Single-enantiomer spin polarisers in superconducting junctions

By utilizing superconducting scanning tunneling microscopy with manganese-functionalized tips to avoid ferromagnetic electrodes, this study provides unambiguous experimental evidence that single enantiomers of heptahelicene act as effective spin polarizers, confirming the chirality-induced spin selectivity effect and ruling out electrostatic artifacts.

The Gist

Imagine you have a crowd of people (electrons) trying to walk through a narrow hallway. In the world of quantum physics, these people have a secret trait called "spin," which acts like a tiny internal compass pointing either "up" or "down."

For years, scientists have been trying to build a "turnstile" that only lets people with a specific compass direction pass through, based on the shape of the hallway itself. This phenomenon is called the Chirality-Induced Spin Selectivity (CISS) effect. "Chirality" just means the object is handed—like a left hand or a right hand. The idea is that if you make a hallway shaped like a left-handed spiral, it should only let "spin-up" people through, and a right-handed spiral should only let "spin-down" people through.

However, the scientific community has been arguing about this. Previous experiments were messy. They used magnetic walls (ferromagnets) to try to detect the effect, but critics said, "Wait, maybe it's not the shape of the hallway doing the work; maybe it's just the magnetic walls changing their electrical properties." It was like trying to hear a whisper in a noisy room.

The New Experiment: A Silent, Magnetic Detective

This paper presents a new, much cleaner way to test the theory. The researchers built a tiny, ultra-precise tunnel using a Scanning Tunneling Microscope (STM). Here is how they set up their "hallway":

1. The Floor (The Sample): They placed a single layer of spiral-shaped molecules (called heptahelicene) on a lead surface. Some molecules were left-handed spirals, and some were right-handed. Crucially, they arranged them so the left-handed ones were in one group and the right-handed ones in another, like sorting red and blue marbles into separate piles.
2. The Ceiling (The Tip): Instead of a normal metal tip, they used a superconducting lead tip (a material where electricity flows without resistance) and stuck a tiny cluster of magnetic manganese atoms on the very end.
3. The Magic (YSR States): Because the tip is magnetic and superconducting, it creates special "ghostly" energy states inside the tunnel. Think of these as sensitive tripwires. These tripwires are tuned to only react if a specific type of electron (spin-up or spin-down) tries to cross them.

The Discovery

The researchers sent electrons through the tunnel and measured how easily they passed. They found a clear difference:

• When they sent electrons through the left-handed molecules, the "tripwires" for one type of spin lit up brightly, while the other stayed dark.
• When they sent electrons through the right-handed molecules, the pattern flipped. The other type of spin lit up, and the first one went dark.

This proves that the shape of the molecule itself acts as a spin polarizer. It's not just filtering out the "wrong" people; it's actively sorting them based on their internal compass.

Why This Matters (According to the Paper)

• No More Noise: By avoiding magnetic walls and magnet reversal, they removed the "noise" that made previous experiments confusing. They proved the effect comes from the molecule, not from the electrodes changing their electrical properties.
• Direction Matters: The experiment showed that the sorting effect depends on which way the electrons are traveling. This suggests the molecules act like active spin-polarizers (sorting the traffic) rather than just passive filters (blocking the traffic).
• Location is Key: They also found that the effect is strongest at the very tip of the molecule and weaker in the middle. This explains why some previous experiments failed: if you average the signal over the whole molecule (like taking a blurry photo of the whole hallway), the effect disappears. You have to look at the specific spot where the sorting happens.

In Summary

The paper claims to have finally caught the "ghost" of the CISS effect in a single molecule. They used a superconducting, magnetic detective tip to show that a single left-handed spiral molecule sorts electrons differently than a single right-handed one. This confirms that the shape of the molecule is indeed the key to controlling electron spin, without needing any external magnetic tricks.

Technical Summary

Problem Statement
The Chirality-Induced Spin Selectivity (CISS) effect, which posits that chiral molecules can act as spin-selective devices in electron transport, remains a subject of intense debate. While previous experimental reports suggested spin selectivity in chiral matter, these findings have been questioned. Critics argue that observed magnetoresistance changes in chiral materials on ferromagnets may result from electrostatic effects—such as work function changes upon magnetization reversal or variations in molecular electric dipoles—rather than genuine chiral spin locking. Furthermore, recent break junction studies have reported an apparent absence of the CISS effect in single-molecule experiments, raising doubts about whether single chiral molecules can function as spin polarizers at all. The core challenge is to provide unambiguous experimental evidence for the CISS effect in single enantiomers while eliminating confounding factors like ferromagnetic electrodes and magnetization reversal.

