To illustrate a simple mean-field-like approach for examining quantum phase transitions we consider
the ##IMG## [http://ej.iop.org/images/0143-0807/38/3/033002/ejpaa5c71ieqn1.gif]
{$J\mbox{–}{J}^{\prime }$} quantum Heisenberg antiferromagnet on a square lattice. The exchange
couplings J and ##IMG## [http://ej.iop.org/images/0143-0807/38/3/033002/ejpaa5c71ieqn2.gif]
{${J}^{\prime }$} are competing with each other. The ratio ##IMG##
[http://ej.iop.org/images/0143-0807/38/3/033002/ejpaa5c71ieqn3.gif] {${J}^{\prime }/J$} 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
…