946 research outputs found
Current induced light emission and light induced current in molecular tunneling junctions
The interaction of metal-molecule-metal junctions with light is considered
within a simple generic model. We show, for the first time, that light induced
current in unbiased junctions can take place when the bridging molecule is
characterized by a strong charge-transfer transition. The same model shows
current induced light emission under potential bias that exceeds the molecular
excitation energy. Results based on realistic estimates of molecular-lead
coupling and molecule-radiation field interaction suggest that both effects
should be observable.Comment: 5 pages, 3 figures, RevTeX
Coherent and incoherent dynamics in excitonic energy transfer: correlated fluctuations and off-resonance effects
We study the nature of the energy transfer process within a pair of coupled
two-level systems (donor and acceptor) subject to interactions with the
surrounding environment. Going beyond a standard weak-coupling approach, we
derive a master equation within the polaron representation that allows for
investigation of both weak and strong system-bath couplings, as well as
reliable interpolation between these two limits. With this theory, we are then
able to explore both coherent and incoherent regimes of energy transfer within
the donor-acceptor pair. We elucidate how the degree of correlation in the
donor and acceptor fluctuations, the donor-acceptor energy mismatch, and the
range of the environment frequency distribution impact upon the energy transfer
dynamics. In the resonant case (no energy mismatch) we describe in detail how a
crossover from coherent to incoherent transfer dynamics occurs with increasing
temperature [A. Nazir, Phys. Rev. Lett. 103, 146404 (2009)], and we also
explore how fluctuation correlations are able to protect coherence in the
energy transfer process. We show that a strict crossover criterion is harder to
define when off-resonance, though we find qualitatively similar population
dynamics to the resonant case with increasing temperature, while the amplitude
of coherent population oscillations also becomes suppressed with growing site
energy mismatch.Comment: 14 pages, 7 figures, builds upon PRL 103, 146404 (2009)
(arXiv:0906.0592). Comments welcome. V2 - Section IV shortened to improve
presentation, references updated, new Imperial College affiliation added for
A. Nazir. Published versio
Molecular Wires Acting as Coherent Quantum Ratchets
The effect of laser fields on the electron transport through a molecular wire
being weakly coupled to two leads is investigated. The molecular wire acts as a
coherent quantum ratchet if the molecule is composed of periodically arranged,
asymmetric chemical groups. This setup presents a quantum rectifier with a
finite dc-response in the absence of a static bias. The nonlinear current is
evaluated in closed form within the Floquet basis of the isolated, driven wire.
The current response reveals multiple current reversals together with a
nonlinear dependence (reflecting avoided quasi-energy crossings) on both, the
amplitude and the frequency of the laser field. The current saturates for long
wires at a nonzero value, while it may change sign upon decreasing its length.Comment: 4 pages, 4 figures, RevTeX
Quantum dynamics of a vibronically coupled linear chain using a surrogate Hamiltonian approach
Vibronic coupling between the electronic and vibrational degrees of freedom has been reported to play an important role in charge and exciton transport in organic photovoltaic materials, molecular aggregates and light-harvesting complexes. Explicitly accounting for effective vibrational modes rather than treating them as a thermal environment has been shown to be crucial to describe the effect of vibronic coupling. We present a methodology to study dissipative quantum dynamics of vibronically coupled systems based on a surrogate Hamiltonian approach, which is in principle not limited by Markov approximation or weak system-bath interaction, using a vibronic basis. We apply vibronic surrogate Hamiltonian method to a linear chain system and discuss how different types of relaxation process, intramolecular vibrational relaxation and intermolecular vibronic relaxation, influence population dynamics of dissipative vibronic systems
Transport and optical response of molecular junctions driven by surface plasmon-polaritons
We consider a biased molecular junction subjected to external time-dependent
electromagnetic field. The field for two typical junction geometries (bowtie
antennas and metal nanospheres) is calculated within finite-difference
time-domain technique. Time-dependent transport and optical response of the
junctions is calculated within non-equilibrium Green's function approach
expressed in a form convenient for description of multi-level systems. We
present numerical results for a two-level (HOMO-LUMO) model, and discuss
influence of localized surface plasmon polariton modes on transport.Comment: 9 pages, 6 figure
Molecular transport junctions: Current from electronic excitations in the leads
Using a model comprising a 2-level bridge connecting free electron reservoirs
we show that coupling of a molecular bridge to electron-hole excitations in the
leads can markedly effect the source-drain current through a molecular
junction.In some cases, e.g. molecules that exhibit strong charge transfer
transitions, the contribution from electron-hole excitations can exceed the
Landauer elastic current and dominate the observed conduction.Comment: 4 pages, 2 figures, submitted to PR
Quantum master equation for electron transport through quantum dots and single molecules
A quantum master equation (QME) is derived for the many-body density matrix
of an open current-carrying system weakly coupled to two metal leads. The
dynamics and the steady-state properties of the system for arbitrary bias are
studied using projection operator techniques, which keep track of number of
electrons in the system. We show that coherences between system states with
different number of electrons, n, (Fock space coherences) do not contribute to
the transport to second order in system-lead coupling.
