On the thermodynamic limit at a quantum critical point
Open questions regarding the uniqueness and existence of the thermodynamic limit at a quantum critical point are discussed in the context of the Lipkin model, a popular model in many-body theory. This model illustrates a quantum phase transition, including spontaneous symmetry breaking. The thermodynamic limit seemingly yields different answers depending on the approach used. The discussion is based on recent developments, including a joint publication by the authors.