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 (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)approximate to0.75. This result is compared with several theoretical predictions as well as recent experiments on the S=1/2 SLQHA.