A Multi-Modal Spectroscopy Technique to Probe Emergent Quantum Order using Magnetic Resonance

The quadrupolar interaction between a nuclear spin and its local electric environment describes the local point-group symmetry and imprints information into the spin's dynamics. In traditional magnetic resonance, this information is encoded in the spectra. But it is easily obscured by multiple sources of spectral broadening, including non-trivial multipolar exchange interactions and zero local values of susceptibility. This is particularly relevant for materials that exhibit an emergence of exotic quantum phases that possess unusual symmetries and orderings, whose order parameters have large variations near phase transitions, and oftentimes zero mean.

Here, we present results from a multi-modal spectroscopy technique applied to a Mott insulator with strong spin-orbit coupling, showcasing a general probe of emergent quantum order that can be used in other materials. We can distinguish if orbital or magnetic order drives a phase transition by extracting the temperature-dependence of their associated fluctuations. This represents an ideal method for evaluating the distribution of disorder above phase transitions, identifying relevant parameters for “order from disorder” transitions, which are characterized by fluctuations of an order parameter with zero mean.