, spin polarization and spin diffusion length, and obtain the conditions for efficient spin injection, spin accumulation and spin current in the device. It is demonstrated that the spin Hall effect is caused by spin–orbit scattering in nonmagnetic conductors and gives rise to the conversion between spin and charge currents in a nonlocal device. A method of evaluating spin–orbit coupling in nonmagnetic metals is proposed.     

Band Structures of Narrow Gap Semiconductors in Terms of Coulombic Polaron Effect

We consider the formation of a polaron in an interacting Fermi gas due to Coulomb polarization of electron gas by a single electron. We show that the polarization effect due to the Coulomb field in an interacting Fermi gas leads to the Coulomb polaron caused by the exchange-correlation hole and the electron spectrum shows an insulating behavior with the dispersion law of quasiparticles similar to the Kane kp-model of a band structure for narrow gap semiconductors. We apply the polaronic model to the case of inhomogeneous periodic and interacting electron Fermi gas of crystals. We show that the formation of the Coulomb polaron leads to the transition to the semiconductor with the energy bands similar to the well-known semiempirical Kane kp-model of the band structure of narrow gap semiconductors. We also discuss the impact of the results on band structure calculations.     

Few electron limit of n-type metal oxide semiconductor single electron transistors

We report the electronic transport on n-type silicon single electron transistors (SETs) fabricated in complementary metal oxide semiconductor (CMOS) technology. The n-type metal oxide silicon SETs (n-MOSSETs) are built within a pre-industrial fully depleted silicon on insulator (FDSOI) technology with a silicon thickness down to 10 nm on 200 mm wafers. The nominal channel size of 20 × 20 nm(2) is obtained by employing electron beam lithography for active and gate level patterning. The Coulomb blockade stability diagram is precisely resolved at 4.2 K and it exhibits large addition energies of tens of meV. The confinement of the electrons in the quantum dot has been modeled by using a current spin density functional theory (CS-DFT) method. CMOS technology enables massive production of SETs for ultimate nanoelectronic and quantum variable based devices.