Neutron Scattering Study of Sr2Cu3O4Cl2

We report a neutron scattering study on the tetragonal compound Sr2Cu3O4Cl2, which has two-dimensional (2D) interpenetrating CuI and CuII 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 CuII subsystem. Due to quantum fluctuations a dramatic increase of the CuI 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 CuI and CuII ions and between the CuII ions are determined experimentally to be JI−II=−10(2) meV and JII=10.5(5) meV, respectively. Due to its small exchange interaction JII, the 2D dispersion of the CuII 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 CuII SLQHA is obtained up to a very high temperature, T/JII≈0.75. This result is compared with several theoretical predictions as well as recent experiments on the S=1/2 SLQHA. Disciplines Physics | Quantum Physics Author(s) Youngjune Kim, Robert J. Birgeneau, Fangcheng Chou, Martin Greven, Marc A. Kastner, Youngsu Lee, Barrett O. Wells, Amnon Aharony, Ora Entin-Wohlman, I. Ya Korenblit, A. Brooks Harris, Ross W. Erwin, and Gen Shirane This journal article is available at ScholarlyCommons: http://repository.upenn.edu/physics_papers/340 Neutron scattering study of Sr2Cu3O4Cl2 Y. J. Kim* Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 and Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 R. J. Birgeneau, F. C. Chou, M. Greven, M. A. Kastner, Y. S. Lee, and B. O. Wells Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 A. Aharony, O. Entin-Wohlman, and I. Ya. Korenblit School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel A. B. Harris Department of Physics, University of Pennsylvania, Philadelphia, Pennsylvania 19104 R. W. Erwin Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 G. Shirane Department of Physics, Brookhaven National Laboratory, Upton, New York 11973 ~Received 20 September 2000; published 22 June 2001! We report a neutron scattering study on the tetragonal compound Sr2Cu3O4Cl2, which has two-dimensional ~2D! interpenetrating CuI and CuII subsystems, each forming a S51/2 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 CuII subsystem. Due to quantum fluctuations a dramatic increase of the CuI 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 CuI and CuII ions and between the CuII ions are determined experimentally to be J I-II5210(2) meV and J II 510.5(5) meV, respectively. Due to its small exchange interaction J II , the 2D dispersion of the CuII 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 CuII 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 S51/2 SLQHA. DOI: 10.1103/PhysRevB.64.024435 PACS number~s!: 75.30.Ds, 75.10.Jm, 75.25.1z, 75.45.1j