Abstract
We investigate the nature of the ground state of quantum spin-(1/2) models on a 2D square lattice using a Jastrow variational wave function. The effects of quantum frustration in a spin-(1/2) Mott insulator doped with holes are represented by a second-neighbor antiferromagnetic Heisenberg coupling. We find disappearance of 2D antiferromagnetic long-range order above a critical hole concentration on comparing the ground-state energy for a Jastrow function with off-diagonal long-range order with that computed using a power law form. We also investigate the nature of the superfluidity of 4He monolayer on a periodic substrate. We find that in a limit where the substrate potential is sufficiently weak, represented by a 2D spin-(1/2) XY model, the effects of quantum fluctuations destroy the ODLRO of the 4He film.