Conventional phase transitions occur at nonzero temperature when the growth of random thermal fluctuations leads to a change in the physical state of a system.
Second-order phase transitions are marked by the growth of fluctuations on ever-longer length-scales.
At a nonzero temperature phase transition, the fluctuations that develop at a critical point are governed by classical physics, because the characteristic energy of quantum fluctuations is always smaller than the characteristic Boltzmann thermal energy
A wide variety of metallic ferromagnets and antiferromagnets have been observed to develop quantum critical behavior when their magnetic transition temperature is driven to zero through the application of pressure, chemical doping or magnetic fields.
There is particular interest in these unusual metallic states, which are believed to exhibit a marked preponderance towards the development of superconductivity.
Most commonly this is done by taking a system with a second-order phase transition which occurs at nonzero temperature and tuning it—for example by applying pressure or magnetic field or changing its chemical composition.
First-order transitions do not normally show critical fluctuations as the material moves discontinuously from one phase into another.