We consider the exchanged Hamiltonian H-ST = -J Sigma((rr')) (2S(r). S-r' -
1/2)( 2T(r'). T-r' -1/2), describing two isotropic spin-1/2 Heisenberg anti
ferromagnets coupled by a quartic term on equivalent bonds. The model is re
levant for systems with orbital degeneracy and strong electron-vibron coupl
ing in the large Hubbard repulsion limit. To investigate the ground state p
roperties we use a Green's Function Monte Carlo, calculating energy gaps an
d correlation functions, the latter through the forward walking technique.
In one dimension we find that the ground state is a "crystal" of valence bo
nd dimers. In two dimensions, the spin gap appears to remain finite in the
thermodynamic limit, and, consistently, the staggered magnetization-signal
of Neel long range order-seems to vanish. From the analysis of dimer-dimer
correlation functions, however, we find no sign of a valance bond crystal.
A spin liquid appears as a plausible scenario compatible with our findings.
(C) 1999 American Institute of Physics. [S0021-8979(99)53908-X].