Quantum phase transitions

beimu1009

cite:http://physicsworld.com/cws/article/print/1225

Phase transitions are normally associated with changes of temperature but a new type of transition - caused by quantum fluctuations near absolute zero - is possible, and can tell us more about the properties of a wide range of systems in condensed-matter physics.

Nature abounds with phase transitions. The boiling and freezing of water are everyday examples of phase transitions, as are more exotic processes such as superconductivity and superfluidity. The universe itself is thought to have passed through several phase transitions as the high-temperature plasma formed by the big bang cooled to form the world as we know it today.

Phase transitions are traditionally classified as first or second order.  In first-order transitions the two phases co-exist at the transition temperature - e.g. ice and water at 0 °C, or water and steam at 100 °C. In second-order transitions the two phases do not co-exist. In the last decade, attention has focused on phase transitions that are qualitatively different from the examples noted above: these are quantum phase transitions and they occur only at the absolute zero of temperature. The transition takes place at the "quantum critical" value of some other parameter such as pressure, composition or magnetic field strength. A quantum phase transition takes place when co-operative ordering of the system disappears, but this loss of order is driven solely by the quantum fluctuations demanded by Heisenberg's uncertainty principle.

The physical properties of these quantum fluctuations are quite distinct from those of the thermal fluctuations responsible for traditional, finite-temperature phase transitions. In particular, the quantum system is described by a complex-valued wave function, and the dynamics of its phase near the quantum critical point requires novel theories that have no analogue in the traditional framework of phase transitions.

In the April issue of Physics World, Subir Sachdev of Yale University, US describes the history of quantum phase transitions.

Further reading：

G Aeppli, S Hayden and T Perring 1997 Seeing the spinsin solids Physics World December pp33­37

G Aeppli etal.1998 Nearly singular magnetic fluctuationsin the normal state of a high-Tc cuprate superconductor Science 278 1432

D Bitko, T F Rosenbaum and G Aeppli 1996 Quantumcritical behaviour for a model magnet Phys. Rev. Lett. 77 940

S Das Sarma, S Sachdev and L Zheng 1998 Cantedantiferromagnetic and spin singlet quantum Hall states in double-layer systems Phys.Rev. B58 4672

T Imai et al. 1993 Low frequency spin dynamicsinundoped and Sr-doped La2CuO4 Phys. Rev. Lett. 70 1002

S Sachdev 1999 Quantum Phase Transitions(Cambridge University Press)

S L Sondhi et al. 1997 Continuous quantum phasetransitions Rev. Mod. Phys. 69 315

D Voss 1998 How matter can melt at absolute zero Science 282 221