Ni-O hybridization-driven electronic reconstruction across the superconducting dome in an infinite-layer nickelate

This study utilizes x-ray absorption spectroscopy to demonstrate that a Ni-O orbital-selective crossover, characterized by a redistribution of spectral weight from Ni 3d to O 2p states near optimal doping, drives transport anomalies and governs the superconducting dome in infinite-layer La$_{1-x}$Ca$_x$NiO$_2$.

The Gist

Imagine a material that acts like a superhighway for electricity, allowing current to flow with zero resistance. This is superconductivity. For decades, scientists have been trying to understand how to make these "superhighways" work at higher temperatures, looking closely at a family of materials called cuprates (copper-based). Recently, they discovered a new family of materials that look and act very similar to cuprates, but instead of copper, they use nickel. These are called infinite-layer nickelates.

This paper is like a detective story where the researchers are trying to figure out exactly what happens inside these nickel materials as they change their chemical recipe to become superconductors.

The Recipe: Mixing Calcium into Nickel

Think of the base material, LaNiO₂, as a plain cake that doesn't conduct electricity well. To make it a superconductor, the scientists add a special ingredient: Calcium. They gradually increase the amount of Calcium (a process called "doping"), changing the recipe from pure nickel to a mix like La₁₋ₓCaₓNiO₂.

They found that the "super-conducting cake" only works when the Calcium amount is just right, specifically between 18% and 27%. Too little, and it's not a superconductor; too much, and the superconductivity fades away.

The Investigation: Taking an X-Ray Snapshot

To see what's happening inside the cake, the researchers used a powerful tool called X-ray Absorption Spectroscopy (XAS). You can think of this as taking a high-resolution X-ray photo of the material's "empty seats" (unoccupied electronic states) to see which atoms are sitting there.

They looked at two specific characters in the material:

1. Nickel (Ni): The main actor.
2. Oxygen (O): The supporting actor that holds hands with Nickel.

In the world of these materials, the "hand-holding" between Nickel and Oxygen is called hybridization. It's like a dance where the two atoms share energy.

The Big Discovery: A Change in the Dance

The researchers discovered that as they added more Calcium, the "dance" between Nickel and Oxygen changed dramatically, right in the middle of the superconducting zone.

• The Early Days (Low Calcium): The energy states were mostly dominated by Nickel. Imagine the dance floor was crowded with Nickel atoms doing their own thing.
• The Sweet Spot (Optimal Doping, ~20-23% Calcium): Something interesting happened. The Nickel atoms started to step back, and the Oxygen atoms stepped forward, taking a more active role in the dance. The material shifted from being "Nickel-heavy" to being a strong partnership where Oxygen and Nickel shared the energy equally.
• The Overdoped Zone (High Calcium): As they added even more Calcium, the Oxygen influence grew even stronger, but the superconductivity started to die out.

Connecting the Dots: The Hall Effect and the "Sign Flip"

The paper also looked at how electricity moves through the material (transport properties). They noticed a strange event: the Hall coefficient (a measurement that tells you the direction and type of charge carriers) suddenly flipped its sign right around the same time the Oxygen started taking over the dance floor.

Think of it like a traffic light changing from green to red at the exact moment the crowd on the dance floor changes its rhythm. This coincidence suggests that the change in the "dance" (the electronic structure) is the cause of the traffic changes, not just a side effect.

Why This Matters

The authors conclude that the secret to superconductivity in these nickel materials isn't just about adding more "charge carriers" (like adding more cars to a highway). Instead, it's about rearranging the dance partners.

• When the dance shifts from being Nickel-led to a balanced Nickel-Oxygen partnership, superconductivity thrives.
• When the dance shifts too far toward Oxygen (in the overdoped zone), the superconductivity breaks down.

The Bottom Line

This paper provides a clear map of the "electronic phase diagram" for these nickel materials. It tells us that the strength of the bond between Nickel and Oxygen is the key knob to turn. If you can control how much these two atoms mix and share energy, you might be able to engineer better superconductors.

In short: The superconductivity in these nickel materials is driven by a specific reorganization of the electronic dance floor, where Oxygen atoms take a more central role, and this shift happens right when the material becomes a superconductor and when the electrical properties start to behave strangely.

Technical Summary

Technical Summary: Ni-O Hybridization-Driven Electronic Reconstruction Across the Superconducting Dome in an Infinite-Layer Nickelate

Problem Statement
While infinite-layer nickelates (e.g., $Nd_{1-x}Sr_xNiO_2$) have emerged as promising analogues to cuprates for studying high-temperature superconductivity, the evolution of their electronic structure with hole doping remains unresolved. A central question is how the orbital character of low-energy states changes across the superconducting dome. Specifically, it is unclear whether Ni $3d$-dominated states give way to increasingly O $2p$-hybridized states near the compositions where superconductivity emerges and subsequently weakens. Furthermore, the relationship between these orbital evolutions and transport anomalies, such as the sign reversal of the Hall coefficient ($R_H$), requires direct experimental verification beyond simple carrier counting models.

