Parity
matters in a wire (23-7-2003)
(Observation of a length-dependent oscillation of conductance in atomic
wires. R.H.M. Smit, C. Untiedt, G. Rubio-Bollinger,
R.C. Segers, J.M. van Ruitenbeek. Phys. Rev. Lett 91,076805
(2003) and cond-mat/0303039)
The common experience tells us that the electrical
conductance (the inverse of the resistance) of a macroscopic wire is dependent
of its length. This can be easily understood when we think that the electrical
current is carried by electrons and the longer these have to move, more
chances they will have to find an obstacle that delays their movement. If
we reduce the size of a wire below the characteristic length that an electron
moves without finding such an obstacle, its resistance will no longer depend
on the length of the wire in what is called the ballistic regime of the
electrical conduction. However a recent collaboration of researchers of
the AMC group of the University of Leiden (The Netherlands) and the Laboratorio
de bajas temperaturas of the Universidad Autónoma de Madrid (Spain)
have shown that this is not the case for an atomic chain.
The thinnest wire possible is the chain of atoms. These can be formed
at the final stages of breaking a macroscopic wire and their conductance
can be measured wile increasing their length atom by atom. In the case of
chains made out of gold the conductance is close to the quantum of conductance
and small variations in its value are observed wile pulling the chain. From
these variations, the new research has shown dependence between the conductance
of the wire and the number of atoms which form the chain. This time the
dependence is of a new kind, in such a way that it depends on whether the
number of atoms in the chain is even or if it is odd.
The even-odd behavior is a consequence of connecting the chain to
the electrodes, the extremes of the chain will act as a barrier to the electrons
in the chain and this will make the chain to work as an interferometer of
electrons. This interference effect will be the responsible of the alternating
conductance with the number of atoms in the chain. The conductance of a
chain containing an even number of atoms will be generally smaller than
the one containing an even number which will give the maximum conductance
possible.
This behavior was predicted for metallic wires made of materials with
a simple electronic structure were the conduction was carried by electrons
of “s” type (electrons residing in a spherical electron cloud) but this
research shows that this holds for all the metals which forms chains (Au,
Pt and Ir). On top of that, for the last two which are not “s”-type, an
unpredicted continuous decrease of conductance is also observed while increasing
the chain length.
These new phenomena show how different things are when going to small.
(More information at www.physics.leidenuniv.nl/sections/cm/amc/
and http://www.ua.es/personal/untiedt/ )