Critical slowdown of spontaneous fluctuations in the vicinity of metal-insulator transition in rare earth nickelates

Fluctuations of the order parameter near the phase transition boundary in complex oxides could be key to designing probabilistic bits and stochastic synapses. Direct probes of fluctuations how- ever are non-trivial due to the requirement of coherent spatio-temporal probes. Here, we report magnetic and lattice fluctuation dynamics in prototypical perovskite nickelate systems of NdNiO3 and SmNiO3 using X-ray photon correlation spectroscopy. For NdNiO3, our studies show strong coupling of structural and magnetic order parameters with slowdown of fluctuation timescales by one order of magnitude near Néel temperature irrespective of the epitaxial strain of the film. For SmNiO3, no such slowdown is observed emphasizing the distinct nature of structural fluctuations. Our measurements of diffraction wavevector dependence, indicate that short range structural fluctuations are surprisingly slow (3-5x slower) compared to long range fluctuations. Our results reveal novel insights into fluctuations enabled by coherent x-rays, expanding the unique capability of spatio-temporal probes to investigate quantum materials.