Neutron scattering study ofSr2Cu3O4Cl2

Abstract

We report a neutron scattering study on the tetragonal compound ${\mathrm{Sr}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{4}{\mathrm{Cl}}_{2},$ which has two-dimensional (2D) interpenetrating ${\mathrm{Cu}}_{\mathrm{I}}$ and ${\mathrm{Cu}}_{\mathrm{II}}$ 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 ${\mathrm{Cu}}_{\mathrm{II}}$ subsystem. Due to quantum fluctuations a dramatic increase of the ${\mathrm{Cu}}_{\mathrm{I}}$ 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 ${\mathrm{Cu}}_{\mathrm{I}}$ and ${\mathrm{Cu}}_{\mathrm{II}}$ ions and between the ${\mathrm{Cu}}_{\mathrm{II}}$ ions are determined experimentally to be ${J}_{\mathrm{I}\ensuremath{-}\mathrm{I}\mathrm{I}}=\ensuremath{-}10(2)$ meV and ${J}_{\mathrm{II}}=10.5(5)$ meV, respectively. Due to its small exchange interaction ${J}_{\mathrm{II}},$ the 2D dispersion of the ${\mathrm{Cu}}_{\mathrm{II}}$ 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 ${\mathrm{Cu}}_{\mathrm{II}}$ SLQHA is obtained up to a very high temperature, ${T/J}_{\mathrm{II}}\ensuremath{\approx}0.75.$ This result is compared with several theoretical predictions as well as recent experiments on the $S=1/2$ SLQHA.