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Berlin 2008 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 32: Correlated Electrons: Poster Session

TT 32.71: Poster

Thursday, February 28, 2008, 14:00–18:00, Poster B

The phase diagram of the J1-J2-Heisenberg model using auxiliary fermions — •Johannes Reuther, Jan Brinckmann, and Peter Woelfle — Institut für Theorie der Kondensierten Materie, Universität Karlsruhe

We consider the two-dimensional spin-1/2 J1-J2-Heisenberg model: In addition to the nearest neighbor coupling J1>0 we introduce frustration by a next nearest neighbor coupling J2>0. It is well known that for J2=0, J1≠ 0 the model features a Néel-like ground state, while for very large J2 the system shows antiferromagnetic order on both sublattices. This is called collinear order. In between, i.e., in the vicinity of the ratio J1/J2=1/2 a state without magnetic order is expected.

We use the auxiliary fermion-formulation of spin operators in conjunction with a method proposed by Popov and Fedotov. The latter enables us to take exactly into account the auxiliary-particle constraint (i.e. the projection onto the physical Hilbert space).

The ground state phase diagram of the model is calculated using a simple diagrammatic approximation for the fermion’s self energy. We find a Néel state at small J1 and a continuous transition into a non-magnetic state if J1 is increased beyond a critical value. On the other hand for large J2 we find the expected collinear state. The magnetic excitation spectrum and correlation length are studied in detail, in particular for the non-magnetic phase around J1/J2=1/2.

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