Over the past few years, condensed-matter physicists have become fascinated by the phenomenon of quantum phase transitions. We know that new forms of order develop within a material by way of phase transitions and that they can drastically change its properties.
Classical examples of quantum phase transitions, such as superconductivity and magnetism occur at finite temperatures, but when the phase-transition temperature is suppressed to near absolute zero, quantum effects become important. Recent experiments in the UK and US have shown that these weird states of “quantum criticality” can be achieved in that most common of materials chromium.
Pioneering work by Lev Landau in the 1940s proposed that the development of phases in a material can be described by the emergence of an “order parameter”. This quantity describes the state of order, such as the local magnetic polarization of a ferromagnet, as it develops at each point in the material. A material that is close to a classical phase transition senses thermal fluctuations that develop in the order parameter over successively larger regions. This is known as a “critical state” and its understanding is one of the triumphs of condensed-matter physics in the last century.
Now, however, a new revolution is taking place with the advent of quantum criticality. Once thought to be of purely academic interest, the phenomenon of quantum phase transitions has emerged as a major challenge to our understanding of condensed matter.
In the January issue of Physics World, Piers Coleman in the Department of Physics and Astronomy at Rutgers University in the USA, discusses some recent developments in this field.
