  To illustrate a simple mean-field-like approach for examining quantum phase
transitions we consider the $J-J^\prime$ quantum Heisenberg antiferromagnet on
a square lattice. The exchange couplings $J$ and $J^\prime$ are competing with
each other. The ratio $J^\prime/J$ is the control parameter and its change
drives the transition. We adopt a variational ansatz, calculate the
ground-state energy as well as the order parameter and describe the quantum
phase transition inherent in the model. This description corresponds completely
to the standard Landau theory of phase transitions. We also discuss how to
generalize such an approach for more complicated quantum spin models.
