Bipolar-doped superconducting infinite-layer cuprates

This study achieves controllable bipolar doping in infinite-layer cuprate thin films, revealing a hole-doped superconducting phase with a transition temperature exceeding 60 K that coexists with antiferromagnetic order, thereby establishing a definitive platform to investigate the intrinsic mechanism of high-temperature superconductivity.

The Gist

The Big Picture: Cleaning Up the "Superconductor Kitchen"

Imagine high-temperature superconductors (materials that conduct electricity with zero resistance) as a complex kitchen where a chef is trying to bake the perfect cake (superconductivity). For decades, scientists have known that the "cake" happens in the CuO₂ planes (the actual baking pan), but the recipe has always been cluttered with extra ingredients in the "charge-reservoir layers" (the pantry and the oven walls). These extra layers supply the necessary ingredients (electrons or holes), but they also create a mess, making it hard to see exactly how the baking pan works on its own.

This paper is about finally clearing out the pantry and the oven walls to get a pristine, isolated baking pan. The researchers successfully created a "clean kitchen" using a special type of material called infinite-layer cuprates. In these materials, the baking pans (CuO₂ planes) are stacked directly on top of each other with nothing in between, allowing scientists to study the superconductivity in its purest form.

The Challenge: The "One-Way Street" Problem

For a long time, scientists could easily add electrons (negative charge) to these clean pans to make them superconduct. It was like adding sugar to a cake batter; it worked well. However, adding holes (positive charge, or missing electrons) to these same clean pans was a nightmare. It was like trying to add salt to a cake without making it crumble; the structure would fall apart or become uneven. Because they couldn't control the "hole" side, they couldn't compare the two sides fairly to understand the full recipe.

The Breakthrough: A New Cooking Technique

The team at Southern University of Science and Technology developed a new method called Gigantic-Oxidative Atomic-Layer-by-Layer Epitaxy (GAE). Think of this as a robotic chef that builds the material one single atom at a time in a super-sterile, oxygen-rich environment.

• For the Electron Side: They swapped some Strontium atoms with Europium atoms to add electrons.
• For the Hole Side: They used a very delicate trick, adjusting the amount of ozone (a super-charged oxygen gas) during the growth process to add holes. They had to be so careful that they had to transport the finished films in a special "cryogenic suitcase" (a vacuum-sealed, freezing cold box) to the lab to ensure the surface didn't get ruined by air.

The result? They successfully created two types of perfect, single-crystal films: one with extra electrons and one with extra holes.

The Discovery: Two Sides of the Same Coin

Once they had these clean films, they used a powerful microscope called ARPES (Angle-Resolved Photoemission Spectroscopy) to take a "snapshot" of the electrons moving inside. Here is what they found:

1. It's Flat, Not Round: They confirmed that the electricity flows in flat, 2D sheets (like a stack of paper) rather than in a 3D block. This proves the "infinite-layer" design works perfectly.
2. The Magic of "Folding": In the electron-doped side, scientists already knew that the electrons' paths "folded" over themselves due to magnetic order (like a piece of paper being folded in half). They expected the hole-doped side to be different.
   • The Surprise: Even on the hole-doped side, they saw this "folding" happening! But here is the kicker: this folding appeared right at the very tips of the "Fermi arcs" (the electron paths) at a very low level of doping.
   • The Analogy: Imagine a river (the electron path). On one side, the river flows straight. On the other, scientists thought the river would just curve. Instead, they found that even in the hole-doped river, the water was folding back on itself, creating a complex pattern right where the river was just starting to flow.

The "Goldilocks" Zone

The most exciting finding is that this "hole-doped" film, which is still in a very "underdoped" state (meaning it hasn't reached its full potential yet), already starts conducting electricity with zero resistance at temperatures above 60 Kelvin (about -213°C).

• Why this matters: The electron-doped side only reached about 30 K. The hole-doped side is already twice as hot (in superconductor terms) despite being less "filled up." This suggests that the magnetic order (the folding) and the superconductivity are deeply intertwined, working together even at very low doping levels.

The "Single Surface" Secret

In older, more complex cuprate materials (like multi-layer cakes), scientists saw different electron patterns in the top layers versus the bottom layers, making it hard to know what was really happening.

In this new "clean kitchen" (the infinite-layer films), there is only one single electron surface. There is no confusion between top and bottom layers. This means the strange mix of "Fermi arcs" and "antiferromagnetic folding" they saw is an intrinsic property of the material itself, not an accident caused by messy layers.

Summary

This paper solves a long-standing puzzle by creating a pristine, uncontaminated version of a superconductor. By successfully doping it with both electrons and holes, the researchers showed that:

1. The material behaves like a perfect, flat 2D sheet.
2. Magnetic order (the "folding") and superconductivity coexist even in the hole-doped side, challenging previous theories.
3. This clean platform allows scientists to finally study the "intrinsic physics" of high-temperature superconductivity without the noise of extra chemical layers.

They haven't built a new power grid or a clinical device yet; they have simply built the perfect, clean laboratory model to finally understand how these materials work at a fundamental level.

