Berlin 2005 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 11: Correlated Electrons - Spin Systems and Itinerant Magnets: Theory

TT 11.1: Vortrag

Samstag, 5. März 2005, 08:45–09:00, TU H2053

Derivation of the quantum-nonlinear-σ-model for antiferromagnets in a uniform magnetic field — •Nils Hasselmann and Peter Kopietz — Institut für Theoretische Physik, Universität Frankfurt, Robert-Mayer-Strasse 8, 60054 Frankfurt/Main

We reconsider the derivation of the quantum-nonlinear-σ-model for antiferromagnets in a magnetic field. The standard assumption is that the term (∂_τn)² in the action is replaced in presence of a magnetic field H by (∂_τn− i H× n)². Here, n (with n²=1) represents the staggered moments while τ is the imaginary time. We show that this result is incorrect in that it does not take account of the fact that a uniform magnetic field generates a finite uniform magnetization. We present the correct form of the non-linear-σ-model describing the gapless spin excitations near the antiferromagnetic ordering wavevector.