Stabilizing and Tuning Superconductivity in La$_3$Ni$_2$O$_{7-δ}$ Films: Oxygen Recycling Protocol Reveals Hole-Doping Analogue

TL;DR

This paper reports superconductivity in La3Ni2O7-delta thin films under compressive strain and presents an oxygen recycling protocol to restore superconductivity after degradation. The method uses an initial oxygen-removal step followed by ozone-assisted annealing, enabling reversible switching between insulating and superconducting states and allowing systematic tuning by oxygen content that mimics hole doping via La/Sr substitution. X-ray absorption spectroscopy indicates holes in Ni 3d-derived bonding states in as-grown films, while superconducting samples show a rearrangement of Ni electronic states; an ozone-dosing phase diagram reveals insulating, metallic, and superconducting regions. The findings offer a practical route to stabilize and optimize superconductivity in these nickelate films and provide a framework for understanding the role of oxygen and interface effects in the doping mechanism.

Abstract

The recent achievement of superconductivity in La$_3$Ni$_2$O$_{7-δ}$ with transition temperatures exceeding 40 K in thin films under compressive strain and 80 K in bulk crystals under high pressure opens new avenues for research on high-temperature superconductivity. The realization of superconductivity in thin films requires delicate control of growth conditions, which presents significant challenges in the synthesis process. Furthermore, the stability of superconducting La$_3$Ni$_2$O$_{7-δ}$ films is compromised by oxygen loss, which complicates their characterization. We introduce an effective recycling protocol that involves oxygen removal in a precursor phase followed by ozone-assisted annealing, which restores superconducting properties. By tuning the oxygen content, we construct an electronic phase diagram that highlights oxygen addition as a potential analogue to hole doping via La substitution with Sr, providing insights into the doping mechanism and guiding future material optimization.

Figures (4)

• Figure 1: Schematic structural and electrical characterization of La$_3$Ni$_2$O$_{7-\delta}$ films on LaAlO$_3$(001)(LAO) and SrLaAlO$_4$(001)(SLAO) substrates.a, the schematic structure of La$_3$Ni$_2$O$_{7-\delta}$ films grown on LAO and SLAO substrates. b, $\rho$(T) curves for three typical La$_3$Ni$_2$O$_{7-\delta}$ films with a thickness of 36 nm on LAO substrates with varying oxygen pressure during the growth, where the $\rho$(T) curve of the film grown under 150 mTorr and annealed in ozone is shown in red. c, $\rho$(T) curves for three typical 5.5 nm-thick La$_3$Ni$_2$O$_{7-\delta}$ films grown on SLAO substrates with different ozone annealing treatments, where the curve in orange shows that superconducting transition occurs at $\sim$ 40 K.
• Figure 2: Transport properties of La$_3$Ni$_2$O$_{7-\delta}$ thin films on SLAO substrates under a magnetic field.a, b,$\rho$(T) curves under various magnetic fields applied perpendicular to the $ab$ plane of films with different thicknesses, 8.5 nm and 3.5 nm, respectively; the arrow in Fig. 2a indicates that a second transition emerges at $\sim$18 K, and the inset shows a zoomed-in view of resistivity measurements demonstrating that zero resistance occurs below 5.5 K. c,d, Solid circles and squares represent the upper critical fields ($H_{c,\perp}^{50\%}$ and $H_{c,\perp}^{90\%}$) extracted by the $T_{c,50\%}$ and $T_{c,90\%}$. Dotted lines are Ginzburg-Landau fits.
• Figure 3: Recycle of degraded superconducting La$_3$Ni$_2$O$_{7-\delta}$ thin films.a,$\rho$(T) curves for a typical superconducting La$_3$Ni$_2$O$_{7-\delta}$ film with a thickness of 3.5 nm, and the same film stored in an argon-filled glove box for 15 days. b, Illustration of the two-step process for recycling the degraded superconducting La$_3$Ni$_2$O$_{7-\delta}$ films; Reannealing the degraded superconducting films under ozone tends to destroy the superconductivity. However, the two-step process, i.e., removing oxygen followed by an annealing process under ozone, can effectively restore the superconducting signature in the film. c, d,The out-of-plane XRD pattern and $\rho$(T) curves for a typical 3.5 nm-thick La$_3$Ni$_2$O$_{7-\delta}$ thin film under recycling.The mixed-gas volume is estimated by multiplying the flow rate by the total annealing time. It flows continuously into a chamber of about 0.5 L for annealing, as ozone lifetime above 100$^\circ$C is short.
• Figure 4: Phase diagram, STEM image, and XAS of La$_3$Ni$_2$O$_{7-\delta}$ thin film.a,The solid squares and triangles represent the $T_{c,onset}$, and the $T_{MIT}$, extracted from $\rho$(T) curves after each annealing with different $O_3$ flow rates. We caution that the horizontal coordinate indicates the total volume of $O_3$ used during the annealing process, which might not precisely represent the actual oxygen content in the film. Instead, it indicates that oxygen gradually enters the film, meaning $\delta$ decreases as it is annealed step-by-step in ozone. b, c, ADF-STEM images of 3.5 nm cycled La$_3$Ni$_2$O$_{7-\delta}$ thin film on SLAO at various regions along the [010] axis of the substrate. The La$_3$Ni$_2$O$_{7-\delta}$ thin film-substrate interface is marked by the white dashed line. The reduced intensity near the surface may result from damage during sample preparation for STEM. The positions of La, Ni, La/Sr, and Al ions are indicated by different colors. At the film-substrate interface, the bilayer (LaSr)$_3$Ni$_2$O$_{7-\delta}$ and monolayer La$_2$NiO$_4$ structures are clearly visible. The octahedra in La$_2$NiO$_4$ are highlighted in red. Scale bars are 5 nm in (b-c). d, XAS measurements on a typical film with $\sim$7 nm thickness grown on SLAO, suggests the presence of holes in Ni $d_{z^2}$ derived bonding band, as illustated in e. f, XAS measurements on a superconducting film with $\sim$8.5 nm thickness, reveal only two components corresponding to $d_{x^2-y^2}$ and $d_{z^2}$ anti-bonding states, indicating the absence of holes in Ni $d_{z^2}$ derived bonding band, as illustrated in g.