Probing the evolution of electronic phase-coexistence in complex systems by terahertz radiation

In complex oxides, electrons under the influence of competing energetics determine the coexistence or phase-separation of two or more electronic or magnetic phases within the same structural configuration. Probing the growth and evolution of such phase-coexistence state is crucial to determine the correct mechanism of related phase transition. Here, we demonstrate the combination of terahertz (THz) time-domain spectroscopy and DC transport as a strategy to probe the electronic phase-coexistence. This is demonstrated in disorder-controlled phase-separated rare-earth nickelate thin films, which exhibit a temperature induced metal-insulator transition in DC conductivity but lack this transition in THz dynamic conductivity. Such pronounced disparity exploits two extreme attributes, namely, the large sensitivity of THz radiation to a spatial range of the order of its wavelength-compatible electronic inhomogeneities, and its insensitivity to a range beyond the size of its wavelength. This feature is generic in nature, depending solely on the size of insulating and metallic clusters. Therefore, our strategy offers a high-sensitivity methodology to investigate electronic phase-coexistence and phase transition in a wide range of complex material systems.