Andersen Group | El.-Phon. | QMC | C60 | GW | Resistivity saturation |
Coulomb pseudopotential |
The effects of the Coulomb repulsion on the superconductivity
is described by the Coulomb pseudopotential mu*. The strong
Coulomb repulsion is reduced by retardation and screening
effects. For conventional superconductors retardation effects
are believed to be very important in view of the very different
energy scales for the electrons and phonons. An important question
for the Fullerenes is then what the relevant energy scales are. If
the t1u band width W is the appropriate energy
scale, one would have to assume that the retardation effects are
small. This would then raise the question if screening effects
can reduce mu* sufficiently to allow for the large experimental
Tc, or if the superconductivity should be
understood in a different framework, e.g., in terms of an
electronic mechanism.
We have found that the the t1u band width W~1/2 eV
indeed is a reasonable electronic energy scale and that
retardation effects on the
Coulomb pseudopotential should not be very important for these systems
in view of W~w_{ph}.
Assuming RPA screening, it was found that
screening reduces mu* sufficiently to allow for a T_{c}
of the right order of magnitude.
Since correlation effects are believed to be important in these
systems, it is not clear that RPA is a good approximation.
A quantum Monte-Carlo calculation ( Phys.
Rev. Lett. , 83 , 620 (1999). ) for a Hubbard-like model
of the static screening showed, however, that RPA is a fairly good
approximation until the system is rather close to a Mott transition,
while for larger values of U/W RPA breaks down completely.
For systems very close to the Mott transition, the screening becomes
inefficient and mu* becomes large. This may explain why Tc
increases under pressure in Cs3C60 and
NH3K3C60
while it decreases in A3C60
(A= K, Rb). In the former systems,
being close to a metal-insulator transition, the dominating effect
of applying pressure is probably a reduction of mu*, while in the latter
systems a reduction of lambda dominates.
Andersen Group |
Max-Planck-Institut für Festkörperforschung Heisenbergstraße 1 D-70569 Stuttgart |