When the field Molecular Electronics was emerging at the
end of the last century its main drive was towards building new devices
for applications in information and sensor technology. This still is
important motivation driving the field.
However, it soon became clear that Molecular Electronics is much more
than this. It is a field of fundamental sciences, where Chemistry,
Surface and Condensed Matter Experts collaborate in order to understand
fundamental processes at interfaces in and out of equilibrium. Questions
range from the flow of charge and heat around single atoms in molecular
switches up to biological scales, where sticking of specific viruses
can be gate-controlled on certain semi-conductor surfaces.
Enormous challenges exist in the field concerning method development but
also with respect to analyzing fascinating experiments on single
molecule or even single atom transport. Activities in our group concern
both directions, supported by numerous international collaborations.
A very distinct feature of molecules, that makes them
very special as compared to, e.g., quantum dots familiar from
semi-conductor devices, originates from their symmetries, e.g. related
to space groups. Roughly speaking, such symmetries can produce
degeneracies which in turn lead to the competition of ground states, say
magnetic vs non-magnetic ones. As a consequence, by proper chemical
design a molecular structure can be tuned close to an instability which
then can be addressed experimentally, say by a gate voltage or strain.
In this way, a molecular bistability can be constructed, that can
exhibit, e.g., a switchable Kondo-effect.
See an example of a switchable Kondo effect in our
Physical Review Letters article.
The application of density functional theory (DFT) to
transport problems has a long history in the field of Molecular
Electronics. The DFT-technology is so important, because it can connect
the molecular structure to its observable properties, which often are
sensitive to changing just a single atom. DFT-transport theories rely
upon two central ingredients. Future improvements require work in both
directions and we are active there.
Nonequilibrium Green's Function Approach: AitransS
The results of any first principles calculation need to be converted
into transport information, like transmission functions, charge and
heat conductances etc.. To this end we employ the nonequilibrium
Green's functions approach.
Our particular methodology
is implemented in a transport module --
-- that currently is running with standard DFT-codes
and FHI-aims .
Improvements and extensions of this transport module are continuously
under way. This includes the calculation of local current densities,
orbital magnetism or spin-orbit torques out-of-equilibrium.
Current pattern in large functionalized graphene flakes
Using our transport tool, we also investigate the effect of
functionalization on conductance and the current pattern through large
mesoscopic flakes. Usually these currents vary greatly on microscopic
length-scales. Our DFT-approach includes effects like lattice
distortion, e.g. strain, and cross-talk between different impurities.
Current flows in loops in graphene sheets: find more in our recent Physical Review Letters paper.
Fundamental Aspects and Functionals
At the heart of any DFT-calculation is the density functional. All
DFT-calculations rely upon certain approximations of this functional,
the best known one being the local density approximation (LDA). We aim
at the better understanding and, in particular, also at improving
existent functionals with a special emphasis on the development of
post-DFT procedures like the GW-technology for applications in Molecular
Electronics. This fundamental research can be expected to have broad
impact in all nano-sciences that rely upon ab-initio calculations.