Quantum criticality in FeSc2S4
Our group has been studying spinel antiferromagnets, with the chemical formula AB2X4, for some time, focusing on those materials in which the A-site is magnetic. Amongst those materials studied experimentally, FeSc2S4 stands out for its notable lack of magnetic ordering or freezing transition, even at temperatures approximately 1000 times smaller than its Curie-Weiss temperature. It has been discussed as an example of a "spin-orbital liquid", owing to a two-fold orbital degeneracy of the Fe2+ ions which is not present in the other well-studied A-site spinels.
^ TOPOur work:
We propose a model for FeSc2S4, in which the physics is controlled by a competition between spin-orbit coupling and exchange interactions. The former tends to stabilize a non-magnetic Spin-Orbital Singlet (SOS) state, while the latter favors a magneticaly ordered ground state. Between these two phases lies a Quantum Critical Point (QCP), and we argue that FeSc2S4 lies near this QCP on the SOS side. This proposal naturally explains the small (2K) gap, which is much smaller than both the magnetic bandwidth and spin orbit interaction, both of order of several tens of Kelvin. We study the QCP by mean field theory and other techniques, and discuss its physical consequences.
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