Chirality-Induced Spin Selectivity (CISS)

• CISS is a phenomenon where chiral structures induce spin polarization in electrons during transmission, transport, or chemical reactions.
• The effect arises from diverse mechanisms such as spin–orbit coupling, vibronic interactions, and polaronic transport that amplify spin selectivity.
• Recent studies demonstrate CISS in varied systems—from molecular donor–bridge–acceptor setups to twisted quantum materials—linking chirality to advanced spintronic applications.

Chirality-induced spin selectivity (CISS) is the phenomenon in which electrons moving through a chiral molecular or crystalline environment emerge with a preferred spin orientation. In the classic picture, the direction of spin polarization depends on the handedness of the chiral structure and on the direction of electron motion, but the microscopic origin of the effect remains unsettled: the literature contains transport-based, dynamical, vibronic, orbital-to-spin-conversion, and non-Hermitian or pseudo-Hermitian formulations, and no single theory yet explains all reported observations quantitatively across transmission, transport, and chemical settings (Evers et al., 2021).

1. Definition, scope, and phenomenology

CISS is used as an umbrella term for spin-dependent electron processes in non-magnetic chiral systems. The established experimental domains discussed in the theoretical literature are electron transmission, electron transport, and chemical reactions. In transmission, unbound electrons cross a chiral layer and acquire spin polarization; in transport, current through a chiral junction becomes spin selective; in chemistry, spin-polarized electrons or spin-dependent interfacial effects can modify reaction rates, bond cleavage, adsorption, water splitting, crystallization, and enantioselective interactions (Evers et al., 2021).

A persistent empirical feature is that the sign of the spin response tracks chirality. Opposite enantiomers generally produce opposite polarization, and in many settings the signal also depends on the direction of charge flow. The same review literature emphasizes that CISS is real and widespread but not universal in the sense of appearing under all conditions, and that the decisive microscopic ingredients can depend on contacts, non-equilibrium driving, dissipation, molecular length, and environmental coupling (Evers et al., 2021).

The standard scalar observable in the transport literature is a spin polarization of transmitted current or conductance. A representative definition in a two-terminal scattering formulation is

$$P_t(E)\equiv \frac{\sigma_t(E)}{\mathrm{Tr}[\rho_{\mathbb t}(E)]}\in[-1,1],$$

where $\sigma_t(E)$ is the spin-resolved transmission conductance-like quantity and $\rho_{\mathbb t}(E)$ is the transmission probability matrix (Menichetti et al., 2023). In molecular and solid-state studies alike, the central question is what microscopic structure makes $P_t(E)$ nonzero and, in some systems, large.

2. Symmetry constraints, reciprocity, and transport geometry

A major strand of the modern CISS literature treats the effect as a symmetry problem rather than only a molecular one. In coherent two-terminal transport with nonmagnetic leads, the conventional spin-filter picture—one spin preferentially transmitted and the opposite spin preferentially reflected—conflicts with the constraint that equilibrium spin current must vanish. In a scattering-matrix description, unitarity yields

$$\sigma_t+\sigma_{r'}=0,\qquad \sigma_r+\sigma_{t'}=0,$$

and the resulting interpretation is that a chiral device acts as a spin polarizer rather than a spin filter: transmitted and reflected electrons for the same injection side can carry the same spin sign, while reversing the transport direction reverses the polarization sign (2208.00043).

A related group-theoretic analysis shows that the relevant object is the symmetry of the full junction—electrodes plus molecule or spacer—not the isolated molecule alone. A longitudinal mirror plane $\sigma_l$ forces the longitudinal spin polarization to vanish, and longitudinal rotations constrain transverse components. Conversely, even an achiral molecule can exhibit spin polarization along the transport direction if the whole junction is chiral in a specific way. This formalism also implies that rotating one electrode relative to the other can be sufficient to generate spin polarization in standalone metallic nanocontacts, provided the symmetry reduction removes the mirror or rotation operations that otherwise constrain the scattering matrix (Dednam et al., 2022).

These reciprocity constraints have direct experimental implications. In a linear-response Landauer–Büttiker framework, a CISS molecule modeled as a spin-selective element must also include spin-flip reflection, and more than two terminals are required in order to probe the CISS effect in the linear regime. Four-terminal nonlocal geometries were proposed precisely because two-terminal linear conductance does not provide a robust CISS signature as a pure charge signal (Yang et al., 2018).

