A 2026 first-principles study (Han et al., arXiv:2608.07305) predicts that interlayer sliding in two-dimensional magnetic sliding ferroelectrics carrying a 'fractional spin ferroelectric' order generates a symmetry-protected pure spin current — a flow of spin angular momentum with no accompanying net charge current — because the ionic and valence-electron contributions to in-plane charge transfer cancel during ferroelectric switching while the spin transfer, carried solely by valence electrons, persists. Estimated spin-current densities reach about 10810^8–109 (ℏ/2e) A/m210^9\,(\hbar/2e)\,\mathrm{A/m^2} for nanosecond switching in HH-stacked bilayer CrI3_3 and RR-stacked 2H-VX2_2 (XX = S, Se, Te), with up to 1012 (ℏ/2e) A/m210^{12}\,(\hbar/2e)\,\mathrm{A/m^2} projected if picosecond light-induced switching is achieved. This prediction is currently a single group's theory result: no experiment has detected a sliding-generated spin current, the picosecond regime is an untested extrapolation, and the material scope beyond the proposed candidates is unexplored. The open problem is to establish the sliding spin current as a real physical mechanism — experimentally detect and quantify the pure spin current generated by electrically driven interlayer sliding in a magnetic sliding ferroelectric.

Background

Spintronics encodes information in the spin degree of freedom of electrons, and the generation of pure spin currents — flows of spin angular momentum with no net transport of electric charge — is a central enabling task, because such currents transfer angular momentum without the Joule heating associated with charge flow. The established generation mechanisms each carry material or process constraints. The spin Hall effect (SHE) converts a longitudinal charge current into a transverse spin current via spin-orbit coupling, and therefore requires charge flow through a material with strong spin-orbit coupling, typically a heavy metal (Sinova et al. 2015). Spin pumping injects spin angular momentum from precessing magnetization of a ferromagnet into an adjacent normal metal (Tserkovnyak et al. 2005), and therefore requires sustained magnetization dynamics, usually microwave-driven. Newer proposals, such as pure spin currents generated in insulating antiferromagnets (Chen et al. 2025), add platforms but not the basic architecture of a driven magnetic or heavy-metal component.

In parallel, sliding ferroelectrics (SFEs) have emerged as a distinct class of two-dimensional van der Waals (vdW) ferroelectrics — layered crystals held together by weak interlayer forces — in which the switchable out-of-plane electric polarization (a separation of bound charge) is reversed not by displacing ions within each layer but by rigidly sliding one layer over the other between two energetically degenerate stacking arrangements. Interfacial ferroelectricity by vdW sliding was established experimentally in parallel-stacked bilayer boron nitride and related systems (Vizner Stern et al. 2021), and sliding-ferroelectric devices have since demonstrated fast (sub-nanosecond) and fatigue-resistant switching (Yasuda et al. 2024). In magnetic SFEs the sliding degree of freedom couples to magnetic order: in polar hexagonally stacked (HH-stacked, a stacking sequence without inversion symmetry) few-layer CrI3_3, sliding ferroelectricity coexists with magnetism and switches it at gate voltages as low as 0.4 V (Fox et al. 2025).

