Phase transitions have fascinated physicists for a very long time. The transitions of water to ice, or an electrical current from resistive flow into a super conducting flow, have proven to be surprisingly difficult to describe accurately. For example, at low pressure, water molecules separate and become a vapor. However, as the pressure increases, the molecules are forced closer together and the water liquefies. In doing so, the capacity of the material to store heat changes suddenly. On either side of the transition, water and vapor are quite easy to describe by the pressure and temperature but at the phase transition, everything falls apart. If the pressure and temperature are chosen to fall at a critical point, then the water and vapor can no longer be distinguished. Islands of water form throughout the volume of the container and within those islands are pockets of vapor. Within those pockets are still more islands of water, and this continues from the scale of the size of the vessel down to a single water molecule. This gives some hint as to how complicated things become at the phase transition point. To get around these difficulties, a key physical property is approximated and the complexity of that approximation is described as an an order parameter. The phase transition is described by how various properties change with that property and parameter.
Much to everyone's surprise, this classical parameter approach seemed to work quite well for quantum systems, too. For instance, Bose-Einstein condensation, superfluidity, and superconductivity all have a classical order parameter (e.g. temperature). However, quantum critical points are harder to test because they occur at absolute zero. Now a team of researchers from many institutions have published a paper in Science, where they examine the onset of a magnetic critical point in a metal alloy by cooling it to a few tens of milliKelvin. They have shown that the order parameter, which is energy in this case, is not sufficient to describe the behavior as the metal cools down towards the critical point. The researchers believe that a second energy parameter that is related to the correlations between the separate spins that generate the macroscopic magnetic field.
Phase transitions have been one of the hottest areas of physics for decades. This new discovery will, hopefully, shed yet more light on the mystery of entanglement.