Global control and fast solid-state donor electron spin quantum computing

We propose a scheme for quantum information processing based on donor electron spins in semiconductors, with an architecture complementary to the original Kane proposal. We show that a naive implementation of electron spin qubits provides only modest improvement over the Kane scheme, however through the introduction of global gate control we are able to take full advantage of the fast electron evolution timescales. We estimate that the latent clock speed is 100-1000 times that of the nuclear spin quantum computer with the ratio T-2/T-ops approaching the 10(6) level.

Coexistence of spin polarization and pairing correlations in metallic grains

We investigate the competition between magnetic depairing interactions, due to spin-exchange mechanism and∕or to spin-dependent asymmetric bandwidths, and pairing coupling in metallic grains. We present a detailed analysis of the quantum ground state in different regimes arising from the interplay between ferromagnetic and pairing correlations for different fillings. We find out that the occurrence of a ground state with coexisting spin-polarization and pairing correlations is enhanced when the asymmetric spin-dependent distribution of the single-particle energies is considered. The mechanisms leading to such a stable quantum state are finally clarified.

Optical third harmonic generation in the magnetic semiconductor EuSe

Third harmonic generation (THG) has been studied in europium selenide EuSe in the vicinity of the band gap at 2.1-2.6 eV and at higher energies up to 3.7 eV. EuSe is amagnetic semiconductor crystalizing in centrosymmetric structure of rock-salt type with the point group m3m. For this symmetry the crystallographic and magnetic-field-induced THG nonlinearities are allowed in the electric-dipole approximation. Using temperature, magnetic field, and rotational anisotropy measurements, the crystallographic and magnetic-field-induced contributions to THG were unambiguously separated. Strong resonant magnetic-field-induced THG signals were measured at energies in the range of 2.1-2.6 eV and 3.1-3.6 eV for which we assign to transitions from 4 f(7) to 4 f(6)5d(1) bands, namely involving 5d(t(2g)) and 5d(e(g)) states.

Magneto-optical probe of quantum Hall states in a wide parabolic well modulated by random potential

Polarized photoluminescence from weakly coupled random multiple well quasi-three-dimensional electron system is studied in the regime of the integer quantum Hall effect. Two quantum Hall ferromagnetic ground states assigned to the uncorrelated miniband quantum Hall state and to the spontaneous interwell phase coherent dimer quantum Hall state are observed. Photoluminescence associated with these states exhibits features caused by finite-size skyrmions: dramatic reduction of the electron spin polarization when the magnetic field is increased past the filling factor nu = 1. The effective skyrmion size is larger than in two-dimensional electron systems.

Vielkanal-Spinpolarimeter für energie- und winkeldispersive Photoemissionsmessungen

Spinpolarisationsmessungen an freien Elektronen sind seit ihrer ersten Verwirklichung durch Mott anspruchsvoll geblieben. Die relevante Größe eines Spinpolarimeters ist seine Gütefunktion, FoM = S^2*I/I_0, mit der Asymmetriefunktion S und dem Verhältnis aus Streu- und Primärintensität I/I_0. Alle bisherigen Geräte basieren auf einer einkanaligen Streuung (Spin-Bahn- oder Austauschwechselwirkung), die durch FoM = 10^(-4) charakterisiert ist. Moderne Halbkugelanalysatoren ermöglichen hingegen eine effiziente Vielkanaldetektion der spinintegralen Intensität mit mehr als 10^4 simultan erfassten Datenpunkten. Im Vergleich zwischen spinaufgelöster und spinintegraler Elektronenspektroskopie findet man somit einen Effizienzunterschied von 8 Größenordnungen.rnDie vorliegende Arbeit beschäftigt sich mit der Entwicklung und Untersuchung eines neuartigen Verfahrens zur Effizienzsteigerung in der spinaufgelösten Elektronenspektroskopie unter Ausnutzung von Vielkanaldetektion. Der Spindetektor wurde in eine µ-metallgeschirmte UHV-Kammer integriert und hinter einem konventionellen Halbkugelanalysator angebracht. Durch elektronenoptische Simulationen wurde die Geometrie des elektrostatischen Linsensystems festgelegt. Das Grundkonzept basiert auf der k(senkrecht)-erhaltenden elastischen Streuung des (0,0)-Spekularstrahls an einem W(100)- Streukristall unter einem Einfallswinkel von 45°. Es konnte gezeigt werden, dass etwarn960 Datenpunkte (15 Energie- und 64 Winkelpunkte) in einem Energiebereich vonrnetwa 3 eV simultan auf einen Delayline-Detektor abgebildet werden können. Dies führt zu einer zweidimensionalen Gütefunktion von FoM_2D = 1,7. Verglichen mit konventionellen Spindetektoren ist der neuartige Ansatz somit durch einen Effizienzgewinn von 4 Größenordnungen gekennzeichnet.rnDurch Messungen einer Fe/MgO(100) und O p(1x1)/Fe(100)-Probe konnte die Funktionstüchtigkeit des neuen Spinpolarimeters nachgewiesen werden, indem die aus der Literatur bekannten typischen UPS-Ergebnisse mit stark verkürzter Messzeit reproduziert wurden. Durch die hohe Effizienz ist es möglich, besonders reaktive Oberflächen in kurzer Zeit zu vermessen. Dieser Vorteil wurde bereits für eine erste grundlagenorientierte Anwendung genutzt: Als Test für die Gültigkeit von Bandstrukturrechnungen für die Heusler-Verbindung Co_2MnGa, wobei eine gute Übereinstimmung zwischen Theorie und Experiment gefunden wurde.rnMit dem Vielkanal-Spinfilter wurde die Grundlage zu einem um Größenordnungen verbesserten Messverfahren der Elektronenspin-Polarisation geschaffen und damit der Zugang zu Experimenten eröffnet, die mit den bisherigen einkanaligen Detektoren nicht möglich sind.rn