Technical Summary

Technical Summary: Bipolar-doped superconducting infinite-layer cuprates

Problem Statement
A central challenge in high-temperature superconductivity is isolating the intrinsic physics of the superconducting CuO₂ planes from the structural and chemical complexities introduced by charge-reservoir layers. While the infinite-layer structure offers an ideal configuration where CuO₂ planes are stacked directly via alkaline-earth cations without intervening reservoirs, stabilizing this phase has been difficult. Although electron-doped infinite-layer cuprates have been synthesized, achieving controllable and uniform hole doping within this pristine structure has remained elusive. This limitation has prevented comprehensive spectroscopic characterization, particularly regarding the interplay between antiferromagnetism and superconductivity on the hole-doped side, and has obscured whether observed phenomena in multilayer systems are intrinsic or proximity-induced.

Methodology
The authors address these challenges by growing single-crystalline thin films of infinite-layer cuprates using gigantic-oxidative atomic-layer-by-layer epitaxy (GAE). This method maintains an exceptionally oxidative environment throughout the growth process.

• Electron Doping: Rare-earth element Europium (Eu) is doped into SrCuO₂ films grown on GdScO₃ substrates. Carrier concentration is tuned by varying the Eu content.
• Hole Doping: Alkali metal Lithium (Li) is doped into CaCuO₂₊δ films grown on NdGaO₃ substrates. Carrier concentration is tuned by adjusting the ozone partial pressure during growth to control oxygen content.
• Sample Handling: To preserve the delicate surface oxygen stoichiometry of the hole-doped films, samples are transferred from the growth laboratory to the synchrotron facility in a cryogenic ultrahigh-vacuum suitcase, maintaining temperatures below 150 K to prevent surface degradation.
• Characterization: The study employs transport measurements (resistance vs. temperature under various magnetic fields and angles), angle-resolved photoemission spectroscopy (ARPES), X-ray diffraction (XRD), and cross-sectional scanning transmission electron microscopy (STEM).

Key Contributions and Results

1. Realization of a Bipolar Phase Diagram: The authors successfully mapped the electronic phase diagram for both electron- and hole-doped infinite-layer cuprates starting from the common 3d⁹ insulating parent.

   • Electron Side: A dome-shaped superconducting region was resolved with Eu concentrations of ~0.05–0.17, peaking near 0.11 with an onset transition temperature ($T_{c}^{onset}$) of ~30 K.
   • Hole Side: Superconductivity is governed by ozone pressure. $T_{c}^{onset}$ rises monotonically with pressure, reaching ~80 K at 0.06 mbar. Notably, this high transition temperature is achieved in the underdoped regime (hole doping $p \approx 0.07$), exceeding the maximum $T_c$ of the electron-doped side by more than a factor of two.

2. Quasi-Two-Dimensional Superconductivity: Transport measurements of magnetoresistance reveal pronounced anisotropy in both doping regimes. The transition is suppressed far more strongly by out-of-plane magnetic fields than in-plane fields. Analysis of the upper critical fields and angular dependence of $T_c$ confirms that the data fits the two-dimensional Ginzburg–Landau and Tinkham models, while failing the anisotropic 3D-GL model. This indicates the films behave as weakly coupled CuO₂ planes.

3. Unified Electronic Structure and Coexistence of Orders: ARPES measurements on atomically flat surfaces reveal a single Fermi surface in both doping cases, confirming negligible doping gradients across the film thickness.

   • Electron Doped: The Fermi surface shows a main band and a folded band across the antiferromagnetic zone boundary (AFZB), consistent with $n \approx 0.12$ electrons per Cu.
   • Hole Doped: Remarkably, at a low hole doping of $p \approx 0.07$, the system exhibits Fermi arcs that coexist with antiferromagnetic band folding. The folded bands emerge specifically at the tips of the Fermi arcs where the main band meets the AFZB. This observation confirms the intrinsic coexistence of superconductivity and antiferromagnetic order within a single Fermi surface, distinct from the complex layer-dependent behaviors seen in multilayer cuprates.

4. Structural Integrity: XRD and STEM confirm phase-pure, epitaxial growth with atomically sharp interfaces. A specific buffer strategy was employed for hole-doped films: a 3-unit-cell SrCuO₂ buffer (which adopts a chain-type polymorph due to strain and thickness constraints) acts as a chemical template to enable the layer-by-layer growth of the metastable infinite-layer CaCuO₂ phase.

Significance
The paper claims that these findings settle a long-standing materials bottleneck by providing a definitive platform to investigate the intrinsic mechanism of high-temperature superconducting cuprates. By isolating the CuO₂ planes from charge-reservoir complexities, the study reconciles the historical electron-hole dichotomy regarding the interplay of antiferromagnetism and superconductivity. The observation of Fermi arcs and antiferromagnetic folding coexisting at ultra-low hole doping challenges prevailing understandings, suggesting that magnetic order is intrinsically intertwined with superconductivity at the microscopic level. This work establishes a clear pathway to decode the fundamental pairing mechanism of high-temperature superconductivity without the ambiguities of multilayer systems.