7,954 research outputs found
Theoretical studies of spin-dependent electronic transport in ferromagnetically contacted graphene flakes
Based on a tight-binding model and a recursive Green's function technique,
spin-depentent ballistic transport through tinny graphene sheets (flakes) is
studied. The main interest is focussed on: electrical conductivity, giant
magnetoresistance (GMR) and shot noise. It is shown that when graphene flakes
are sandwiched between two ferromagnetic electrodes, the resulting GMR
coefficient may be quite significant. This statement holds true both for zigzag
and armchair chiralities, as well as for different aspect (width/length)
ratios. Remarkably, in absolute values the GMR of the armchair-edge graphene
flakes is systematically greater than that corresponding to the zigzag-edge
graphene flakes. This finding is attributed to the different degree of
conduction channel mixing for the two chiralities in question. It is also shown
that for big aspect ratio flakes, 3-dimensional end-contacted leads, very much
like invasive contacts, result in non-universal behavior of both conductivity
and Fano factor.Comment: to appear in PR
Inverted polymer fullerene solar cells exceeding 10% efficiency with poly(2-ethyl-2-oxazoline) nanodots on electron-collecting buffer layers
Polymer solar cells have been spotlighted due to their potential for low-cost manufacturing but their efficiency is still less than required for commercial application as lightweight/flexible modules. Forming a dipole layer at the electron-collecting interface has been suggested as one of the more attractive approaches for efficiency enhancement. However, only a few dipole layer material types have been reported so far, including only one non-ionic (charge neutral) polymer. Here we show that a further neutral polymer, namely poly(2-ethyl-2-oxazoline) (PEOz) can be successfully used as a dipole layer. Inclusion of a PEOz layer, in particular with a nanodot morphology, increases the effective work function at the electron-collecting interface within inverted solar cells and thermal annealing of PEOz layer leads to a state-of-the-art 10.74% efficiency for single-stack bulk heterojunction blend structures comprising poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-alt-3-fluorothieno[3,4-b]thiophene-2-carboxylate] as donor and [6,6]-phenyl-C71-butyric acid methyl ester as acceptor
Extraction of higher-order nonlinear electronic response to strong field excitation in solids using high harmonic generation
State-of-the-art experiments employ strong ultrafast optical fields to study
the nonlinear response of electrons in solids on an attosecond time-scale.
Notably, a recent experiment retrieved a 3rd order nonlinear susceptibility by
comparing the nonlinear response induced by a strong laser field to a linear
response induced by the otherwise identical weak field. In parallel,
experiments have demonstrated high harmonic generation (HHG) in solids, a
highly nonlinear process that until recently had only been observed in gases.
The highly nonlinear nature of HHG has the potential to extract even higher
order nonlinear susceptibility terms, and thereby characterize the entire
response of the electronic system to strong field excitation. However, up till
now, such characterization has been elusive due to a lack of direct
correspondence between high harmonics and nonlinear susceptibilities. Here, we
demonstrate a regime where such correspondence can be clearly made, extracting
nonlinear susceptibilities (7th, 9th, and 11th) from sapphire of the same order
as the measured high harmonics. The extracted high order susceptibilities show
angular-resolved periodicities arising from variation in the band structure
with crystal orientation. Nonlinear susceptibilities are key to ultrafast
lightwave driven optoelectronics, allowing petahertz scaling manipulation of
the signal. Our results open a door to multi-channel signal processing,
controlled by laser polarization
Forecasting with an adaptive control algorithm
We construct a parsimonious model of the U.S. macro economy using a state space representation and recursive estimation. At the core of the estimation procedure is a prediction/correction algorithm based on a recursive least squares estimation with exponential forgetting. The algorithm is a Kalman filter-type update method which minimizes the sum of discounted squared errors. This method reduces the contribution of past errors in the estimate of the current period coefficients and thereby adapts to potential time variation of parameters. The root mean square errors of out-of-sample forecast of the model show improvement over OLS forecasts. One period ahead in-sample forecasts showed better tracking than OLS in-sample forecasts.Forecasting
Single-photon detection timing jitter in a visible light photon counter
Visible light photon counters (VLPCs) offer many attractive features as
photon detectors, such as high quantum efficiency and photon number resolution.
We report measurements of the single-photon timing jitter in a VLPC, a critical
performance factor in a time-correlated single-photon counting measurement, in
a fiber-coupled closed-cycle cryocooler. The measured timing jitter is 240 ps
full-width-at-half-maximum at a wavelength of 550 nm, with a dark count rate of
25 000 counts per second. The timing jitter increases modestly at longer
wavelengths to 300 ps at 1000 nm, and increases substantially at lower bias
voltages as the quantum efficiency is reduced
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