Abstract
Conventional phase transitions, such as ice melting to water or ferromagnetic iron losing its strong intrinsic magnetization, are driven by thermal effects. A different class of transitions, called quantum phase transitions, can occur at absolute zero, driven by quantum fluctuations. In their Perspective, Mackenzie and Grigera discuss results reported in the same issue by Lévy et al. in which quantum critical phenomena are found to play a role in a surprising new kind of superconductivity. At low magnetic fields, the material URhGe is slightly superconducting. Increasing the field strength destroys the superconductivity, which then returns in even stronger form at very high magnetic field. As a result of this and related experiments, quantum criticality is becoming recognized as providing a new framework for discovering exotic superconductivity and other collective quantum states.