Spin-polarized transport in a lateral two-dimensional diluted magnetic semiconductor electron gas

The transport property of a lateral two-dimensional paramagnetic diluted magnetic semiconductor electron gas under a spatially periodic magnetic field is investigated theoretically. We find that the electron Fermi velocity along the modulation direction is highly spin dependent even if the spin polarization of the carrier population is negligibly small. It turns out that this spin-polarized Fermi velocity alone can lead to a strong spin polarization of the current, which is still robust against the energy broadening effect induced by the impurity scattering. (c) 2006 American Institute of Physics.

Quasiequilibrium optical nonlinearities from spin-polarized carriers in GaAs

Semiconductor Bloch equations, which microscopically describe the dynamics of a Coulomb interacting, spin-unpolarized electron-hole plasma, can be solved in two limits: the coherent and the quasiequilibrium regimes. These equations have been recently extended to include the spin degree of freedom and used to explain spin dynamics in the coherent regime. In the quasiequilibrium limit, one solves the Bethe-Salpeter equation in a two-band model to describe how optical absorption is affected by Coulomb interactions within a spin unpolarized plasma of arbitrary density. In this work, we modified the solution of the Bethe-Salpeter equation to include spin polarization and light holes in a three-band model, which allowed us to account for spin-polarized versions of many-body effects in absorption. The calculated absorption reproduced the spin-dependent, density-dependent, and spectral trends observed in bulk GaAs at room temperature, in a recent pump-probe experiment with circularly polarized light. Hence, our results may be useful in the microscopic modeling of density-dependent optical nonlinearities due to spin-polarized carriers in semiconductors.

Unconventional spin texture of a topologically nontrivial semimetal Sb(110)

9 p.

Temperature-dependent transport and spin accumulation in a quantum wire with Rashba spin-orbit interaction

We study the theory of temperature-dependent electron transport, spin polarization, and spin accumulation in a Rashba spin-orbit interaction (RSOI) quantum wire connected nonadiabatically to two normal conductor electrode leads. The influence of both the wire-lead connection and the RSOI on the electron transport is treated analytically by means of a scattering matrix technique and by using an effective free-electron approximation. Through analytical analysis and numerical examples, we demonstrate a simple way to design a sensitive spin-transfer switch that operates without applying any external magnetic fields or attaching ferromagnetic contacts. We also demonstrate that the antisymmetry of the spin accumulation can be destroyed slightly by the coupling between the leads and the wire. Moreover, temperature can weaken the polarization and smear out the oscillations in the spin accumulation.

Effect of electron-electron scattering on spin dephasing in a high-mobility low-density two-dimensional electron gas

By utilizing time-resolved Kerr rotation techniques, we have investigated the spin dynamics of a high-mobility low density two-dimensional electron gas in a GaAs/Al0.35Ga0.65As heterostructure in the dependence on temperature from 1.5 to 30 K. It is found that the spin relaxation/dephasing time under a magnetic field of 0.5 T exhibits a maximum of 3.12 ns around 14 K, which is superimposed on an increasing background with rising temperature. The appearance of the maximum is ascribed to that at the temperature where the crossover from the degenerate to the nondegenerate regime takes place, electron-electron Coulomb scattering becomes strongest, and thus inhomogeneous precession broadening due to the D'yakonov-Perel' mechanism becomes weakest. These results agree with the recent theoretical predictions [J. Zhou et al., Phys. Rev. B 15, 045305 (2007)], which verify the importance of electron-electron Coulomb scattering to electron spin relaxation/dephasing.

Spin dynamics of electrons in the first excited subband of a high-mobility low-density two-dimensional electron system

We report on time-resolved Kerr rotation measurements of spin coherence of electrons in the first excited subband of a high-mobility low-density two-dimensional electron system in a GaAs/Al0.35Ga0.65As heterostructure. While the transverse spin lifetime (T-2(*)) of electrons decreases monotonically with increasing magnetic field, it has a nonmonotonic dependence on the temperature and reaches a peak value of 596 ps at 36 K, indicating the effect of intersubband electron-electron scattering on the electron-spin relaxation.

Spin Relaxation of Electrons in Single InAs Quantum Dots

By using polarization-resolved photoluminescence spectra, we study the electron spin relaxation in single InAs quantum dots (QDs) with the configuration of positively charged excitons X+ (one electron, two holes). The spin relaxation rate of the hot electrons increases with the increasing energy of exciting photons. For electrons localized in QDs the spin relaxation is induced by hyperfine interaction with the nuclei. A rapid decrease of polarization degree with increasing temperature suggests that the spin relaxation mechanisms are mainly changed from the hyperfine interaction with nuclei into an electron-hole exchange interaction.