45 research outputs found
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
Interpretations of High-Order Transient Absorption Spectroscopies
Transient absorption (TA) spectroscopy has long been an invaluable tool for
determining the energetics and dynamics of excited states in atomic, molecular,
and solid-state systems. When pump pulse intensities are sufficiently high, the
resulting TA spectra include both the generally desired third-order response of
the studied material as well as responses that are higher order in the electric
field amplitudes of the pulses. It has recently been shown that
pump-intensity-dependent TA measurements allow separating the various orders of
response of the TA signal, but the information content available in those
higher orders has not been described. We give a general framework, intuition,
and nomenclature for understanding the information contained in high-order TA
spectra. Standard TA spectra are generally interpreted in terms of three
fundamental processes: ground-state bleach (GSB), stimulated emission (SE), and
excited state absorption (ESA), and we extend those concepts to higher order.
Each order introduces two new processes: SE and ESA from highly excited states
that were not accessible in lower orders. In addition, each order contain
negations of lower-order processes, just as GSB is a negation of the linear
absorption. We show the new spectral and dynamical information that is
introduced at each order and show how the relative signs of the signals in
different orders can be used to identify which processes are dominant.Comment: 15 pages, 7 figure
Estimating causal networks in biosphere–atmosphere interaction with the PCMCI approach
Local meteorological conditions and biospheric activity are tightly coupled. Understanding these links is an essential prerequisite for predicting the Earth system under climate change conditions. However, many empirical studies on the interaction between the biosphere and the atmosphere are based on correlative approaches that are not able to deduce causal paths, and only very few studies apply causal discovery methods. Here, we use a recently proposed causal graph discovery algorithm, which aims to reconstruct the causal dependency structure underlying a set of time series. We explore the potential of this method to infer temporal dependencies in biosphere-atmosphere interactions. Specifically we address the following questions: How do periodicity and heteroscedasticity influence causal detection rates, i.e. the detection of existing and non-existing links? How consistent are results for noise-contaminated data? Do results exhibit an increased information content that justifies the use of this causal-inference method? We explore the first question using artificial time series with well known dependencies that mimic real-world biosphere-atmosphere interactions. The two remaining questions are addressed jointly in two case studies utilizing observational data. Firstly, we analyse three replicated eddy covariance datasets from a Mediterranean ecosystem at half hourly time resolution allowing us to understand the impact of measurement uncertainties. Secondly, we analyse global NDVI time series (GIMMS 3g) along with gridded climate data to study large-scale climatic drivers of vegetation greenness. Overall, the results confirm the capacity of the causal discovery method to extract time-lagged linear dependencies under realistic settings. The violation of the method's assumptions increases the likelihood to detect false links. Nevertheless, we consistently identify interaction patterns in observational data. Our findings suggest that estimating a directed biosphere-atmosphere network at the ecosystem level can offer novel possibilities to unravel complex multi-directional interactions. Other than classical correlative approaches, our findings are constrained to a few meaningful set of relations which can be powerful insights for the evaluation of terrestrial ecosystem models
Dynamic Nuclear Polarization in Double Quantum Dots
We theoretically investigate the controlled dynamic polarization of lattice
nuclear spins in GaAs double quantum dots containing two electrons. Three
regimes of long-term dynamics are identified, including the build up of a large
difference in the Overhauser fields across the dots, the saturation of the
nuclear polarization process associated with formation of so-called "dark
states," and the elimination of the difference field. We show that in the case
of unequal dots, build up of difference fields generally accompanies the
nuclear polarization process, whereas for nearly identical dots, build up of
difference fields competes with polarization saturation in dark states. The
elimination of the difference field does not, in general, correspond to a
stable steady state of the polarization process.Comment: 4 pages, 2 figure
Methodology for vetting heavily doped semiconductors for intermediate band photovoltaics: A case study in sulfur-hyperdoped silicon
We present a methodology for estimating the efficiency potential for candidate impurity-band photovoltaic materials from empirical measurements. This methodology employs both Fourier transform infrared spectroscopy and low-temperature photoconductivity to calculate a “performance figure of merit” and to determine both the position and bandwidth of the impurity band. We evaluate a candidate impurity-band material, silicon hyperdoped with sulfur; we find that the figure of merit is more than one order of magnitude too low for photovoltaic devices that exceed the thermodynamic efficiency limit for single band gap materials.National Science Foundation (U.S.) (Energy, Power, and Adaptive Systems Grant Contract ECCS-1102050)National Science Foundation (U.S.) (United States. Dept. of Energy NSF CA EEC-1041895)Center for Clean Water and Clean Energy at MIT and KFUP
Spin polarized current generation from quantum dots without magnetic fields
An unpolarized charge current passing through a chaotic quantum dot with
spin-orbit coupling can produce a spin polarized exit current without magnetic
fields or ferromagnets. We use random matrix theory to estimate the typical
spin polarization as a function of the number of channels in each lead in the
limit of large spin-orbit coupling. We find rms spin polarizations up to 45%
with one input channel and two output channels. Finite temperature and
dephasing both suppress the effect, and we include dephasing effects using a
new variation of the third lead model. If there is only one channel in the
output lead, no spin polarization can be produced, but we show that dephasing
lifts this restriction.Comment: 7 pages, 3 figures. Revised version with reference to Bardarson,
Adagideli, and Jacquod, Phys Rev Lett 98, 196601 (2007), who performed an
analogous calculation in a different geometr
Coherent Exciton Dynamics in Supramolecular Light-Harvesting Nanotubes Revealed by Ultrafast Quantum Process Tomography
Long-lived exciton coherences have been recently observed in photosynthetic complexes via ultrafast spectroscopy, opening exciting possibilities for the study and design of coherent exciton transport. Yet, ambiguity in the spectroscopic signals has led to arguments for interpreting them in terms of the exciton dynamics, demanding more stringent tests. We propose a novel strategy, Quantum Process Tomography (QPT) for ultrafast spectroscopy, to reconstruct the evolving quantum state of excitons in double-walled supramolecular light-harvesting nanotubes at room temperature. The protocol calls for eight transient grating experiments with varied pulse spectra. Our analysis reveals unidirectional energy transfer from the outer to the inner wall excitons, absence of nonsecular processes, and an unexpected coherence between those two states lasting about 150 femtoseconds, indicating weak electronic coupling between the walls. Our work constitutes the first experimental QPT in a “warm” and complex system, and provides an elegant scheme to maximize information from ultrafast spectroscopy experiments.Chemistry and Chemical BiologyPhysic