Quantum Criticality

By raising or lowering the temperature of a liquid, one can induce evaporation into a gas, or freezing into a solid. Physicists refer to the states of matter (solid, liquid, gas) as phases, and the changes of state between them as phase transitions. In the usual case of melting or evaporation, the transition occurs due to heating of the lower temperature phase, which increases the average kinetic energy of the constituent particles. At a continuous phase transition, the low and high temperature phases "dissolve" into a third, highly fluctuating transition state in which energy is exchanged on all length scales. In a universe described by classical mechanics, all such fluctuations would be frozen out at sufficiently low temperatures. In reality, phase transitions are possible between states of matter even at zero temperature, due to quantum mechanics. A continuous zero temperature transition is driven not by quantum fluctuations. This is the scenario of quantum criticality.

Quantum critical points separate metal, insulator, superconducting, and magnetic phases of electrons in many solids. In strongly interacting systems like the high temperature cuprate superconductors, an intriguing possibility is that quantum critical fluctuations enhance or perhaps facilitate superconductivity. This means that quantum criticality might hold the key to high transition temperatures relevant to technological applications. Rice researchers have made fundamental contributions to the theory of quantum criticality in solids with heavy elements (heavy fermion materials), and are leading theoretical and experimental efforts to understand its role in the recently-discovered high temperature superconductors. Rice researchers are also pioneering the study of the strange electronic fluids that emerge at finite temperature above a quantum critical point.