Spin-polarized current generated by magneto-electrical gating

We theoretically study spin-polarized current through a single electron tunneling transistor (SETT), in which a quantum dot (QD) is coupled to non-magnetic source and drain electrodes via tunnel junctions, and gated by a ferromagnetic (FM) electrode. The I–V characteristics of the device are investigated for both spin and charge currents, based on the non-equilibrium Green's function formalism. The FM electrode generates a magnetic field, which causes a Zeeman spin-splitting of the energy levels in the QD. By tuning the size of the Zeeman splitting and the source–drain bias, a fully spin-polarized current is generated. Additionally, by modulating the electrical gate bias, one can effect a complete switch of the polarization of the tunneling current from spin-up to spin-down current, or vice versa.     

Electron-spin polarization in a non-magnetic heterostructure

The spin dependent electron transmission through a non-magnetic III-V semiconductor symmetric well is studied theoretically so as to investigate the output transmission current polarization at zero magnetic field. Transparency of electron transmission is calculated as a function of electron energy as well as the well width, within the one electron band approximation along with the spin-orbit interaction. Enhanced spin-polarized resonant tunneling in the heterostructure due to Dresselhaus and Rashba spin-orbit coupling induced splitting of the resonant level is observed. We predict that a spin-polarized current spontaneously emerges in this heterostructure. This effect could be employed in the fabrication of spin filters, spin injectors and detectors based on non-magnetic semiconductors.     

Charge and spin currents tunnelling through a toroidal carbon nanotube side-coupled with a quantum dot

We have investigated the mesoscopic transport through the system with a quantum dot (QD) side-coupled to a toroidal carbon nanotube (TCN) in the presence of spin-flip effect. The coupled QD contributes to the mesoscopic transport significantly through adjusting the gate voltage and Zeeman field applied to the QD. The compound TCN-QD microstructure is related to the separate subsystems, the applied external magnetic fields, as well as the combination of subsystems. The spin current component I_z^s is independent on time, while the spin current components I_x^s and I_y^s evolve with time sinusoidally. The rotating magnetic field induces novel levels due to the spin splitting and photon absorption procedures. The suppression and enhancement of resonant peaks, and semiconductor-metal phase transition are observed by studying the differential conductance through tuning the source-drain bias and photon energy. The magnetic flux induces Aharonov-Bohm oscillation, and it controls the tunnelling behavior due to adjusting the flux. The Fano type of multi-resonant behaviors are displayed in the conductance structures by adjusting the gate voltage V_g and the Zeeman field applied to the QD.     

Magnetic Field Effects on Quantum-Dot Spin Valves

We study the magnetic field effects on the spin-polarized transport of the quantum dot (QD) spin valve in the sequential tunneling regime. A set of generalized master equation is derived. Based on that, we discuss the collinear and noncollinear magnetic field effects, respectively. In the collinear magnetic field case,we find that the Zeeman splitting can induce a negative differential conductance (NDC), which is quite different from the one found in previous studies. It has a critical polarization in the parallel arrangement and will disappear in the antiparallelconfiguration. In the noncollinear magnetic field case, the current shows two plateaus and their angular dependence is analyzed. Although sometimes the two current plateaus have similar angular dependence, their mechanisms are different. Our formalism is also suitable for calculating the transport in magnetic molecules, in which the spin splitting is induced not by a magnetic field but by the intrinsic magnetization.     

Flux-dependent anti-crossing of resonances in parallel non-coupled double quantum dots

We present novel resonant phenomena through parallel non-coupled double quantum dots (QDs) embedded in each arm of an Aharonov-Bohm (AB) ring with magnetic flux passing through its center. The electron transmission through this AB ring with each QD formed by two short-range potential barriers is calculated using a scattering matrix at each junction and a transfer matrix in each arm. We show that as the magnetic flux modulates, a distortion of the grid-like square transmission occurs and an anti-crossing of the resonances appears. Hence, the modulation of magnetic flux in this system can have an equivalent effect to the control of inter-dot coupling between the two QDs.     

Giant anisotropy of Zeeman splitting of quantum confined acceptors in Si/Ge

Shallow acceptor levels in Si/Ge/Si quantum well heterostructures are characterized by resonant-tunneling spectroscopy in the presence of high magnetic fields. In a perpendicular magnetic field we observe a linear Zeeman splitting of the acceptor levels. In an in-plane field, on the other hand, the Zeeman splitting is strongly suppressed. This anisotropic Zeeman splitting is shown to be a consequence of the huge light-hole--heavy-hole splitting caused by a large biaxial strain and a strong quantum confinement in the Ge quantum well.     

