# Synopsis: In the strangest of places

Behavior particular to quantum critical metals is, surprisingly, found in a class of insulators.

Metals containing magnetic ions can be driven to the quantum critical regime at zero temperature by tuning the strength of the interaction between the magnetic moments on the ions and the conduction electrons. Typically, this interaction can be tuned by deforming the metal via chemical substitution.

The hallmark of quantum criticality in this type of system is the appearance of scaling laws, such as the so-called $E/T$ scaling: the dynamical response of the system to a perturbation of energy $E$ at temperature $T$ depends only on the ratio $E/T$—in other words, temperature is the only relevant energy scale.

In an article published in Physical Review B, Thomas Heitmann and collaborators from the Technical University of Delft in the Netherlands, the University of Missouri, and the Missouri University of Science and Technology in the US, show that the magnetic scattering intensity measured by neutrons in a family of spinel insulators, otherwise unrelated to quantum-critical metals, exhibits $E/T$ scaling at moderate temperatures. The researchers show that in this class of materials, the origin of the scaling is due to the formation of a network of magnetic clusters. They speculate that similar physics could also be at play in the prototypical quantum-critical metals: the substantial chemical doping could lead to the formation of a network of magnetic clusters, which would then result in $E/T$ scaling, similarly to the spinel case. –Alex Klironomos

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