Methodology
The authors investigated the electronic structure of infinite-layer $La_{1-x}Ca_xNiO_2$ thin films spanning the undoped to the overdoped regime ($0 \le x \le 0.35$). The samples were prepared via pulsed laser deposition (PLD) of perovskite precursors followed by a soft-chemistry topotactic reduction to stabilize the infinite-layer phase.

The study employed a combination of:

1. X-ray Absorption Spectroscopy (XAS): Measurements were conducted at the Oxygen K-edge ($1s \to 2p$) and Nickel L-edge ($2p \to 3d$) using linearly polarized X-rays at the Singapore Synchrotron Light Source (SSLS). Spectra were collected in total electron yield (TEY) mode to probe unoccupied states above the Fermi level.
2. X-ray Linear Dichroism (XLD): Polarization-dependent measurements at the Ni L-edge were performed at both normal and grazing incidence angles to resolve orbital anisotropy between in-plane ($3d_{x^2-y^2}$) and out-of-plane ($3d_{z^2}$) states.
3. Temperature Dependence: Spectra were acquired at both room temperature (300 K) and low temperature (30 K) to analyze thermal effects on spectral weight.
4. Correlation with Transport: The spectroscopic data were cross-referenced with previously reported electrical transport measurements, specifically the Hall coefficient and superconducting critical temperature ($T_c$).

Key Results

• Superconducting Range: Superconductivity was confirmed in the doping range $0.18 \le x \le 0.27$.
• O K-edge Evolution:
  • In the perovskite precursor, a strong pre-peak at $\sim 529.0$ eV (ligand-hole character) was observed, which disappeared upon reduction to the infinite-layer phase, consistent with the removal of apical oxygen.
  • In the infinite-layer films, a distinct, weaker pre-peak ("feature a") emerged at $\sim 530.7$ eV, attributed to O $2p$–Ni $3d$ hybridization. Its intensity increased with doping, saturating near the optimal composition ($x \approx 0.23$).
  • A second, weaker feature ("feature b") appeared at $\sim 531.6$ eV near the onset of superconductivity ($x \approx 0.18$). This feature reached maximum intensity at $x = 0.23$ and diminished in the overdoped regime ($x \ge 0.27$), correlating with the suppression of superconductivity.
• Ni L-edge Evolution: The Ni L$_3$-edge intensity increased with doping up to $x \approx 0.18$ and then decreased, tracking the effective Ni valence change from $Ni^{1+}$ toward $Ni^{(1+x)+}$.
• Spectral Weight Redistribution: A systematic redistribution of low-energy spectral weight was identified. As doping increased toward the optimal regime, spectral weight shifted from Ni $3d$-dominated states toward more strongly O $2p$-hybridized states. This crossover was most rapid near $x \approx 0.20–0.23$.
• Temperature Dependence: The intensity difference ($\Delta I_T = I_{300K} - I_{30K}$) in the pre-edge region increased with doping, indicating that O $2p$ hybridization with Ni- and La-derived states is stronger at room temperature than at low temperature.
• Linear Dichroism: Unlike Nd-based nickelates, La-based films exhibited only weak dichroism between in-plane and out-of-plane polarizations. This suggests a smaller energy splitting between the $3d_{x^2-y^2}$ and $3d_{z^2}$ orbitals in the square-planar NiO$_2$ environment of the La-based system.

Significance and Claims
The paper claims to provide a direct link between the superconducting dome, transport anomalies, and a measurable Ni–O orbital reorganization. The authors assert that the observed crossover—where Ni $3d$-dominated states decrease and O $2p$-hybridized states increase—coincides with the sign reversal of the Hall coefficient and precedes the reduction of $T_c$ at higher doping levels.

The significance of this work lies in:

1. Orbital-Resolved Phase Diagram: It refines the electronic phase diagram of infinite-layer nickelates by connecting transport anomalies (specifically the Hall sign change) to a hybridization-driven evolution of the electronic structure, moving beyond simple carrier-density descriptions.
2. Hybridization Control: It establishes Ni–O covalency and its doping evolution as central to the electronic structure changes across the superconducting dome, suggesting that hybridization control is a practical route to engineer superconducting behavior.
3. Material Specificity: It highlights distinct electronic behaviors in La-based nickelates compared to their Nd-based counterparts, particularly regarding orbital anisotropy and the specific nature of the low-energy spectral weight redistribution.

The authors conclude that these findings represent a key step toward a unified, orbital-resolved understanding of infinite-layer nickelates and their potential as alternative platforms for uncovering the ingredients of high-temperature superconductivity.