Quantum fluctuations in the frustrated antiferromagnet Sr2Cu3O4Cl2 - art. no. 024436

Sr2Cu3O4Cl2 is an antiferromagnet consisting of weakly coupled CuO planes w hich comprise two weakly interacting antiferromagnetic subsystems I and II which order at respective temperatures T-I approximate to 390 K and T-II ap proximate to 40 K. Except asymptotically near the ordering temperature, the se systems are good representations of the two-dimensional quantum spin-1/2 Heisenberg model. For T< T-II there are four low-energy modes at zero wave vector, three of whose energies are dominated by quantum fluctuations. For T-II< T< T-I there are two low-energy modes. The mode with lower energy is dominated by quantum fluctuations. Our calculations of the energies of the se modes (including dispersion for wave vectors perpendicular to the CuO pl anes) agree extremely well with the experimental results of inelastic neutr on scattering (in the accompanying paper) and for modes in the sub-meV rang e observed by electron spin resonance. The parameters needed to describe qu antum fluctuations are either calculated here or are taken from the literat ure. These results show that we have a reasonable qualitative understanding of the band structure of the lamellar cuprates needed to calculate the ani sotropic exchange constants used here.