Bulk superconductivity up to 96 K in pressurized nickelate single crystals

This study reports the synthesis of high-quality, ambient-pressure flux-grown bilayer nickelate single crystals that exhibit bulk superconductivity with a transition temperature reaching up to 96 K under high pressure, establishing a correlation between in-plane lattice distortion and enhanced superconductivity.

The Gist

The Big Picture: Chasing the "Holy Grail" of Superconductors

Imagine electricity as water flowing through a pipe. Usually, the pipe has rough walls that slow the water down and create heat (this is electrical resistance). Superconductors are like "magic pipes" where the water flows with zero friction and zero heat loss. This is incredibly useful for things like MRI machines, maglev trains, and lossless power grids.

However, there's a catch: most magic pipes only work when they are frozen solid (near absolute zero, -273°C). To make them practical, scientists are hunting for a material that becomes a superconductor at "warm" temperatures—specifically, above the boiling point of liquid nitrogen (-196°C or 77 K). If we find one, it changes the world.

Recently, scientists found a new family of materials called nickelates (made of nickel and oxygen) that act like superconductors at high temperatures, but only if you squeeze them with the pressure of a mountain (about 14,000 times the atmospheric pressure). This paper is about making these materials easier to grow and finding a version that works at even higher temperatures.

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1. The Problem: Growing Crystals is Like Baking a Cake in a Pressure Cooker

Previously, to grow these special nickel crystals (specifically a "bilayer" structure called La3Ni2O7), scientists had to use a technique called "floating zone" growth.

• The Analogy: Imagine trying to bake a perfect cake, but you have to do it inside a pressure cooker at 18 bars of pressure. It's dangerous, expensive, and hard to control. If you mess up the pressure, the cake turns out lumpy, has impurities, or doesn't rise properly.
• The Result: The old crystals were often messy, mixed with other unwanted phases (like a cake that accidentally turned into a cookie), and were very small.

2. The Breakthrough: The "Flux" Method (The Slow Cooker Approach)

The team at Shandong University and other labs developed a new way to grow these crystals at normal room pressure.

• The Analogy: Instead of a pressure cooker, they used a "flux" method. Think of this like making rock candy. You dissolve sugar in hot water (the flux) and let it cool slowly. The sugar crystals grow out of the solution as the water evaporates.
• What they did: They melted the ingredients (Lanthanum, Nickel, Oxygen) in a special liquid salt (Potassium Carbonate) at high heat and let it cool down slowly.
• The Win: They successfully grew large, high-quality crystals (up to 220 micrometers, which is about the width of a human hair) without needing a pressure cooker. It's like moving from a high-tech pressure cooker to a simple, reliable slow cooker.

3. The Secret Ingredient: Chemical Pressure (The "Shrink Ray")

The team wanted to know: Can we make the superconducting temperature even higher?

• The Theory: In the world of superconductors, squeezing atoms closer together (physical pressure) usually helps. The scientists tried to mimic this squeezing without using a machine. They did this by swapping some of the big Lanthanum atoms with smaller rare-earth atoms (like Samarium).
• The Analogy: Imagine a dance floor filled with big dancers (Lanthanum). If you swap some big dancers for smaller ones (Samarium), the remaining dancers get squeezed closer together naturally. This is called "Chemical Pressure."
• The Discovery: They found that swapping in Samarium worked best. It created a specific crystal structure (La2SmNi2O7) that was cleaner and more perfect than the original.

4. The Result: A New Record (91 K)

They took their new, high-quality Samarium-doped crystals and put them in a high-pressure machine (a Diamond Anvil Cell) just to test them.

• The Outcome: The crystals started conducting electricity with zero resistance at 91 Kelvin (-182°C).
• Why it matters: This is the highest temperature ever recorded for a nickel-based superconductor. It's a new world record.
• The Catch: They still need high pressure (about 22 GPa) to reach this temperature. They haven't quite solved the "ambient pressure" superconductivity puzzle yet, but they have found the best candidate material and the best way to grow it.