However, coherences between states with the same n may effect transport
properties when the damping rate is of the order or faster then the system Bohr
frequencies.
For large bias, when all the system many-body states lie between the chemical
potentials of the two leads, we recover previous results. In the rotating wave
approximation (when the damping is slow compared to the Bohr frequencies of the
system), the dynamics of populations and the coherences in the system
eigenbasis are decoupled. The QME then reduces to a birth and death master
equation for populations.Comment: 22 pages, 8 figures, paper accepted in Phys. Rev.
Self-induced parametric amplification arising from nonlinear elastic coupling in a micromechanical resonating disk gyroscope
Parametric amplification, resulting from intentionally varying a parameter in a resonator at twice its resonant frequency, has been successfully employed to increase the sensitivity of many micro- and nano-scale sensors. Here, we introduce the concept of self-induced parametric amplification, which arises naturally from nonlinear elastic coupling between the degenerate vibration modes in a micromechanical disk-resonator, and is not externally applied. The device functions as a gyroscope wherein angular rotation is detected from Coriolis coupling of elastic vibration energy from a driven vibration mode into a second degenerate sensing mode. While nonlinear elasticity in silicon resonators is extremely weak, in this high quality-factor device, ppm-level nonlinear elastic effects result in an order-of-magnitude increase in the observed sensitivity to Coriolis force relative to linear theory. Perfect degeneracy of the primary and secondary vibration modes is achieved through electrostatic frequency tuning, which also enables the phase and frequency of the parametric coupling to be varied, and we show that the resulting phase and frequency dependence of the amplification follow the theory of parametric resonance. We expect that this phenomenon will be useful for both fundamental studies of dynamic systems with low dissipation and for increasing signal-to-noise ratio in practical applications such as gyroscopes
Exciton- and Light-induced Current in Molecular Nanojunctions
We consider exciton- and light-induced current in molecular nanojunctions.
Using a model comprising a two two-level sites bridge connecting free electron
reservoirs we show that the exciton coupling between the sites of the molecular
bridge can markedly effect the source-drain current through a molecular
junction. In some cases when excited and unexcited states of the sites are
coupled differently to the leads, the contribution from electron-hole
excitations can exceed the Landauer elastic current and dominate the observed
conduction. We have proposed an optical control method using chirped pulses for
enhancing charge transfer in unbiased junctions where the bridging molecule is
characterized by a strong charge-transfer transition.Comment: 8 pages, 2 figures,submitted to the Canadian J. Phy
Quantum Transition State Theory for proton transfer reactions in enzymes
We consider the role of quantum effects in the transfer of hyrogen-like
species in enzyme-catalysed reactions. This study is stimulated by claims that
the observed magnitude and temperature dependence of kinetic isotope effects
imply that quantum tunneling below the energy barrier associated with the
transition state significantly enhances the reaction rate in many enzymes. We
use a path integral approach which provides a general framework to understand
tunneling in a quantum system which interacts with an environment at non-zero
temperature. Here the quantum system is the active site of the enzyme and the
environment is the surrounding protein and water. Tunneling well below the
barrier only occurs for temperatures less than a temperature which is
determined by the curvature of potential energy surface near the top of the
barrier. We argue that for most enzymes this temperature is less than room
temperature. For physically reasonable parameters quantum transition state
theory gives a quantitative description of the temperature dependence and
magnitude of kinetic isotope effects for two classes of enzymes which have been
claimed to exhibit signatures of quantum tunneling. The only quantum effects
are those associated with the transition state, both reflection at the barrier
top and tunneling just below the barrier. We establish that the friction due to
the environment is weak and only slightly modifies the reaction rate.
Furthermore, at room temperature and for typical energy barriers environmental
degrees of freedom with frequencies much less than 1000 cm do not have a
significant effect on quantum corrections to the reaction rate.Comment: Aspects of the article are discussed at
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