Electric-field tuning s-d exchange interaction in quantum dots

We investigate theoretically the electron-hole pair states in CdTe quantum dot (QD) containing a single Mn2+ ion by the magneto-optical spectrum tuned by the electric field. It is shown that the electric field does not only tune the spin splitting via the sp-d exchange interaction but also affect significantly the anticrossing behavior in the photoluminescence spectrum. This anticrossing is caused by the s-d exchange interaction and/or the hole mixing effect, which depends sensitively on the shape of the QD. (C) 2008 American Institute of Physics.

Spin precession and electron spin polarization wave in [001]-grown quantum wells

We theoretically study the spatial behaviors of spin precessions modulated by an effective magnetic field in a two-dimensional electron system with spin-orbit interaction. Through analysis of interaction between the spin and the effective magnetic field, we find some laws of spin precession in the system, by which we explain some previous phenomena of spin precession, and predict a controllable electron spin polarization wave in [001]-grown quantum wells. The shape of the wave, like water wave, mostly are ellipse-like or circle-like, and the wavelength is anisotropic in the quantum wells with two unequal coupling strengths of the Rashba and Dresselhaus interactions, and is isotropic in the quantum wells with only one spin orbit interaction.

Strong Spin-Orbit Interactions in an InAlAs/InGaAs/InAlAs Two-Dimensional Electron Gas by Weak Antilocalization Analysis

Spin-orbit interactions in a two-dimensional electron gas were studied in an InAlAs/InGaAs/InAlAs quantum well. Since weak anti localization effects take place far beyond the diffusive regime, (i.e., the ratio of the characteristic magnetic field, at which the magnetoresistance correction maximum occurs, to the transport magnetic field is more than ten) the experimental data are examined by the Golub theory, which is applicable to both diffusive regime and ballistic regime. Satisfactory fitting lines to the experimental data have been achieved using the Golub theory. In the strong spin-orbit interaction two-dimensional electron gas system, the large spin splitting energy of 6.08 meV is observed mainly due to the high electron concentration in the quantum well. The temperature dependence of the phase-breaking rate is qualitatively in agreement with the theoretical predictions. (C) 2009 The Japan Society of Applied Physics

Spin Hall effect on the kagome lattice with Rashba spin-orbit interaction

We study the spin Hall effect in the kagome lattice with Rashba spin-orbit coupling. The conserved spin Hall conductance sigma(s)(xy) (see text) and its two components, i.e., the conventional term sigma(s0)(xy) and the spin-torque-dipole term sigma(s tau)(xy), are numerically calculated, which show a series of plateaus as a function of the electron Fermi energy epsilon(F). A consistent two-band analysis, as well as a Berry-phase interpretation, is also given. We show that these plateaus are a consequence of various Fermi-surface topologies when tuning epsilon(F). In particular, we predict that compared to the case with the Fermi surface encircling the Gamma point in the Brillouin zone, the amplitude of the spin Hall conductance with the Fermi surface encircling the K points is twice enhanced, which makes it highly meaningful in the future to systematically carry out studies of the K-valley spintronics.

Spin precession induced by an effective magnetic field in a two-dimensional electron gas

We theoretically study the spatial behaviors of the spin precession in a two-dimensional electron system with spin-orbit interaction. Through analysis of interaction between the spin and the effective magnetic field in the system, we obtain the general conditions to generate a persistent spin helix and predict a persistent spin helix pattern in [001]-grown quantum wells. Particularly, we demonstrate that the phase of spin can be locked to propagate in a quantum well with SU(2) symmetry.

Electron spin relaxation by nuclei and holes in single InAs quantum dots

Electron spin relaxation of charged excitons X+ and X2+ are investigated by time-resolved and polarization-resolved photoluminescence spectroscopy. For X+ configuration, the electron spin relaxation shows a typical decay curve induced by hyperfine interaction with nuclei, whereas for X2+ state the electron spin relaxation is affected not only by nuclei but also by electron-hole exchange interaction, leading to a power-law time dependence.

Electronic structure of self-assembled InAs quantum disks in an axial magnetic field and two-electron quantum-disk qubit

We have studied the single-electron and two-electron vertically assembled quantum disks in an axial magnetic field using the effective mass approximation. The electron interaction is treated accurately by the direct diagonalization of the Hamiltonian matrix. We calculate the six energy levels of the single-electron quantum disks and the two lowest energy levels of the two-electron quantum disks in an axial magnetic field. The change of the magnetic field strongly modifies the electronic structures as an effective potential, leading to the splitting of the levels and the crossings between the levels. The effect of the vertical alignment on the electronic structures is discussed. It is demonstrated that the switching of the ground-state spin exists between S=0 and S=1. The energy difference DeltaE between the lowest S=0 and S=1 states is shown as a function of the axial magnetic field. It is also found that the variation of the energy difference between the lowest S=0 and S=1 states in the strong-B S=0 state is fairly linear. Our results provide a possible realization for a qubit to be fabricated by current growth techniques. (C) 2004 American Institute of Physics.

Electron and hole states in diluted magnetic semiconductor quantum dots

The electronic structure of a diluted magnetic semiconductor (DMS) quantum dot (QD) is studied within the framework of the effective-mass theory. We find that the energies of the electron with different spin orientation exhibit different behavior as a function of magnetic field at small magnetic fields. The energies of the hole decreases rapidly at low magnetic fields and saturate at higher magnetic field due to the sp-d exchange interaction between the carriers and the magnetic ions. The mixing effect of the hole states in the DMS QD can be tuned by changing the external magnetic field. An interesting crossing behavior of the hole ground state between the heavy-hole state and the light-hole state is found with variation of the QD radius. The strength of the interband optical transition for different circular polarization exhibts quite different behavior with increasing magnetic field and QD radius.