Quantum Switch and Efficient Spin-Filter in a System Consisting of Multiple Three-Quantum-Dot Rings

Using the non-equilibrium Keldysh Green's function technique, we investigate electron transport properties of a system consisting of multiple three-quantum-dot rings. The conductance as a function of the electron energy is numerically calculated. An antiresonance point emerges in the conductance spectra and evolves into a welldefined insulating band with the increasing number of three-quantum-dot rings. The position of the well-defined insulating band can be modulated by varying the tunneling coupling strength between adjacent three-quantumdot rings. When an external magnetic flux is introduced, several to 100% spin-polarized windows will occur due to the Zeeman splitting. These results strongly suggest that this device may realize multiple functions including quantum switch and efficient spin filtering.     

Nuclear ground states dressed with rf photons in Mössbauer–Zeeman 57Fe spectroscopy

We have applied the “dressed” state concept, developed in quantum electronics, to the situation in which spin 1/2 ground-state nuclear levels are coupled by rf photons. In particular, we have studied Mössbauer spectroscopy when there is Zeeman splitting of the nuclear levels and a further interaction due to an applied rf-radiation field when the rf frequency is in the neighborhood of the ground-state splitting. The dressed-state approach treats the coupling of the ground nuclear Zeeman levels, due to a radio frequency field, by considering the total system made up of: nucleus, static magnetic field, and rf field as one global quantum system. The energy levels and corresponding eigenstates of the system are calculated as a function of the rf frequency and the magnitude of the rf magnetic flux density. Mössbauer spectra are calculated for the ⁵⁷Fe case in which the source is subjected to both the static and radiation fields while the absorber nuclear levels are unsplit.

     

Spin edge states in two-dimensional electron systems in the presence of Zeeman effect

We study the spin edge states, induced by the combined effect of Bychkov-Rashba spinorbit and Zeeman interactions or of Dresselhaus spin-orbit and Zeeman interactions in a twodimensional electron system, exposed to a perpendicular quantizing magnetic field and restricted by a hard-wall confining potential. We derive an exact analytical formula for the dispersion relations of spin edge states and analyze their energy spectrum versus the momentum and the magnetic field. We calculate the average spin components and the average transverse position of electron. It is shown that by removing the spin degeneracy, spin-orbit interaction splits the spin edge states not only in the energy but also induces their spatial separation. Depending on the type of spin-orbit coupling and the principal quantum number, the Zeeman term in the combination with spin-orbit interaction increases or decreases essentially the splitting of bulk Landau levels while it has a weak influence on the spin edge states.     

Electron states in diluted magnetic semiconductors

One of the remarkable properties of the II–VI diluted magnetic semiconductor (DMS) quantum dot (QD) is the giant Zeeman splitting of the carrier states under application of a magnetic field. This splitting reveals strong exchange interaction between the magnetic ion moment and electronic spins in the QD. A theoretical study of the electron spectrum and of its relaxation to the ground state via the emission of a longitudinal optical (LO) phonon, in a CdSe/ZnMnSe self-assembled quantum dot, is proposed in this work. Numerical calculations showed that the strength of this interaction increases as a function of the magnetic field to become more than 30 meV and allows some level crossings. We have also shown that the electron is more localized in this DMS QD and its relaxation to the ground state via the emission of one LO phonon is allowed.     

Temperature dependence of the spin polarization of composite fermions

It is found that at a critical value of the magnetic field in which a system of composite fermions becomes completely spin-polarized, the temperature dependence of the electronic spin polarization is a linear function at low temperatures. It is shown that the slope of this dependence is determined by the Fermi energy of the composite fermions. This made it possible to measure the Fermi energy and the Zeeman splitting of the composite fermions. A large amplification of the spin splitting of composite fermions for complete spin polarization of the system is found. This makes it possible to measure the strength of the interaction between composite fermions. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 11, 722–726 (10 December 1999)     

Perfect spin filtering and conditions for Fano antiresonance and Dicke resonance in a parallel coupled triple quantum-dot array

Electronic transport through a parallel coupled triple quantum dot (tQD) array has been studied by means of nonequilibrium Green’s function formalism. By producing an energy difference between the site energy in the upper QDs and down ones, we find that the linear conductance spectrum of this tQD array displays Fano antiresonance and Dicke resonance effects. As the energy difference increases or the tQD chain length increases to a not very large value, the antiresonance valley in the conductance changes to a well-defined insulating band with very steep edges. Meanwhile, the relations of the Fano antiresonance and the well-defined insulating band are explored, and the conditions for the Fano antiresonance and the Dicke resonance are presented. By introducing a Zeeman splitting due to an external magnetic field, the spin-splitting conductance spectrum shows some highly to 100% spin-polarized windows (SPWs). If a gate voltage runs in these SPWs, we can achieve an entirely spin-polarized current, indicating that such a tQD array can be used as a perfect spin filter and a quantum-signal generator. Moreover, the intradot Coulomb repulsion on the electronic transport is also investigated. The results show that the intradot Coulomb repulsion does not affect the device applications for this system mentioned above.     

