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 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.

Spin-polarized ballistic transport in diluted magnetic semiconductor quantum wire systems

The energy dispersion of an electron in a double quantum wire with a diluted magnetic semiconductor barrier in between is calculated. An external magnetic field modifies significantly the energy dispersion of the electron which is different for the two spin states. The conductance exhibits many interesting peaks and dips which are directly related to the energy dispersions of the different electron spin states. These phenomena are attributed to the interwell coupling which can be tuned by the magnetic field due to the s-d exchange interaction.

Mean-field study of Fe2+- and Co2+-doped diluted magnetic semiconductors

Based on a modified mean-field model, we calculate the Curie temperatures of Fe2+- and Co2+-doped diluted magnetic semiconductors (DMSs) and their dependence on the hole concentration. We find that the Curie temperatures increase with an increase in hole concentration and the relationship T(C)proportional to p(1/3) also approximately holds for Fe2+- and Co2+-doped systems with moderate hole concentration. For either low or high hole concentrations, however, the p(1/3) law is violated due to the anomalous magnetization of the Fe2+ and Co2+ ions, and the nonparabolic nature of the hole bands. Further, the values of T-C for Fe2+- and Co2+-doped DMSs are significantly higher than those for Mn2+-doped DMSs, due to the larger exchange interaction strength.

Spin relaxation in diluted magnetic semiconductor quantum dots

Electron spin relaxation induced by phonon-mediated s-d exchange interaction in a II-VI diluted magnetic semiconductor quantum dot is investigated theoretically. The electron-acoustic phonon interaction due to piezoelectric coupling and deformation potential is included. The resulting spin lifetime is typically on the order of microseconds. The effectiveness of the phonon-mediated spin-flip mechanism increases with increasing Mn concentration, electron spin splitting, vertical confining strength, and lateral diameter, while it shows nonmonotonic dependence on the magnetic field and temperature. An interesting finding is that the spin relaxation in a small quantum dot is suppressed for strong magnetic field and low Mn concentration at low temperature.

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.

Spin states and persistent currents in mesoscopic rings: Spin-orbit interactions

We investigate theoretically electron spin states in one-dimensional and two-dimensional (2D) hard-wall mesoscopic rings in the presence of both the Rashba spin-orbit interaction (RSOI) and the Dresselhaus spin-orbit interaction (DSOI) in a perpendicular magnetic field. The Hamiltonian of the RSOI alone is mathematically equivalent to that of the DSOI alone using an SU(2) spin rotation transformation. Our theoretical results show that the interplay between the RSOI and DSOI results in an effective periodic potential, which consequently leads to gaps in the energy spectrum. This periodic potential also weakens and smoothens the oscillations of the persistent charge current and spin current and results in the localization of electrons. For a 2D ring with a finite width, higher radial modes destroy the periodic oscillations of persistent currents.

Magnetic properties of tin-doped magnetite nanoparticles

Structural and magnetic characteristics of Fe3-xSnxO4 (x < 0.3) nanoparticles synthesized using the precipitation exchange method have been investigated by X-ray diffraction, transmission electron microscope, Mossbauer spectra, X-ray photoelectron spectroscopy and magnetization measurement. The mean particle dimension decreases from 8 to 6 nm, the lattice parameters enlarge, the saturation magnetization decreases, as well as the magnetization and the coercive field increase, with increasing tin-content. The paramagnetic property of the specimens indicates that the replacement of Fe3+ by Sn4+ on the octahedral sites of Fe3O4 causes a progressive lowering of the Curie temperature and the Curie temperatures of the materials are all lower than that of crystallite tin-doped magnetite. This striking debasing is due to the lessening of the grain size. This is the smallest size reported thus far for paramagnetic tin-doped magnetite particles. (c) 2006 Elsevier B.V. All rights reserved.

