At https://eurekalert.org/pub_releases/2019-07/ru-pff071519.php … apparently, there is no way to prove that a three dimensional 'quantum spin liquid' exists. Physicists have come up with another way to persuade their fellows of such a discovery – single crystals of cerium zirconium pyrochlore that contained the right kind of stuff to qualify as the first 3D version of the long sought after state of matter. In a research paper in Nature Physics (July 2019) a lot of experimental evidence was on offer – including neutron scattering experiments at the Oak Ridge National Laboratory and muon spin relaxation experiments at Switzerland's Paul Scherrar Institute. They have produced the first material that might qualify as a 3D quantum spin liquid (or that is the argument put forward).
However, a quantum spin liquid (funnily enough, a solid) is something that scientists define based on what they DON'T see (their words). No long range order in the arrangement of the spins – but neither do you see disorder. Or anything else. It is not this – or that. There is no conclusive evidence identification as the paper points out. In spite of all the effort the team say it is impossible to definitively say cerium zirconium 227 is a spin liquid. This is partly because the definition of a quantum spin liquid is a state that exists at absolute zero temperature – not a fraction above or a fraction below zero. Does zero exist?
A copy of the paper can be read at www.nature.com/articles/s41567-019-0577-6