Chiral Induced Spin Selectivity (CISS)

• CISS is a phenomenon where unpolarized electrons become spin-polarized when transmitted through chiral media, with the polarization sign determined by the material's handedness.
• Experiments reveal that CISS operates at room temperature across diverse systems such as molecular monolayers, chiral crystals, and quantum wells, often showing spin polarizations up to 80%.
• Theoretical models of CISS invoke mechanisms like dipole-induced Rashba SOC, many-body interactions, vibronic coupling, and dephasing to explain its unexpectedly high spin selectivity.

Searching arXiv for recent and foundational CISS papers to ground the article. {} Chiral induced spin selectivity (CISS) denotes the emergence of spin-polarized electron transport or transfer in a chiral medium, typically without magnetic fields or magnetic constituents. In its canonical formulation, unpolarized carriers traversing a chiral molecule acquire a spin polarization whose sign depends on the handedness of the molecular structure; more recent work extends the phenomenon to chiral solids, donor–bridge–acceptor systems, cavity-dressed conductors, engineered quantum wells, and reciprocal spin-to-charge conversion. The effect is experimentally prominent at room temperature and is theoretically notable because the observed spin polarization is often much larger than what bare atomic spin–orbit coupling in light-element systems would naively suggest, so the microscopic origin of the effective spin selectivity remains a central issue (Li et al., 2020).

1. Definition, observables, and empirical scope

In transport language, CISS is usually expressed through a spin polarization

$$P=\frac{I_\uparrow-I_\downarrow}{I_\uparrow+I_\downarrow},$$

or, equivalently in scattering form, through a spin-resolved transmission imbalance between spin channels (Phuc, 2022). In donor–bridge–acceptor settings the same quantity is written in terms of the asymptotic spin populations in the acceptor, while in scanning-probe and magnetoresistance measurements it is often reported as

$$\mathrm{SP}(\%)=\frac{I^\uparrow-I^\downarrow}{I^\uparrow+I^\downarrow}\cdot100$$

(Zhang et al., 4 Sep 2025, Nalakath et al., 18 Mar 2026).

The empirical domain of CISS is broader than the original molecular-transport setting. The phenomenon has been studied in helical molecules and self-assembled monolayers, in trigonal selenium and tellurium, in chiral crystals such as RhSi and TaSi$_2$, in multilayer quantum-dot assemblies coupled by chiral linkers, in chiral-cavity geometries, and in artificial topological quantum wells (Yang et al., 2023, Fridman et al., 8 Jan 2026, Phuc, 2022, Liu et al., 23 Mar 2026). Reciprocal effects have also been formulated: inverse CISS (ICISS) describes spin-to-charge conversion in chiral layers under pure spin injection, while hybrid spin-pumping experiments report chiral-induced unidirectional spin-to-charge conversion at metal/chiral interfaces (Zhang et al., 4 Sep 2025, Moharana et al., 2024).

Two features recur across these studies. First, the sign of the response reverses with molecular or structural handedness. Second, sizable signals can persist at room temperature, which is why CISS is regularly discussed in relation to spintronics, quantum information, and spin-controlled chemistry (Li et al., 2020).

2. Symmetry constraints, reciprocity, and the meaning of “spin selectivity”

The symmetry content of CISS is subtler than a simple “spin filter” picture suggests. In the standard narrative, one spin state is transmitted and the opposite spin is reflected. However, several transport analyses emphasize that in a two-terminal linear-response device, time-reversal symmetry and unitarity impose strong reciprocity constraints. In particular, more than two terminals are required in order to probe the CISS effect in the linear regime, and spin-flip reflection accompanies chirality-dependent spin transmission in coherent scattering descriptions (Yang et al., 2018).

A sharper criticism is that a naive spin-filter scenario, in which transmitted and reflected electrons have opposite spin polarization, contradicts the principle that equilibrium spin current must vanish in a two-terminal device. An alternative formulation is that chiral molecules act as spin polarizers rather than spin filters: both transmitted and reflected electrons acquire the same type spin polarization, with the direction set by the molecule chirality and the electron incident direction (2208.00043). This does not remove spin selectivity; it reformulates it so that equilibrium constraints are respected.

These constraints explain why many transport models introduce some mechanism that breaks effective reciprocity in the electronic subspace or suppresses exact cancellation. One route is non-unitarity through leakage into side leads, which generates complex self-energies and spin-dependent evanescent decay lengths in a helical molecule (Matityahu et al., 2015). Another is explicit dephasing via Büttiker probes, as in engineered InAs/GaSb quantum wells, where geometric chirality and dephasing can be introduced controllably; achiral configurations exhibit no spin selectivity, whereas a chiral geometry plus edge-selective dephasing produces a finite polarization whose sign reverses under chirality reversal (Liu et al., 23 Mar 2026).

