We now look at a MW system that displays antiresonances that
are predicted by the formula derived in Sec. II. Eq. (8)
was derived for an idealized MW system where there was
only one incident electron mode on the molecule. Thus in
designing the MW system for the numerical study we tried to
approximate the ideal system as closely as possible. The MW
system we suggest consists of left and right 
conjugated
chain molecules attached to what we will call the ``active''
molecule. The purpose of these conjugated chains is to act as
filters to the many modes that will be incident from the
metallic leads. For appropriate energies they will restrict
the propagating electron mode to be only
like. This
backbone will only interact with the
orbitals of the
molecule if they are bonded in an appropriated fashion.
Figure 4:
Atomistic diagram of leads attached to filter
molecules and active molecule. The filter molecules consist of
C8H8 chains. The active molecule contains 2
levels
which interact with the filter molecules. The leads are (111)
gold.
|
|
An atomic diagram of our system is shown in Fig. 4.
The leads are oriented in the (111) direction. The chain
molecules are bonded to clusters of gold atoms that form the
tips of the leads. The carbon atom nearest to the gold tip
binds over the hollow site of the tip.
The chains each have eight CH groups, which for the
energies considered will only admit a
like state to
propagate along them. This gives rise to the filtering process
mentioned above. The active molecule is chosen to have two
states and two
states. The chain's
like
orbitals will only couple to the two
states of the active
molecule.
The Fermi energy for our gold leads is around -10 eV which lies
within the
band. We would like an antiresonance to occur
somewhere near this energy. The parameters entering
Eq. 8 (i.e. the molecular orbital energies and
their overlaps with the chains) are chosen so that one of the
roots of the equation yields a value near -10 eV. The
numerically calculated electron transmission probability for
this model is shown in Fig. 5a; an antiresonance is
seen in the plot at the predicted energy of -10.08 eV. The
differential conductance at room temperature was calculated
using Eq. (1). Two different conductance
calculations are shown in Fig. 5b. The solid curve
corresponds to a choice of Fermi energy of -10.2 eV. Because
it lies to the left of the antiresonance in a region of strong
transmission, the conductance is strong at 0 V. It then drops
at around 0.2 V when the antiresonance is crossed. The dashed
curve was calculated using a Fermi energy of -10.0 eV. It
starts in a region of lower transmission and thus the
antiresonance suppresses the increase in current. After 0.2 V
the large transmission to the left of the antiresonance is
sampled and the current rises sharply. So in both cases the
antiresonance has lowered the conductance. It is conceivable to
think of utilizing more antiresonances in a narrow energy range
to create a more pronounced conductance drop.
|