Quantum Criticality and the Stability of Magnetic Order

There is great interest in finding ways to destabilize magnetic order while retaining magnetic character, present in the form of quantum fluctuations and long-ranged entanglement. Such quantum spin liquids were first sought in insulating compounds where geometrical frustration prevents long-ranged and long-lived order. The next step would be to demonstrate that the metallic analogs of such systems could host superconductivity, possibly unconventional, that would be mediated by those magnetic fluctuations. The confluence of unconventional superconductivity and the suppression of magnetic order at quantum critical points is well documented in virtually every class of correlated metal. Remaining less clear is the extent to which the geometrical frustration and low dimensionality are responsible for the quantum critical fluctuations, since in those correlated metals the moments themselves can become unstable at the QCP.
I will compare here the scaling properties of two pristine quantum critical systems: Sc3Mn3Al5Si7, where Mn moments form on the kagome lattice, and YFe2Al10 where Fe moments form a 2D XY lattice. In both, the moment obtained from the Curie-Weiss susceptibility is substantially smaller than the Hunds rule value, and no magnetic order is found to temperatures as small as 0.05 K. Both display power law divergencies in the magnetic susceptibility, as well as field-temperature scaling. The universality implied by those scaling relations is compared to those found in their insulating analogs, as well as other correlated electron systems where the role of geometric frustration is less well understood.