It is shown that the ground state of a spin-1/2 antiferromagnetic ladder model is characterized by a string order parameter which is similar to but differs from that introduced by den Nijs and Rommelse. This reveals the presence of a hidden antiferromagnetic order and symmetry breaking even in the ground state which relates to high-${\mathit{T}}_{\mathit{c}}$ superconductors. The inverse correlation length is further calculated for any ratio of the interchain and intrachain couplings under shifted boundary conditions.

• Received 6 July 1995

DOI:https://doi.org/10.1103/PhysRevB.52.12508