Charge Distribution across Capped and Uncapped Infinite‐Layer Neodymium Nickelate Thin Films

• Published in: Small (Weinheim an der Bergstrasse, Germany) Germany), 2023-12, Vol.19 (49), p.e2304872-n/a
• Main Authors: Raji, Aravind, Krieger, Guillaume, Viart, Nathalie, Preziosi, Daniele, Rueff, Jean‐Pascal, Gloter, Alexandre
• Format: Article
• Language: English
• Subjects: Capping; Charge distribution; charge order; Charge transfer; Condensed Matter; Electron energy loss spectroscopy; Electrons; hard X‐ray photoemission spectroscopy; infinite‐layer nickelates; Nanotechnology; Neodymium; Oxygen; Parameters; Photoelectric emission; Physics; Scanning transmission electron microscopy; Spectrum analysis; STEM‐EELS; Strongly Correlated Electrons; Superconductivity; Thin films
• ISSN: 1613-6810; 1613-6829
• DOI: 10.1002/smll.202304872

Charge ordering (CO) phenomena have been widely debated in strongly‐correlated electron systems mainly regarding their role in high‐temperature superconductivity. Here, the structural and charge distribution in NdNiO2 thin films prepared with and without capping layers, and characterized by the absence and presence of CO are elucidated. The microstructural and spectroscopic analysis is done by scanning transmission electron microscopy‐electron energy loss spectroscopy (STEM‐EELS) and hard X‐ray photoemission spectroscopy (HAXPES). Capped samples show Ni1+, with an out‐of‐plane (o‐o‐p) lattice parameter of around 3.30 Å indicating good stabilization of the infinite‐layer structure. Bulk‐sensitive HAXPES on Ni‐2p shows weak satellite features indicating large charge‐transfer energy. The uncapped samples evidence an increase of the o‐o‐p parameter up to 3.65 Å on the thin film top with a valence toward Ni2+ in this region. Here, 4D‐STEM demonstrates (303)‐oriented stripes which emerge from partially occupied apical oxygen. Those stripes form quasi‐2D coherent domains viewed as rods in the reciprocal space with Δqz ≈ 0.24 reciprocal lattice units (r.l.u.) extension located at Q = (±13,0,±13$\pm \frac{1}{3},0,\pm \frac{1}{3}$) and (±23,0,±23$\pm \frac{2}{3},0,\pm \frac{2}{3}$) r.l.u. The stripes associated with oxygen re‐intercalation concomitant with hole doping suggest a possible link to the previously reported CO in infinite‐layer nickelate thin films. Understanding superconductivity is quite challenging due to its intertwined existence with competing orders such as charge ordering (CO) or lattice instability. Here, a real‐space structural and electronic study between capped and uncapped nickelate samples that evidences a differential charge distribution and modulation across them is carried out. This provides a new perspective on the origin of CO in nickelates.