Zeeman splitting in ballistic hole quantum wires

We have studied the Zeeman splitting in ballistic hole quantum wires formed in a (311)A quantum well by surface gate confinement. Transport measurements clearly show lifting of the spin degeneracy and crossings of the subbands when an in-plane magnetic field B is applied parallel to the wire. When B is oriented perpendicular to the wire, no spin splitting is discernible up to B = 8.8 T. The observed large Zeeman splitting anisotropy in our hole quantum wires demonstrates the importance of quantum confinement for spin splitting in nanostructures with strong spin-orbit coupling.     

Measurements of strongly anisotropic g factors for spins in single quantum states

We have measured the full angular dependence, as a function of the direction of magnetic field, for the Zeeman splitting of individual energy states in copper nanoparticles. The g factors for spin splitting are highly anisotropic, with angular variations as large as a factor of 5. The angular dependence fits well to ellipsoids. Both the principal-axis directions and g-factor magnitudes vary between different energy levels within one nanoparticle. The variations agree quantitatively with random-matrix theory predictions which incorporate spin-orbit coupling.     

Effects of external fields on a two-dimensional Klein-Gordon particle under pseudo-harmonic oscillator interaction

We study the effects of the perpendicular magnetic and Aharonov-Bohm(AB) flux fields on the energy levels of a two-dimensional(2D) Klein-Gordon(KG) particle subjected to an equal scalar and vector pseudo-harmonic oscillator(PHO).We calculate the exact energy eigenvalues and normalized wave functions in terms of chemical potential parameter,magnetic field strength,AB flux field,and magnetic quantum number by means of the Nikiforov-Uvarov(NU) method.The non-relativistic limit,PHO,and harmonic oscillator solutions in the existence and absence of external fields are also obtained.     

Transport properties of a single-quantum dot Aharonov-Bohm interferometer

We consider a two-terminal Aharonov-Bohm (AB) interferometer with a quantum dot inserted in one path of the AB ring. We investigate the transport properties of this system in and out of the Kondo regime. We utilize perturbation theory to calculate the electron self-energy of the quantum dot with respect to the intradot Coulomb interaction. We show the expression of the Kondo temperature as a function of the AB phase together with its dependence on other characteristics such as the linewidth of the ring and the finite Coulomb interaction and the energy levels of the quantum dot. The current oscillates periodically as a function of the AB phase. The amplitude of the current oscillation decreases with increasing Coulomb interaction. For a given temperature, the electron transport through the AB interferometer can be selected to be in or out of the Kondo regime by changing the magnetic flux threading perpendicular to the AB ring of the system.     

External Electric Field Effect on Hydrogenic Donor Impurity in Zinc-Blende InGaN Quantum Dot

The binding energy of a hydrogenic donor impurity in zinc-blende (ZB) InGaN quantum dot (QD) is calculated in the framework of effective-mass envelope-function theory using the plane wave basis. It is shown that the donor binding energy is highly dependent on the impurity position, QD size and the external electric field. The symmetry of the electron probability distribution is broken and the maximum of the donor binding energy is shifted from the centre of QD in the presence of the external electric field. The degenerating energy levels for symmetrical positions with respect to the centre of QD are split. The splitting increases with the increase of QD height while the splitting increases up to a maximum value and then decreases with the increase of QD radius.     

Fano-type spin-flip mesoscopic transport through an Aharonov-Bohm interferometer

We have investigated the mesoscopic transport properties of a quantum dot embedded Aharonov-Bohm (AB) interferometer applied with a rotating magnetic field. The spin-flip effect is induced by the rotating magnetic field, and the tunneling current is sensitive to the spin-flip effect. The spin-flipped electrons tunneling from the direct channel and the resonant channel interfere with each other to form spin-polarized tunneling current components. The non-resonant tunneling (direct transmission) strength and the AB phase φ play important roles. When the non-resonant tunneling (background transmission) exists, the spin and charge currents form asymmetric peaks and valleys, which exhibit Fano-type line shapes by varying the source-drain bias voltage, or gate voltage. The AB oscillations of the spin and charge currents exhibit distinct dependence on the magnetic flux and direct tunneling strength.     

Spin-polarized current in double quantum dots

We analyze the transport through asymmetric double quantum dots with an inhomogeneous Zeeman splitting in the presence of crossed dc and ac magnetic fields.A strong spin-polarized current can be obtained by changing the dc magnetic field.It is mainly due to the resonant tunnelling.But for the ferromagnetic right electrode,the electron spin resonance also plays an important role in transport.We show that the double quantum dots with three-level mixing under crossed dc and ac magnetic fields can act not only as a bipolar spin filter but also as a spin inverter under suitable conditions.     

Adiabatic quantum pump of spin-polarized current

We propose a mechanism by which an open quantum dot driven by two ac (radio frequency) gate voltages in the presence of a moderate in-plane magnetic field generates a spin-polarized, phase-coherent dc current. The idea combines adiabatic, nonquantized (but coherent) pumping through periodically modulated external parameters and the strong fluctuations of the electron wave function existent in chaotic cavities. We estimate that the spin polarization of the current can be observed for temperatures and Zeeman splitting energies of the order of the single-particle mean level spacing.