Ambient-pressure 151-K superconductivity in HgBa2Ca2Cu3O8+{\delta} via pressure quench

This paper reports the achievement of a record ambient-pressure superconducting transition temperature of 151 K in HgBa2Ca2Cu3O8+{\delta} by utilizing a novel pressure-quench protocol to stabilize pressure-induced superconducting states at ambient conditions.

The Gist

The Big Idea: Catching a "Super-Shape" and Freezing It

Imagine you have a lump of clay. If you squeeze it really hard with a giant press, it changes shape and becomes incredibly strong and useful. But the moment you let go of the press, it snaps back to its original, weaker shape.

For decades, scientists have been trying to find materials that conduct electricity with zero resistance (superconductivity) at high temperatures. This would revolutionize the world, allowing for loss-free power grids, super-fast trains, and powerful quantum computers.

The problem? The best superconductors we know of only work when they are under immense pressure (like being crushed deep inside the Earth). Once you take that pressure away, they stop working.

This paper reports a breakthrough: The scientists found a way to squeeze a special material, squeeze it until it becomes a "super-superconductor," and then quickly release the pressure while keeping the new, powerful shape. It's like molding the clay, then instantly flash-freezing it so it stays in that strong shape even after you take your hands off.

The Material: The "Super-Clay" (Hg1223)

The material they used is a complex crystal called HgBa₂Ca₂Cu₃O₈+δ (let's call it Hg1223 for short).

• At normal pressure: It conducts electricity without resistance at -140°C (133 Kelvin). This is already very cold, but it's the current world record for this type of material.
• Under high pressure: If you squeeze it hard, it gets even better, conducting electricity at -109°C (164 Kelvin). That's much warmer and much more useful.

The Trick: The "Pressure Quench" (PQP)

The scientists developed a new technique called Pressure Quenching. Think of it like this:

1. The Squeeze: They put the Hg1223 crystal inside a tiny diamond cell (a machine that can squeeze things with the force of a mountain on a speck of dust). They squeezed it until it reached a "super-state" where it could work at very high temperatures.
2. The Freeze: While the crystal is in this super-state, they quickly drop the temperature to near absolute zero (using liquid helium). This "locks" the atoms in place.
3. The Snap: They suddenly release the pressure. Because the atoms were frozen in that high-pressure shape, they don't have time to snap back to their original form. They get "stuck" in the super-state.
4. The Result: They pull the crystal out of the machine. It is now at normal room pressure, but it still acts like it's under immense pressure.

The Results: A New World Record

By using this "Pressure Quench" trick, the scientists achieved something amazing:

• They created a superconductor that works at -122°C (151 Kelvin) at normal atmospheric pressure.
• This is a new world record, beating the previous record by 18 degrees.

Why does this matter?
Imagine a world where we don't need giant, expensive liquid-nitrogen cooling systems to run superconductors. If we can get them to work at higher temperatures, we can use them in everyday technology. This discovery is a giant step toward making superconductors practical for real life.

What's Actually Happening Inside? (The Science Bit)

The scientists wanted to know why the material stayed in this new shape.

• No Structural Change: They looked at the crystal with powerful X-rays and saw that the basic building blocks (the atoms) didn't rearrange into a new pattern. The "blueprint" of the house is the same.
• The "Wrinkles" Theory: However, they noticed the crystal was a bit "wrinkled" or strained. It's as if you squeezed a sponge, froze it, and let it go. It doesn't return to a perfect sphere; it stays slightly squished.
• The Metaphor: Imagine a crowd of people (electrons) in a room. Under normal pressure, they are jumbled and can't move efficiently. When you squeeze the room (pressure), they organize into a perfect line and move instantly. The "Pressure Quench" is like taking a photo of that perfect line and freezing the people in place. Even when you open the doors (release pressure), they stay in that perfect line for a while because they are "stuck" in that formation by the strain and tiny defects in the crystal.

The Future: What's Next?

The scientists admit this is just the beginning.

• Stability: The "frozen" state isn't permanent forever. If you heat it up too much, it eventually relaxes back to normal. They need to figure out how to make it last longer.
• Even Higher Temperatures: In one experiment, they saw a hint of superconductivity at an even higher temperature (172 K), but they couldn't repeat it yet. They hope to refine their "freezing" technique to catch that even higher state.
• Beyond Superconductors: This "Pressure Quench" trick isn't just for electricity. It could be used to trap other weird, useful states of matter that usually only exist under extreme conditions, opening the door to entirely new technologies.

Summary

In short, these scientists figured out how to squeeze a material into a super-powerful state and then "snap-freeze" it so it keeps that power even when the pressure is gone. They broke the world record for how warm a superconductor can be at normal pressure, bringing us one step closer to a future with super-efficient energy and super-fast computers.