Han et al. (2026) combined symmetry analysis with density-functional-theory calculations to propose that magnetic SFEs can host a 'fractional spin ferroelectric' (FSFE) order: during ferroelectric switching by interlayer sliding, the change of the in-plane component of the spin electronic polarization — the difference Ps=P↑−P↓\boldsymbol{P}^s = \boldsymbol{P}^{\uparrow} - \boldsymbol{P}^{\downarrow} between the electric polarizations carried by spin-up and spin-down valence electrons — equals a fractional polarization quantum, while the change of the total electric polarization vanishes modulo the lattice polarization quantum (the polarization change is only defined up to a quantum set by the lattice; 'fractional' means a nontrivial rational fraction of that quantum). The mechanism is that the ionic and valence-electron contributions to the in-plane charge transfer cancel exactly, whereas the spin transfer, which stems solely from valence electrons, survives symmetry-protected. The sliding motion therefore pumps a pure spin current, with no net charge current, whenever the stacking state is switched. In the proposed materials — HH-stacked bilayer CrI3_3 and RR-stacked (rhombohedrally stacked) bilayers 2H-VX2_2 (XX = S, Se, Te), with VSi2X4_2X_4 (XX = N, P), NiI2_2, Cr2_2Ge2_2Te6_6, and ScI2_2 analyzed in the supplement — the estimated spin-current density is 1.5×109 (ℏ/2e) A/m21.5\times10^9\,(\hbar/2e)\,\mathrm{A/m^2} for bilayer CrI3_3 and 3.72×108 (ℏ/2e) A/m23.72\times10^8\,(\hbar/2e)\,\mathrm{A/m^2} for bilayer VS2_2 at a typical 1 ns switching time (the unit (ℏ/2e) A/m2(\hbar/2e)\,\mathrm{A/m^2} expresses spin current density in charge-current units, one ℏ/2\hbar/2 of angular momentum per electron charge). Because light-induced complete reversal of ferroelectric polarization in SFEs on picosecond timescales has been predicted theoretically (Yang and Meng 2024), the authors project densities as high as 1012 (ℏ/2e) A/m210^{12}\,(\hbar/2e)\,\mathrm{A/m^2} for picosecond switching — an unexplored regime.

What remains unresolved is precisely the physical reality and scope of this mechanism. The prediction rests on one group's first-principles and symmetry analysis; no independent calculation has verified it, and no experiment has detected a spin current generated by the sliding motion itself. Detection is nontrivial: a pure spin current produces no direct voltage, so an experiment must transduce it, for example via the inverse spin Hall effect (the reciprocal of the SHE, in which a spin current in a heavy-metal detector is converted into a measurable transverse voltage), while excluding the displacement and leakage charge transients that unavoidably accompany ferroelectric switching. The experimental premise is in place — magnetic SFE switching is established in HH-stacked CrI3_3 — but the sliding spin current itself, its magnitude, its fast-switching regime, and its material generality are all open.

Problem Statement

Establish whether the sliding spin current predicted by Han et al. (arXiv:2608.07305) is a real physical effect, by experimentally realizing and quantifying it in a two-dimensional magnetic sliding ferroelectric. Concretely: detect, during electrically driven interlayer sliding (ferroelectric switching) in a magnetic sliding ferroelectric such as HH-stacked bilayer CrI3_3 or RR-stacked bilayer 2H-VX2_2 (XX = S, Se, Te), a spin-current signal that carries no net charge current, whose sign tracks the direction of the stacking-state change, and whose density is consistent with the predicted relation js=ΔPs/Δtj_s = \Delta P^s/\Delta t (about 10810^8–109 (ℏ/2e) A/m210^9\,(\hbar/2e)\,\mathrm{A/m^2} at nanosecond switching times). This detection serves the overarching objective of verifying that interlayer sliding is a genuine all-electrical mechanism for pure spin current generation. A signal with this signature in at least one real material would establish the effect, while a measurement whose sensitivity places a quantitative upper bound on any sliding-generated spin current below the predicted magnitude would rule it out. Reaching the picosecond-switching regime that the source projects conditionally — spin-current densities of order 1012 (ℏ/2e) A/m210^{12}\,(\hbar/2e)\,\mathrm{A/m^2} if picosecond light-induced switching is achieved — and identifying fractional spin ferroelectric order in magnetic sliding ferroelectrics beyond those already analyzed by Han et al. would extend this verification.

