Cubic Dresselhaus Spin-Orbit Coupling in 2D Electron Quantum Dots

We study effects of the oft-neglected cubic Dresselhaus spin-orbit coupling (i.e., $\propto p^3$) 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, $\gamma$, 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