240 research outputs found
FX technical trading rules can be profitable sometimes!
This paper investigates the profitability of technical trading rules in the foreign exchange market taking into account data snooping bias and transaction costs. A universe of 7650 trading rules is applied to six currencies quoted in U.S. dollars over the 1994:3?2014:12 period. The Barras, Scaillet, and Wermers (2010) false discovery rate method is employed to deal with data snooping and it detects almost all outperforming trading rules while keeping the proportion of false discoveries to a pre-specified level. The out-of-sample results reveal a large number of outperforming rules that are profitable over short periods based on the Sharpe ratio. However, they are not consistently profitable and so the overall results are more consistent with the adaptive markets hypothesis
36 degree step size of proton-driven c-ring rotation in FoF1-ATP synthase
Synthesis of the biological "energy currency molecule" adenosine triphosphate
ATP is accomplished by FoF1-ATP synthase. In the plasma membrane of Escherichia
coli, proton-driven rotation of a ring of 10 c subunits in the Fo motor powers
catalysis in the F1 motor. While F1 uses 120 degree stepping, Fo models predict
a step-by-step rotation of c subunits 36 degree at a time, which is here
demonstrated by single-molecule fluorescence resonance energy transfer.Comment: 8 pages, 1 figur
Direct observation of trap-assisted recombination in organic photovoltaic devices
Trap-assisted recombination caused by localised sub-gap states is one of the most important first-order loss mechanism limiting the power-conversion efficiency of all solar cells. The presence and relevance of trap-assisted recombination in organic photovoltaic devices is still a matter of some considerable ambiguity and debate, hindering the field as it seeks to deliver ever higher efficiencies and ultimately a viable new solar photovoltaic technology. In this work, we show that trap-assisted recombination loss of photocurrent is universally present under operational conditions in a wide variety of organic solar cell materials including the new non-fullerene electron acceptor systems currently breaking all efficiency records. The trap-assisted recombination is found to be induced by states lying 0.35-0.6 eV below the transport edge, acting as deep trap states at light intensities equivalent to 1 sun. Apart from limiting the photocurrent, we show that the associated trap-assisted recombination via these comparatively deep traps is also responsible for ideality factors between 1 and 2, shedding further light on another open and important question as to the fundamental working principles of organic solar cells. Our results also provide insights for avoiding trap-induced losses in related indoor photovoltaic and photodetector applications
A universal Urbach rule for disordered organic semiconductors
In crystalline semiconductors, absorption onset sharpness is characterized by
temperature dependent Urbach energies. These energies quantify the static,
structural disorder causing localized exponential-tail states, and dynamic
disorder from electron-phonon scattering. Applicability of this
exponential-tail model to disordered solids has been long debated. Nonetheless,
exponential fittings are routinely applied to sub-gap absorption analysis of
organic semiconductors. Herein, we elucidate the sub-gap spectral line-shapes
of organic semiconductors and their blends by temperature-dependent quantum
efficiency measurements. We find that sub-gap absorption due to singlet
excitons is universally dominated by thermal broadening at low photon energies
and the associated Urbach energy equals the thermal energy, regardless of
static disorder. This is consistent with absorptions obtained from a
convolution of Gaussian density of excitonic states weighted by Boltzmann-like
thermally activated optical transitions. A simple model is presented that
explains absorption line-shapes of disordered systems, and we also provide a
strategy to determine the excitonic disorder energy. Our findings elaborate the
meaning of the Urbach energy in molecular solids and relate the photo-physics
to static disorder, crucial for optimizing organic solar cells for which we
present a new radiative open-circuit voltage limit
Detecting substeps in the rotary motors of FoF1-ATP synthase by Hidden Markov Models
FoF1-ATP synthase is the enzyme that provides the 'chemical energy currency'
adenosine triphosphate, ATP, for living cells. The formation of ATP is
accomplished by a stepwise internal rotation of subunits within the enzyme. We
monitor subunit rotation by a single-molecule fluorescence resonance energy
transfer (FRET) approach using two fluorophores specifically attached to the
enzyme. To identify the stepsize of rotary movements by the motors of ATP
synthase we simulated the confocal single-molecule FRET data of freely
diffusing enzymes and developed a step finder algorithm based on 'Hidden Markov
Models' (HMM). The HMM is able to find the proximity factors, P, for a
three-level system and for a five-level system, and to unravel the dwell times
of the simulated rotary movements. To identify the number of hidden states in
the system, a likelihood parameter is calculated for the series of one-state to
eight-state HMMs applied to each set of simulated data. Thereby, the basic
prerequisites for the experimental single-molecule FRET data are defined that
allow for discrimination between a 120 degree stepping mode or a 36 degree
substep rotation mode for the proton-driven Fo motor of ATP synthase.Comment: 12 pages, 5 figure
Online unit clustering in higher dimensions
We revisit the online Unit Clustering and Unit Covering problems in higher
dimensions: Given a set of points in a metric space, that arrive one by
one, Unit Clustering asks to partition the points into the minimum number of
clusters (subsets) of diameter at most one; while Unit Covering asks to cover
all points by the minimum number of balls of unit radius. In this paper, we
work in using the norm.
We show that the competitive ratio of any online algorithm (deterministic or
randomized) for Unit Clustering must depend on the dimension . We also give
a randomized online algorithm with competitive ratio for Unit
Clustering}of integer points (i.e., points in , , under norm). We show that the competitive ratio of
any deterministic online algorithm for Unit Covering is at least . This
ratio is the best possible, as it can be attained by a simple deterministic
algorithm that assigns points to a predefined set of unit cubes. We complement
these results with some additional lower bounds for related problems in higher
dimensions.Comment: 15 pages, 4 figures. A preliminary version appeared in the
Proceedings of the 15th Workshop on Approximation and Online Algorithms (WAOA
2017
Quantifying the Excitonic Static Disorder in Organic Semiconductors
Organic semiconductors are disordered molecular solids, and as a result, their internal charge generation dynamics, charge transport dynamics, and ultimately, the performance of the optoelectronic devices they constitute, are governed by energetic disorder. This is particularly pertinent for emerging photovoltaic technology where the extractable power is directly dependent on these dynamics. To ascertain how energetic disorder impacts charge generation, exciton transport, charge transport, and the performance of organic semiconductor devices, an accurate approach is first required to measure this critical parameter. In this work, it is shown that the static disorder of an organic semiconductor can be obtained from its photovoltaic external quantum efficiency spectrum at wavelengths near the absorption onset. A detailed methodology is presented, alongside a computational framework, for quantifying the static energetic disorder associated with singlet excitons. Moreover, the authors show that minimizing the limiting effects of optical interference is crucial for achieving high-accuracy quantifications. Finally, transparent devices are employed to estimate the excitonic static disorder in several technologically relevant organic semiconductor donor–acceptor blends, including the high-efficiency organic photovoltaic system PM6:Y6
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