116 research outputs found
An efficient tool to calculate two-dimensional optical spectra for photoactive molecular complexes
We combine the coherent modified Redfield theory (CMRT) with the equation of
motion-phase matching approach (PMA) to calculate two-dimensional photon echo
spectra for photoactive molecular complexes with an intermediate strength of
the coupling to their environment. Both techniques are highly efficient, yet
they involve approximations at different levels. By explicitly comparing with
the numerically exact quasi-adiabatic path integral approach, we show for the
Fenna-Matthews-Olson complex that the CMRT describes the decay rates in the
population dynamics well, but final stationary populations and the oscillation
frequencies differ slightly. In addition, we use the combined CMRT+PMA to
calculate two-dimensional photon-echo spectra for a simple dimer model. We find
excellent agreement with the exact path integral calculations at short waiting
times where the dynamics is still coherent. For long waiting times, differences
occur due to different final stationary states, specifically for strong
system-bath coupling. For weak to intermediate system-bath couplings, which is
most important for natural photosynthetic complexes, the combined CMRT+PMA
gives reasonable results with acceptable computational efforts
Nonequilibrium Landau-Zener-Stuckelberg spectroscopy in a double quantum dot
We study theoretically nonequilibrium Landau-Zener-St\"uckelberg (LZS)
dynamics in a driven double quantum dot (DQD) including dephasing and,
importantly, energy relaxation due to environmental fluctuations. We derive
effective nonequilibrium Bloch equations. These allow us to identify clear
signatures for LZS oscilations observed but not recognized as such in
experiments [Petersson et al., Phys. Rev. Lett. 105, 246804, 2010] and to
identify the full environmental fluctuation spectra acting on a DQD given
experimental data as in [Petersson et al., Phys. Rev. Lett. 105, 246804, 2010].
Herein we find that super-Ohmic fluctuations, typically due to phonons, are the
main relaxation channel for a detuned DQD whereas Ohmic fluctuations dominate
at zero detuning.Comment: 5 pages, 4 figure
Elastic response of [111]-tunneling impurities
We study the dynamic response of a [111] quantum impurity, such as lithium or
cyanide in alkali halides, with respect to an external field coupling to the
elastic quadrupole moment. Because of the particular level structure of a
eight-state system on a cubic site, the elastic response function shows a
biexponential relaxation feature and a van Vleck type contribution with a
resonance frequency that is twice the tunnel frequency . This
basically differs from the dielectric response that does not show relaxation.
Moreover, we show that the elastic response of a [111] impurity cannot be
reduced to that of a two-level system. In the experimental part, we report on
recent sound velocity and internal friction measurements on KCl doped with
cyanide at various concentrations. At low doping (45 ppm) we find the dynamics
of a single [111] impurity, whereas at higher concentrations (4700 ppm) the
elastic response rather indicates strongly correlated defects. Our theoretical
model provides a good description of the temperature dependence of
and at low doping, in particular the relaxation peaks, the absolute
values of the amplitude, and the resonant contributions. From our fits we
obtain the value of the elastic deformation potential eV.Comment: 19 pages, 5 figure
In Vivo Time- Resolved Microtomography Reveals the Mechanics of the Blowfly Flight Motor
Dipteran flies are amongst the smallest and most agile of flying animals. Their wings are driven indirectly by large power muscles, which cause cyclical deformations of the thorax that are amplified through the intricate wing hinge. Asymmetric flight manoeuvres are controlled by 13 pairs of steering muscles acting directly on the wing articulations. Collectively the steering muscles account for <3% of total flight muscle mass, raising the question of how they can modulate the vastly greater output of the power muscles during manoeuvres. Here we present the results of a synchrotron-based study performing micrometre-resolution, time-resolved microtomography on the 145 Hz wingbeat of blowflies. These data represent the first four-dimensional visualizations of an organism's internal movements on sub-millisecond and micrometre scales. This technique allows us to visualize and measure the three-dimensional movements of five of the largest steering muscles, and to place these in the context of the deforming thoracic mechanism that the muscles actuate. Our visualizations show that the steering muscles operate through a diverse range of nonlinear mechanisms, revealing several unexpected features that could not have been identified using any other technique. The tendons of some steering muscles buckle on every wingbeat to accommodate high amplitude movements of the wing hinge. Other steering muscles absorb kinetic energy from an oscillating control linkage, which rotates at low wingbeat amplitude but translates at high wingbeat amplitude. Kinetic energy is distributed differently in these two modes of oscillation, which may play a role in asymmetric power management during flight control. Structural flexibility is known to be important to the aerodynamic efficiency of insect wings, and to the function of their indirect power muscles. We show that it is integral also to the operation of the steering muscles, and so to the functional flexibility of the insect flight motor
Modeling visual-based pitch, lift and speed control strategies in hoverflies
