Neutron scattering study of Sr2Cu3O4Cl2 - art. no. 024435

We report a neutron scattering study on the tetragonal compound Sr2Cu3O4Cl2 , which has two-dimensional (2D) interpenetrating Cu-I and Cu-II subsystems , each forming a S=1/2 square lattice quantum Heisenberg antiferromagnet (S LQHA). The mean-field ground state is degenerate, since the intersubsystem interactions are geometrically frustrated. Magnetic neutron scattering expe riments show that quantum fluctuations lift the degeneracy and cause a 2D I sing ordering of the Cu-II subsystem. Due to quantum fluctuations a dramati c increase of the Cu-I out-of-plane spin-wave gap is also observed. The tem perature dependence and the dispersion of the spin-wave energy are quantita tively explained by spin-wave calculations which include quantum fluctuatio ns explicitly. The values for the nearest-neighbor superexchange interactio ns between the Cu-I and Cu-II ions and between the Cu-II ions are determine d experimentally to be J(I-II) = -10(2) meV and J(II) = 10.5(5) meV, respec tively. Due to its small exchange interaction J(II), the 2D dispersion of t he Cu-II SLQHA can be measured over the whole Brillouin zone with thermal n eutrons, and a dispersion at the zone boundary, predicted by theory, is con firmed. The instantaneous magnetic correlation length of the Cu-II SLQHA is obtained up to a very high temperature, T/J(II)approximate to0.75. This re sult is compared with several theoretical predictions as well as recent exp eriments on the S=1/2 SLQHA.