🔬 condensed matter

Marginal-Fermi-Liquid-like Behavior without Pseudogap in Infinite-Layer Nickelates

Using angle-resolved photoemission spectroscopy, this study reveals that infinite-layer nickelates exhibit momentum-selective marginal-Fermi-liquid-like scattering without the pseudogap formation typically seen in cuprates, suggesting that a pseudogap is not an essential prerequisite for high-temperature superconductivity.

Original authors: Yu Fan, Zhitong An, Xiang Ding, Xingtian Sun, Yutong Chen, Zhihui Chen, Shenglin Tang, Chihao Li, Jiahao Ye, Timur Kim, Haichao Xu, Rui Peng, Donglai Feng

Published 2026-07-21

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CC BY 4.0

Original authors: Yu Fan, Zhitong An, Xiang Ding, Xingtian Sun, Yutong Chen, Zhihui Chen, Shenglin Tang, Chihao Li, Jiahao Ye, Timur Kim, 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

Imagine the world of electrons as a bustling city. In most materials, these tiny travelers move in an orderly, predictable way, like commuters on a well-scheduled train system. Physicists call this orderly state a "Fermi liquid." But in some special materials, like the copper-based compounds known as cuprates, the city descends into chaos. The trains stop running on schedule, and the commuters start behaving in strange, unpredictable ways. This chaotic state is called a "strange metal," and it often comes with a mysterious phenomenon called a "pseudogap." Think of the pseudogap as a sudden, invisible fog that rolls in over parts of the city, blocking certain paths for the electrons even before the city freezes into a superconductor (a state where electricity flows with zero resistance). For decades, scientists have been trying to figure out if this fog is the secret ingredient that makes high-temperature superconductivity possible, or if it's just a side effect of the chaos.

Now, enter a new player in this electronic drama: infinite-layer nickelates. These are materials made of nickel and oxygen that look a lot like the copper-based cuprates but have a slightly different recipe. Because they are so similar yet distinct, they offer a perfect "control group" for scientists. If the fog (pseudogap) appears in both, it might be a universal rule of this type of material. If it only appears in the copper ones, then the fog might be a specific quirk of copper. The big question is: Can you have the chaotic, strange-metal behavior and the potential for superconductivity without the fog?

In this study, a team of researchers from Fudan University and international collaborators decided to take a closer look at these nickelate materials using a powerful camera called angle-resolved photoemission spectroscopy (ARPES). This tool is like a high-speed, 3D camera that can snap pictures of electrons as they move, revealing their energy and direction. However, taking these pictures of nickelates is notoriously difficult because the surface of the material is often rough and damaged, like trying to take a clear photo of a reflection in a cracked mirror. The team first had to engineer a way to make the nickelate films incredibly smooth and clean, essentially polishing the mirror until it was perfect.

Once they had these pristine samples, they took pictures of two specific types of nickelate films: one that was "doped" (mixed with calcium to create extra charge carriers) and one that was the "parent" compound (undoped). They were looking for two things: signs of the mysterious fog (pseudogap) and signs of the strange, chaotic electron behavior.

What they found was a surprise. In the doped nickelate, the electrons were indeed behaving strangely. The researchers measured how much the electrons were scattering off each other and found that this scattering increased in a very specific way as they moved across the material's surface. It was linear, not curved, which is a hallmark of "marginal-Fermi-liquid" behavior—the same kind of chaotic, strange-metal signature seen in the famous copper-based superconductors. This suggests that the nickelates are just as "strange" as the cuprates.

However, when they looked for the fog—the pseudogap—they found absolutely nothing. In the copper materials, the fog usually blocks the electrons at certain angles, creating a gap in the energy spectrum. But in these nickelates, the electrons could move freely in all directions. There was no missing energy, no blocked paths, and no "back-bending" of the electron paths that usually signals a gap. Even in the undoped parent material, where the fog is usually strongest in copper compounds, the nickelates remained clear and gapless.

The team also noticed that while the electrons were free to move, they were still being slowed down more in some directions than others. The "traffic" was heaviest near the edges of the material's electronic map, causing the electrons to lose energy faster there, but the road itself was never closed.

This discovery suggests that the chaotic, strange-metal behavior and the potential for superconductivity do not require the mysterious pseudogap fog to exist. The nickelates prove that you can have the wild, non-stop party of a strange metal without the fog rolling in. This helps scientists realize that the fog might not be the essential ingredient for high-temperature superconductivity after all. Instead, the key might lie in the specific way electrons scatter and interact in a momentum-dependent way, a feature that nickelates share with cuprates even though they lack the fog. It's a crucial clue that helps physicists rewrite the rulebook on how these amazing materials work, showing that the path to superconductivity might be more flexible than previously thought.