Taken together, these analyses shift the conceptual center of gravity away from the naive statement that a chiral molecule simply transmits one spin and blocks the other. The more precise statement is that chiral structure, together with the symmetry class of the device and the spin-dependent scattering channels available to it, determines whether measurable spin polarization can appear and how it must transform under reversal of chirality, current direction, or junction geometry.

3. Microscopic mechanisms proposed in theory

The literature contains several non-equivalent microscopic mechanisms for CISS. A widely used baseline is that chirality first induces orbital polarization or chiral orbital motion, and local spin-orbit coupling (SOC) converts that orbital imbalance into spin polarization. This logic appears explicitly in transport reinterpretations of CISS and in first-principles transport studies of chiral crystals, where spin polarization is proportional to intrinsic SOC while orbital polarization is insensitive to SOC (2208.00043, Yang et al., 2023).

One route to strong selectivity in light-atom molecules is pseudo Jahn-Teller coupling. In a $\mathrm{C}_3$ molecular model, electron translation along the molecular axis and rotation around the axis are coupled by nuclear vibrational modes, and chirality is encoded by the condition $V_+\ne V_-$. In that framework, nuclear vibrations do not merely perturb transport; they mediate translation–rotation hybridization and can enhance the spin-filtering efficiency so that it is nearly independent of the bare SOC strength in the favorable regime (Kato et al., 2021).

A different interaction-driven route is polaronic transport. In a helical molecular chain with strong electron–phonon coupling, the current carriers become polarons, and polaron fluctuations create a new energy scale for CISS. In this picture, band narrowing makes SOC comparatively more important, while phonon-assisted hopping broadens the energy range over which strong spin polarization and asymmetric magnetoresistance can appear (Klein et al., 2022).

Several recent works recast CISS as an explicitly dynamical problem. Time-dependent quantum-transport simulations on a chiral tight-binding chain attribute nonzero spin polarization inside the molecule to a spin-dependent group velocity of electrons, with the occupancy polarization defined as

$p_m(t)=\frac{n_{m,+}(t)-n_{m,-}(t)}{n_m(t)}.$

In that framework, a one-lead geometry supports only transient polarization, whereas a two-lead geometry can sustain a steady-state spin imbalance because continuous transport prevents complete cancellation by reflections (Stuermer et al., 31 Oct 2025). Closely related Lindblad-type donor–bridge–acceptor modeling reaches the conclusion that molecular SOC generates unequal spin velocities and that intermediate dephasing converts the slow spin into the fast spin, yielding steady acceptor polarization in isolated donor–chiral molecule–acceptor systems (Zhang et al., 4 Sep 2025).

Other proposals move further away from conventional coherent transport. One electrochemical study argues that molecular vibrations in chiral molecules primarily drive spin polarization and that electric current is mainly a probe of this chiral spin state rather than its cause; in that account, a vibration-induced dynamical spin polarization couples magnetically to a ferromagnetic multilayer and governs CISS-related magnetoconductance and enantiomer separation (Miwa et al., 2024). A cavity-QED extension shows that spin selectivity can be realized in achiral materials by coupling electrons to a single mode of a chiral optical cavity, and that combining a helical molecule with a chiral cavity can enhance spin polarization and broaden the operational window (Phuc, 2022).

These mechanisms are not mutually identical, and the field does not presently treat them as a closed taxonomy. What they share is the view that bare atomic SOC in an otherwise featureless chiral path is generally insufficient to account for the full phenomenology, and that orbital structure, vibrations, many-body dressing, dephasing, or external electromagnetic structure may be needed to amplify or stabilize the effect.

4. Direct observation in isolated donor–chiral bridge–acceptor molecules

A notable conceptual advance was the direct observation of CISS in isolated covalent donor–chiral bridge–acceptor molecules, rather than in molecules attached to metallic, semiconducting, or magnetic substrates. In that study, the system is a donor–chiral bridge–acceptor architecture $D\!-\!B_\chi\!-\!A$ in which the chiral bridge is an axially chiral dimer connecting a photoactive donor $\sigma_t(E)$0 (peri-xanthenoxanthene, PXX) and an acceptor $\sigma_t(E)$1 (NDI). Selective donor photoexcitation triggers the sequence

$\sigma_t(E)$2

and the final long-lived radical-pair state is the spin-correlated radical pair $\sigma_t(E)$3 (Eckvahl et al., 2023).

