The researchers experimentally confirmed the predicted linear evolution of the gap with the magnetic field, which allowed them to pinpoint the location of the quantum critical point. At the critical field, the observable is expected to display a power-law temperature dependence, another hallmark of quantum criticality, with a characteristic power of -3/4 in this case—precisely what they observed. Even more, a rigorous experimental analysis allowed them to estimate the prefactor to this behavior, which they found to correspond nicely to the theoretically predicted one. And finally, they observed this behavior to persist to as high a temperature as almost half of the exchange coupling, which sets the global energy scale of the problem. This answers an essential question about how far away from the absolute zero temperature quantum criticality reaches or how measurable it really is. The experiment constitutes the first quantitative confirmation of the quantum critical behavior predicted by any of the few existing theories.
Nice! Not surprisingly (at least, not to me), the clearest confirmation of this exotic quantum phenomenon is first found in a condensed matter system.
A few background reading for those who want to have more info on quantum criticality can be found here and here.
Zz.
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