Unveiling the control of quenched disorder in rare earth nickelates

Abstract

The role of quenched disorder, a key control to obtain novel phases and functionalities, has not yet been determined in the complex phase diagram of $\mathrm{RNi}{\mathrm{O}}_{3}$ (R = rare-earth ion) perovskites. Here we present such a study by investigating $(\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{E}{\mathrm{u}}_{0.5})\mathrm{Ni}{\mathrm{O}}_{3}$ (LENO) having large R-site cation disorder. We show that in the presence of quenched disorder, (i) the resistivity drops by a few orders of magnitude across the metal-insulator transition (MIT) but the MIT shows only a subtle decrease, (ii) compressive films are completely metallic while largely tensile films are completely insulating sans a MIT, (iii) orthorhombic distortion promotes sharp MIT, and (iv) a Fermi liquid behavior even as high temperature resistivity exceeds the Mott-Ioffe-Regel limit with a bad metallic state. The low-energy terahertz conductivity dynamics obey Drude and Drude-Smith models for compressive and tensile films, respectively. All these features of disordered LENO, which are not typical of prototype ordered $\mathrm{NdNi}{\mathrm{O}}_{3}$, reveal an extraordinary sensitivity to slight structural perturbations. This study depicts the ease with which a variety of electronic phases can be tuned in disordered nickelates and emphasize the need to incorporate quenched disorder as a key control in the phase diagram of nickelates.