Unconventional superconductivity emerging along with the strange-metal behavior in UAs2 under pressure

This study reports the discovery of pressure-induced unconventional superconductivity with a maximum critical temperature of 4 K in UAs2, which emerges alongside strange-metal behavior and exhibits a robust upper critical field far exceeding the Pauli limit, suggesting a quantum critical origin involving 5f-band electrons.

The Gist

Imagine a world where electricity flows without any resistance at all. This is the dream of superconductivity. Usually, this happens when materials are cooled down to near absolute zero. But scientists are always hunting for a special kind of superconductor—one that works in a "weird" way, potentially allowing for future quantum computers.

Recently, a team of scientists discovered a new candidate for this "weird" superconductivity in a material called UAs₂ (Uranium Arsenide), but only when they squeezed it incredibly hard.

Here is the story of their discovery, explained simply:

1. The Starting Point: A Grumpy Metal

At normal pressure (like the air in your room), UAs₂ is a bit of a grump. It's a metal that conducts electricity, but it has a "bad temper" at a specific temperature (about 274 Kelvin, or just above freezing). At this point, the atoms inside line up in a specific magnetic pattern called antiferromagnetism. Think of this like a crowd of people where everyone is standing still and facing opposite directions from their neighbors. This magnetic "stiffness" stops the material from becoming a superconductor.

2. The Squeeze: Changing the Rules

The scientists put this material inside a tiny diamond press (a Diamond Anvil Cell) and started squeezing it. Imagine squeezing a sponge; as you apply pressure, the sponge changes shape and properties.

• The Tipping Point: When they squeezed the material to about 20 times the pressure of the atmosphere (20 Gigapascals), something dramatic happened. The magnetic "stiffness" broke down. The material underwent a structural change, shifting from a square-like arrangement to a rectangular one.
• The Magic Appears: Once that magnetic order was crushed, the material suddenly became a superconductor! It started conducting electricity with zero resistance at temperatures up to 4 Kelvin (about -269°C). This is the highest temperature ever recorded for superconductivity in this specific family of uranium-based materials.

3. The "Strange Metal" Clue

Here is the most fascinating part. Usually, when a metal gets colder, its resistance drops in a predictable curve. But in this squeezed UAs₂, right before it becomes a superconductor, the electricity behaves strangely.

The scientists found that the resistance dropped in a perfectly straight line as the temperature went down. In the world of physics, this is called a "strange-metal" state.

• The Analogy: Imagine driving a car. Normally, as you slow down, the friction changes in a complex way. But in this "strange metal," the friction slows down in a perfectly straight, predictable line, no matter how fast you are going. This straight-line behavior is a famous "fingerprint" of the mysterious, unconventional superconductors that scientists have been chasing for decades.

4. The Magnetic Shield

To test if this was a "special" kind of superconductor, they hit it with strong magnets.

• The Pauli Limit: There is a theoretical "speed limit" for how much magnetism a normal superconductor can handle before it breaks. It's like a dam that can only hold back a certain amount of water.
• The Result: The UAs₂ superconductor didn't just hold back the water; it broke the dam. It withstood magnetic fields twice as strong as the theoretical limit for normal superconductors. This suggests the electrons inside are pairing up in a very unusual way (possibly "spin-triplet" pairing), similar to the recently famous material UTe₂.

5. The Quantum Critical Point

The scientists noticed that this "strange metal" behavior and the superconductivity appeared right at the exact moment the magnetic order was being crushed by pressure.

• The Metaphor: Think of a tightrope walker. The "Quantum Critical Point" is the exact moment the tightrope walker is about to fall. In this material, the "falling" (the collapse of magnetism) creates a chaotic, energetic environment that actually helps the superconductivity form. The "strange metal" behavior is the sign that the material is teetering on this edge.

Summary

The paper claims that by squeezing Uranium Arsenide (UAs₂) to extreme pressures, they:

1. Crushed its magnetic order.
2. Created a new state where it becomes a superconductor at 4 Kelvin.
3. Found that it behaves like a "strange metal" (with linear resistance), a hallmark of exotic physics.
4. Discovered it can withstand magnetic fields far beyond the normal limits, suggesting a rare type of electron pairing.

This discovery adds a new member to the family of mysterious uranium-based materials and gives scientists a new playground to study how magnetism, pressure, and strange metals interact to create superconductivity.

