A quantum critical point

This figure summarises one of the key ideas associated with quantum phase transitions: a quantum critical region.

Diagrams such as this appear in many papers (particularly theory ones) but unless I am mistaken there is an actual paucity of real materials for which there is clear unambiguous evidence for actual quantum critical behaviour (i.e. no quasi-particles and only a single energy scale, k_B T).

I welcome readers posting their best examples.