Dynamic Doping of Nickelates with Lithium Reveals a Widely Tunable Insulator-Metal Transition with Charge Filling and...

The insertion of electron-donating ions has emerged as a powerful technique to manipulate the electronic structure of correlated oxides. However, the resulting electronic structure remains poorly understood, with challenges in quantifying dopant concentration, unexplained differences with substitutionally doped films, and a poor understanding of how dopant atoms interact with insulator-metal transitions (IMTs). Here, these issues are addressed in the context of the rare earth nickelates, a prototypical correlated oxide family with widely tunable electronic behavior under the insertion of protons and alkali metals as interstitial dopants. RNiO₃ (R = Pr, Nd) epitaxial thin films are synthesized, lithium dopants are introduced and quantified using electrochemical and synchrotron-based techniques, and the resulting electronic structure is studied. From electronic transport measurements of Li_xLiRNiO₃, lithium is found to affect the metal-insulator transition, causing more than an order of magnitude reduction in ground-state resistivity at fractions x_Li < 0.18, a systematic lowering of transition temperature, and successively smaller ON/OFF ratios over 0.00 < x_Li < 0.25. At larger fractions x_Li > 0.25, the transition is destroyed, and insulating behavior is observed over T = 5-300 K. Angle-resolved photoemission (ARPES) confirms transport results and reveals band renormalization occurring over 0.10 < x_Li ≤ 0.71. ARPES and X-ray absorption spectroscopy (XAS) combined with density functional theory indicate that rigid band filling models are generally insufficient to explain doping from lithium, especially at low temperatures, but could approximate room temperature effects in the low doping regime (x_Li < 0.10). Broadly, the results indicate that interstitial dopants lead to complex interactions with metal-insulator transitions and the emergence of an exciting family of correlated electronic phases.