5. Proving the Quality: The "X-Ray Passport"

Before celebrating, they had to prove the crystals were actually good. They used several "tests":

• X-Ray Diffraction: Like taking a passport photo to check the identity of the crystal structure. It confirmed the atoms were arranged perfectly.
• Nuclear Quadrupole Resonance (NQR): A sensitive test that listens to the "hum" of the atoms. If the crystal was messy (mixed phases), the hum would be noisy. Their crystals hummed clearly, proving they were pure.
• Electron Microscopy: They took a picture of the crystal at the atomic level, showing that the layers were stacked perfectly, like a neat stack of pancakes, with no crumbs or gaps.

Summary: What Does This Mean for the Future?

1. Easier Manufacturing: They proved you don't need a high-pressure furnace to grow these crystals. You can do it at normal pressure, which makes it cheaper and easier for other scientists to try.
2. Better Materials: By swapping atoms (Chemical Pressure), they found a version of the material that is cleaner and has a higher potential for superconductivity.
3. The Path Forward: While they still need high pressure to get the 91 K superconductivity, they have provided the "blueprint" and the "best ingredients." Now, scientists can focus on tweaking these specific crystals to see if they can achieve superconductivity at normal pressure.

In short: They built a better factory to make the world's most promising superconducting material and found a recipe that pushes the temperature record higher than ever before. We are one step closer to the "magic pipes" that could revolutionize our energy grid.

Technical Summary

1. Problem Statement

The discovery of high-temperature superconductivity (near 80 K) in bilayer Ruddlesden-Popper nickelate La$_3$Ni$_2$O$_7$ under high pressure (14–43.5 GPa) has sparked significant interest. However, the field faces three critical bottlenecks:

• Crystal Quality: Previous synthesis methods (e.g., floating zone under high oxygen pressure) often yield crystals with impurities, intergrowth of different phases (e.g., La$_2$NiO$_4$·La$_4$Ni$_3$O$_{10}$), and inhomogeneity.
• Synthesis Constraints: High-quality single crystals have historically required high-pressure gas environments, making them difficult to access and scale.
• T$_c$ Limits: The highest reported onset temperatures ($T_{c,onset}$) for nickelates are around 83 K (La$_3$Ni$_2$O$_7$) and 82 K (La$_2$PrNi$_2$O$_7$). It remains unclear if higher $T_c$ can be achieved in bulk samples and if chemical pressure (cation substitution) can further enhance superconductivity, as theoretical predictions for trilayer systems have been disappointing.

2. Methodology

The authors employed a novel ambient-pressure flux growth method to synthesize high-quality bilayer nickelate single crystals.

• Synthesis Strategy:
  • Flux System: Used anhydrous K$_2$CO$_3$ as a flux with a mass ratio of 1:15 (sample:flux).
  • Process: Mixed rare earth oxides (La, Pr-Er) and NiO were heated to 1000–1050°C for 72 hours, followed by furnace cooling. This was performed entirely at ambient pressure, eliminating the need for high-pressure gas chambers used in previous floating-zone techniques.
  • Chemical Pressure: To optimize the structure and suppress phase intergrowth, the authors substituted La with smaller rare earth elements (R = Pr, Nd, Sm, Eu, Er) to form La$_{3-x}$R$_x$Ni$_2$O$_{7-\delta}$.
• Characterization Techniques:
  • Structural Analysis: Single-crystal X-ray diffraction (SC-XRD), Powder X-ray diffraction (PXRD) with Rietveld refinement.
  • Microscopy & Spectroscopy: Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) for elemental mapping, and Scanning Transmission Electron Microscopy (STEM) for local structure imaging.
  • Phase Purity: Nuclear Quadrupole Resonance (NQR) to detect intergrowth of different Ruddlesden-Popper phases.
  • Superconductivity Tests: High-pressure electrical resistivity measurements using a Diamond Anvil Cell (DAC) with Helium as a pressure-transmitting medium (up to ~24 GPa).

