## Abstract

We have studied the ground state phase diagram of the quantum spin-1/2 frustrated Heisenberg antiferromagnet on a square lattice by using the framework of the differential operator technique. The Hamiltonian is solved by using an effective-field theory for a cluster with two spins (EFT-2). The model is described using the Heisenberg Hamiltonian with two competing antiferromagnetic interactions: nearest neighbor (NN) with different coupling strengths J _{1} and J_{1}′ along the x and y directions and next nearest neighbor (NNN) with coupling J_{2}. We propose a functional for the free energy (similar to the Landau expansion) and using Maxwell construction we obtain the phase diagram in the (λ, α) space, where λ = J_{1}′/J_{1} and α = J_{2}/J_{1}. We obtain three different states depending on the values of λ and α: antiferromagnetic (AF), collinear antiferromagnetic (CAF) and quantum paramagnetic (QP). For an intermediate region λ_{1} < λ < 1 we observe a QP state between the ordered AF and CAF phases, which disappears for λ above some critical value . We find a second-order phase transition between the AF and QP phases and a first-order transition between the CAF and QP phases. The boundaries between these ordered phases merge at the quantum triple point (QTP). Below this QTP there is again a direct first-order transition between the AF and CAF phases, with a behavior approximately described by the classical line .

Original language | English (US) |
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Article number | P06022 |

Journal | Journal of Statistical Mechanics: Theory and Experiment |

Volume | 2010 |

Issue number | 6 |

DOIs | |

State | Published - Jul 6 2010 |

## Keywords

- phase diagrams (theory)
- quantum phase transitions (theory)

## ASJC Scopus subject areas

- Statistical and Nonlinear Physics
- Statistics and Probability
- Statistics, Probability and Uncertainty

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