🔬 condensed matter

A Disconnected Superconducting Regime at the Parent Limit of Infinite-Layer Nickelates

This paper reports the discovery of a distinct, intrinsic superconducting regime in pristine and lightly substituted infinite-layer PrNiO2 that is separated from the conventional doping-induced superconducting dome by a non-superconducting region, challenging the view of nickelates as simple cuprate analogs.

Original authors: Chihao Li, Yutong Chen, Yaolong Bian, Yihao Zhang, Jiahao Ye, Zhitong An, Xingtian Sun, Yu Fan, Zhihui Chen, Zhanze Wang, Jinglei Zhang, Haichao Xu, Rui Peng, Donglai Feng

Published 2026-07-21

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Chihao Li, Yutong Chen, Yaolong Bian, Yihao Zhang, Jiahao Ye, Zhitong An, Xingtian Sun, Yu Fan, Zhihui Chen, Zhanze Wang, Jinglei Zhang, Haichao Xu, Rui Peng, Donglai Feng

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Search for the Perfect Super-Flow

Imagine you are trying to build a highway where cars can drive forever without ever slowing down, burning fuel, or hitting a traffic jam. In the world of physics, this is called superconductivity. It's a magical state where electricity flows with zero resistance, and materials can even float above magnets. For decades, scientists have been obsessed with a specific family of materials called cuprates (copper-based ceramics) because they become superconductors at relatively "warm" temperatures. These cuprates have a very specific rulebook: they start as insulators (materials that block electricity) and only turn into superconductors when you "dope" them—meaning you sneak in a few extra atoms to change the number of electric charges inside. This creates a sweet spot, often called a "dome," where superconductivity thrives.

Recently, scientists discovered a new family of materials called nickelates (nickel-based ceramics) that look a lot like cuprates. They have the same basic shape and even the same electron arrangement on paper. Naturally, everyone assumed they would follow the same rulebook: start as an insulator, add some doping, and boom—superconductivity. But nature loves to surprise us. The big question hanging over this field was: Do nickelates play by the same rules as cuprates, or do they have their own secret playbook? Understanding this isn't just about making better magnets; it's about cracking the code of how these materials work, which could one day lead to room-temperature superconductors that revolutionize our power grids and computers.

The Surprise in the "Pure" Nickel

In this study, a team of researchers decided to test the nickelate family at its most "pure" state. Usually, to get these materials to superconduct, you have to swap out some of the atoms for different ones (doping) to create the right amount of charge. But the researchers asked a bold question: What if the pure, undoped version of the material is already a superconductor?

They grew high-quality films of a material called PrNiO₂ (Praseodymium Nickel Oxide). Think of this as the "parent" version of the material, with no extra atoms added or removed. To their shock, they found that this pure, stoichiometric film didn't just act like a good conductor; it became a true superconductor. When they cooled it down, the electrical resistance dropped all the way to zero, and the material started pushing away magnetic fields (a property called diamagnetism). This is the "holy grail" proof of superconductivity.

But wait, could this be a trick? Maybe the superconductivity was just happening at the surface where the film touched the substrate (the base it was grown on), or maybe it was caused by a protective layer they usually put on top. To solve this mystery, the team played a game of "cut and paste." They grew a sandwich: a layer of pure PrNiO₂, then a thin, non-superconducting buffer layer, and then the base. If the superconductivity was just a surface effect from the base, the middle buffer should have killed it. But it didn't. The top layer still superconducted perfectly. This proved that the superconductivity is an intrinsic property of the pure nickelate itself, not a side effect of the experiment.

The "Goldilocks" Zone is Tiny

Here is where the story gets really weird. In the cuprate world, if you have a superconductor at a low doping level, adding a little bit more doping usually makes it even better, or at least keeps it going. But in these nickelates, the rules are strict.

The researchers tried to add just a tiny bit of "doping" by swapping some Praseodymium atoms with Strontium or Calcium. They expected the superconductivity to continue or get stronger. Instead, it vanished almost instantly.

With just 1% Strontium doping, the material was still a superconductor.
At 2%, it started to struggle.
By 3%, the superconductivity was completely gone, and the material acted like an insulator again.

