Unusual terahertz spectral weight and conductivity dynamics of the insulator-metal transition in Pr_0.5Nd_0.5NiO₃ thin films

The metal-insulator transition (MIT) in correlated systems is a central phenomenon that possesses potential for several emerging technologies. We investigate the kinetics of such MIT in perovskite nickelates by studying the terahertz (THz) low-energy charge dynamics in orthorhombic and tetragonal symmetries of Pr_0.5Nd_0.5NiO₃ thin films. The THz conductivity of the orthorhombic thin film is dominated by Drude behavior in the entire temperature range, albeit a dominant anomaly at and around the MIT region. The tetragonal thin film exhibits different overall THz conductivity dynamics though, i.e. of a Drude–Smith (DS) type in the entire temperature range, the DS coefficient signifying dominant backscattering peaks in the MIT region. While the overall THz dynamics profile is different for the two films, a unique yet similar sensitivity of the I–M transition regions of both films to THz frequencies underlines the fundamental origin of the bi-critical phase around MIT of the nickelates. The peculiar behavior around the I–M transition, as evaluated in the framework of a percolative path approximation based Dyre expression, emphasizes the importance of critical metallic volume fraction (f _c) for the percolation conduction, as an f _c of ∼0.645 obtained for the present case, along with evidence for the absence of super-heating. (paper)