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TT: Fachverband Tiefe Temperaturen

TT 32: Correlated Electrons: Poster Session

TT 32.71: Poster

Donnerstag, 28. Februar 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.