Hole-doped superconductivity above 100 K in infinite-layer cuprate thin films

This paper reports the first observation of hole-doped superconductivity in infinite-layer Sr$_{1-x}$Rb$_x$CuO$_2$ thin films with an onset temperature above 100 K, achieved through a synergistic mechanism of rubidium substitution and apical oxygen incorporation.

The Gist

Imagine a world where electricity flows without any resistance at all, like a car driving on a perfectly frictionless highway that never slows down. This is called superconductivity. Scientists have been chasing a "holy grail" in this field: finding materials that can do this at temperatures high enough to be useful without needing expensive, super-cold liquid helium.

For decades, a specific family of materials called cuprates (copper-oxide based) has been the star player. They are like a complex orchestra with many different sections (layers of atoms) working together to create music (superconductivity). However, this complexity makes it hard for scientists to understand exactly how the music is made.

The Missing Piece: The "Minimalist" Instrument

About 40 years ago, physicists proposed a "minimalist" version of this orchestra. They imagined stripping away all the extra layers and keeping only the absolute essentials: a single sheet of copper and oxygen atoms (the "CuO2 plane") separated by simple spacer ions. They called this the infinite-layer cuprate.

Think of it like trying to understand a symphony by listening to just the violin section, ignoring the drums, brass, and choir. If you could make just the violins play the superconducting song, you would finally understand the core physics.

The Problem: For 40 years, scientists could build this minimalist structure, but it refused to superconduct. They tried adding "holes" (missing electrons, which act as positive charge carriers) by swapping some atoms, but it always resulted in an insulator (a material that blocks electricity). It was like trying to tune a violin that kept snapping its strings.

The Breakthrough: A Synergistic "Double-Whammy"

In this new paper, a team of researchers finally cracked the code. They didn't just try one trick; they used a synergistic combination of two methods to get the material to sing:

1. The Big Swap (Rubidium): Instead of using small atoms to swap into the structure, they used Rubidium, a large atom. Imagine trying to fit a large suitcase into a small locker. The paper suggests that using a "big" dopant helps avoid the problems that smaller dopants caused (like creating unwanted gaps or vacancies in the structure).
2. The Oxygen Boost (Apical Oxygen): They also carefully added extra oxygen atoms to the "top" and "bottom" of the copper layers (called apical oxygen). Think of this as adding a specific type of lubricant that helps the charge carriers move freely.

By combining the large Rubidium atoms with extra Oxygen, they successfully created a hole-doped superconductor.

The Results: A Hot New Record

The results were impressive:

• The Temperature: The material started superconducting at a "high" temperature of 100 Kelvin (about -173°C). While this is still very cold, it is a massive leap for this specific type of material. The "onset" (where the magic starts) was around 75 K, with full zero-resistance flow at 23 K.
• The Proof: They didn't just see the electricity flow; they proved it was truly superconducting.
  • Magnetic Shielding: When they cooled the material, it pushed magnetic fields away (the Meissner effect), acting like a perfect magnetic shield.
  • Positive Charge: They confirmed that the electricity was carried by "holes" (positive charges), not electrons, which was the specific type of superconductivity they were trying to achieve.

Why This Matters (According to the Paper)

The authors explain that this discovery is a "unique platform" for science, not necessarily for immediate consumer gadgets. Here is why they are excited:

• Simplicity: Because this material has the simplest possible structure of all cuprates, it removes the "noise" of complex layers. It allows scientists to study the fundamental rules of high-temperature superconductivity without the distraction of extra atomic blocks.
• The "Strange Metal" Mystery: The material showed a weird behavior where its resistance increased in a straight line as it got hotter. This is a hallmark of "strange metals," a state of matter that physicists are still trying to understand.
• The Nickelate Connection: Recently, scientists found superconductivity in "nickelates" (a cousin of cuprates). This new hole-doped cuprate acts as a bridge, helping scientists compare the two families to see if they follow the same rules.

In Summary

The paper reports that after 40 years of failure, scientists have finally made the simplest possible cuprate structure superconduct by using a clever mix of large Rubidium atoms and extra Oxygen. It works at surprisingly high temperatures (up to 100 K onset) and provides a clean, stripped-down laboratory to solve the biggest mysteries of how high-temperature superconductivity works.