A recurring misconception is therefore that chirality alone in a strictly coherent two-terminal linear-response geometry is sufficient for an unambiguous measured polarization. The literature instead separates the symmetry-allowed existence of spin-dependent amplitudes from the experimental extraction of a net spin-polarized current.

3. Microscopic mechanisms proposed for CISS

The central theoretical problem is that the required effective spin–orbit scale appears unexpectedly large if one starts only from bare light-element SOC. Current work therefore offers several nonexclusive microscopic routes.

One analytic route treats a chiral molecule as an anisotropic wire with an internal dipole field. In this model, helical structure of the molecule is not necessary to observe CISS; what matters is broken mirror symmetry in three dimensions, anisotropic confinement, and Rashba-type SOC generated by the internal electrostatic field. The polarization arises only at second order in the Born expansion through mixed $V_D$–$V_{\rm SOC}$ terms, and reversing the enantiomer flips the sign of the polarization (Ghazaryan et al., 2020).

A very different route is many-body. A multi-orbital theory with an $s$-like valence Wannier state and a degenerate $(p_x,p_y)$ conduction doublet proposes that an effective SOI emerges from spontaneous formation of electron-hole pairing caused by many-body correlation. The crucial symmetry input is that the valence and conduction Wannier functions belong respectively to one- and two-dimensional representations of the spatial rotation symmetry around the molecular axis. In this theory the emergent first-quantized term is

$$H_{\rm SO}^{\rm eff}=\lambda_{\rm eff}\,\hat\sigma_z\,\hat L_z/2,$$

and the resulting scale can be tens of meV up to 100 meV, sufficient to support large room-temperature spin polarization (Li et al., 2020).

Vibronic scenarios provide another class of mechanisms. In a pseudo Jahn–Teller $C_3$ molecule, electronic translational and rotational degrees of freedom are coupled via nuclear vibrations. The molecule can then act as an efficient spin filter, and the efficiency can be nearly independent of the SOC strength in the relevant regime, while nuclear vibrations enhance the spin-filtering efficiency by increasing the relevant level splitting (Kato et al., 2021).

Dynamical transport theories emphasize unequal spin velocities. In a donor–bridge–acceptor system governed by a Lindblad-type master equation, the molecular spin-orbit coupling generates unequal spin velocities and achieves steady spin polarization with the help of dephasing (Zhang et al., 4 Sep 2025). Time-dependent quantum-transport simulations of a finite chiral molecule similarly attribute a nonzero spin polarization throughout the molecule to a spin-dependent group velocity of electrons; in a two-lead geometry that polarization persists into the steady state (Stuermer et al., 31 Oct 2025).

The mechanisms differ in what they elevate from “auxiliary” to “primary”: anisotropic electrostatics, collective correlations, vibronic translation–rotation coupling, or dynamical velocity splitting. This suggests that CISS is best viewed as a family of chirality-enabled spin-selective phenomena rather than a single settled microscopic mechanism.

Mechanism class	Core ingredient	Representative source
Analytic scattering	Anisotropy + dipole-induced Rashba SOC	(Ghazaryan et al., 2020)
Many-body emergent SOC	Electron–hole condensate in multi-orbital bands	(Li et al., 2020)
Vibronic enhancement	Pseudo Jahn–Teller translation–rotation coupling	(Kato et al., 2021)
Dynamical transport	Unequal spin velocities with dephasing	(Zhang et al., 4 Sep 2025, Stuermer et al., 31 Oct 2025)

4. Decoherence, vibrations, and scattering-assisted spin build-up

Dephasing and inelastic scattering occupy a special place in the CISS literature because they often provide the operative departure from exact two-terminal reciprocity. A minimal one-dimensional barrier model with Rashba-type SOC and a Büttiker voltage probe shows that spin-independent decoherence can act as an order-of-magnitude polarization enhancement mechanism. In that formulation, polarization arises by disruption of spin precession around the spin-orbit magnetic field with a new spin component along the field direction, and under-the-barrier decoherence is the efficient regime (Mena et al., 2024). The broader minimal-model review identifies four ingredients that are necessarily present in all experiments to date: chirality, spin–orbit coupling, decoherence, and tunneling (Mena et al., 2024).

A first-principles study of trigonal selenium makes the role of scattering more explicit. There the Hamiltonian $H=H_0+H_{\rm SOC}+H_{e\text{–}ph}$ is combined with a density-matrix evolution equation and a Lindblad-like collision integral for electron–phonon scattering. Charge transport along the chiral axis induces chirality-dependent spin and orbital polarization; orbital polarization arises from the helical symmetry alone and is present even at $$\mathrm{SP}(\%)=\frac{I^\uparrow-I^\downarrow}{I^\uparrow+I^\downarrow}\cdot100$$0, whereas spin polarization vanishes when SOC is turned off. Most importantly, the scattering rates become spin dependent in the presence of SOC, $$\mathrm{SP}(\%)=\frac{I^\uparrow-I^\downarrow}{I^\uparrow+I^\downarrow}\cdot100$$1, so spin polarization grows with distance $$\mathrm{SP}(\%)=\frac{I^\uparrow-I^\downarrow}{I^\uparrow+I^\downarrow}\cdot100$$2, a device-length dependence identified as a key difference from the collinear Edelstein effect (Gupta et al., 5 Aug 2025).

