Ordering due to Quantum Fluctuations inSr2Cu3O4Cl2

Abstract

Sr{sub 2}Cu {sub 3}O{sub 4}Cl{sub 2} has Cu{sub I } and Cu{sub II } subsystems, forming interpenetrating S=1/2 square lattice Heisenberg antiferromagnets. The classical ground state is degenerate, due to frustration of the intersubsystem interactions. Magnetic neutron scattering experiments show that quantum fluctuations cause a two dimensional Ising ordering of the Cu{sub II } `s, lifting the degeneracy, and a dramatic increase of the Cu{sub I } out-of-plane spin-wave gap, unique for {ital order out of disorder}. The spin-wave energies are quantitatively predicted by calculations which include quantum fluctuations. {copyright} {ital 1999} {ital The American Physical Society }