Ordering due to quantum fluctuations in Sr2Cu3O4Cl2

Sr2Cu3O4Cl2 has Cu-I and Cu-II subsystems, forming interpenetrating S=1/2 s quare lattice Heisenberg antiferromagnets. The classical ground state is de generate, due to frustration of the intersubsystem interactions. Magnetic n eutron scattering experiments show that quantum fluctuations cause a two di mensional Ising ordering of the Cu-II's, lifting the degeneracy, and a dram atic increase of the Cu-I out-of-plane spin-wave gap, unique for order out of disorder. The spin-wave energies are quantitatively predicted by calcula tions which include quantum fluctuations.