Dresden 2003 – wissenschaftliches Programm
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TT: Tiefe Temperaturen
TT 2: Systeme korrelierter Elektronen I
TT 2.6: Vortrag
Montag, 24. März 2003, 11:15–11:30, HSZ304
Spin fluctuations in undoped cuprates at finite temperature: auxiliary-fermion approach. — •Jan Brinckmann and Peter Wölfle — Institut für Theorie der Kondensierten Materie, Universität Karlsruhe, D-76128 Karlsruhe
The structure factor S(q) of the antiferromagnetic spin-1/2 Heisenberg model in 2D is considered at small finite temperature. The model is relevant for cuprate superconductors in the 1/2-filled (undoped) case. Neutron scattering experiments revealed a magnetic correlation length in these systems, that grows exponentially with 1/T .
In mean-field theories using Schwinger bosons, a correlation length ξ(T)∼exp(A/T) comes out naturally, since bosons do not condense in 2D. Alternatively, spin operators can be re-written in auxiliary fermions, which is part of the usual ‘slave boson’ approach to doped cuprates (t-J-model). For 1/2-filling, however, this mean-field theory gives a Néel state with finite TN∼ J . We propose a self-consistent approximation beyond mean field, where the susceptibility is coupled back into the self energy of auxiliary fermions. We obtain the correct behaviour of ξ(T) and a suppression of TN→ 0 , in accordance with the theorem of Mermin and Wagner.
From studies of the QNLσM and the Schwinger-boson theory it is known that the dynamical structure factor S(q,ω) shows spin-wave like peaks far off the Néel wave-vector even at T>0 (short-range order). Our simple approximation is able to reproduce these physics, including the so-called dynamical scaling behaviour.