=(π,0) and (π/2,π/2) occur.
The postscript file shows correlation functions, Cij=〈 n_i n_j〉-〈 n_i〉 〈 n_j〉,
where i and j stand for orbital and spin indices. The results are obtained for the first DCA iteration,
when the bath is fully metallic. For small U the (π,0)up(π,0)down
curve bends down as expected when (π,0) forms a Kondo state with its bath.
For larger U the curve turns upwards. This is due to the formation of
a bound, nondegenerate, state in the (π,0)-(0,π) space, as in the four level
model. This is supported by other correlation functions in this space. At this point
a pseudogap starts to form for (π,0). For larger U a similar behavior is seen
in the (π/2,π/2) space, and a pseudogap forms also in this space.
The decisive difference is that the coupling to the bath
is much stronger in the (π/2,π/2) space and therefore
a Kondo state remains favorable up to larger U. The stronger coupling is due
to the stronger dispersion in the band structure at (π/2,π/2).
neutral cluster state is a singlet with a substantial local d-wave pairing. The
excited neutral states forming the pseudogap are triplets or singlets with a smaller
d-wave pairing. The pseudogap is then related to breaking preformed d-wave pairs.
Although the pseudogap is due to electronic correlation, we find that there is
nevertheless a substantial isotope effect.
G. Sangiovanni and O. Gunnarsson:
Isotope effect in the pseudogap state
of high-temperature copper-oxide superconductors,
Phys. Rev. B, Rapid Commun.
84, 100505 (2011).
J. Merino and O. Gunnarsson:
Pseudogap in cuprate and organic superconductors,
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For further information contact Olle Gunnarsson (O.Gunnarsson@fkf.mpg.de),
Jaime merino (email@example.com) or
Giorgio Sangiovanni (firstname.lastname@example.org).
Max-Planck Institut für Festkörperforschung
Postfach 800 665
Last Update: September 2012