[Paper Review] Discovery of Giant Unit-Cell Super-Structure in the Infinite-Layer Nickelate PrNiO$_2$

Jens Oppliger, J. Küspert+11 more|arXiv (Cornell University)|Apr 27, 2024

Magnetic and transport properties of perovskites and related materialsMaterials Science3 citations

TL;DR

This study discovers a giant, stable superlattice structure in the infinite-layer nickelate PrNiO₂ through in-situ temperature annealing, revealing a rare sixfold in-plane and fourfold out-of-plane periodicity. The superstructure, attributed to diffusive oxygen ordering, forms above room temperature and suggests a pathway to ultra-small Brillouin zone electronics distinct from Moiré systems.

ABSTRACT

Spectacular quantum phenomena such as superconductivity often emerge in flat-band systems where Coulomb interactions overpower electron kinetics. Engineering strategies for flat-band physics is therefore of great importance. Here, using high-energy grazing-incidence x-ray diffraction, we demonstrate how in-situ temperature annealing of the infinite-layer nickelate PrNiO$_2$ induces a giant superlattice structure. The annealing effect has a maximum well above room temperature. By covering a large scattering volume, we show a rare period-six in-plane (bi-axial) symmetry and a period-four symmetry in the out-of-plane direction. This giant unit-cell superstructure likely stems from ordering of diffusive oxygen. The stability of this superlattice structure suggests a connection to an energetically favorable electronic state of matter. As such, our study provides a new pathway - different from Moiré structures - to ultra-small Brillouin zone electronics.

Motivation & Objective

• To investigate the emergence of long-range superstructures in the infinite-layer nickelate PrNiO₂ under thermal treatment.
• To determine the origin and stability of a previously unobserved giant unit-cell superstructure in PrNiO₂ thin films.
• To explore whether oxygen diffusion or capping layer structure governs the formation of this superlattice.
• To assess the implications of this superstructure for flat-band physics and ultra-small Brillouin zone electronics.
• To establish a new engineering pathway for quantum materials beyond Moiré heterostructures.

Proposed method

• High-energy grazing-incidence x-ray diffraction (GIXD) was employed to probe large scattering volumes across multiple Brillouin zones.
• In-situ temperature annealing was applied to PrNiO₂ thin films grown on SrTiO₃ substrates, with measurements conducted at 386 K.
• Two film systems were studied: one with crystalline SrTiO₃ capping and one with amorphous SrTiO₃ capping, to isolate capping effects.
• Reciprocal space maps were reconstructed from detector images, with peak intensities analyzed using split Gaussian functions and quadratic backgrounds.
• Correlation lengths were extracted from the standard deviations of split Gaussians to assess long-range order.
• Data were collected at the P07 beamline (PETRA III, DESY) using 73 keV x-rays at grazing incidence (μ = 0.05°) and a Pilatus3 X CdTe 2M detector.

Experimental results

Research questions

• RQ1 What structural superstructures emerge in PrNiO₂ upon thermal annealing above room temperature?
• RQ2 Is the observed superlattice structure dependent on the crystallinity of the SrTiO₃ capping layer?
• RQ3 What is the origin of the giant 6×6×4 superstructure—oxygen ordering, cation displacement, or intercalation?
• RQ4 How does the stability and periodicity of this superstructure relate to electronic flat-band physics?
• RQ5 Can this superstructure serve as a platform for ultra-small Brillouin zone electronics distinct from Moiré systems?

Key findings

• A stable giant superlattice with a 6×6×4 unit cell was observed in PrNiO₂, with a period-six in-plane translational symmetry and a period-four out-of-plane stacking order.
• The superstructure emerges above room temperature and remains stable over a broad thermal range, indicating high thermodynamic stability.
• The superstructure forms regardless of whether the SrTiO₃ capping layer is crystalline or amorphous, indicating it is intrinsic to the PrNiO₂ film.
• The superlattice reflections were clearly resolved in reciprocal space, with intensity maps showing distinct peaks at (h,k,1.75), (h,1,ℓ), and (1,k,ℓ) planes.
• The correlation length derived from split Gaussian fits indicates long-range order, supporting the stability of the superstructure.
• The results suggest that oxygen diffusion during annealing is the likely origin of the superstructure, potentially stabilizing a low-energy electronic state.