Coexisting first and second-order electronic phase transitions in a correlated oxide

Understanding and controlling phase transitions is a cornerstone of contemporary physics. Landau provided an invaluable insight by formulating the thermodynamics of complex systems in terms of a local order parameter (Φ), wherein second-order transitions are described by continuous evolution of Φ from zero, whereas in first-order transitions, Φ changes discontinuously. Here we show that the temperature-tuned insulator-to-metal transition (IMT) in the prototypical correlated electron system NdNiO₃ defies this established binary classification. By harnessing a nano-scale optical probe of the local electronic conductivity, we observed two physically distinct, yet concurrent phase transitions in different regions of a 7nm NdNiO₃ epitaxial film.In the bulk of the material, we resolve a discrete, first-order transition between metal and insulator phases. Meanwhile, we visualize anomalous nano-scale “domains walls” in the insulating state that undergo a continuous IMT, with hallmarks of a second-order transition, distinct from the bulk behavior of our specimen. The accurate reproduction of our experimental findings within Landau theory confirms that interaction between concurrent orders forms a crucial organizing principle in the complex phase transition of NdNiO₃.