A truly quantum phase transition

Rajiv Singh has a nice clear Viewpoint in Physics, Does quantum mechanics play a role in critical phenomena?

It discusses a recent PRL by Anders Sandvik that found some unexpected behaviour (logarithmic corrections to scaling) near the quantum critical point of a Heisenberg model which is the best candidate for deconfined quantum criticality.

Many quantum critical points in d spatial dimensions are expected to have properties similar to corresponding classical critical points in d+1 dimensions.

So when does quantum physics really matter?

Rajiv gives a nice discussion of how quantum interference effects associated with Berry's phases may lead to qualitatively different behaviour.

For example, the existence of a critical point where the order parameters O1 and O2 of two physically distinct phases both vanish (as in the figure above) are not expected in classical Landau theory unless parameters are finely tuned.