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Project summary |
News & Views |
Recent results
Electron transport through molecules: The role of coherent
excitations, vibrational modes, Coulomb interaction, noise-induced dynamics,
shot noise, charge and heat transfer
Principal investigators: P. Hänggi, S. Kohler
Project summary
The field of molecular electronics deals with the electron transport
through electrode-molecule-electrode settings and its possible
technological applications. Within this proposal, we will investigate
charge and heat transport through molecular wires focussing on current
noise, energy transfer, and heating phenomena, and the influence of
time-dependent fields and gate voltages. In particular, we plan to
explore the influence of Coulomb interaction, coupling to vibrational
degrees of freedom, and stochastic fields on charge and
heat transfer. As a working model for the molecule, a tight-binding
Hamiltonian is employed which includes interactions with metallic leads,
with vibrational degrees of freedom, with electrons, and with external
fields as well.
Thereby, we will contribute to both theoretical understanding of
experimental observations and the prediction of novel phenomena.
Moreover, our calculations will serve for testing the approximations
on which the far more complex ab-initio methods are based on.
News & Views
Recent results
- Coulomb repulsion and shot noise in driven molecular wires
Eur. Phys. J. B 54, 201 (2006)
Ann. Phys. (Leipzig) 16, 702 (2007)
Electron transport is not only characterised by the average current, but also
by the current noise and the counting statistics. As an essential tool to
achieve the goals listed in our proposal, we developed a master equation
formalism for interacting electrons under the influence of time-dependent
fields. A decomposition of the reduced density matrix into a Floquet basis
provides an efficient numerical implementation. This formalism is particularly
suitable for studying the orbital degrees of freedom in extended conductors
like molecular chains. As a first application of our formalism, we showed that
for a recently studied electron pump, Coulomb interaction has only a minor
influence on the Fano factor.
- Electron heat pump
Phys. Rev. B 76, 085337 (2007)
The investigation of thermoelectric effects being a central part of the
project, requires to compute the energy balance of quantum transport.
It turned out that here a meaningful measure is the rate at which the
total energy of the leads changes. This quantity is dominated by
photon absorption of the transported electrons and by inelastic
reflection.
We showed that the latter always heats up the leads.
Nevertheless, for a weakly coupled conductor with proper energy
levels, the energy balance in one lead can be negative which amounts
to cooling the lead; see Figure.
Moreover, it turned out that heat transport in nanoscale conductors is
maximal when a lead at finite temperature is coupled to a lead at zero
temperature. Thus the cooling rate dQ/dt is
limited by a "quantum of cooling power".
The influence of a coupling of the wire electrons to substrate phonons
is presently under investigation.
- Shot noise in spin pumps
Physica E 40, 1276 (2008)
In the presence of a magnetic field, the current generally depends on
the electron spin. Consequently, a pump mechanism can prefer
electrons with a certain spin state and, thus, act as a spin pump.
The Fano factor of the resulting spin current
depends sensitively on the ratio between driving amplitude and
frequency. Adjusting this ratio, one can tune the Fano factor to
values between 0.5 and 2.
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DFG SPP 1243: Quantum transport at the molecular scale
Theoretical physicists have predicted that ultrashort laser pulses can be used
to drive electrical currents through single molecules, and also to stop
currents in molecular junctions.