Role of interactions in electronic structure of a two-electron quantum dot molecule

We have studied a two-electron quantum dot molecule in a magnetic field. The electron interaction is treated accurately by the direct diagonalization of the Hamiltonian matrix. We calculate two lowest energy levels of the two-electron quantum dot molecule in a magnetic field. Our results show that the electron interactions are significant, as they can change the total spin of the two-electron ground state of the system by adjusting the magnetic field between S = 0 and S = 1. The energy difference DeltaE between the lowest S = 0 and S = 1 states is shown as a function of the axial magnetic field. We found that the energy difference between the lowest S = 0 and S = 1 states in the strong-B S = 0 state varies linearly. Our results provide a possible realization for a qubit to be fabricated by current growth techniques.

Effect of interaction between periodic delta-doping in both well and barrier layers on modulation of superlattice band structure

The modulation of superlattice band structure via periodic delta-doping in both well and barrier layers have been theoretically investigated, and the importance of interaction between the delta-function potentials in the well layers and those in the barrier layers on SL band structure have been revealed. It is pointed out that the energy dispersion relation Eq. (3) given in [G. Ihm, S.K. Noh, J.I. Lee, J.-S. Hwang, T.W. Kim, Phys. Rev. B 44 (1991) 6266] is an incomplete one, as the interaction between periodic delta-doping in both well and barrier layers had been overlooked. Finally, we have shown numerically that the electron states of a GaAs/Ga0.7Al0.3As superlattice can be altered more efficiently by intelligent tuning the two delta-doping's positions and heights. (c) 2007 Elsevier B.V. All rights reserved.

An n-type SiGe/Ge QC structure utilizing the deep Ge quantum well for electron at the Gamma point

In this paper, an n-type Si1-xGex/Ge (x >= 0.85) quantum cascade (QC) structure utilizing a deep Ge quantum well for electrons at the Gamma point is proposed. Based on linear interpolation, a conduction band offset at the Gamma point in a Si1-xGex/Ge ( x >= 0.85) heterostructure is presented, which is suitable for designing a QC laser. This approach has the advantages of a large conduction band offset at the Gamma point, a low lattice mismatch between the Si1-xGex/Ge ( x >= 0.85) active layers and the Si1-yGey ( y > x) virtual substrate, a small electron effective mass in the Gamma band, simple conduction energy band structures and a simple phonon scattering mechanism in the Ge quantum well. The theory predicts that if high-energy electrons are continuously injected into the Gamma band, a quasi-equilibrium distribution of electrons between the Gamma and L bands can be reached and held, i.e., electrons with a certain density will be kept in the Gamma band. This result is supported by the intervalley scattering experiments. In n-type Si1-xGex/Ge ( x >= 0.85) QC structures, population inversion between the laser's upper and lower levels is demonstrated.

Spin-polarized transport through an Aharonov-Bohm interferometer with Rashba spin-orbit interaction

We study electron transport through an Aharonov-Bohm (AB) interferometer with a noninteracting quantum dot in each of its arms. Both a magnetic flux phi threading through the AB ring and the Rashba spin-orbit (SO) interaction inside the two dots are taken into account. Due to the existence of the SO interaction, the electrons flowing through different arms of the AB ring will acquire a spin-dependent phase factor in the tunnel-coupling strengths. This phase factor, as well as the influence of the magnetic flux, will induce various interesting interference phenomena. We show that the conductance and the local density of states can become spin polarized by tuning the magnetic flux and the Rashba interaction strength. Under certain circumstances, a pure spin-up or spin-down conductance can be obtained when a spin-unpolarized current is injected from the external leads. Therefore, the electron spin can be manipulated by adjusting the Rashba spin-orbit strength and the structure parameters. (c) 2006 American Institute of Physics.

Charge-density and spin-density excitations in a two-dimensional electron gas with Rashba spin-orbit coupling

We study theoretically the charge-density and spin-density excitations in a two-dimensional electron gas in the presence of a perpendicular magnetic field and a Rashba type spin-orbit coupling. The dispersion and the corresponding intensity of excitations in the vicinity of cyclotron resonance frequency are calculated within the framework of random phase approximation. The dependence of excitation dispersion on various system parameters, i.e., the Rashba spin-orbit interaction strength, the electron density, the Zeeman spin splitting, and the Coulomb interaction strength is investigated.

Interplay between s-d exchange interaction and Rashba effect: Spin-polarized transport

The authors investigate the spin-polarized transport properties of a two-dimensional electron gas in a n-type diluted magnetic narrow gap semiconductor quantum well subjected to perpendicular magnetic and electric fields. Interesting beating patterns in the magnetoresistance are found which can be tuned significantly by varying the electric field. A resonant enhancement of spin-polarized current is found which is induced by the competition between the s-d exchange interaction and the Rashba effect [Y. A. Bychkov and E. I. Rashba, J. Phys. C 17, 6039 (1984)]. (c) 2006 American Institute of Physics.

RKKY interaction between quantum dot spins tuned by the quantum dot level

We have investigated the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two quantum dot (QD) spins mediated by a two-dimensional electron gas in the simplest case. The oscillation of the RKKY interaction versus the distance between the two QDs consists of two ingredients with different periods. The RKKY interaction undulates with the variation of the singly occupied QD level, which provides us a way to tune the magnitude and the sign of the RKKY interaction by pushing the QD level up and down. These conclusions are quite different from the usual result obtained by replacing the s-d exchange interaction with its value at the Fermi level. The influence on the RKKY interaction brought about under more realistic conditions is also discussed.