Soft spin-amplitude fluctuations in a Mott-insulating ruthenate

A. Jain; M. Krautloher; J. Porras; G. H. Ryu; D. P. Chen; D. L. Abernathy; J. T. Park; A. Ivanov; J. Chaloupka; G. Khaliullin; B. Keimer; B. J. Kim

arXiv · 1510.07011

Soft spin-amplitude fluctuations in a Mott-insulating ruthenate

A. Jain·M. Krautloher·J. Porras·G. H. Ryu·D. P. Chen·D. L. Abernathy·J. T. Park·A. Ivanov·J. Chaloupka·G. Khaliullin·B. Keimer·B. J. Kim

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Abstract

Magnetism in transition-metal compounds (TMCs) has traditionally been associated with spin degrees of freedom, because the orbital magnetic moments are typically largely quenched. On the other hand, magnetic order in 4f- and 5d-electron systems arises from spin and orbital moments that are rigidly tied together by the large intra-atomic spin-orbit coupling (SOC). Using inelastic neutron scattering on the archetypal 4d-electron Mott insulator Ca$_2$RuO$_4$, we report a novel form of excitonic magnetism in the intermediate-strength regime of the SOC. The magnetic order is characterized by ``soft'' magnetic moments with large amplitude fluctuations manifested by an intense, low-energy excitonic mode analogous to the Higgs mode in particle physics. This mode heralds a proximate quantum critical point separating the soft magnetic order driven by the superexchange interaction from a quantum-paramagnetic state driven by the SOC. We further show that this quantum critical point can be tuned by lattice distortions, and hence may be accessible in epitaxial thin-film structures. The unconventional spin-orbital-lattice dynamics in Ca$_2$RuO$_4$ identifies the SOC as a novel source of quantum criticality in TMCs.

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A. Jain, M. Krautloher, J. Porras, G. H. Ryu, D. P. Chen, D. L. Abernathy, J. T. Park, A. Ivanov, J. Chaloupka, G. Khaliullin, B. Keimer, B. J. Kim. 2015-10-23. Soft spin-amplitude fluctuations in a Mott-insulating ruthenate. https://arxiv.org/abs/1510.07011

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