Gregorio Ponti, UT-Austin

"Metal-Insulator transition in yttrium- and neodymium-doped lanthanum nickelates"

Abstract: Discovery of high-temperature superconductivity (SC) in (La,Ba)₂CuO₄^[1] opened many lines of investigation into similar perovskite-related compounds, such as the square-planar "infinite-layer" superconductor, Sr_1-xR_xCuO₂ (R = rare-earth element) ^[2]. Until recently, SC had not yet been found in any nickelates, including the isostructural infinite-layer RNiO₂. Presently, chemical reduction of RNiO₃/STO thin films (R = Nd_0.8Sr_0.2) using CaH₂ into the RNiO₂ infinite-layer phase was found to have a zero-resistivity measurement below T_c = 9 − 15 K^[3]. This study left many open questions, mainly regarding the effect on superconductivity of oxygen defects (vacancies and interstitials, e.g., apical) and of strain (due to cation substitutions and substrate-film mismatch).  The study also re-prompted the search for SC in the bulk material. The parent RNiO₃ bulk compounds are known to have a rich structural phase diagram and a metal-insulator transition T_M-I that varies based on the rare-earth element.

This work investigates steric and doping effects in bulk nickelates La_1−xR_xNiO₃ (R = Y, Nd) and both develops and tests the prediction of a universal T→0 Metal-Insulator (M-I) transition for all (La, R)NiO₃ nearly-perovskite materials. Polycrystalline bulk samples were prepared using a sol-gel precursor that was then subject to high oxygen pressure (150 − 200 bar), high temperature (950°C − 1050°C) conditions, or high quasi-hydrostatic pressures using a hot piston-cylinder apparatus (2.0 − 2.5 GPa) with an oxidizer at high temperatures (900°C − 1000°C) for La_1−xY_xNiO₃ (x > 0.2). These compounds can be reduced several ways to produce the infinite layer phase, as well as some other R_nNi_nO_y phases (e.g., “337”). Similarly, thin films of Nd_1−xA_xNiO₃ (A = La, Sr) deposited using Pulsed Laser Deposition can be reduced to compare the effects in the bulk to that in thin films. The synthesized samples are studied using x-ray diffraction, electrical resistivity, and other techniques to correlate structural, transport, and magnetic properties.

[1] Bednorz, J. G. & Müller, K. A., Phys. B 64, 189–193 (1986).

[2] M. G. Smith, A. Manthiram, J. Zhou, J. B. Goodenough, and J. T. Markert, Nature 351, 549–551 (1991).

[3] D. Li et al., Nature 572, 624 (2019).