Observing a Scale Anomaly in Graphene: A Universal Quantum Phase Transition
Classical symmetries broken at the quantum level are called anomalies. Since the discovery of this surprising phenomenon, it has become a very active field of research in physics. 
A remarkable class of anomalies occurs when the continuous scale symmetry (CS) of a scale free quantum system is broken into a discrete scale symmetry (DS) for a critical value of a control parameter. This is an example of a (zero temperature) quantum phase transition (QPT). Examples exist which can be cast in the framework of this anomaly, but surprisingly enough none, as yet, has been presented as a universal quantum phase transition. 
A physical realization of broken continuous scale symmetry is the quantum inverse square potential which plays a role in various systems and more importantly in Efimov physics. This broken CS also appears for a charged and massless Dirac fermion in an attractive 1/r Coulomb potential -- the so-called atomic collapse. The purpose of this letter is to exhibit the underlying universality of this particular QPT and to present convincing experimental evidence of its existence for a charged and massless fermion in an attractive Coulomb potential as realised in graphene.