Regensburg 2002 – scientific programme

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TT: Tiefe Temperaturen

TT 7: Postersitzung I: Anwendungen der Supraleitung (1-6), Dünne Schichten, Josephsonkontakte, SQUIDs (7-17), Neue und unkonv. Supraleiter(18-23), Amorphe- und Tunnelsyst. (24-28), Quantenflüssigkeiten (29-30), Syst. korr. Elektr.: Theorie I (31-45)

TT 7.40: Poster

Monday, March 11, 2002, 14:30–18:00, A

Derivation of effective spin models from three and four band models for CuO₂–planes — •Alexander Reischl and Erwin Müller-Hartmann — Institut für theoretische Physik, Universität zu Köln, Zülpicher Str. 77, 50937 Köln

The derivation of effective spin models describing the low energy magnetic properties of undoped CuO₂–planes is reinvestigated. Starting from a conventional three–band model we determine the exchange couplings for the nearest and next–nearest neighbor Heisenberg exchange as well as for 4–spin exchange terms via a direct perturbation expansion with respect to the copper–oxygen hopping t_pd and oxygen–oxygen hopping t_pp. Our results demonstrate that this perturbation expansion does not converge for hopping parameters of the relevant size. Well behaved extrapolations of the couplings are derived, however, in terms of Padé approximants. We check the significance of these results from the direct perturbation expansion by employing the Zhang–Rice reformulation of the three band model in terms of hybridizing oxygen Wannier orbitals centered at copper ion sites.

Motivated by LDA band structure calculations we study the effect of an additional Cu 4s band. Hopping to this band produces in second order not only the usual pp–hopping between next–nearest oxygen orbitals but also between oxygen orbitals on opposite sides of a Cu. The implications for the magnetic couplings will be discussed.