Quantitatively, the same selenium study reports, for coherent transport under $$\mathrm{SP}(\%)=\frac{I^\uparrow-I^\downarrow}{I^\uparrow+I^\downarrow}\cdot100$$3 eV, a maximum $$\mathrm{SP}(\%)=\frac{I^\uparrow-I^\downarrow}{I^\uparrow+I^\downarrow}\cdot100$$4 and $$\mathrm{SP}(\%)=\frac{I^\uparrow-I^\downarrow}{I^\uparrow+I^\downarrow}\cdot100$$5, with $$\mathrm{SP}(\%)=\frac{I^\uparrow-I^\downarrow}{I^\uparrow+I^\downarrow}\cdot100$$6 and $$\mathrm{SP}(\%)=\frac{I^\uparrow-I^\downarrow}{I^\uparrow+I^\downarrow}\cdot100$$7 nearly constant versus SOC. Under explicit electron–phonon scattering, $$\mathrm{SP}(\%)=\frac{I^\uparrow-I^\downarrow}{I^\uparrow+I^\downarrow}\cdot100$$8 grows nearly linearly over hundreds of nm in the bulk region, and the spin-diffusion length is $$\mathrm{SP}(\%)=\frac{I^\uparrow-I^\downarrow}{I^\uparrow+I^\downarrow}\cdot100$$9m (Gupta et al., 5 Aug 2025).

Strong driving can further regulate the scattering landscape. A non-perturbative Floquet treatment shows that light can have opposite effects on the CISS, because the steady states of nuclei coupled to spin electrons differ for spin-up and spin-down, and those differences arise from light-induced Lorentz forces associated with Floquet-renormalized SOC (Liu et al., 2023). This places decoherence, vibronic dynamics, and nonequilibrium dressing on the same conceptual footing: they are not merely corrections to a static spin filter, but can determine whether a measurable steady spin polarization exists at all.

5. Materials realizations and quantitative benchmarks

Molecular monolayers remain the most visible experimental platform. A notable recent example is the self-assembled monolayer of the axially chiral organophosphoric acid derivative 1,1'-binaphthyl-2,2'-diyl hydrogenphosphate (BNP) on NiOx/Ni. The monolayers are roughly 1 nm thick, preserve chirality according to circular dichroism, and show a CISS response with a high magnetoresistance and a spin polarization of 50–80% when measured using magnetic-conductive atomic force microscopy. For $_2$0 V, the reported values are: R-BNP, $_2$1; S-BNP, $_2$2; rac-BNP, $_2$3 (Nalakath et al., 18 Mar 2026). Above 0.5 V, the magnetoresistance curves can be fit by a Fowler–Nordheim tunneling model, from which effective barrier ratios $_2$4 are extracted.

This result is also important conceptually because BNP is axially chiral rather than helical, which supports the more general claim that helicity is not mandatory for CISS if true three-dimensional chirality and the relevant spin-active couplings are present (Ghazaryan et al., 2020, Nalakath et al., 18 Mar 2026).

Chiral solids broaden the materials landscape. First-principles transport in chiral crystals predicts simultaneous spin and orbital polarization of transmitted electrons, both increasing with thickness before saturating. The spin polarization is proportional to intrinsic SOC while the orbital polarization is insensitive to SOC. Representative values include $_2$5 and $_2$6 for RhSi at $_2$7, $_2$8 over a broad valence-band window in RhBiS, and $_2$9 at the valence-band edge of Te (Yang et al., 2023). These values are smaller than some molecular-monolayer reports, but the conductivity and carrier density are much larger.

Trigonal selenium provides a bridge between molecular and crystalline settings. The first-principles density-matrix dynamics study reports chirality-dependent spin and orbital polarization generated during transport, with monotonic increase versus a chirality measure $V_D$0, and a clear separation between SOC-sensitive spin polarization and weakly SOC-dependent orbital polarization (Gupta et al., 5 Aug 2025).

Quantum-optical and excitonic platforms probe another regime. In multilayer CdSe quantum-dot assemblies coupled by all-L or all-D heptapeptide polyalanine linkers, the photoluminescence lifetime difference $V_D$1 shows an oscillatory dependence on magnetic field and field angle, consistent with spin precession driven by the transverse field component relative to the chiral axis. This is presented as a room-temperature platform for probing quantum coherent manifestations of CISS (Fridman et al., 8 Jan 2026).

