10 research outputs found
Low-temperature spin relaxation in n-type GaAs
Low-temperature electron spin relaxation is studied by the optical
orientation method in bulk n-GaAs with donor concentrations from 10^14 cm^{-3}
to 5x10^17 cm^{-3}.
A peculiarity related to the metal-to-insulator transition (MIT) is observed
in the dependence of the spin lifetime on doping near n_D = 2x10^16 cm^{-3}. In
the metallic phase, spin relaxation is governed by the Dyakonov-Perel
mechanism, while in the insulator phase it is due to anisotropic exchange
interaction and hyperfine interactio
Anisotropic exchange interaction of localized conduction-band electrons in semiconductor structures
The spin-orbit interaction in semiconductors is shown to result in an
anisotropic contribution into the exchange Hamiltonian of a pair of localized
conduction-band electrons. The anisotropic exchange interaction exists in
semiconductor structures which are not symmetric with respect to spatial
inversion, for instance in bulk zinc-blend semiconductors. The interaction has
both symmetric and antisymmetric parts with respect to permutation of spin
components. The antisymmetric (Dzyaloshinskii-Moriya) interaction is the
strongest one. It contributes significantly into spin relaxation of localized
electrons; in particular, it governs low-temperature spin relaxation in n-GaAs
with the donor concentration near 10^16cm-3. The interaction must be allowed
for in designing spintronic devices, especially spin-based quantum computers,
where it may be a major source of decoherence and errors
Encoded Universality for Generalized Anisotropic Exchange Hamiltonians
We derive an encoded universality representation for a generalized
anisotropic exchange Hamiltonian that contains cross-product terms in addition
to the usual two-particle exchange terms. The recently developed algebraic
approach is used to show that the minimal universality-generating encodings of
one logical qubit are based on three physical qubits. We show how to generate
both single- and two-qubit operations on the logical qubits, using suitably
timed conjugating operations derived from analysis of the commutator algebra.
The timing of the operations is seen to be crucial in allowing simplification
of the gate sequences for the generalized Hamiltonian to forms similar to that
derived previously for the symmetric (XY) anisotropic exchange Hamiltonian. The
total number of operations needed for a controlled-Z gate up to local
transformations is five. A scalable architecture is proposed.Comment: 11 pages, 4 figure
Spin relaxation and antisymmetric exchange in n-doped III-V semiconductor
Recently K. Kavokin [Phys. Rev. B 64, 075305 (2001)] suggested that the
Dzyaloshinskii-Moriya interaction between localized electrons governs slow spin
relaxation in -doped GaAs in the regime close to the metal-insulator
transition. We derive the correct spin Hamiltonian and apply it to the
determination of spin dephasing time using the method of moments expansion. We
argue that the proposed mechanism is insufficient to explain the observed
values of the spin relaxation time.Comment: 5 pages, 1 figure
Temperature dependence of polarization relaxation in semiconductor quantum dots
The decay time of the linear polarization degree of the luminescence in
strongly confined semiconductor quantum dots with asymmetrical shape is
calculated in the frame of second-order quasielastic interaction between
quantum dot charge carriers and LO phonons. The phonon bottleneck does not
prevent significantly the relaxation processes and the calculated decay times
can be of the order of a few tens picoseconds at temperature K,
consistent with recent experiments by Paillard et al. [Phys. Rev. Lett.
{\bf86}, 1634 (2001)].Comment: 4 pages, 4 figure
Electron spin relaxation by nuclei in semiconductor quantum dots
We have studied theoretically the electron spin relaxation in semiconductor
quantum dots via interaction with nuclear spins. The relaxation is shown to be
determined by three processes: (i) -- the precession of the electron spin in
the hyperfine field of the frozen fluctuation of the nuclear spins; (ii) -- the
precession of the nuclear spins in the hyperfine field of the electron; and
(iii) -- the precession of the nuclear spin in the dipole field of its nuclear
neighbors. In external magnetic fields the relaxation of electron spins
directed along the magnetic field is suppressed. Electron spins directed
transverse to the magnetic field relax completely in a time on the order of the
precession period of its spin in the field of the frozen fluctuation of the
nuclear spins. Comparison with experiment shows that the hyperfine interaction
with nuclei may be the dominant mechanism of electron spin relaxation in
quantum dots
Symmetry of anisotropic exchange interactions in semiconductor nanostructures
The symmetry of exchange interaction of charge carriers in semiconductor
nanostructures (quantum wells and quantum dots) is analysed. It is shown that
the exchange Hamiltonian of two particles belonging to the same energy band can
be universally expressed via pseudospin operators of the particles. The
relative strength of the anisotropic exchange interaction is shown to be
independent of the binding energy and the isotropic exchange constant
Spin-polarized current amplification and spin injection in magnetic bipolar transistors
The magnetic bipolar transistor (MBT) is a bipolar junction transistor with
an equilibrium and nonequilibrium spin (magnetization) in the emitter, base, or
collector. The low-injection theory of spin-polarized transport through MBTs
and of a more general case of an array of magnetic {\it p-n} junctions is
developed and illustrated on several important cases. Two main physical
phenomena are discussed: electrical spin injection and spin control of current
amplification (magnetoamplification). It is shown that a source spin can be
injected from the emitter to the collector. If the base of an MBT has an
equilibrium magnetization, the spin can be injected from the base to the
collector by intrinsic spin injection. The resulting spin accumulation in the
collector is proportional to , where is the proton
charge, is the bias in the emitter-base junction, and is the
thermal energy. To control the electrical current through MBTs both the
equilibrium and the nonequilibrium spin can be employed. The equilibrium spin
controls the magnitude of the equilibrium electron and hole densities, thereby
controlling the currents. Increasing the equilibrium spin polarization of the
base (emitter) increases (decreases) the current amplification. If there is a
nonequilibrium spin in the emitter, and the base or the emitter has an
equilibrium spin, a spin-valve effect can lead to a giant magnetoamplification
effect, where the current amplifications for the parallel and antiparallel
orientations of the the equilibrium and nonequilibrium spins differ
significantly. The theory is elucidated using qualitative analyses and is
illustrated on an MBT example with generic materials parameters.Comment: 14 PRB-style pages, 10 figure
Exciton Spin Dynamics in Semiconductor Quantum Wells
In this paper we will review Exciton Spin Dynamics in Semiconductor Quantum
Wells. The spin properties of excitons in nanostructures are determined by
their fine structure. We will mainly focus in this review on GaAs and InGaAs
quantum wells which are model systems.Comment: 55 pages, 27 figure