Research on the controllable growth and electronic phase transitions for metastable perovskite rare-earth nickelate films

Abstract

The multiple electronic phase transitions as achieved in the metastable perovskite (<i>Re</i>NiO₃, <i>Re</i> denotes as the lanthanide rare-earth elements) by using a critical temperature, hydrogenation, electrical field and interfacial strain arouse considerable attentions in the field of condensed matter physics and material science that enable the promising applications in the field of critical temperature thermistor, artificial intelligence, energy conversion and weak electric field sensing. Nevertheless, the above abundant applications are still bottlenecked by the intrinsically thermodynamic metastability associated to <i>Re</i>NiO₃. Herein, we synthesized the atomically flat <i>Re</i>NiO₃ film material with thermodynamic metastability by using laser molecular beam epitaxy (LMBE) that exhibits excellent thermally-driven electronic phase transitions. Noting the similar lattice constant between LaAlO₃ substrate and <i>Re</i>NiO₃ film, this is attributed to the interfacial heterogeneous nucleation as induced by the template effect of as-used (001)-oriented LaAlO₃ substrates. In addition, we clarify the critical role of <i>in situ</i> annealing upon an oxygen-enriched atmosphere in stabilizing the distorted perovskite structure associated to <i>Re</i>NiO₃. Apart from the depositing process associated to LMBE, the <i>Re</i>NiO₃ with heavy rare-earth composition exhibits a more distorted NiO₆ octahedron and a higher Gibbs free energy that is rather difficult to be synthesized by using physical vacuum deposition. As a representative case, the <i>in situ</i> annealing-assisted LMBE process cannot be utilized to deposit the SmNiO₃ film, in which the impurity peaks associated to <i>Re</i>₂O₃ and NiO are observed in its XRD spectra. Assisted by the X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure, the valence state of nickel for <i>Re</i>NiO₃ was revealed to be +3, with the ^t_2g⁶<i>e</i>_g¹ configuration being observed. Considering the highly tunable electronic orbital configuration of <i>Re</i>NiO₃ associated to the NiO₆ octahedron, co-occupying the A-site of perovskite structure with Nd and Sm elements regulates the transition temperature (TMIT) for <i>Re</i>NiO₃ within a broad temperature range. Furthermore, we demonstrate the anisotropy in the electronic phase transitions for Nd_1-xSm_xNiO₃, in which case the TMIT as achieved in the Nd_1-xSm_xNiO₃/LaAlO₃ (111) heterostructure exceeds the one deposited on the (001)-oriented LaAlO₃ substrate. The presently observed anisotropy in the electrical transportation for Nd_1-xSm_xNiO₃ film materials is associated to the anisotropic in-plane NiO₆ octahedron configuration as triggered by differently oriented LaAlO₃ substrates. The present work is expected to introduce a new freedom to regulate the electronic phase transition and explore new electronic phase within <i>Re</i>NiO₃ material system, and pave ways toward growing atomically flat <i>Re</i>NiO₃ film material with expected electronic phase transition functionality.