The key experimental methods were time-resolved electron paramagnetic resonance spectroscopy (TREPR) and out-of-phase electron spin echo envelope modulation (OOP-ESEEM). The molecules were embedded in aligned nematic liquid crystal 5CB and frozen, allowing the long molecular axis—and therefore the chiral axis and the electron-transfer axis—to be oriented either parallel or perpendicular to the applied magnetic field. This geometry is essential because the observed CISS contribution depends on the relative orientation of the charge-transfer direction and the field (Eckvahl et al., 2023).

The decisive observation is an orientation-dependent spectral signature. When the molecular axis is parallel to the field, the chiral molecules and the achiral reference behave similarly. When the axis is perpendicular to the field, the chiral molecules show additional “wing” features in the TREPR spectra that are absent in the achiral control. Those wings indicate that the initial radical-pair spin state is not purely singlet: chirality has mixed triplet character into the initial state. In the high-field description used in that work, the singlet and triplet sublevels are mixed into field-dependent eigenstates, and the CISS contribution biases the spin state created during ultrafast charge separation so that the precursor is better described as a chiral mixture of singlet and triplet character along the chiral axis (Eckvahl et al., 2023).

The evidentiary structure is also important. Transient absorption spectroscopy confirms ultrafast sequential charge separation and long radical-pair lifetimes; OOP-ESEEM measures donor–acceptor distances and verifies good alignment in the frozen liquid crystal; orientation-dependent TREPR isolates the features unique to the chiral molecules; and spectral simulations reproduce the data only when a substantial CISS contribution is included. The central implication is that chirality alone can influence spin dynamics in the molecule itself, without requiring a substrate to generate or amplify the effect (Eckvahl et al., 2023).

This result repositions molecular CISS within spin chemistry and quantum-state control. It shows that chirality can act during the generation and evolution of a radical-pair state, not only during transmission across a junction, and therefore links CISS to radical-pair photophysics, molecular spin initialization, and related proposals for molecular quantum information science.

5. Chiral crystals, tellurium, and twisted quantum materials

CISS is not confined to organic molecules. In chiral crystals with good conductivity and intrinsic SOC, first-principles transport calculations show that electrons passing through the crystal acquire both spin and orbital polarization, and that both increase with material thickness before saturating to bulk values. In this formulation, opposite enantiomers yield opposite spin and orbital polarization, spin polarization is proportional to intrinsic SOC, and orbital polarization is largely insensitive to SOC strength (Yang et al., 2023).

The materials dependence is nontrivial. In the $\sigma_t(E)$4–$\sigma_t(E)$5–$\sigma_t(E)$6 series, the calculated $\sigma_t(E)$7 at a representative peak energy around $\sigma_t(E)$8 eV rises from $\sigma_t(E)$9 in $\rho_{\mathbb t}(E)$0 to $\rho_{\mathbb t}(E)$1 in $\rho_{\mathbb t}(E)$2 and $\rho_{\mathbb t}(E)$3 in $\rho_{\mathbb t}(E)$4, consistent with stronger atomic SOC from $\rho_{\mathbb t}(E)$5 to $\rho_{\mathbb t}(E)$6. At the same time, $\rho_{\mathbb t}(E)$7 remains comparatively robust, reaching $\rho_{\mathbb t}(E)$8 in $\rho_{\mathbb t}(E)$9. Tellurium provides the most dramatic example in that work, with a giant spin polarization of about $P_t(E)$0 near the valence-band edge (Yang et al., 2023).

Twisted van der Waals bilayers offer a distinct, atomically thin realization of structural chirality. In twisted homobilayer transition metal dichalcogenides, chirality arises from the twist angle $P_t(E)$1, and the transmission spin polarization vanishes in the untwisted case: $P_t(E)$2 Reversing the twist, $P_t(E)$3, reverses the sign of the polarization. Detailed calculations for $P_t(E)$4 show that $P_t(E)$5 exceeds $P_t(E)$6 at $P_t(E)$7 and that $P_t(E)$8 can be reached at $P_t(E)$9, identifying twisted TMD homobilayers as a giant, tunable CISS platform (Menichetti et al., 2023).