6. Engineered extensions and reciprocal effects

A striking extension is that spin selectivity can be realized in achiral materials by coupling electrons to a single mode of a chiral optical cavity. In a nonequilibrium Green’s-function treatment, the spin polarization in a two-terminal setup is demonstrated to approach unity if the rate of dephasing is sufficiently small and the average chemical potential lies in an appropriate, narrow range. When an intrinsically chiral molecule is strongly coupled to the same cavity mode, the operational energy window broadens through the combined action of cavity sidebands and the molecule’s intrinsic spin selectivity (Phuc, 2022).

An engineered solid-state realization appears in the InAs/GaSb topological quantum well. There, a chiral structure plus dephasing electrodes yields a clear spin polarization whose sign reverses when the chirality is flipped, while achiral configurations exhibit no spin selectivity. Within the bulk gap and with a single dephasing lead, $V_D$2 and $V_D$3, so $V_D$4; for $V_D$5 dephasing leads the plateau is $V_D$6, and for $V_D$7 it is $V_D$8 (Liu et al., 23 Mar 2026). This is a controlled demonstration of the cooperative role of chirality, SOC, and dephasing in a lithographically defined platform.

Reciprocal conversion effects further expand the field. The ICISS proposal models a chiral layer as a tight-binding slab of intertwined helical chains under a pure spin bias and finds a transverse charge current along the molecular axis, $V_D$9, because $V_{\rm SOC}$0 when the chiral axis and spin quantization are collinear. The calculated voltage reverses sign with chirality, survives strong disorder, and differs microscopically from a conventional inverse spin Hall response (Zhang et al., 4 Sep 2025). Hybrid spin-pumping experiments on YIG/Au/chiral-monolayer heterostructures report a chiral-induced unidirectionality in spin-to-charge conversion, with the effect maximal when the spin angular momentum is aligned with the molecular chiral axis (Moharana et al., 2024).

These developments show that CISS is no longer confined to “electron transmission through a molecule.” It now includes cavity-mediated spin selection, topological implementations, reciprocal spin-to-charge conversion, and coherent optical signatures.

7. Design principles, unresolved issues, and current directions

Several design rules recur across otherwise different models. In the many-body emergent-SOC theory, strong CISS is predicted only in molecules whose HOMO–LUMO gap is small and whose inter-orbital Coulomb/Hund interactions are large; the effective SOC decreases when the band gap increases, and small-gap systems can generate the large effective SOI needed for room-temperature selectivity (Li et al., 2020). In trigonal selenium, one should maximize structural chirality, maintain moderate to strong SOC, and exploit extended transport lengths or repeated scatterings so that spin-dependent electron–phonon processes can build up polarization with device length (Gupta et al., 5 Aug 2025). The minimal-model program instead emphasizes the combined necessity of chirality, SOC, decoherence, and tunneling (Mena et al., 2024).

The main unresolved issue is not whether CISS exists, but which microscopic ingredients are universal and which are platform specific. Some theories require dephasing, leakage, or electron–phonon scattering to obtain a measurable steady-state polarization in transport (Matityahu et al., 2015, Mena et al., 2024, Gupta et al., 5 Aug 2025). Others propose that many-body order, pseudo Jahn–Teller coupling, or spin-dependent group velocity can already generate the relevant selectivity and that environmental coupling mainly stabilizes or reveals it (Li et al., 2020, Kato et al., 2021, Stuermer et al., 31 Oct 2025). A plausible implication is that “CISS” collects several symmetry-compatible spin-selective mechanisms that become dominant in different materials classes.

A second unresolved point concerns the relation between spin and orbital polarization. Multiple studies now separate them sharply: orbital polarization can be generated by chirality alone and remain weakly dependent on SOC, whereas spin polarization requires SOC to convert orbital angular momentum into spin (Gupta et al., 5 Aug 2025, Yang et al., 2023). This suggests that orbital observables may provide a cleaner probe of structural chirality, while spin observables are more sensitive to interfacial fields, dissipation, and many-body enhancement.

Finally, the field has moved beyond the assumption that CISS is a purely molecular curiosity. Chiral crystals, cavity QED settings, artificial topological devices, and reciprocal conversion experiments suggest a broader program: engineering chiral symmetry breaking, spin-active couplings, and controlled decoherence to realize spin-polarized transport without conventional magnets (Phuc, 2022, Liu et al., 23 Mar 2026, Zhang et al., 4 Sep 2025). The outstanding task is to connect these platforms through a quantitatively predictive framework that respects reciprocity constraints, identifies the operative effective spin–orbit scale, and distinguishes universal symmetry requirements from mechanism-specific amplification channels.