605 research outputs found
General no-go condition for stochastic pumping
The control of chemical dynamics requires understanding the effect of
time-dependent transition rates between states of chemo-mechanical molecular
configurations. Pumping refers to generating a net current, e.g. per period in
the time-dependence, through a cycle of consecutive states. The working of
artificial machines or synthesized molecular motors depends on it. In this
paper we give short and simple proofs of no-go theorems, some of which appeared
before but here with essential extensions to non-Markovian dynamics, including
the study of the diffusion limit. It allows to exclude certain protocols in the
working of chemical motors where only the depth of the energy well is changed
in time and not the barrier height between pairs of states. We also show how
pre-existing steady state currents are in general modified with a
multiplicative factor when this time-dependence is turned on.Comment: 8 pages; v2: minor changes, 1 reference adde
Towards absolute calibration of optical tweezers
Aiming at absolute force calibration of optical tweezers, following a
critical review of proposed theoretical models, we present and test the results
of MDSA (Mie-Debye-Spherical Aberration) theory, an extension of a previous
(MD) model, taking account of spherical aberration at the glass/water
interface. This first-principles theory is formulated entirely in terms of
experimentally accessible parameters (none adjustable). Careful experimental
tests of the MDSA theory, undertaken at two laboratories, with very different
setups, are described. A detailed description is given of the procedures
employed to measure laser beam waist, local beam power at the transparent
microspheres trapped by the tweezers, microsphere radius and the trap
transverse stiffness, as a function of radius and height in the (inverted
microscope) sample chamber. We find generally very good agreement with MDSA
theory predictions, for a wide size range, from the Rayleigh domain to large
radii, including the values most often employed in practice, and at different
chamber heights, both with objective overfilling and underfilling. The results
asymptotically approach geometrical optics in the mean over size intervals, as
they should, and this already happens for size parameters not much larger than
unity. MDSA predictions for the trapping threshold, position of stiffness peak,
stiffness variation with height, multiple equilibrium points and `hopping'
effects among them are verified. Remaining discrepancies are ascribed to focus
degradation, possibly arising from objective aberrations in the infrared, not
yet included in MDSA theory.Comment: 15 pages, 20 figure
Molecular motor traffic in a half-open tube
The traffic of molecular motors which interact through mutual exclusion is
studied theoretically for half-open tube-like compartments. These half-open
tubes mimic the shapes of axons. The mutual exclusion leads to traffic jams or
density plateaus on the filaments. A phase transition is obtained when the
motor velocity changes sign. We identify the relevant length scales and
characterize the jamming behavior using both analytical approximations and
Monte Carlo simulations of lattice models.Comment: 14 pages, 5 postscript figure
Interaction of molecular motors can enhance their efficiency
Particles moving in oscillating potential with broken mirror symmetry are
considered. We calculate their energetic efficiency, when acting as molecular
motors carrying a load against external force. It is shown that interaction
between particles enhances the efficiency in wide range of parameters. Possible
consequences for artificial molecular motors are discussed.Comment: 6 pages, 8 figure
Control of fine-structure splitting and excitonic binding energies in selected individual InAs/GaAs quantum dots
A systematic study of the impact of annealing on the electronic properties of
single InAs/GaAs quantum dots (QDs) is presented. Single QD cathodoluminescence
spectra are recorded to trace the evolution of one and the same QD over several
steps of annealing. A substantial reduction of the excitonic fine-structure
splitting upon annealing is observed. In addition, the binding energies of
different excitonic complexes change dramatically. The results are compared to
model calculations within eight-band k.p theory and the configuration
interaction method, suggesting a change of electron and hole wave function
shape and relative position.Comment: 4 pages, 4 figure
Interrelation of structural and electronic properties of InGaN/GaN quantum dots using an eight-band k.p model
We present an eight-band k.p model for the calculation of the electronic
structure of wurtzite semiconductor quantum dots (QDs) and its application to
indium gallium nitride (InGaN) QDs formed by composition fluctuations in InGaN
layers. The eight-band k.p model accounts for strain effects, piezoelectric and
pyroelectricity, spin-orbit and crystal field splitting. Exciton binding
energies are calculated using the self-consistent Hartree method. Using this
model, we studied the electronic properties of InGaN QDs and their dependence
on structural properties, i.e., their chemical composition, height, and lateral
diameter. We found a dominant influence of the built-in piezoelectric and
pyroelectric fields, causing a spatial separation of the bound electron and
hole states and a redshift of the exciton transition energies. The
single-particle energies as well as the exciton energies depend heavily on the
composition and geometry of the QDs
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