81 research outputs found
Cubic Dresselhaus Spin-Orbit Coupling in 2D Electron Quantum Dots
We study effects of the oft-neglected cubic Dresselhaus spin-orbit coupling
(i.e., ) in GaAs/AlGaAs quantum dots. Using a semiclassical
billiard model, we estimate the magnitude of the spin-orbit induced avoided
crossings in a closed quantum dot in a Zeeman field. Using these results,
together with previous analyses based on random matrix theory, we calculate
corresponding effects on the conductance through an open quantum dot. Combining
our results with an experiment on conductance through an 8 um^2 quantum dot [D
M Zumbuhl et al., Phys. Rev. B 72, 081305 (2005)] suggests that 1) the GaAs
Dresselhaus coupling constant, , is approximately 9 eVA^3,
significantly less than the commonly cited value of 27.5 eVA^3 and 2) the
majority of the spin-flip component of spin-orbit coupling can come from the
cubic Dresselhaus term.Comment: 4 pages plus supplementary tabl
Inhomogeneous Nuclear Spin Flips
We discuss a feedback mechanism between electronic states in a double quantum
dot and the underlying nuclear spin bath. We analyze two pumping cycles for
which this feedback provides a force for the Overhauser fields of the two dots
to either equilibrate or diverge. Which of these effects is favored depends on
the g-factor and Overhauser coupling constant A of the material. The strength
of the effect increases with A/V_x, where V_x is the exchange matrix element,
and also increases as the external magnetic field B_{ext} decreases.Comment: 5 pages, 4 figures (jpg
Scaling and localization lengths of a topologically disordered system
We consider a noninteracting disordered system designed to model particle
diffusion, relaxation in glasses, and impurity bands of semiconductors.
Disorder originates in the random spatial distribution of sites. We find strong
numerical evidence that this model displays the same universal behavior as the
standard Anderson model. We use finite-size-scaling to find the localization
length as a function of energy and density, including localized states away
from the delocalization transition. Results at many energies all fit onto the
same universal scaling curve.Comment: 5+ page
Nonradiative lifetimes in intermediate band materials - absence of lifetime recovery
Intermediate band photovoltaics hold the promise of being highly efficient
and cost effective photovoltaic cells. Intermediate states in the band gap,
however, are known to facilitate nonradiative recombination. Much effort has
been dedicated to producing metallic intermediate bands in hopes of producing
lifetime recovery -- an increase in carrier lifetime as doping levels increase.
We show that lifetime recovery induced by the insulator-to-metal transition
will not occur, because the metallic extended states will be localised by
phonons during the recombination process. Only trivial forms of lifetime
recovery, e.g., from an overall shift in intermediate levels, are possible.
Future work in intermediate band photovoltaics must focus on optimizing subgap
optical absorption and minimizing recombination, but not via lifetime recovery.Comment: 8 page
Emergent percolation length and localization in random elastic networks
We study, theoretically and numerically, a minimal model for phonons in a
disordered system. For sufficient disorder, the vibrational modes of this
classical system can become Anderson localized, yet this problem has received
significantly less attention than its electronic counterpart. We find rich
behavior in the localization properties of the phonons as a function of the
density, frequency and the spatial dimension. We use a percolation analysis to
argue for a Debye spectrum at low frequencies for dimensions higher than one,
and for a localization/delocalization transition (at a critical frequency)
above two dimensions. We show that in contrast to the behavior in electronic
systems, the transition exists for arbitrarily large disorder, albeit with an
exponentially small critical frequency. The structure of the modes reflects a
divergent percolation length that arises from the disorder in the springs
without being explicitly present in the definition of our model. Within the
percolation approach we calculate the speed-of-sound of the delocalized modes
(phonons), which we corroborate with numerics. We find the critical frequency
of the localization transition at a given density, and find good agreement of
these predictions with numerical results using a recursive Green function
method adapted for this problem. The connection of our results to recent
experiments on amorphous solids are discussed.Comment: accepted to PR
A witness for coherent electronic oscillations in ultrafast spectroscopy
We report a conceptually straightforward witness that isolates coherent
electronic oscillations from their vibronic counterparts in nonlinear optical
spectra of molecular aggregates: Coherent oscillations as a function of waiting
time in broadband pump/broadband probe spectra correspond to coherent
electronic oscillations. Oscillations in individual peaks of 2D electronic
spectra do not necessarily yield this conclusion. Our witness is simpler to
implement than quantum process tomography and potentially resolves a
long-standing controversy on the character of oscillations in ultrafast spectra
of photosynthetic light harvesting systems.Comment: 4 pages, 2 figures, plus Supplementary Information. Work presented by
the first author on March 1, 2012 at APS, Boston, Session W41, Focus Session
on Quantum Coherence in Biological System
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