Spin-liquid ground state in a two-dimensional nonfrustrated spin model

We consider an exchange model describing two isotropic spin-1/2 Heisenberg antiferromagnets coupled by a quartic term on the square lattice. The model is relevant for systems with orbital degeneracy and strong electron-vibron coupling in the large Hubbard repulsion limit, and is known to show a spin -Peierls-like dimerization in one dimension. In two dimensions we calculate energy gaps, susceptibilities, and con elation functions with a Green's fu nction Monte Carlo method. We find a finite tripler and singlet gap, with n o evidence of order of any kind. We conclude that the ground state is, most likely, a spin liquid of resonating valence bonds.