Quantum Monte Carlo study of weakly coupled spin ladders

We report a quantum Monte Carlo study of the thermodynamic properties of arrays of spin ladders with various widths (n), coupled via a weak interladder exchange coupling alpha J, where J is the intraladder coupling both along and between the chains. This coupled ladder system serves as a simplified model for the magnetism of presumed ordered spin and charge stripes in the two-dimensional CuO2 planes of hole-doped copper oxides. Our results for n = 3 with weak interladder coupling alpha = 0.05, estimated from the t-t'-t "-J model, show good agreement with the ordering temperature of the recently observed spin-density-wave condensation in La2CuO4+y. We show that there exists a quantum critical point at alpha(c) similar or equal to 0.07 for n = 4, and determine the phase diagram. Our data at this quantum critical point agree quantitatively with the universal scaling predicted by the quantum nonlinear a model. We also report results on random mixtures of n = 2 and n = 3 ladders, which correspond to the doping region near but above 1/8. Our study of the magnetic static structure factor reveals a saturation of the incommensurability of the spin correlations around 1/8, while the incommensurability of the charge stripes grows linearly with hole concentration. The implications of this result for the interpretation of neutron-scattering experiments on the dynamic spin fluctuations in La2-xSrxCuO4 are discussed. [S0163-1829(99)14829-X].