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Adding or removing one triangulene molecule changes a chain’s parity, and that single-unit change determines whether its edge spins interact with a superconducting condensate. In the reported 2026 experiment, odd-length chains coupled to the condensate and produced Yu–Shiba–Rusinov (YSR) bound states inside the superconducting gap; even-length chains formed a spin-zero ground state and showed no corresponding coupling.
What the researchers built
The experiment uses triangulene, a small triangular graphene fragment with spin S=1. When several triangulene units are linked through antiferromagnetic interactions, they form a molecular Haldane chain. A defining feature of the Haldane phase is that each end of the chain carries a fractional spin-1/2 state—half the spin of an individual triangulene unit.
To study those molecular spins next to a superconductor, the team created a custom Au(111)/Nb(110) platform. Gold is suitable for on-surface molecular synthesis but is not intrinsically superconducting. The researchers therefore grew a 25–30-atomic-layer gold film on a niobium crystal. Through the proximity effect, the gold acquired superconducting electronic properties while retaining a chemically useful surface for assembling the chains.
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Why one molecule changes the physics
“Parity” here means whether the chain contains an even or odd number of triangulene units. The edge states at the two ends interact differently depending on that count. The sign and strength of the inter-edge interaction alternate with chain length, producing distinct total-spin ground states.
| Chain property | Odd number of triangulene units | Even number of triangulene units |
|---|---|---|
| Chain parity | Odd | Even |
| Total ground-state spin | S=1 | S=0 |
| Coupling to the superconducting condensate | Present | Absent in the measured parity-dependent behavior |
| Spectroscopic signature | YSR bound states inside the superconducting gap | No corresponding YSR coupling |
| Role in a length-controlled experiment | Coupled molecular spin state | Decoupled reference state |
Odd chains retain an uncompensated global spin and can exchange spin information with the condensate. Even chains pair their contributions into an S=0 ground state, leaving no equivalent magnetic moment for the observed superconducting coupling.
How the coupling was detected
The researchers used scanning tunnelling microscopy and scanning tunnelling spectroscopy (STM/STS) to resolve both the molecular structures and their electronic excitations. In an odd chain, the characteristic signal was a Yu–Shiba–Rusinov bound state: a state created inside a superconductor’s energy gap by a magnetic interaction.
Those in-gap states provide a spectroscopic marker that the molecular spin is interacting with the superconducting condensate. Even chains did not show the corresponding parity-dependent YSR coupling, consistent with their spin-zero ground state.
The four-unit-chain measurement
For a four-unit chain, terminal tunnelling spectra showed peaks at about 8.5 meV. The reported singlet–triplet excitation energy associated with that chain was 6.3 meV (Nature, 2026). These measurements connect the end-state spectrum to the underlying interaction between the two fractional edge spins.
What is new about the result
The key advance is control by molecular length rather than by changing a gate voltage, magnetic field or chemical environment. Adding or removing exactly one triangulene unit switches the chain between the odd and even cases, turning the measured interaction with the superconducting condensate on or off.
CIC nanoGUNE researcher Nacho Pascual described the result as “a method for turning the interaction of the chains with the superconductor on or off by changing their length by a single unit.” The team says the same design principle can be used to explore how magnetic states, quantum correlations and topological phases interact with a superconducting condensate, molecule by molecule.
What the experiment does—and does not—demonstrate
Demonstrated
- Triangulene units with S=1 can be assembled into molecular Haldane chains on a proximitized gold surface.
- The chain’s even/odd length changes its total-spin ground state between the measured S=1 and S=0 cases.
- Odd chains couple to the superconducting condensate and exhibit YSR in-gap states.
- Even chains provide a spin-zero, decoupled comparison within the same material platform.
- STM/STS can resolve the parity-dependent electronic signatures at the chain ends.
Not yet demonstrated
- A consumer quantum device or commercial molecular qubit.
- Long-term storage, processing or readout of quantum information.
- A scalable manufacturing process for arrays of these chains.
- A complete topological-computing architecture.
Potential applications in molecular spin qubits and controlled studies of quantum correlations remain research directions. The reported platform is a specialized laboratory system built around a custom thin film, synthesized molecules and STM/STS instrumentation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why the parity switch matters for future experiments
Having coupled and decoupled chains that differ by only one molecular unit gives researchers an internal control. An odd chain can probe how a localized molecular spin modifies superconducting states, while an even chain can serve as a nearby reference with the same constituent building block but a different total spin.
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That controlled contrast may help isolate magnetic exchange, edge-state interactions and superconducting effects without changing the substrate or measurement setup. It also offers a way to test theoretical descriptions of Haldane chains and their fractional edge spins under superconducting proximity.
The broader significance is methodological: molecular design supplies a discrete, atom-by-atom switch for coupling to a quantum material. The experiment shows that in this system, the difference between “on” and “off” is one triangulene unit.
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