A preprint by Lee, Lee, Jeong, Dalayoan, Shin, Jeong, Jin, Yan, Park and Namgung (arXiv:2509.21558) reports the first direct spatial visualisation of current-induced spin polarisation in a chirality-induced spin selectivity (CISS) device. The experiment uses chiral tellurium nanowires contacted by graphene electrodes — a design chosen specifically to eliminate the spin-orbit coupling (SOC) ambiguities that have clouded earlier CISS measurements made with heavy-metal electrodes. The principal finding is that spin polarisation in the nanowire and in both graphene electrodes shares the same sign, contradicting the widely assumed "spin filter" model and instead supporting what the authors call a "spin polariser" mechanism.
The work comes from the Ulsan National Institute of Science and Technology (UNIST), with Binghai Yan of the Weizmann Institute of Science and Pennsylvania State University as a corresponding author.
Why tellurium and graphene
Most CISS experiments have used organic chiral molecules on heavy-metal substrates. This design makes it difficult to determine whether the observed spin effects originate in the chiral molecule or are induced by proximity to the metal's strong SOC — a long-standing criticism of the field. The Lee et al. study inverts this arrangement: tellurium nanowires have intrinsically strong SOC from the heavy chalcogen atoms, while graphene has negligible SOC and a spin relaxation length of several micrometres. This means any spin polarisation detected in the graphene electrodes must have been injected from the nanowire rather than generated locally.
The tellurium nanowires were synthesised hydrothermally from Na₂TeO₃ and polyvinylpyrrolidone, producing single-crystalline wires tens of micrometres long and roughly 150 nm in height. The crystal structure consists of covalently bonded helical atomic chains arranged in a trigonal lattice, with right-handed (space group P3₁21) and left-handed (P3₂21) enantiomorphs. Chirality was confirmed by aberration-corrected scanning transmission electron microscopy (STEM) of cross-sections cut along the c-axis.
Two measurement techniques
Circular photogalvanic effect
The authors first confirmed chirality-dependent electronic behaviour using the Circular Photogalvanic Effect. A 532 nm laser at oblique incidence (45°) generated a photocurrent along the nanowire at zero bias that reversed sign between right- and left-circularly polarised light. Crucially, the sign of this circular-polarisation-dependent photocurrent was opposite for nanowires of opposite handedness, consistent with the spin-momentum-locked band structure computed by density functional theory (DFT). The CPGE determination of chirality matched the STEM structural determination.
Reflective magnetic circular dichroism
The core experiment measured reflective magnetic circular dichroism (RMCD) — the difference in reflectivity between left- and right-circularly polarised light — while DC current flowed through the device at 3.5 K. RMCD probes localised spin polarisation, because the asymmetric spin population modifies optical absorption differently for the two circular polarisations.
The laser was incident at 30° from normal, in the plane containing the nanowire axis. Four key observations are reported:
- Linear current dependence. RMCD scales linearly with applied current, indicating that the spin polarisation is proportional to the charge current.
- Current-reversal symmetry. When the current direction is reversed, the sign of RMCD reverses — the spin polarisation flips.
- Chirality reversal. Devices made from nanowires of opposite handedness show opposite RMCD slopes as a function of current.
- Longitudinal alignment. RMCD vanishes at normal incidence (θ = 0°), confirming that the spin is polarised along the nanowire axis, not perpendicular to it.
These four features — linear scaling, current-sign reversal, chirality-sign reversal and longitudinal orientation — are the hallmarks of the CISS effect in this system.
The spin-filter vs. spin-polariser question
The most consequential result concerns the spatial distribution of spin polarisation across the device. By scanning the RMCD probe over the entire nanowire-plus-electrodes structure, the authors map spin polarisation in real space. Two devices of opposite chirality were measured.
In the widely assumed spin filter model, a chiral conductor preferentially transmits one spin channel and reflects the other. This implies that the two electrodes should show opposite spin polarisation — one accumulates spin-up, the other spin-down — producing a spin current through the chiral material.