Photoinduced spin alignment of the magnetic ions in (Ga,Mn)As

Time-resolved Kerr rotation measurement in the (Ga,Mn)As diluted magnetic semiconductor allows direct observation of the dynamical properties of the spin system of the magnetic ions and the spin-polarized holes. Experimental results show that the magnetic ions can be aligned by the polarized holes, and the time scales of spin alignment and relaxation take place in tens and hundreds of picoseconds, respectively. The Larmor frequency and effective g factor obtained in the Voigt geometry show an unusual temperature dependence in the vicinity of the Curie temperature due to the exchange coupling between the photoexcited holes and magnetic ions. Such a spin coherent precession can be amplified or destructed by two sequential excitation pulses with circularly copolarized or oppositely polarized helicity, respectively. (c) 2006 American Institute of Physics.

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.

Thermal entanglement in a two-spin-qutrit system under a nonuniform external magnetic field

The thermal entanglement in a two-spin-qutrit system with two spins coupled by exchange interaction under a magnetic field in an arbitrary direction is investigated. Negativity, the measurement of entanglement is calculated. We find that for any temperature the evolvement of negativity is symmetric with respect to magnetic field. The behavior of negativity is presented for four different cases. The results show that for different temperature; different magnetic field give maximum entanglement. Both the parallel and antiparallel magnetic field cases are investigated qualitatively (not quantitatively) in detail, we find that the entanglement may be enhanced under an antiparallel magnetic field.

Magnetic properties of silicon doped with gadolinium

The magnetic semiconductor GdxSi1-x was prepared by low-energy dual ion-beam epitaxy. GdxSi1-x shows excellent magnetic properties at room temperature. A high magnetic moment of 10 mu(B) per Gd atom is observed. The high atomic magnetic moment is interpreted as being a result of the RKKY mechanism. The indirect exchange interaction between ions is strong at large distances due to the low state density of the carriers in the magnetic semiconductor.

Magneto-optical properties of diluted magnetic semiconductor quantum dots

The magnetoexciton polaron (MP) is investigated theoretically in a diluted magnetic semiconductor quantum dot (QD), with the Coulomb interaction and the sp-d exchange interaction included. The MP energy decreases rapidly with increasing magnetic field at low magnetic field and saturates at high magnetic field for small QDs, and the dependences of the MP energy on magnetic field are quite different for different QD radii due to the different carrier-induced magnetic fields B-MP. The competition between the sp-d exchange interaction and the band gap shrinkage results in there being a maximum exhibited by the MP energy With increasing temperature. Our numerical results are in good agreement with experiment (Maksimov A A, Bacher G, MacDonald A, Kulakovskii V D, Forchel A, Becker C R, Landwehr G and Molenkamp L W 2000 Phys. Rev. B 62 R7767).

Electronic structure of diluted magnetic semiconductor superlattices: In-plane magnetic field effect

The electronic structure of diluted magnetic semiconductor (DMS) superlattices under an in-plane magnetic field is studied within the framework of the effective-mass theory; the strain effect is also included in the calculation. The numerical results show that an increase of the in-plane magnetic field renders the DMS superlattice from the direct band-gap system to the indirect band-gap system, and spatially separates the electron and the hole by changing the type-I band alignment to a type-II band alignment. The optical transition probability changes from type I to type II and back to type I like at large magnetic field. This phenomenon arises from the interplay among the superlattice potential profile, the external magnetic field, and the sp-d exchange interaction between the carriers and the magnetic ions. The shear strain induces a strong coupling of the light- and heavy-hole states and a transition of the hole ground states from "light"-hole to "heavy"-hole-like states.

Longitudinal spin transport in diluted magnetic semiconductor superlattices: The effect of the giant Zeeman splitting

Longitudinal spin transport in diluted magnetic semiconductor superlattices is investigated theoretically. The longitudinal magnetoconductivity (MC) in such systems exhibits an oscillating behavior as function of an external magnetic field. In the weak magnetic-field region the giant Zeeman splitting plays a dominant role that leads to a large negative magnetoconductivity. In the strong magnetic-field region the MC exhibits deep dips with increasing magnetic field. The oscillating behavior is attributed to the interplay between the discrete Landau levels and the Fermi surface. The decrease of the MC at low magnetic field is caused by the s-d exchange interaction between the electron in the conduction band and the magnetic ions. The spin polarization increases rapidly with increasing magnetic field and the longitudinal current becomes spin polarized in strong magnetic field. The effect of spin-disorder scattering on MC is estimated numerically for low magnetic fields and found to be neglectible for our system.