Technical Summary

1. Problem Statement

Since 1993, the record for the highest superconducting transition temperature ($T_c$) at ambient pressure has remained stagnant at 133 K for the cuprate compound HgBa$_2$Ca$_2$Cu$_3$O$_{8+\delta}$ (Hg1223). While higher $T_c$ values (up to 164 K for Hg1223 and 260 K for hydrides) have been achieved, they require extreme pressures (up to 190 GPa), rendering them impractical for widespread technological application. The central challenge is to stabilize these high-pressure-induced, metastable high-$T_c$ phases at ambient pressure without losing their enhanced superconducting properties.

2. Methodology: The Pressure-Quench Protocol (PQP)

The authors developed and applied a novel Pressure-Quench Protocol (PQP) to trap high-pressure metastable states at ambient pressure. The process involves three critical stages:

1. Target Identification: Applying high pressure to the sample to reach a specific target $T_c$ (above the ambient record).
2. Rapid Quenching: Rapidly releasing the pressure (quenching) while maintaining the sample at a low temperature ($T_Q$) to "freeze" the metastable phase.
3. Retrieval and Characterization: Removing the sample from the Diamond Anvil Cell (DAC) with minimal disturbance and characterizing its properties at ambient pressure.

Key Experimental Parameters:

• Material: High-quality single crystals of Hg1223 synthesized via the self-flux method.
• Pressure ($P_Q$): Ranged from 10.1 GPa to 29.7 GPa.
• Quench Temperature ($T_Q$): Primarily 4.2 K (liquid helium), with some experiments at 77 K (liquid nitrogen).
• Quench Rate ($v_Q$): Rapid release of pressure screws to achieve a fast $dP/dt$.
• Characterization: Transport measurements (resistivity), DC magnetization, Synchrotron X-ray diffraction (XRD), and Density Functional Theory (DFT) calculations.

3. Key Contributions

• New Record: Achieved a reproducible ambient-pressure $T_c$ of 151 K in Hg1223, breaking the 30-year-old record of 133 K.
• Protocol Validation: Demonstrated that PQP can stabilize pressure-induced metastable phases in cuprates, extending the applicability of this technique beyond previous successes in materials like Sb and FeSe.
• Mechanistic Insight: Provided evidence that the retention of the high-$T_c$ phase is linked to strain and structural defects (nanostructures) rather than a bulk structural phase transition, supported by XRD line broadening and DFT calculations showing a Lifshitz transition.

4. Key Results

• Superconducting Transition Temperatures:
  • 151 K: Achieved by quenching from 18.9 GPa at 4.2 K. This represents an 18 K increase over the pristine ambient-pressure record.
  • 147–149 K: Achieved with quenching pressures between 10.1 GPa and 10.9 GPa at 4.2 K.
  • 139 K: Achieved when quenching at a higher temperature ($T_Q = 77$ K), indicating that lower $T_Q$ is crucial for retaining higher $T_c$ phases.
  • 172 K (Unreproduced): One sample showed a transient transition up to 172 K, but this was not reproducible and may involve complex multi-phase relaxation.
• Phase Stability:
  • The retained high-$T_c$ phase is bulk superconducting (approx. 78% volume fraction), confirmed by DC magnetization.
  • The phase is stable for at least 3 days at 77 K but degrades when heated above 200 K, confirming its metastable nature.
• Structural Analysis (XRD):
  • Synchrotron XRD confirmed that the crystal structure remains tetragonal (P4mmm) at ambient pressure after quenching; no new structural phase was formed.
  • However, the Full Width at Half Maximum (FWHM) of diffraction peaks increased significantly (0.20° vs. 0.12°), indicating the presence of micro-strain and structural defects induced by the pressure quench.
• Theoretical Support:
  • DFT calculations revealed a Lifshitz transition around 12.5 GPa, characterized by the appearance of new Fermi pockets from apical oxygen 2p electrons.
  • This transition causes a sudden rise in the electronic density of states (DOS), explaining the sharp increase in $T_c$ under pressure. The authors hypothesize that the energy barrier created by this electronic anomaly allows the high-pressure state to be trapped at ambient pressure via the quench.

5. Significance

• Technological Impact: This breakthrough demonstrates a viable pathway to achieve high-temperature superconductivity without the need for continuous high-pressure apparatus, potentially enabling applications in energy transmission, quantum computing, and fusion technology.
• Scientific Paradigm: The PQP establishes a new non-equilibrium strategy for exploring quantum states that are otherwise inaccessible at ambient conditions. It bridges the gap between fundamental high-pressure physics and practical material science.
• Future Directions: The study suggests that optimizing the quench parameters (pressure, temperature, rate) and understanding the role of oxygen vacancies and defect engineering could push the ambient-pressure $T_c$ even higher, potentially exceeding 151 K. It opens new avenues for investigating other quantum materials where pressure enhances properties.