<div><p>To avoid crashing onto the floor, a free falling fly needs to trigger its wingbeats quickly and control the orientation of its thrust accurately and swiftly to stabilize its pitch and hence its speed. Behavioural data have suggested that the vertical optic flow produced by the fall and crossing the visual field plays a key role in this anti-crash response. Free fall behavior analyses have also suggested that flying insect may not rely on graviception to stabilize their flight. Based on these two assumptions, we have developed a model which accounts for hoverflies´ position and pitch orientation recorded in 3D with a fast stereo camera during experimental free falls. Our dynamic model shows that optic flow-based control combined with closed-loop control of the pitch suffice to stabilize the flight properly. In addition, our model sheds a new light on the visual-based feedback control of fly´s pitch, lift and thrust. Since graviceptive cues are possibly not used by flying insects, the use of a vertical reference to control the pitch is discussed, based on the results obtained on a complete dynamic model of a virtual fly falling in a textured corridor. This model would provide a useful tool for understanding more clearly how insects may or not estimate their absolute attitude.</p></div
Functional Subsystems and Quantum Redundancy in Photosynthetic Light Harvesting
The Fenna-Matthews-Olson (FMO) antennae complex, responsible for light
harvesting in green sulfur bacteria, consists of three monomers, each with
seven chromophores. Here we show that multiple subsystems of the seven
chromophores can transfer energy from either chromophore 1 or 6 to the reaction
center with an efficiency matching or in many cases exceeding that of the full
seven chromophore system. In the FMO complex these functional subsystems
support multiple quantum pathways for efficient energy transfer that provide a
built-in quantum redundancy. There are many instances of redundancy in nature,
providing reliability and protection, and in photosynthetic light harvesting
this quantum redundancy provides protection against the temporary or permanent
loss of one or more chromophores. The complete characterization of functional
subsystems within the FMO complex offers a detailed map of the energy flow
within the FMO complex, which has potential applications to the design of more
efficient photovoltaic devices
HACE1 deficiency causes an autosomal recessive neurodevelopmental syndrome
Background: The genetic etiology of neurodevelopmental defects is extremely diverse, and the lack of distinctive phenotypic features means that genetic criteria are often required for accurate diagnostic classification. We aimed to identify the causative genetic lesions in two families in which eight affected individuals displayed variable learning disability, spasticity and abnormal gait. Methods: Autosomal recessive inheritance was suggested by consanguinity in one family and by sibling recurrences with normal parents in the second. Autozygosity mapping and exome sequencing, respectively, were used to identify the causative gene. Results: In both families, biallelic loss-of-function mutations in HACE1 were identified. HACE1 is an E3 ubiquitin ligase that regulates the activity of cellular GTPases, including Rac1 and members of the Rab family. In the consanguineous family, a homozygous mutation p.R219* predicted a truncated protein entirely lacking its catalytic domain. In the other family, compound heterozygosity for nonsense mutation p.R748* and a 20-nt insertion interrupting the catalytic HECT domain was present; Western analysis of patient cells revealed an absence of detectable HACE1 protein. Conclusion: HACE1 mutations underlie a new autosomal recessive neurodevelopmental disorder. Previous studies have implicated HACE1 as a tumour suppressor gene; however, since cancer predisposition was not observed either in homozygous or heterozygous mutation carriers, this concept may require re-evaluation
Origin of Long Lived Coherences in Light-Harvesting Complexes
A vibronic exciton model is developed to investigate the origin of long lived
coherences in light-harvesting complexes. Using experimentally determined
parameters and uncorrelated site energy fluctuations, the model predicts
oscillations in the nonlinear spectra of the Fenna-Matthews-Olson (FMO) complex
with a dephasing time of 1.3 ps at 77 K. These oscillations correspond to the
coherent superposition of vibronic exciton states with dominant contributions
from vibrational excitations on the same pigment. Purely electronic coherences
are found to decay on a 200 fs timescale.Comment: 4 pages, 2 figure
Relating Neuronal to Behavioral Performance: Variability of Optomotor Responses in the Blowfly
Behavioral responses of an animal vary even when they are elicited by the same stimulus. This variability is due to stochastic processes within the nervous system and to the changing internal states of the animal. To what extent does the variability of neuronal responses account for the overall variability at the behavioral level? To address this question we evaluate the neuronal variability at the output stage of the blowfly's (Calliphora vicina) visual system by recording from motion-sensitive interneurons mediating head optomotor responses. By means of a simple modelling approach representing the sensory-motor transformation, we predict head movements on the basis of the recorded responses of motion-sensitive neurons and compare the variability of the predicted head movements with that of the observed ones. Large gain changes of optomotor head movements have previously been shown to go along with changes in the animals' activity state. Our modelling approach substantiates that these gain changes are imposed downstream of the motion-sensitive neurons of the visual system. Moreover, since predicted head movements are clearly more reliable than those actually observed, we conclude that substantial variability is introduced downstream of the visual system
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