Achieving superconductivity in infinite-layer nickelate thin films by aluminum sputtering deposition

TL;DR

This work introduces a practical aluminum-sputtering reduction method to synthesize superconducting infinite-layer nickelate thin films, specifically Pr0.8Sr0.2NiO2, from the perovskite precursor Pr0.8Sr0.2NiO3. Systematic optimization shows that in situ aluminum reduction yields the best crystallinity and transport properties, achieving a maximum $T_{c}^{onset}$ of 17 K and robust superconductivity with sharp transitions, while ex situ reduction exhibits greater disorder and sample-to-sample variability. The method, which can operate entirely in situ or after air exposure, preserves surface cleanliness and enables surface-sensitive probes (ARPES/STM), potentially accelerating understanding of nickelate superconductivity. Overall, aluminum sputtering reduction provides a simpler, more reproducible, and accessible pathway to high-quality IL nickelate superconductors, expanding participation across research groups. The findings suggest that minimizing surface contamination and optimizing topotactic transformation are key to achieving consistent superconducting behavior in IL nickelates.

Abstract

The recent discovery of superconductivity in infinite-layer (IL, ABO$_2$) nickelates has opened a new avenue to deepen the understanding of high-temperature superconductivity. However, progress in this field is slowed by significant challenges in material synthesis and the scarcity of research groups capable of producing high quality superconducting samples. IL nickelates are obtained from a reduction of the perovskite ABO$_3$ phase, typically achieved by annealing using CaH$_2$ as a reducing agent. Here, we present a new method to synthesize superconducting infinite-layer nickelate Pr$_{0.8}$Sr$_{0.2}$NiO$_2$ thin films using an aluminum overlayer deposited by sputtering as a reducing agent. We systematically optimized the aluminum deposition parameters and obtained superconducting samples reduced either in situ or ex situ (after air exposure of the precursor ABO$_3$ films). A comparison of their crystalline quality and transport properties shows that in situ Al reduction enhances the quality of the superconducting Pr$_{0.8}$Sr$_{0.2}$NiO$_2$ thin films, achieving a maximum superconducting transition temperature $T_{c}^{onset}$ of 17 K, in agreement with the optimum value reported for this compound. This simple synthesis route, much more accessible than existing methods, offers better control and reproducibility over the topotactic transformation, opening new opportunities to gain insights into the physics of superconductivity in nickelates.

Figures (13)

• Figure 1: a) Schematic diagram of the Al reduction process in nickelates. First, an aluminum overlayer is sputtered onto the precursor PSNO3 perovskite thin film at moderate temperatures (280 - 380°C). This is followed by a post-deposition annealing at the same temperature for a specified time (60 - 180 min). Al deposition temperature and post-annealing time are selected depending on the thickness of the precursor perovskite film. b) X-Ray Diffraction $\theta$ - 2$\theta$ symmetric scans of a 8-nm thick parent PSNO3 thin film reduced under different Al deposition temperatures, 350, 380 and 390°C. The post-annealing time was fixed to 120 minutes in all the cases. c) Resistivity as a function of the temperature for the samples showed in panel b). d) Temperature dependence of resistivity as a function of the post-annealing time for a representative series of 8 nm-thick films when Al is deposited at 380°C. The inset shows the same data near the superconducting transition. e) Evolution of the Al deposition temperature (grey circles) and post-annealing time (purple circles) as a function of the perovskite PSNO3 film thickness. The Al thickness was set at 3.5 nm. f) Evolution of the c-axis lattice parameter (Å) of the thin film as a function of the aluminum deposition rate in Å/s. The deposition rate optimization was performed on a 6 nm-thick sample (10x10 mm$^{2}$, later cut in four pieces for the experiments), using 2.5 nm Al thickness, at a fixed reduction temperature of 290°C and a post-annealing time of 60 mins.
• Figure 2: a) X-Ray Diffraction $\theta$ - 2$\theta$ scans of aluminum reduced Pr0.8Sr0.2NiO2 samples, one exposed to air before aluminum deposition (ex situ, red), and another reduced without air exposure (in situ, blue). b) Resistivity comparison as a function of the temperature for infinite-layer phase after ex situ aluminum reduction (red line), showing $T_{c}^{onset}$ = 10 K and $T_{c}^{zero}$ = 4 K, and after in situ aluminum reduction, showing $T_{c}^{onset}$ = 15 K and $T_{c}^{zero}$ = 9.5 K. c) Enlarged view of the $\rho$ versus T curves from panel b) in the temperature range of 2 to 25 K (around the superconducting transition). $T_{c}^{onset}$ is defined as the intersection of the linear extrapolations of the normal state and the superconducting transition regions. d) Critical temperature onset ($T_{c}^{onset}$) as a function of the resistivity ($\rho$) at 20 K for in situ and ex situ samples, respectively. Blue shaded area is a guide to the eye indicating the most common values for in situ reduced samples. e) HAAF-STEM image of a Pr0.8Sr0.2NiO2 film reduced via ex situ Al deposition. The nominal thickness of the initial perovskite film was 8 nm with 3.5 nm of Al used for the reduction. f,g) Geometrical phase analysis the of STEM image in panel e) showing the out-of-plane (f) and in-plane (g) lattice contraction relative to the SrTiO3 substrate. h) (left panel) Atomic-resolution HAAF-STEM image from the region near the bottom interface, marked in panel e) (orange rectangle), and (right panel) corresponding 4D-STEM dCOM image showing the absence of oxygen atoms at the apical sites (indicated by orange arrows). In both images, green circles represent Pr/Sr atom sites, orange circles represent Ni sites and red circles represent O sites.
• Figure 3: a) X-Ray Diffraction $\theta$ - 2$\theta$ scans of a 8-nm thick parent Pr0.8Sr0.2NiO3 thin film (grey) and in situ aluminum reduced sample (Pr0.8Sr0.2NiO2 film) (blue). b) (left panel) AFM topography images of the parent perovskite phase (average steps height $\approx$ 0.595 nm, average steps width $\approx$ 324 nm) and (right panel) the in situ reduced sample after aluminum deposition by DC magnetron sputtering (average roughness $\approx$ 0.517 nm). c) High resolution RSM around the ($\bar{1}$03) asymmetric reflection of an in situ reduced infinite-layer nickelate thin film on SrTiO3 substrate, indices are taken with respect to the pseudocubic unit cell.
• Figure 4: a) Resistivity as a function of temperature for the superconducting infinite-layer phase obtained after in situ aluminum reduction, with a $T_{c}^{onset}$ = 17 K and $T_{c}^{zero}$ = 15 K. The inset shows an expanded view around the superconducting transition. b) Temperature-dependent resistivity measurements of an aluminum reduced Pr0.8Sr0.2NiO2 sample around the $T_{c}^{onset}$ for different externally applied out-of-plane magnetic fields up to 9 T. c) Normal state Hall coefficient (RH) as a function of temperature for in situ Al reduced PSNO$\textsubscript{2}$ thin films.
• Figure S1: a) RHEED intensity oscillations observed during the PLD growth of Pr0.8Sr0.2NiO3 perovskite thin films on SrTiO$_3{}$ (001) substrate. b) RHEED diffraction patterns observed before (STO substrate) and c) after deposition of the perovskite thin film.