Andersen Group | El.-Phon. | QMC | C60 | GW | Resistivity saturation |
Resistivity |
In a semi-classical theory,
the resistivity in a metal can be interpreted in terms of the
mean free path l, the average distance an electron travels
before it is scattered.
For a good metal, l is typically very much larger than
the distance d between two atoms. For some systems,
e.g., some transition metal compounds, it is found that
l and d can become comparable. Many such compounds
were studied in the 70's and 80's, and it was found that the
resistivity saturated when l became comparable
to d. This behaviour was therefor considered as universal.
As an example, the resistivity of Nb3Sn
and Cu are compared in a postscript file.
The resistivity corresponding to l=d (Ioffe-Regel) is also shown.
Later some apparent exceptions were found, e.g., i) some high temperature
superconductors and ii) the alkali-doped fullerenes
(A3C60)( see
postscript file).
These systems could, however, be in exotic states,
where only a small fraction of the nominal conduction electrons
actually contribute to the conductivity. In that case one
would have to conclude that l is correspondingly larger,
and perhaps even larger than d.
Our aim has therefore been to construct a microscopic model
of the alkali-doped fullerenes and then to solve this model
essentially exactly by using a quantum Monte-Carlo method.
If such a model shows a resistivity which becomes so large
that l is much smaller than d (the separation
between two fullerene molecules), this would then provide
a counter example to the assumed universality of the
resistivity saturation when l is similar to d.
We find that a model including the sattering of the electrons from
the intramolecular phonons indeed has such a behavior
Nature 405 , 1027 (2000) .
This illustrates that saturation when l and d
are comparable is not a universal phenomenon,
since it would then also have to happen in our model.
A general discussion is also given
by P.B. Allen in "Misbehaviour in metals", News and Views,
Nature 405 , 1007 (2000).
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