227 research outputs found
Accuracy of circular polarization as a measure of spin polarization in quantum dot qubits
A quantum dot spin LED provides a test of carrier spin injection into a
qubit, as well as a means of analyzing carrier spin injection in general and
local spin polarization. The polarization of the observed light is, however,
significantly influenced by the dot geometry so the spin may be more polarized
than the emitted light would naively suggest. We have calculated carrier
polarization-dependent optical matrix elements using 8-band strain-dependent
k.p theory for InAs/GaAs self-assembled quantum dots (SAQDs) for electron and
hole spin injection into a range of quantum dot sizes and shapes, and for
arbitrary emission directions. The observed circular polarization does not
depend on whether the injected spin-polarized carriers are electrons or holes,
but is strongly influenced by the SAQD geometry and emission direction.
Calculations for typical SAQD geometries with emission along [110] show light
that is only ~5% circularly polarized for spin states that are 100% polarized
along [110]. Therefore observed polarizations [Chye et al. PRB 66, 201301(R)]
of ~1% imply a spin polarization within the dot of ~20%. We also find that
measuring along the growth direction gives near unity conversion of spin to
photon polarization, and is the least sensitive to uncertainties in SAQD
geometry.Comment: 4 pages, 6 figure
Strong spin relaxation length dependence on electric field gradients
We discuss the influence of electrical effects on spin transport, and in
particular the propagation and relaxation of spin polarized electrons in the
presence of inhomogeneous electric fields. We show that the spin relaxation
length strongly depends on electric field gradients, and that significant
suppression of electron spin polarization can occur as a result thereof. A
discussion in terms of a drift-diffusion picture, and self-consistent numerical
calculations based on a Boltzmann-Poisson approach shows that the spin
relaxation length in fact can be of the order of the charge screening length.Comment: 4 pages, 3 figures, to be presented at PASPSI
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