I study the order/disorder transition due to singlet formation
in a quantum spin system. The systems is build by spin 1/2 on a
two-dimensional square lattice with two different kinds of
antiferromagnetic Heisenberg interactions. If one of the interactions
becomes sufficiently larger then the other the purely quantum effect of
singlet formation drives the system into a disordered phase with only
short range correlations in the plaquettes and a spin gap. I study the
transition point by evaluating different exact diagonalization data as
spin gap and spin-spin correlation. I compare the result of different
methods and confirm the critical value for the
transition point.
©Andreas Voigt