References

  1. Yilin Han, Lei Li, Chaoxi Cui, Run-Wu Zhang, Zhi-Ming Yu, and Yugui Yao, "Fractional Spin Ferroelectric and Sliding Spin Current in Magnetic Sliding Ferroelectrics", arXiv:2608.07305 (2026), DOI 10.48550/arXiv.2608.07305, https://arxiv.org/abs/2608.07305
  2. Carter Fox, Jose D. Mella, Jack Rollins, Yangchen He, Yulu Mao, Haotian Jiang, Alaina Drew, Hongrui Ma, Takashi Taniguchi, Kenji Watanabe, Ying Wang, Daniel Rhodes, Salvador Barraza-Lopez, and Jun Xiao, "Sliding multiferroicity in hexagonal stacked CrI3", arXiv:2510.03220 (2025), DOI 10.48550/arXiv.2510.03220, https://arxiv.org/abs/2510.03220
  3. Jairo Sinova, Sergio O. Valenzuela, J. Wunderlich, C. H. Back, and T. Jungwirth, "Spin Hall effects", Reviews of Modern Physics 87, 1213 (2015), DOI 10.1103/RevModPhys.87.1213, https://doi.org/10.1103/RevModPhys.87.1213
  4. Yaroslav Tserkovnyak, Arne Brataas, Gerrit E. W. Bauer, and Bertrand I. Halperin, "Nonlocal magnetization dynamics in ferromagnetic heterostructures", Reviews of Modern Physics 77, 1375 (2005), DOI 10.1103/RevModPhys.77.1375, https://doi.org/10.1103/RevModPhys.77.1375
  5. M. Vizner Stern, Y. Waschitz, W. Cao, I. Nevo, K. Watanabe, T. Taniguchi, E. Sela, M. Urbakh, O. Hod, and M. Ben Shalom, "Interfacial ferroelectricity by van der Waals sliding", Science 372, 1462 (2021), DOI 10.1126/science.abe8177, https://doi.org/10.1126/science.abe8177
  6. Kenji Yasuda, Evan Zalys-Geller, Xirui Wang, Daniel Bennett, Suraj S. Cheema, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras, Pablo Jarillo-Herrero, and Raymond Ashoori, "Ultrafast high-endurance memory based on sliding ferroelectrics", Science 385, 53 (2024), DOI 10.1126/science.adp3575, https://doi.org/10.1126/science.adp3575
  7. Qing Yang and Sheng Meng, "Light-Induced Complete Reversal of Ferroelectric Polarization in Sliding Ferroelectrics", Physical Review Letters 133, 136902 (2024), DOI 10.1103/PhysRevLett.133.136902, https://doi.org/10.1103/PhysRevLett.133.136902
  8. Yingwei Chen, Junyi Ji, Liangliang Hong, Xiangang Wan, and Hongjun Xiang, "Generation of Pure Spin Current with Insulating Antiferromagnetic Materials", Physical Review Letters 135, 146703 (2025), DOI 10.1103/n8d2-hjnd, https://doi.org/10.1103/n8d2-hjnd
  9. Haoxiang Dong and Jian Zhou, "Current-Induced Spin and Orbital Polarization in Magnetic Sliding Ferroelectrics", physica status solidi (RRL) – Rapid Research Letters 18, 2400062 (2024), DOI 10.1002/pssr.202400062, https://doi.org/10.1002/pssr.202400062
  10. Chao Wu, Pengqiang Dong, Kai Wei, Hanbo Sun, and Ping Li, "Sliding ferroelectricity tunable conventional and anomalous spin Hall effects in bilayer 1T'-WTe2", Applied Physics Letters 129, 011901 (2026), DOI 10.1063/5.0323597, https://doi.org/10.1063/5.0323597
  11. Zhenzhou Guo, Jiangtao Yu, Shibo Fang, Jin Cao, Xiaodong Zhou, Yee Sin Ang, Wenhong Wang, Zhenxiang Cheng, and Xiaotian Wang, "Hybrid-parity sliding multiferroics", arXiv:2607.24337 (2026), DOI 10.48550/arXiv.2607.24337, https://arxiv.org/abs/2607.24337
  12. Yuxuan Sheng, Jia Zhang, Junwei Liu, and Menghao Wu, "Ubiquitous van der Waals altermagnetism with sliding/moire ferroelectricity", Science China Physics, Mechanics & Astronomy 68, 297511 (2025), DOI 10.1007/s11433-025-2698-5, https://doi.org/10.1007/s11433-025-2698-5
  13. Martin Latorre, Sebastian Allende, and Alvaro S. Nunez, "Electrically Switchable Spintronics in a Multiferroic Altermagnet", arXiv:2608.15953 (2026), DOI 10.48550/arXiv.2608.15953, https://arxiv.org/abs/2608.15953