Technical Summary

Technical Summary: Unconventional Superconductivity and Strange-Metal Behavior in UAs₂ under High Pressure

Problem and Motivation
Unconventional superconductivity, particularly with spin-triplet pairing, remains a central pursuit in condensed matter physics due to its potential for topological quantum computation. While the uranium-based compound UTe₂ has recently attracted significant attention as a candidate for spin-triplet superconductivity, intrinsic examples are rare. The authors investigate the uranium di-pnictide UAs₂, a sibling compound to UTe₂, to explore the interplay between magnetism and superconductivity in uranium-based systems. UAs₂ is known to be an antiferromagnet (AFM) with a high Néel temperature ($T_N \approx 274$ K) at ambient pressure. The study aims to determine if suppressing this magnetic order via high pressure can induce superconductivity and whether such a state exhibits signatures of unconventional pairing, such as strange-metal behavior and an upper critical field exceeding the Pauli limit.

Methodology
The researchers utilized high-quality single crystals of UAs₂ grown via the chemical vapor transport method. The study employed a combination of ambient-pressure characterization and in situ high-pressure electrical transport measurements.

• Ambient Pressure: Single-crystal X-ray diffraction (XRD), DC magnetization (SQUID-VSM), and resistivity measurements were performed to establish the baseline structural and magnetic properties.
• High Pressure: Resistivity measurements were conducted using a BeCu-type diamond anvil cell (DAC) with a four-probe van der Pauw configuration. Soft KBr was used as the pressure-transmitting medium, and pressure was calibrated via ruby fluorescence. Measurements extended up to 40.5 GPa.
• Analysis: The temperature dependence of resistivity ($R(T)$) was analyzed using the power-law formula $R(T) = R_0 + AT^n$ to distinguish between Fermi liquid ($n \approx 2$), strange-metal ($n \approx 1$), and other transport behaviors. Upper critical fields ($\mu_0H_{c2}$) were determined by measuring $R(T)$ under various magnetic fields and fitting the data to the Ginzburg-Landau equation.

Key Results

1. Suppression of Antiferromagnetism and Structural Transition: At ambient pressure, UAs₂ exhibits metallic behavior with an AFM transition at 274 K. As pressure increases, $T_N$ decreases monotonically. A sudden drop in room-temperature resistance occurs around 20 GPa, attributed to a structural phase transition from tetragonal to orthorhombic symmetry. Above approximately 22.5 GPa, the long-range AFM order is completely suppressed.
2. Emergence of Superconductivity: Upon further increasing pressure beyond the suppression of AFM order, a superconducting state emerges. The superconducting transition temperature ($T_c$) increases with pressure, reaching a maximum onset $T_c$ of approximately 4.05 K and a zero-resistance $T_c$ of 3.27 K at 26.8 GPa. The superconducting dome is relatively narrow, vanishing completely above 31.9 GPa.
3. Strange-Metal Behavior: In the normal state above the superconducting transition, the electrical resistance exhibits a linear temperature dependence ($R \propto T$, or $n \approx 1$) over a wide temperature range. This "strange-metal" behavior is most robust near the optimal pressure for superconductivity (around 26.8 GPa) and persists even when superconductivity is suppressed by high magnetic fields (up to 9 T).
4. Upper Critical Field and Unconventional Pairing: The superconducting state is robust against magnetic fields. The zero-temperature upper critical field, $\mu_0H_{c2}(0)$, is estimated to be approximately 12 T. This value significantly exceeds the Pauli paramagnetic limit ($\mu_0H_p \approx 1.84 T_c \approx 6.6$ T for $T_c = 3.6$ K), suggesting that the pairing mechanism is unconventional and potentially spin-triplet in nature.
5. Quantum Criticality: The phase diagram reveals a quantum critical point (QCP) near 20 GPa where the AFM order collapses. The coexistence of the strange-metal state ($n \approx 1$) and the optimal superconducting dome near this QCP indicates that the superconductivity likely arises from fluctuations associated with quantum criticality.

Significance and Claims
The authors claim that their work demonstrates the first observation of pressure-induced superconductivity in UAs₂, achieving a $T_c$ of ~4 K, which is the highest among uranium-based compounds with 5f-band electrons reported to date. The study establishes a clear link between the suppression of antiferromagnetism, the emergence of a strange-metal state, and unconventional superconductivity in this system.

The paper posits that the combination of a high upper critical field exceeding the Pauli limit and the presence of strange-metal behavior strongly suggests an exotic pairing mechanism, possibly spin-triplet, similar to that proposed for UTe₂. The authors emphasize that UAs₂ under pressure offers a rich phase diagram merging antiferromagnetism, robust superconductivity, strange-metal behavior, and quantum criticality, providing a new avenue for investigating the dual itinerant-localized nature of 5f-electrons in uranium-based materials. They conclude that these findings support the exploration of unconventional superconductors in other compounds containing partially filled f-band and d-band electrons.