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Spin gap in low-dimensional Mott insulators with orbital degeneracy

L. Guidoni·G. Santoro·S. Sorella·A. Parola·E. Tosatti
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Abstract

We consider the exchange Hamiltonian H_ST = -J Sum_{ } (2 S_r S_r' - 1/2) (2 T_r T_r' - 1/2) describing two isotropic spin-1/2 Heisenberg antiferromagnets coupled by a quartic term on equivalent bonds. The model is relevant for systems with orbital degeneracy and strong electron-vibron coupling in the large Hubbard repulsion limit. To investigate the ground state properties we use a Green's Function Monte Carlo, calculating energy gaps and correlation functions, the latter through the forward walking technique. In one dimension we find that the ground state is a ``crystal'' of valence bond 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 valence bond crystal. A spin liquid appears as a plausible scenario compatible with our findings.

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L. Guidoni, G. Santoro, S. Sorella, A. Parola, E. Tosatti. 1998-09-01. Spin gap in low-dimensional Mott insulators with orbital degeneracy. https://doi.org/10.1063/1.370241

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