154 research outputs found
Quantum coherent biomolecular energy transfer with spatially correlated fluctuations
We show that the quantum coherent transfer of excitations between
biomolecular chromophores is strongly influenced by spatial correlations of the
environmental fluctuations. The latter are due either to propagating
environmental modes or to local fluctuations with a finite localization length.
A simple toy model of a single donor-acceptor pair with spatially separated
chromophore sites allows to investigate the influence of these spatial
correlations on the quantum coherent excitation transfer. The sound velocity of
the solvent determines the wave lengths of the environmental modes, which, in
turn, has to be compared to the spatial distance of the chromophore sites. When
the wave length exceeds the distance between donor and acceptor site, we find
strong suppression of decoherence. In addition, we consider two spatially
separated donor-acceptor pairs under the influence of propagating environmental
modes. Depending on their wave lengths fixed by the sound velocity of the
solvent material, the spatial range of correlations may extend over typical
interpair distances, which can lead to an increase of the decohering influence
of the solvent. Surprisingly, this effect is counteracted by increasing
temperature
Ultraslow quantum dynamics in a sub-Ohmic heat bath
We show that the low-frequency modes of a sub-Ohmic bosonic heat bath
generate an effective dynamical asymmetry for an intrinsically symmetric
quantum spin -1/2. An initially fully polarized spin first decays towards a
quasiequilibrium determined by the dynamical asymmetry, thereby showing
coherent damped oscillations on the (fast) time scale of the spin splitting. On
top of this, the dynamical asymmetry itself decays on an ultraslow time scale
and vanishes asymptotically since the global equilibrium phase is symmetric. We
quantitatively study the nature of the initial fast decay to the
quasiequilibrium and discuss the features of ultraslow dynamics of the
quasiequilibrium itself. The dynamical asymmetry is more pronounced for smaller
values of the sub-Ohmic exponent and for lower temperatures, which emphasizes
the quantum many-body nature of the effect. The symmetry breaking is related to
the dynamic crossover between coherent and overdamped relaxation of the spin
polarization and is not connected to the localization quantum phase transition.
In addition to this delocalized phase, we identify a novel phase which is
characterized by damped coherent oscillations in the localized phase. This
allows for a sketch of the zero-temperature phase diagram of the sub-Ohmic
spin-boson model with four distinct phases.Comment: published version (minor changes), 8 pages, 5 figure
Nonequilibrium quantum fluctuation relations for harmonic systems in nonthermal environments
We formulate exact generalized nonequilibrium fluctuation relations for the
quantum mechanical harmonic oscillator coupled to multiple harmonic baths. Each
of the different baths is prepared in its own individual (in general
nonthermal) state. Starting from the exact solution for the oscillator dynamics
we study fluctuations of the oscillator position as well as of the energy
current through the oscillator under general nonequilibrium conditions. In
particular, we formulate a fluctuation-dissipation relation for the oscillator
position autocorrelation function that generalizes the standard result for the
case of a single bath at thermal equilibrium. Moreover, we show that the
generating function for the position operator fullfills a generalized
Gallavotti-Cohen-like relation. For the energy transfer through the oscillator,
we determine the average energy current together with the current fluctuations.
Finally, we discuss the generalization of the cumulant generating function for
the energy transfer to nonthermal bath preparations.Comment: 21 page
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
Laser Control of Dissipative Two-Exciton Dynamics in Molecular Aggregates
There are two types of two-photon transitions in molecular aggregates, that
is, non-local excitations of two monomers and local double excitations to some
higher excited intra-monomer electronic state. As a consequence of the
inter-monomer Coulomb interaction these different excitation states are coupled
to each other. Higher excited intra-monomer states are rather short-lived due
to efficient internal conversion of electronic into vibrational energy.
Combining both processes leads to the annihilation of an electronic excitation
state, which is a major loss channel for establishing high excitation densities
in molecular aggregates. Applying theoretical pulse optimization techniques to
a Frenkel exciton model it is shown that the dynamics of two-exciton states in
linear aggregates (dimer to tetramer) can be influenced by ultrafast shaped
laser pulses. In particular, it is studied to what extent the decay of the
two-exciton population by inter-band transitions can be transiently suppressed.
Intra-band dynamics is described by a dissipative hierarchy equation approach,
which takes into account strong exciton-vibrational coupling in the
non-Markovian regime.Comment: revised version, fig. 8 ne
Competition between relaxation and external driving in the dissipative Landau-Zener problem
We study Landau-Zener transitions in a dissipative environment by means of
the quasiadiabatic propagator path-integral scheme. It allows to obtain
numerically exact results for the full range of the involved parameters. We
discover a nonmonotonic dependence of the Landau-Zener transition probability
on the sweep velocity which is explained in terms of a simple physical picture.
This feature results from a nontrivial competition between relaxation processes
and the external sweep and is not captured by perturbative approaches. In
addition to the Landau-Zener transition probability, we study the excitation
survival probability and also provide a qualitative understanding of the
involved competition of time scales.Comment: 9 pages, 15 figure
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