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We report on comprehensive high-resolution linewidth measurements of critical antiferromagnetic fluctuations in Ca$_2$RuO$_4$ (CRO214) performed at the neutron resonance spin-echo spectrometer TRISP at FRM II and the cold triple-axis spectrometer FLEXX at BER II. CRO214 is structurally related to the unconventional superconductor Sr$_2$RuO$_4$ [1] and hosts a complex interplay between magnetic and electronic correlations leading to a novel type of soft-magnetism with strong single-ion anisotropy, and ‘Higgs’ amplitude fluctuations in the spin-wave spectrum, as revealed by recent neutron experiments [2].
In contrast to conventional magnetic phase transitions, the magnetic ordering in CRO214 below T$_N$ ~ 110 K emerges from exciton condensation [3]. Therefore, since the magnetic fluctuations in proximity to T$_N$ are fundamentally related to the nature of the magnetic correlations in the system, our study can shed new light on the exceptional ‘excitonic’ magnetism in CRO214.
[1] Nat. 372, 532, (1994).
[2] Nat. Phys. 13, 633, (2017).
[3] Phys. Rev. Lett. 111, 197201, (2013).