Methodology
To address these ambiguities, the authors employed a Scanning Tunneling Microscope (STM) junction design that avoids ferromagnetic electrodes entirely. The experimental system consists of:

1. Substrate: A crystalline Pb(111) surface, which serves as a superconducting electrode.
2. Molecules: A racemic mixture of heptahelicene (C30H18, [7]H) molecules, comprising $\Lambda$ and $\Delta$ enantiomers, adsorbed as a monolayer. On Pb(111), these molecules self-assemble into enantiopure domains, allowing for the clear spatial discrimination of individual $\Lambda$ and $\Delta$ molecules via STM imaging based on intramolecular structure.
3. Tip: A superconducting Pb tip functionalized with a magnetic manganese (Mn) atom cluster. This functionalization induces Yu-Shiba-Rusinov (YSR) states within the Bardeen-Cooper-Schrieffer (BCS) energy gap of the tip.
4. Probe Mechanism: The YSR states are fully spin-polarized due to the magnetic Mn cluster. These states serve as spin-sensitive probes for the tunneling current. By measuring the differential conductance ($dI/dV$) spectra, the authors probe the spin polarization of quasielectrons tunneling through the chiral molecules.
5. Controls: The study utilizes difference spectra ($\delta dI/dV$) between the two enantiomers to isolate the chiral effect. Experiments were conducted with and without an external magnetic field (40 mT) to verify that the Mn cluster maintains its magnetic moment independently. The authors also systematically varied tip-molecule distances and adsorption sites to rule out electrostatic artifacts and substrate-induced spin polarization.

Key Results

• Enantiomer Discrimination: The STM successfully resolved the intramolecular structure of the heptahelicene enantiomers, allowing for the unambiguous identification of $\Lambda$ and $\Delta$ domains and individual molecules.
• Spin-Dependent Signal Strength: Spectroscopy of the differential conductance revealed a significant dependence of the YSR signal strength on the handedness of the molecule. Specifically, the $\Lambda$ and $\Delta$ enantiomers exhibited opposite preferences for transmitting quasielectrons to the low-energy versus high-energy pairs of spin-polarized YSR states.
• Current Direction Dependence: The spectral signal dependence on the bias voltage polarity (current direction) was observed. The $\Lambda$ enantiomer preferentially transmitted spin-up quasielectrons at positive sample voltage and spin-down at negative voltage, while the $\Delta$ enantiomer showed the inverse behavior.
• Spatial Variability: The magnitude of the effect was found to depend on the specific site within the molecule (e.g., site 3 vs. site 2), correlating with the effective molecular length.
• Exclusion of Alternatives:
  • Electrostatics: The spectra remained invariant over a large range of tip-molecule separations, ruling out work function changes or dipole modifications as the primary cause.
  • Substrate Effects: The symmetry of the difference spectra contradicted the behavior expected from a spin-polarized substrate density of states.
  • Spin-Orbit Coupling (SOC): Calculations indicated that SOC effects in the Pb substrate are negligible at the Fermi level and relevant wave vectors, supporting the conclusion that the effect originates from the molecule.
  • Magnetic Impurities: The adsorbed molecules did not induce intragap states themselves, confirming they do not act as magnetic impurities.

Significance and Claims
The paper claims to provide unambiguous experimental evidence for the CISS effect at the single-enantiomer level. By utilizing superconducting electrodes and spin-polarized YSR states instead of ferromagnetic contacts, the study successfully isolates the chiral effect from electrostatic artifacts and magnetization reversal issues that have plagued previous research.

The authors conclude that the observed dependence of the signal on current direction demonstrates that individual enantiomers act as spin polarizers rather than simple spin filters. This distinction is crucial for understanding the mechanism of spin-momentum locking in chiral systems. Additionally, the spatial variability of the effect offers an explanation for null results in previous break junction experiments, suggesting that spatial averaging over contacts with undefined molecular orientation may obscure the single-molecule spin polarizer behavior. The work establishes a robust model system for future investigations into the fundamental physics of the CISS effect without the complications of ferromagnetic interfaces.