Spin phase transitions

A spin phase transition (Sun2019?) is the process in which a spin configuration changes, e.g. from singlet to triplet.

While, in the purest sense, it is only applicable for actual eigenfunctions of \(\hat{S}_z\), in the context of GHF, the term is also used related to the expectation value \(\ev{\hat{S}}_z\), or in general the norm of \(\ev{\hat{\vb{S}}}\).