Exact diagonalization study of the frustrated Heisenberg model: A new disordered phase

The spin-(1/2 frustrated Heisenberg model is studied on a square lattice using the modified Lanczos technique. Frustration is introduced in the system through an additional coupling along the diagonals of the plaquettes. On a 4×4 lattice the ground state smoothly interpolates between two asymptotic Néel-like limits. However, on a narrow region of parameter space we find that the lowest-lying levels above the ground state are singlets rather than triplets, suggesting the existence of a new phase in the thermodynamic limit.