3. Key Contributions

• Ambient Pressure Synthesis: Successfully demonstrated a scalable, accessible method to grow large, high-quality bilayer nickelate single crystals without high-pressure gas equipment.
• Phase Purification via Substitution: Identified that substituting La with Sm (forming La$_2$SmNi$_2$O$_{7-\delta}$) effectively suppresses the intergrowth of competing hybrid phases (La$_2$NiO$_4$·La$_4$Ni$_3$O$_{10}$), a major issue in pure La$_3$Ni$_2$O$_7$ growth.
• Record-Breaking $T_c$: Achieved a superconducting onset temperature ($T_{c,onset}$) of 91 K under high pressure, the highest reported for any nickelate superconductor to date.

4. Key Results

A. Crystal Growth and Structural Properties

• Crystal Size: Grew single crystals up to 220 μm on edge for La$_2$SmNi$_2$O$_{7-\delta}$, nearly double the size of pure La$_3$Ni$_2$O$_7$ crystals grown by the same method.
• Structural Integrity:
  • The crystals belong to the monoclinic $P2_1/m$ space group.
  • Site Preference: Sm atoms preferentially occupy the La sites between the bilayers (interlayer sites), rather than randomly.
  • Bond Angles: The out-of-plane Ni-O-Ni bond angle decreased from 168.5° (La$_3$Ni$_2$O$_7$) to 164.2° in La$_2$SmNi$_2$O$_{7-\delta}$, indicating significant structural distortion induced by chemical pressure.
  • Valence State: Bond valence sum calculations confirmed Ni remains in a similar valence state (~2.7) and Sm is 3+, suggesting the substitution does not drastically alter the Ni electronic count but modifies the lattice.

B. Crystal Quality Verification

• Homogeneity: EDS mapping confirmed uniform distribution of La, Sm, and Ni across the crystal.
• Phase Purity (NQR): Unlike La$_3$Ni$_2$O$_7$ which showed four distinct resonance lines indicating phase intergrowth, the La$_2$SmNi$_2$O$_{7-\delta}$ sample showed a single broad resonance peak. This indicates a significant suppression of R-P phase intergrowth, confirming a nearly pure bilayer phase.
• Local Order: STEM imaging revealed perfectly ordered bilayer alternating stacks over tens of nanometers with no visible intergrowth.

C. Superconductivity

• Transition Temperature: Annealed La$_2$SmNi$_2$O$_{7-\delta}$ single crystals exhibited a superconducting transition with $T_{c,onset} = 91$ K at 22 GPa.
• Bulk Nature:
  • Resistance dropped sharply, with residual resistance at 10 K being only 5.5% of the normal state.
  • Measurements along orthogonal directions showed identical behavior, ruling out filamentary superconductivity.
  • Magnetic field suppression of the transition followed standard high-$T_c$ superconductor characteristics.
• Upper Critical Field ($H_{c2}$): Using the Ginzburg-Landau model, the zero-temperature upper critical field was estimated to be as high as 291.7 T (based on 90% resistance criteria), with coherence lengths of 1.1–2.0 nm.
• Medium Effect: Using Helium (hydrostatic medium) instead of paraffin resulted in a sharper transition (~10 K width), confirming the importance of hydrostatic pressure for observing the intrinsic properties.

5. Significance

• New Synthesis Paradigm: The ambient-pressure flux method provides a "low-barrier" route for the global scientific community to produce high-quality nickelate single crystals, accelerating research into the mechanism of high-$T_c$ superconductivity.
• Chemical Pressure Strategy: The work validates that chemical pressure via cation substitution (specifically Sm) can optimize the crystal structure to suppress competing phases and potentially enhance $T_c$.
• Record $T_c$: Achieving 91 K pushes the boundaries of nickelate superconductivity closer to the liquid nitrogen boiling point (77 K), suggesting that nickelates are a viable platform for high-temperature superconductivity comparable to cuprates.
• Future Direction: The results suggest that further optimization of the rare-earth substitution and pressure conditions could lead to even higher $T_c$ values or potentially ambient-pressure superconductivity in bulk nickelates.