Real-space imaging has sharpened the transport interpretation. In planar heterostructures of chiral Te nanowires bridged by graphene electrodes, reflective magnetic circular dichroism (RMCD) maps show that current-induced spin polarization is linear in current, reverses with current direction, reverses with chirality, and appears in both the Te nanowire and the graphene electrodes with the same sign. The extracted spin relaxation lengths in graphene are $$\sigma_t+\sigma_{r'}=0,\qquad \sigma_r+\sigma_{t'}=0,$$0, $$\sigma_t+\sigma_{r'}=0,\qquad \sigma_r+\sigma_{t'}=0,$$1, $$\sigma_t+\sigma_{r'}=0,\qquad \sigma_r+\sigma_{t'}=0,$$2, and $$\sigma_t+\sigma_{r'}=0,\qquad \sigma_r+\sigma_{t'}=0,$$3, consistent with known values of roughly $$\sigma_t+\sigma_{r'}=0,\qquad \sigma_r+\sigma_{t'}=0,$$4–$$\sigma_t+\sigma_{r'}=0,\qquad \sigma_r+\sigma_{t'}=0,$$5 for multilayer graphene on $$\sigma_t+\sigma_{r'}=0,\qquad \sigma_r+\sigma_{t'}=0,$$6. The same-sign polarization in the chiral wire and low-SOC electrodes directly supports a spin-polarizer or spin-scattering interpretation rather than the classic spin-filter intuition (Lee et al., 25 Sep 2025).

These solid-state extensions matter methodologically as much as materially. They replace chemically and structurally heterogeneous molecular junctions with platforms where chirality, thickness, twist angle, carrier density, and SOC can be tuned more systematically, thereby making CISS a condensed-matter problem as well as a molecular one.

6. Controversies, applications, and theoretical frontiers

The central controversy in CISS is not whether chirality and spin couple, but how they do so and under what thermodynamic conditions the coupling is observable. One disputed issue is whether large zero-bias magnetoresistance in chiral molecular junctions can be explained as spin polarization alone. A non-Hermitian transport proposal argues instead that giant CISS magnetoresistance originates from magnetochiral charge pumping: a non-Hermitian skin effect localizes states at one interface, magnetization or chirality changes the occupation of trap states, and the resulting trapped charge modifies the tunneling barrier, thereby producing large resistance changes and apparent Onsager violation (Zhao et al., 2022).

An even more radical proposal attributes CISS to symmetry-enforced many-electron exchange in structurally chiral systems. In that framework, twin-pair exchange in a chiral multi-electron geometry generates an effective non-Hermitian term

$$\sigma_t+\sigma_{r'}=0,\qquad \sigma_r+\sigma_{t'}=0,$$7

which is odd under $$\sigma_t+\sigma_{r'}=0,\qquad \sigma_r+\sigma_{t'}=0,$$8 and $$\sigma_t+\sigma_{r'}=0,\qquad \sigma_r+\sigma_{t'}=0,$$9 separately but even under $\sigma_l$0. Open boundaries then produce spin-dependent boundary localization, interpreted as the source of interfacial spin and charge accumulation in CISS experiments (Theiler et al., 9 May 2025). A related pseudo-Hermitian equilibrium theory argues that structural chirality can support equilibrium spin polarization through a non-local metric coupling spin and motion, and introduces a spin–displacement order parameter $\sigma_l$1 as the signature of a proposed cismagnetic phase (Theiler et al., 28 Oct 2025).

Relativistic current-based approaches pursue a different route. Real-time relativistic four-current simulations on helicenes report curvature-induced helical electron currents that generate handedness-dependent spontaneous magnetic fields aligned along the molecular axis, reaching magnitudes of $\sigma_l$2 Tesla per single helicene strand. A separate fully relativistic Dirac-DFT study of isolated chiral molecules finds that relativistic chirality density and spin-dependent transmission follow qualitative CISS trends but remain too small to account fully for experiment, which suggests the need for a more general treatment of SOC, including geometrical terms or exchange-correlation functionals that depend on spin-current density (Zheng et al., 3 Apr 2025, Behera et al., 2024).

Despite these disagreements, the application space is relatively consistent across the literature. CISS is repeatedly connected to spintronics, molecular spin filtering, magnetoresistance, spin-controlled chemistry, enantiomer separation, and quantum information science. The isolated donor–bridge–acceptor experiments point to chiral molecular building blocks as a route to control electron spin states in molecular quantum devices, while twisted TMDs and chiral crystals provide tunable condensed-matter platforms for spin-selective transport, electrical magnetochiral anisotropy, and spin–orbital conversion (Eckvahl et al., 2023, Menichetti et al., 2023, Yang et al., 2023).

A plausible implication is that the field is converging on a layered rather than singular understanding of CISS. Chirality is the indispensable symmetry ingredient; SOC, orbital structure, vibrations, dephasing, interfacial scattering, and nonequilibrium driving determine how that symmetry breaking becomes an experimentally accessible spin signal. The outstanding task is to identify which subset of those ingredients controls each experimental class without collapsing the phenomenon into a single mechanism prematurely.