## Abstract

We report a neutron scattering study on the tetragonal compound Sr_{2}Cu_{3}O_{4}Cl_{2}, which has two-dimensional (2D) interpenetrating Cu_{I} and Cu_{II} subsystems, each forming a S = 1/2 square lattice quantum Heisenberg antiferromagnet (SLQHA). The mean-field ground state is degenerate, since the intersubsystem interactions are geometrically frustrated. Magnetic neutron scattering experiments show that quantum fluctuations lift the degeneracy and cause a 2D Ising ordering of the Cu_{II} subsystem. Due to quantum fluctuations a dramatic increase of the Cu_{I} out-of-plane spin-wave gap is also observed. The temperature dependence and the dispersion of the spin-wave energy are quantitatively explained by spin-wave calculations which include quantum fluctuations explicitly. The values for the nearest-neighbor superexchange interactions between the Cu_{I} and Cu_{II} ions and between the Cu_{II} ions are determined experimentally to be J_{I-II}= -10(2) meV and J_{II} = 10.5(5) meV, respectively. Due to its small exchange interaction J_{II}, the 2D dispersion of the Cu_{II} SLQHA can be measured over the whole Brillouin zone with thermal neutrons, and a dispersion at the zone boundary, predicted by theory, is confirmed. The instantaneous magnetic correlation length of the Cu_{II} SLQHA is obtained up to a very high temperature, T/J_{II}≈0.75. This result is compared with several theoretical predictions as well as recent experiments on the S = 1/2 SLQHA.

Original language | English |
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Article number | 024435 |

Pages (from-to) | 244351-2443519 |

Number of pages | 2199169 |

Journal | Physical Review B - Condensed Matter and Materials Physics |

Volume | 64 |

Issue number | 2 |

DOIs | |

State | Published - 2001 |

### Funding

Funders | Funder number |
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National Science Foundation | |

Directorate for Mathematical and Physical Sciences | 9423101 |

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