In the spin polariser model, predicted by Wolf, Liu, Xiao, Park and Yan (2022) and by Yang, van der Wal and van Wees (2019), both transmitted and reflected electrons acquire the same spin polarisation. SOC inside the chiral material mixes the spin channels: the favoured spin transmits preferentially, while the disfavoured spin is backscattered with a spin flip, so that the reflected population is also preferentially polarised in the same direction.
Lee et al. observe the same sign of spin polarisation in the nanowire and in both graphene electrodes — for both current polarities and for both chiralities. This is the spatial signature of the spin-polariser mechanism. The authors describe this as "strongly indicative" of the spin-polariser model.
Spin relaxation in graphene
The RMCD signal is strongest at the nanowire–graphene interface and decays exponentially into the graphene. Fitting eight line profiles in each device yields characteristic spin relaxation lengths of 7.74 µm and 6.34 µm for positive and negative current in one device, and 3.74 µm and 5.37 µm in the device of opposite chirality. These values are consistent with independent measurements of spin relaxation in multilayer graphene on SiO₂ (roughly 3–8 µm), obtained by the nonlocal spin-valve method in previous work.
First-principles support
Real-time time-dependent DFT (rt-TDDFT) calculations model the spin dynamics under an applied electric field along the chiral axis. The computed spin–momentum-locked band structure shows opposite spin textures for right- and left-handed wires. When an electric field is applied, the resulting spin polarisation reverses with handedness and with field direction, matching the experimental observations. Crucially, calculations without SOC produce zero spin polarisation, confirming that SOC and chiral geometry are both necessary ingredients.
Relationship to the CISS debate
This study enters a contentious field. The dominant narrative, associated with Ron Naaman and collaborators, treats CISS as spin filtering — chiral molecules selectively pass one spin species. Lee et al. explicitly frame their result as evidence against the spin-filter picture, at least in the tellurium–graphene system.
The finding sits in a complex relationship with the electrostatic alternative proposed by Tirion and van Wees (2024), who argue that the large magnetoresistance in two-terminal CISS devices does not require spin-dependent transport at all. Lee et al. confirm that current-induced spin polarisation is real and spatially resolvable — this is not an artefact of contact potentials — but their spin-polariser conclusion aligns with the Groningen group's earlier theoretical work (Yang, van der Wal and van Wees, 2019), which predicted the spin-polariser mechanism that Lee et al. now report observing.
The study does not address the anomalously large spin polarisation ratios (60 % or higher) commonly reported for organic chiral molecules with negligible intrinsic SOC. The authors note this explicitly: by using tellurium, which has undisputed strong SOC, they sidestep the long-standing puzzle of where the effective SOC comes from in organic CISS systems.
What the study does not establish
The experiment confirms spin polarisation but does not directly measure the magnitude of spin polarisation as a fraction of the total carrier population. RMCD gives a relative signal proportional to spin imbalance, not an absolute percentage. The measurements were performed at 3.5 K; whether the spatial spin distribution persists at room temperature is not reported. The study concerns an inorganic chiral material with strong intrinsic SOC, and the authors acknowledge that its conclusions may not transfer directly to organic chiral molecules where the SOC origin remains debated.
References
- Lee, J.; Lee, S.; Jeong, U.; Dalayoan, D. J. C.; Shin, S.; Jeong, H. Y.; Jin, H.; Yan, B.; Park, N.; Namgung, S. "Real Space Imaging of Spin Scattering in Chirality-Induced Spin Selectivity." arXiv:2509.21558.
- Wolf, Y.; Liu, Y.; Xiao, J.; Park, N.; Yan, B. "Unusual spin polarization in the Chirality-Induced spin selectivity." ACS Nano 16, 18601–18607 (2022).
- Yang, X.; van der Wal, C. H.; van Wees, B. J. "Spin-dependent electron transmission model for chiral molecules in mesoscopic devices." Physical Review B 99, 024418 (2019).
Source notes & attribution
- https://arxiv.org/pdf/2509.21558