🔬 mesoscale physics

Reconfigurable chiral superconductivity

Using nanoscale SQUID-on-tip magnetometry, researchers demonstrate that rhombohedral pentalayer graphene hosts reconfigurable chiral superconductivity driven by isospin-polarized domains, enabling deterministic, ultra-low current switching between opposite chirality states.

Original authors: Surajit Dutta, Nadav Auerbach, Tonghang Han, Yaozhang Zhou, Gal Shavit, Niladri-Sekhar Kander, Yuri Myasoedov, Martin E. Huber, Kenji Watanabe, Takashi Taniguchi, Long Ju, Eli Zeldov

Published 2026-05-14

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CC BY 4.0

Original authors: Surajit Dutta, Nadav Auerbach, Tonghang Han, Yaozhang Zhou, Gal Shavit, Niladri-Sekhar Kander, Yuri Myasoedov, Martin E. Huber, Kenji Watanabe, Takashi Taniguchi, Long Ju, Eli Zeldov

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a piece of graphene (a material made of a single layer of carbon atoms) that has been stacked five times in a specific, diamond-like pattern. Under very specific conditions—extreme cold and strong electric fields—this material doesn't just conduct electricity; it becomes a superconductor, meaning electricity flows through it with absolutely zero resistance.

But this isn't just any superconductor. The researchers discovered it has a "handedness" or chirality. Think of it like a screw: it can be a right-handed screw or a left-handed screw. In this material, the electrons spontaneously choose to spin in one direction (like a right-handed screw) or the opposite direction (left-handed), breaking the natural symmetry of time. This is called Chiral Superconductivity.

Here is the breakdown of what the scientists found, using simple analogies:

1. The "Traffic Jam" of Electrons

Before it becomes a superconductor, this material exists in a state called a "quarter-metal." Imagine a highway where cars (electrons) are forced to pick a specific lane and stay there. They are polarized.

The Discovery: The researchers used a tiny, super-sensitive magnetic camera (called a SQUID-on-tip) to take pictures of this material. They saw that the electrons weren't just flowing; they were forming domains.
The Analogy: Imagine a large field covered in grass. Half the grass is growing "North" and the other half is growing "South." The line where the North-grass meets the South-grass is called a Domain Wall. In this material, these walls separate regions where the electrons are spinning in opposite directions.

2. The "Ghost Wall" That Blocks Traffic

Usually, when electrons move between two regions, they flow freely. But in this material, the wall separating the "North-spin" and "South-spin" regions acts like a massive, invisible brick wall.

The Finding: When the researchers tried to push electricity across this wall, it got stuck. The wall was so resistive that it turned a perfect superconductor (zero resistance) into a high-resistance state.
The Analogy: It's like a highway that is perfectly smooth everywhere, except for one specific lane divider that acts like a concrete barrier. If you try to drive across it, your car stops dead. The researchers measured this "wall" to be incredibly difficult to cross, effectively blocking the flow of electricity.

3. The "Magnetic Switch"

The most exciting part is how they control these walls.

The Finding: By applying a tiny, tiny electric current (so small it's almost nothing), they could push these domain walls around. They could make the "North" grass take over the whole field, or switch it so the "South" grass takes over.
The Analogy: Imagine you have a giant magnet on a table with a line of dominoes. With a gentle breath (a tiny electric current), you can push the dominoes to fall one way or the other. The researchers found they could flip the entire magnetic state of the superconductor with currents thousands of times smaller than what is needed in current computer memory technology.

4. The "Inheritance" Mystery

The researchers wanted to know: Did the superconductor create these spinning patterns, or did it inherit them from the material before it became a superconductor?

The Finding: They found that the "handedness" (chirality) was already there in the normal state before the material even became a superconductor. When the material cooled down and became a superconductor, it kept that same pattern.
The Analogy: It's like a child inheriting their father's eye color. The superconducting state didn't invent the "spin"; it just kept the "spin" that was already present in the parent material.

5. Why This Matters (According to the Paper)

The paper claims this is a unique discovery because:

Direct Proof: They didn't just guess the material was chiral; they took pictures of the magnetic domains to prove it.
Reconfigurable: They can switch the material between different states (left-handed vs. right-handed) using tiny currents.
New Physics: It shows that superconductivity can coexist with these magnetic "traffic jams" (domain walls), which is something not seen in other superconductors.

In Summary:
The scientists found a way to see and control tiny magnetic "walls" inside a superconducting graphene material. They discovered that these walls act like massive barriers to electricity, but they can be moved and switched with incredibly small amounts of energy. This proves the material has a unique "handedness" that it inherited from its normal state, opening up a new way to think about how electricity and magnetism interact in the quantum world.

Technical Summary: Reconfigurable Chiral Superconductivity in Rhombohedral Pentalayer Graphene

Problem and Motivation
Rhombohedral multilayer graphene (RMG) at high displacement fields hosts superconductivity emerging from a spin–valley-polarized quarter-metal (1/4M) phase. Previous transport measurements in this system revealed pronounced magnetic hysteresis and anomalous transport signatures suggestive of time-reversal symmetry (TRS) breaking and chiral superconductivity (CSC). These observations motivated theoretical proposals of topological superconductivity and non-Abelian quasiparticles. However, direct magnetic evidence and microscopic insight into the superconducting state remained lacking. A central unresolved question was whether the superconducting phase exhibits macroscopic orbital TRS breaking (CSC) and whether this chirality originates from the pairing symmetry itself or is inherited from the underlying correlated normal state. Furthermore, the real-space structure and thermodynamic nature of the ordered state, particularly the role of domain walls (DWs) in isospin-polarized systems, had not been directly visualized.