Technical Summary: Marginal-Fermi-Liquid-like Behavior without Pseudogap in Infinite-Layer Nickelates

Problem Statement
In hole-doped cuprates, the interplay between pseudogap formation, strange-metal transport, and unconventional superconductivity remains a central unresolved issue in condensed matter physics. While these phenomena are tightly intertwined in cuprates, it is unclear whether their coexistence reflects a universal principle of hole-doped Mott systems or depends on material-specific electronic structures. Infinite-layer nickelates offer a distinct platform to investigate this question, possessing a cuprate-like $3d^9$ electronic configuration and a $d_{x^2-y^2}$ -derived Fermi surface, yet exhibiting different charge-transfer energy scales and multiband character. However, previous Angle-Resolved Photoemission Spectroscopy (ARPES) studies on nickelates have been limited by poor surface quality following topotactic reduction, preventing the resolution of low-energy spectral functions necessary to determine if pseudogap behavior and marginal-Fermi-liquid (MFL) scattering exist in these systems.

Methodology
The authors address surface quality limitations by fabricating high-quality infinite-layer nickelate thin films ( $La_{0.8}Ca_{0.2}NiO_2$ and parent $LaNiO_2$ ) using oxide molecular beam epitaxy followed by in-situ atomic hydrogen reduction. To mitigate surface charging and damage, they employed a modified reduction setup featuring a perforated metal mask to ensure uniform hydrogen diffusion while suppressing direct $H^+$ bombardment. Additionally, gold was deposited on substrate edges and connected to the sample holder to eliminate photoemission charging effects.

The study utilizes high-resolution ARPES measurements at 10 K (normal state) to resolve the low-energy spectral function. The analysis focuses on:

Spectral Weight Modulation: Mapping intensity across the Brillouin zone to distinguish matrix-element effects from intrinsic Fermi-arc behavior.
Leading-Edge Analysis: Examining Energy Distribution Curves (EDCs) at Fermi momenta to detect gap opening or back-bending indicative of a pseudogap.
Self-Energy Extraction: Calculating the imaginary part of the electron self-energy, $Im\Sigma(\omega)$ , to characterize quasiparticle scattering rates and energy dependence.
Comparative Doping: Analyzing both the superconducting, optimally doped $La_{0.8}Ca_{0.2}NiO_2$ ( $p \sim 0.2$ ) and the parent compound $LaNiO_2$ ( $p \sim 0.09$ ), a regime where pseudogaps are well-developed in cuprates.

Key Results

Absence of Pseudogap: In both $La_{0.8}Ca_{0.2}NiO_2$ and $LaNiO_2$ , the $\alpha$ band (cuprate-like $d_{x^2-y^2}$ derived) and the $\beta$ band (electride-like $s$ derived) cross the Fermi level continuously. There is no leading-edge shift, back-bending, or depletion of spectral weight indicative of a pseudogap opening. The leading edges remain pinned at the Fermi energy ( $E_F$ ) across all momentum cuts, including the antinodal $(\pi, 0)$ region, with no detectable gap down to the experimental limit of $\sim 2$ meV.
Marginal-Fermi-Liquid-like Scattering: In $La_{0.8}Ca_{0.2}NiO_2$ , the extracted imaginary self-energy $Im\Sigma(\omega)$ exhibits an approximately linear dependence on energy over a 150 meV range, deviating from the quadratic dependence expected in a Fermi liquid. This behavior is consistent with MFL phenomenology.
Momentum-Dependent Correlations: The slope of $Im\Sigma(\omega)$ and the value at the Fermi level increase monotonically from the nodal $(\pi/2, \pi/2)$ direction toward the antinodal $(\pi, 0)$ direction. This indicates a strong momentum dependence of electronic correlations and a reduction in quasiparticle lifetime toward the antinodes.
Spectral Weight Suppression: Both films show a progressive suppression of low-energy spectral weight from the diagonal direction toward $(\pi, 0)$ . This suppression is more pronounced in the parent $LaNiO_2$ (reduced to 0.71 of the nodal weight) compared to the doped sample (0.81), suggesting intrinsic anisotropy linked to reduced hole doping and stronger correlations.
Band Structure: The $\beta$ band also crosses the Fermi level without gap formation, confirming the absence of a pseudogap in the multiband system.

Significance and Claims
The authors claim that their results demonstrate that momentum-selective correlations and marginal-Fermi-liquid-like scattering can occur in infinite-layer nickelates without a detectable cuprate-like pseudogap. This finding challenges the notion that a pseudogap is a necessary precursor or defining feature of the normal state in hole-doped $3d^9$ systems that host high-temperature superconductivity.

The paper posits that the absence of a pseudogap in nickelates, despite their cuprate-like Fermi surface and strange-metal transport, may be attributed to their distinct charge-transfer energetics. Unlike hole-doped cuprates where doped holes have significant oxygen $2p$ character (forming Zhang-Rice states), nickelates appear to reside closer to the Mott-Hubbard regime with holes predominantly on Ni $3d$ orbitals. The authors suggest that charge-transfer energy and the orbital character of doped carriers are critical parameters controlling whether momentum-dependent correlations evolve into a pseudogap.

Ultimately, the work provides a benchmark for theoretical models of high-temperature superconductivity, indicating that the specific electronic ingredients required for the pseudogap phase are not universal to all hole-doped Mott systems, and that strange-metal behavior and superconductivity can emerge in the absence of a cuprate-like pseudogap.

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