The explanation and control of phase transitions remain cornerstones of contemporary physics. Landau provided an invaluable insight into the thermodynamics of complex systems by formulating their phase transitions in terms of an order parameter. Within this formulation, continuous evolution of the order parameter away from zero classifies the phase transition as second-order, whereas a discontinuous change signals a first-order transition. Here we show that the temperature-tuned insulator–metal transition in the prototypical correlated electron system NdNiO3 defies this established binary classification. By harnessing a nanoscale optical probe of the local electronic conductivity, we reveal two physically distinct yet concurrent phase transitions in epitaxial NdNiO3 films. Whereas the sample bulk exhibits a first-order transition between metal and insulator phases, we resolve anomalous nanoscale domain walls in the insulating state that undergo a distinctly continuous insulator–metal transition, with hallmarks of second-order behaviour. We ascribe these domain walls to boundaries between antiferromagnetically ordered domains within the charge ordered bulk. The close correspondence of these observations to predictions from a Landau theory of coupled charge and magnetic orders highlights the importance of coupled order parameters in driving the complex phase transition in NdNiO3.
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This research was supported by ARO grant W911NF-17-1-0543. Development of cryogenic nano-optical instrumentation is supported by DE-SC0018218 and DE-SC-0012375. D.N.B. is in receipt of the Gordon and Betty Moore Foundation’s EPiQS Initiative investigator Grant GBMF4533. E.W.C. and Y.W. acknowledge support from NSF DMR-1508236 and Dept. of Education grant no. P116F140459. Financial support from the Deutsche Forschungsgemeinschaft (DFG) under Grant No. SFB/TRR80 G1 is acknowledged by M.H., M.B., G.C., G.L., P.R., M.M., E.B. and B.K.
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Post, K.W., McLeod, A.S., Hepting, M. et al. Coexisting first- and second-order electronic phase transitions in a correlated oxide. Nature Phys 14, 1056–1061 (2018). https://doi.org/10.1038/s41567-018-0201-1
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