Methodology
The authors employed nanoscale SQUID-on-tip (SOT) magnetometry to directly image and manipulate isospin-polarized domains in rhombohedral pentalayer graphene (R5G) devices. The experimental setup involved:

Device Fabrication: Dual-gated R5G devices with graphite gates and hBN encapsulation, allowing independent control of carrier density ( $n$ ) and displacement field ( $D$ ).
Magnetic Imaging: An indium SOT magnetometer was used to detect the local out-of-plane magnetization. To enhance signal-to-noise, small AC modulations of the gate voltage or current were applied to induce oscillations in carrier density, generating an oscillatory stray field ( $B_{z,ac}$ ) proportional to the differential magnetization ( $m_z = dM_z/dn$ ).
Transport Measurements: Four-probe transport measurements were conducted simultaneously with magnetic imaging to correlate resistance states (zero-resistance, low-resistance, high-resistance, and negative-resistance) with specific magnetic textures.
Control: The system was manipulated via magnetic field sweeps and ultra-low DC/AC currents to observe domain dynamics and switching.

Key Contributions and Results

Direct Evidence of Chiral Superconductivity (CSC):
The study provides spatially resolved thermodynamic evidence of TRS breaking in the superconducting phase. SOT imaging revealed ferromagnetic (paramagnetic) responses in the zero-resistance state, contrasting with the expected diamagnetic response of conventional superconductors. By initializing the system with positive or negative magnetic fields, the authors demonstrated the ability to train the system into two degenerate superconducting states of opposite chirality ( $K\uparrow$ and $K'\downarrow$ ), confirming macroscopic orbital TRS breaking.
Inheritance of Chirality from the Parent State:
The research establishes that the chiral domain structure in the superconducting phase is inherited from the parent 1/4M phase. Imaging of the 1/4M phase at high carrier densities showed that the magnetic hysteresis corresponds to spatially uniform isospin-polarized states. The transition to the CSC phase preserves this isospin polarization, indicating that the TRS breaking is intrinsic to the parent state rather than a novel feature solely of the superconducting pairing.
Domain Wall Proliferation and Hidden Symmetry Breaking:
A critical finding is the correlation between the onset of spike-like resistance hysteresis (indicating domain wall proliferation) and the boundary of the CSC phase. This transformation occurs at a specific carrier density ( $n_{DW}$ ) both above and below the critical temperature ( $T_c$ ). The authors propose that the 1/4M phase undergoes a hidden symmetry-breaking transition at $n_{DW}$ that simultaneously enhances superconductivity and reduces the domain wall line tension, facilitating the nucleation of opposite-polarity domains.
Ultra-Low-Current Control of Isospin Textures:
The authors demonstrated deterministic, reversible switching of chiral domains using currents as low as 0.4 nA. This is orders of magnitude lower than currents required for domain wall motion in conventional spintronic systems. The mechanism is identified as momentum transfer from reflected electrons (non-adiabatic regime) rather than spin-transfer torque, driven by the high interfacial resistivity of the isospin domain walls ( $\rho_{DW} \approx 41 \text{ k}\Omega\cdot\mu\text{m}$ ).
Reconfigurable Transport Regimes:
The CSC phase exhibits multiple transport regimes governed by the configuration of chiral domains:
- Zero-Resistance (ZR): Uniform isospin polarization with no domain walls crossing the measurement path.
- Low-Resistance (LR): Small, confined minority domains that do not span the contacts.
- High-Resistance (HR): Extended minority domains separated by highly resistive domain walls that traverse the current path.
- Negative Resistance (NR): Observed in specific configurations where current-driven domain wall motion generates a negative differential response.
  The authors showed that these states can be switched reversibly by controlling the current polarity and magnitude, effectively tuning the domain configuration.

Significance and Claims
The paper claims to establish rhombohedral graphene as a unique platform for reconfigurable chiral superconductivity. The primary significance lies in:

Direct Verification: Providing the first direct, real-space thermodynamic detection of TRS-breaking chiral domains in a candidate chiral superconductor.
Mechanism Insight: Demonstrating that the chiral order is inherited from a spin-valley-polarized parent state, resolving the origin of the TRS breaking.
New Functionality: Introducing a regime of "isospintronics" where electronic functionality (resistance states) is controlled by the real-space manipulation of isospin textures with ultra-low power consumption.
Platform Potential: Establishing R5G as a model system for coupling superconductivity to magnetic textures, offering a pathway to study non-Abelian quasiparticles and topological phenomena in a highly tunable environment.

The authors remain modest regarding the microscopic mechanism of the superconducting pairing itself, noting that while macroscopic TRS breaking is established, determining whether the order parameter is intrinsically chiral requires further investigation. They emphasize that the observed phenomena represent a new state of matter where superconductivity coexists with bistable, magnetically ordered domains.

1. The "Traffic Jam" of Electrons

2. The "Ghost Wall" That Blocks Traffic

3. The "Magnetic Switch"

4. The "Inheritance" Mystery

5. Why This Matters (According to the Paper)

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