## Abstract

We study the zero-temperature phase diagram and the low-lying excitations of a square-lattice spin-half Heisenberg antiferromagnet with two types of regularly distributed nearest-neighbor exchange bonds [J>0 (antiferromagnetic) and -∞<J'<∞] using the coupled cluster method (CCM) for high orders of approximation (up to LSUB8). We use a Ne´el model state as well as a helical model state as a starting point for the CCM calculations. We find a second-order transition from a phase with Ne´el order to a finite-gap quantum disordered

phase for sufficiently large antiferromagnetic exchange constants J'>0. For frustrating ferromagnetic couplings J'<0 we find indications that quantum fluctuations favor a first-order phase transition from the Ne´el order to a quantum helical state, by contrast with the corresponding second-order transition in the corresponding classical model. The results are compared to those of exact diagonalizations of finite systems (up to 32sites) and those of spin-wave and variational calculations. The CCM results agree well with the exact diagonalization data over the whole range of the parameters. The special case of J'=0, which is equivalent to the honeycomb lattice, is treated more closely.

phase for sufficiently large antiferromagnetic exchange constants J'>0. For frustrating ferromagnetic couplings J'<0 we find indications that quantum fluctuations favor a first-order phase transition from the Ne´el order to a quantum helical state, by contrast with the corresponding second-order transition in the corresponding classical model. The results are compared to those of exact diagonalizations of finite systems (up to 32sites) and those of spin-wave and variational calculations. The CCM results agree well with the exact diagonalization data over the whole range of the parameters. The special case of J'=0, which is equivalent to the honeycomb lattice, is treated more closely.

Original language | English |
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Pages (from-to) | 14607-14615 |

Number of pages | 9 |

Journal | Physical Review B (Condensed Matter and Materials Physics) |

Volume | 61 |

DOIs | |

Publication status | Published - 2000 |

## Keywords

- DISORDER TRANSITION; MODEL; MAGNETS; CAV4O9; STATES; HONEYCOMB; FORMALISM; DIAGRAM; CHAINS