1,799 research outputs found
General theory of feedback control of a nuclear spin ensemble in quantum dots
We present a microscopic theory of the nonequilibrium nuclear spin dynamics
driven by the electron and/or hole under continuous wave pumping in a quantum
dot. We show the correlated dynamics of the nuclear spin ensemble and the
electron and/or hole under optical excitation as a quantum feedback loop and
investigate the dynamics of the many nuclear spins as a nonlinear collective
motion. This gives rise to three observable effects: (i) hysteresis, (ii)
locking (avoidance) of the pump absorption strength to (from) the natural
resonance, and (iii) suppression (amplification) of the fluctuation of weakly
polarized nuclear spins, leading to prolonged (shortened) electron spin
coherence time. A single nonlinear feedback function as a "measurement" of the
nuclear field operator in the quantum feedback loop is constructed which
determines the different outcomes of the three effects listed above depending
on the feedback being negative or positive. The general theory also helps to
put in perspective the wide range of existing theories on the problem of a
single electron spin in a nuclear spin bath.Comment: 20 pages, 7 figure
Coherent exciton transport in semiconductors
We review the topic of Bose-Einstein condensation of excitons in
semiconductors, focusing on the signatures of the macroscopic order of the
exciton condensate.Comment: Some references were updated with respect to the published version.
appears as Chapter 19 in Novel Superfluids Volume 2, edited by K. H.
Bennemann and J. B. Ketterson, International Series of Monographs on Physics
no. 157, pages 423-474 (Oxford University Press, Oxford, 2014
Spin extraction theory and its relevance to spintronics
Extraction of electrons from a semiconductor to a ferromagnet as well as the
case of injection in the reverse direction may be formulated as a scattering
theory. However, the presence of bound states at the interface arising out of
doping on the semiconductor side must be taken into account in the scattering
theory. Inclusion of the interface states yields an explanation of a recent
result of spin imaging measurement which contradicts the current understanding
of spin extraction. The importance of an extraction theory to spintronics is
illustrated by an application to a spin switch.Comment: 4 two column pages, 3 figures, major revisions to improve the
presentation. in addition, the new version includes an electrically
controlled spin switc
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