It's as if the superconductivity exists in a tiny, fragile bubble right at the edge of the "pure" state. If you add even a pinch more charge, the bubble pops. This is a massive departure from the cuprate model, where superconductivity usually forms a wide, smooth hill (a "dome") that spans from about 10% to 20% doping. In nickelates, there is a gap. There is a superconducting island near zero doping, then a wide "no-man's-land" where superconductivity is dead, and then, way over at around 20% doping, a second superconducting island appears.

A Different Kind of Magic

The researchers didn't just stop at finding the island; they looked at how it behaved. They measured how strong a magnetic field the superconductor could withstand before breaking. They found that the "pure" superconductor behaves very differently from the "doped" one.

The pure version is incredibly sensitive to magnetic fields depending on the direction. It can handle a massive 35 Tesla field if the field is parallel to the layers, but much less if it's perpendicular.
This "anisotropy" (directional sensitivity) is much stronger than in the doped versions.

Using a technique called ARPES (which is like taking a high-speed photo of electrons to see where they are), they confirmed that the pure material has a specific electron structure with about 9% holes (missing electrons) naturally built-in. When they added the Calcium to kill the superconductivity, they saw that the electron structure changed in a way that didn't just add disorder; it fundamentally altered the electronic landscape, pushing the system out of the superconducting state.

The Takeaway

This paper tells us that infinite-layer nickelates are not just simple copies of cuprates. They host a disconnected superconducting regime. There is a unique, intrinsic superconducting state that exists in the pristine, undoped material, which is separated from the well-known, heavily doped superconducting dome by a region where superconductivity simply does not exist.

The authors suggest that this means the physics driving superconductivity in the pure state might be totally different from the physics in the doped state. It's possible that the pure state is related to a different kind of electron pairing or even an electron-doped regime that we haven't fully understood yet. While the paper doesn't claim to have solved the mystery of high-temperature superconductivity, it has definitely redrawn the map. It shows us that the nickelate family is more complex and interesting than anyone thought, with a secret superconducting mode hiding right at the starting line, waiting for us to figure out how to keep it alive without accidentally adding too much "doping" and killing the magic.

Technical Summary: A Disconnected Superconducting Regime at the Parent Limit of Infinite-Layer Nickelates

Problem Statement
Infinite-layer nickelates have been extensively studied as analogs to cuprate superconductors, where superconductivity is traditionally observed in a "dome" centered around 10–20% divalent cation substitution (e.g., Sr, Ca, Eu). A central question in the field is whether the superconducting phase diagram of nickelates follows a single, cuprate-like dome or if distinct regimes exist, particularly near the parent limit. While pristine infinite-layer nickelates (RENiO $_2$ ) are nominally Ni $^{3+}$ ( $3d^9$ ) like cuprates, they possess an intrinsic self-hole doping (~9%) due to interstitial $s$ electrons, placing them theoretically within the underdoped regime of cuprates. Previous studies reported superconducting signatures near the parent limit, but these lacked definitive evidence of zero resistance in uncapped films and clear diamagnetic responses. Consequently, it remained unclear whether these signatures represented intrinsic superconductivity of the pristine parent phase or arose from interfacial effects (e.g., capping layers or substrate interactions) and unintended carrier doping. Furthermore, the relationship between this potential parent-state superconductivity and the established hole-doped dome remained unresolved.

Methodology
The authors employed high-quality thin-film growth and advanced characterization techniques to investigate the electronic and superconducting properties of PrNiO $_2$ and its derivatives:

Film Growth & Engineering: High-quality, stoichiometric, uncapped 20-unit-cell (uc) PrNiO $_2$ films were grown on SrTiO $_3$ substrates. To rigorously test for interfacial origins, the authors engineered heterostructures inserting a 3-uc non-superconducting Pr $_{0.97}$ Ca $_{0.03}$ NiO $_2$ spacer layer between the PrNiO $_2$ and the SrTiO $_3$ substrate.
Doping Studies: The study systematically varied the chemical composition to probe the doping dependence:
- Isovalent substitution: Partial replacement of Pr $^{3+}$ with La $^{3+}$ to modify the rare-earth environment without changing carrier concentration.
- Aliovalent substitution: Dilute substitution of Pr $^{3+}$ with divalent Sr $^{2+}$ and Ca $^{2+}$ to introduce controlled hole doping.
Transport Measurements: Temperature-dependent resistivity was measured under various magnetic fields (parallel and perpendicular to the $ab$ plane) to identify zero-resistance states and determine upper critical fields ( $H_{c2}$ ).
Magnetic Characterization: A tunnel diode oscillator (TDO) was used to detect diamagnetic shielding, a key hallmark of bulk superconductivity.
Electronic Structure Analysis: In-situ angle-resolved photoemission spectroscopy (ARPES) was performed on superconducting PrNiO $_2$ and non-superconducting Pr $_{0.95}$ Ca $_{0.05}$ NiO $_2$ films to map Fermi surfaces and determine hole concentrations. Hall effect measurements were conducted to characterize carrier types.

Key Results

Intrinsic Superconductivity in Pristine Films: Stoichiometric, uncapped PrNiO $_2$ films exhibit a clear superconducting transition with zero resistance ( $T_{c0} \approx 7$ K, onset $T_c \approx 11$ K) and a distinct diamagnetic response. This confirms that superconductivity is an intrinsic property of the parent infinite-layer nickelate without divalent cation doping.
Exclusion of Interfacial Origins: The robust superconducting transition persists in heterostructures where the PrNiO $_2$ layer is separated from the SrTiO $_3$ substrate by a non-superconducting spacer. This rules out substrate-induced charge transfer or interface effects as the primary mechanism.
Narrow Doping Window: Superconductivity is highly sensitive to hole doping. While isovalent La substitution preserves superconductivity, dilute divalent substitution (Sr or Ca) rapidly suppresses it.
- 1% Sr doping ( $x=0.01$ ) retains a zero-resistance state.
- 2% Sr doping shows only partial resistance drops.
- 3% Sr or Ca doping ( $x=0.03$ ) results in insulating behavior with no superconductivity.
- Superconductivity only re-emerges at higher doping levels ( $\sim$ 15–20%), corresponding to the established hole-doped dome.
Disconnected Phase Diagram: The results reveal a "disconnected" superconducting regime. The parent superconducting phase (near $p \approx 0.09$ self-doping) is separated from the conventional hole-doped dome ( $p \approx 0.2$ ) by a broad non-superconducting region (approx. 3% to 15% doping).
Distinct Electronic and Magnetic Properties:
- Electronic Structure: ARPES confirms that superconducting PrNiO $_2$ retains the self-doped parent electronic structure (hole concentration $p \approx 0.09$ in the Ni $3d_{x^2-y^2}$ orbital) and a negative Hall coefficient, distinct from the positive Hall coefficient observed in the hole-doped dome.
- Anisotropy: The parent superconducting regime exhibits a remarkably strong upper-critical-field anisotropy ( $\gamma = H_{c2}^{ab}/H_{c2}^{c} \approx 68$ ), significantly higher than that of optimally doped nickelates. The in-plane coherence length ( $\xi_{ab} \approx 6.23$ nm) is also larger than in doped samples.

Significance and Claims
The paper establishes that infinite-layer nickelates host a unique, separated superconducting regime at the parent limit, distinct from the previously known hole-doped dome. The authors claim that:

Nickelate superconductivity cannot be fully captured by a simple cuprate-like doping paradigm where superconductivity evolves continuously from the parent compound to the doped dome.
The rapid suppression of superconductivity upon slight hole doping (3%) and the existence of a non-superconducting gap suggest that the parent-state superconductivity and the hole-doped dome are qualitatively different phases, potentially involving different pairing mechanisms or competing orders.
The enhanced anisotropy and distinct electronic structure of the parent regime point toward unconventional physics, possibly involving time-reversal-symmetry breaking or a connection to an electron-doped regime, rather than being merely the low-doping edge of the conventional dome.

These findings necessitate a reassessment of the nickelate phase diagram and the theoretical frameworks used to describe the interplay between electronic correlations, competing orders, and pairing symmetry in these materials.