Technical Summary

Technical Summary: Hole-Doped Superconductivity above 100 K in Infinite-Layer Cuprate Thin Films

Problem Statement
Since the discovery of high-temperature superconductivity in (La,Ba)₂CuO₄, the search for the minimal structural requirements for this phenomenon has driven extensive research. The infinite-layer cuprate (ACuO₂), consisting solely of CuO₂ square planes separated by spacer ions, represents the simplest and most fundamental member of the cuprate family. While electron-doped superconductivity and superconductivity in defective or superlattice variants of infinite-layer cuprates have been observed, hole-doped superconductivity via chemical substitution in the stoichiometric infinite-layer structure has remained elusive for nearly 40 years. Previous attempts to achieve this through doping with small monovalent cations (Li⁺, Na⁺) or oxygen-rich growth resulted in insulating behavior, leaving a fundamental gap in understanding the mechanism of high-Tc superconductivity and the role of charge-reservoir blocks.

Methodology
The authors synthesized single-crystal thin films of the hole-doped infinite-layer cuprate Sr₁₋ₓRbₓCuO₂ using pulsed laser deposition (PLD) followed by oxygen-rich annealing. The study employed a synergistic approach to hole doping:

1. Cation Substitution: Replacing Sr²⁺ with large monovalent Rb⁺ ions (specifically at x = 0.08, 0.10, and 0.12) to introduce holes.
2. Oxygen Incorporation: Utilizing slow-growth techniques and post-annealing in oxygen-rich environments to incorporate apical oxygen (Oap) and minimize oxygen vacancies.

Structural characterization was performed using X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM), including high-angle annular dark-field (HAADF) and annular bright-field (ABF) imaging. Transport properties were measured via resistivity, Hall effect, and magnetoresistance under magnetic fields up to 9 T. Magnetic properties were probed using DC magnetic susceptibility (ZFC/FC) and magnetization-field (M-H) loops.

Key Contributions and Results

• Realization of Hole-Doped Superconductivity: The study reports the first observation of superconductivity in a chemically substituted, hole-doped infinite-layer cuprate. The Sr₀.₉Rb₀.₁CuO₂ films exhibit a superconducting transition with an onset temperature (Tc,onset) of approximately 75 K (with some samples showing onset up to 100 K in the abstract context, though the main text specifies 75 K for the representative film) and a zero-resistance temperature (Tc,0) of about 23 K.
• Synergistic Doping Mechanism: Structural and transport analyses reveal that hole doping is not solely due to Rb substitution. The measured hole concentration (p ≈ 0.15 at 80 K) exceeds the nominal substitution level (x = 0.10). This excess is attributed to the synergistic effect of Rb substitution and apical oxygen incorporation. STEM-ABF imaging confirms the presence of apical oxygen (Oap) in the spacer layers, evidenced by doublet peaks in intensity line profiles.
• Carrier Type Confirmation: Hall coefficient measurements confirm hole-type charge carriers, distinguishing this system from the well-established electron-doped Sr₀.₉La₀.₁CuO₂. The Hall coefficient remains positive and increases as temperature decreases from 180 K to 40 K.
• Superconducting Characteristics:
  • The material exhibits linear-in-temperature resistivity at high temperatures, a signature often associated with strange metal behavior.
  • Magnetoresistance measurements under perpendicular fields yield an upper critical field Hc⊥(0) ≈ 33 T and a Ginzburg–Landau coherence length ξGL(0) ≈ 3 nm, values comparable to other cuprate superconductors.
  • DC magnetic susceptibility and M-H loops demonstrate a clear diamagnetic response and the Meissner effect below ~13 K, confirming bulk superconductivity, albeit with a limited superconducting volume fraction.
• Doping Dependence: Samples with varying Rb content (x = 0.08, 0.10, 0.12) show that both higher and lower doping levels relative to x = 0.10 can yield higher Tc and hole concentrations. The x = 0.08 sample, in particular, suggests that reduced Rb substitution may minimize oxygen vacancies, thereby favoring apical oxygen incorporation and enhancing hole doping.

Significance
The authors posit that this work resolves a long-standing experimental challenge by providing the first realization of hole-doped superconductivity in the infinite-layer cuprate. The significance of this achievement is threefold:

1. Fundamental Physics: As the parent structure of all cuprates, the hole-doped infinite-layer compound demonstrates that charge-reservoir blocks are not essential for the emergence of superconductivity. It offers a unique platform to revisit key puzzles such as electron-hole symmetry and strange metal behavior without the complexity of large charge-reservoir blocks.
2. Temperature Potential: While the highest Tc in cuprates remains 133 K (in HgBa₂Ca₂Cu₃O₈₊δ), achieving an onset temperature around 100 K in the minimal infinite-layer structure opens new avenues for further enhancing Tc through ionic-radius engineering and spin fluctuation tuning.
3. Bridge to Nickelates: The hole-doped infinite-layer cuprate serves as an isostructural analog to recently discovered superconducting infinite-layer nickelates. This system provides a critical bridge for comparing cuprate and nickelate physics, potentially offering insights into the absence of electron-doped superconductivity in nickelates and the role of electron-hole symmetry.