Unusual Power Law Frequency-dependence of THz Complex Conductivity of Disordered La_xEu_1-xNiO₃/LSAT (100) Thin Films

The presence of disorder has a strong influence on the phase diagram of transition metal oxides. Here, we present low energy carrier dynamics of quench disordered La_xEu_1-xNiO₃/LSAT (100) thin films. While DC transport probes only the overall macroscopic electronic state of the films i.e., insulating state, metal-insulator transition and complete metallic state, for x = 0.25, 0.50 and 0.75, respectively in the temperature range of 2-300 K, the terahertz spectroscopy provides much deeper insights about the microscopic states and their responses at low energy (~meV) range. For x = 0.50, the complex conductivity is dominated by Drude-Smith type response while for x = 0.25 and 0.75, the conductivity exhibits strong power-law frequency dependence. The power-law frequency dependence of the conductivity is most for x = 0.25 i.e., for insulating film. The Drude and Drude-Smith type carrier transports in strongly correlated materials are very common however, the power-law frequency dependence of the conductivity is very rare and is a typical characteristics of non-metallic disordered systems. Thus, the power-law frequency dependence of the conductivity of nickelates reveals its strong behavioral resemblance with non-metallic disordered systems. Conversely, it suggests that the disorder-induced electronic inhomogeneity acts as random free energy barriers other than the scattering center where free charge carriers get accumulated. On increasing THz frequency, these accumulated charge carriers contribute to additional conductivity, thus, overall conductivity increases.