Chirality Induced Spin Selectivity (CISS)

• CISS is a phenomenon where chiral molecules or solids induce spin-dependent electron transmission, leveraging spin–orbit coupling and vibrational dynamics to create significant polarization.
• Recent theoretical progress, including pseudo-Hermitian and many-body frameworks, shows that vibrational coupling and electron correlations can enhance spin polarization even at room temperature.
• Optimizing device performance involves symmetry breaking and tailored electrode geometries, as evidenced by experiments and quantum transport models highlighting robust spin polarizer behavior.

Chirality Induced Spin Selectivity (CISS) refers to the spin-dependent transmission or filtering of electrons through chiral molecular or solid-state systems, yielding observable spin polarization even in the absence of magnetic order. First identified more than two decades ago, CISS has become central to spintronic device engineering, enantioselective separation, quantum control, and the mechanistic understanding of symmetry-breaking phenomena in chemistry and condensed matter. While classical models attributed CISS solely to spin–orbit coupling plus molecular chiral geometry, recent research, particularly the vibrationally driven paradigm (Miwa et al., 2024), genetic pseudo-Hermitian frameworks (Theiler et al., 28 Oct 2025), electron correlation effects (Fransson, 2019), dynamical velocity asymmetries (Zhang et al., 4 Sep 2025), and group-theoretic analyses (Dednam et al., 2022), has substantially revised the mechanistic picture and extended the scope of CISS to diverse contexts.

1. Microscopic Mechanisms: Beyond Static SOC

Early models of CISS posited that electron spin polarization arose from the interplay of intrinsic molecular chirality with local spin–orbit coupling, typically modeled as a helical tight-binding chain with an atomic SOI term (e.g., $H_\text{SOI}=\lambda\,\mathbf{L}\cdot\mathbf{S}$) and a non-centrosymmetric molecular geometry (Mena et al., 2024, Behera et al., 2024, Utsumi et al., 2020). However, Miwa et al. (Miwa et al., 2024) demonstrated that in closed-shell chiral molecules, molecular vibrations—specifically circularly polarized (chiral) phonon modes—carry intrinsic angular momentum, which couples to electronic spin via spin–orbit interaction. An external magnetic field $B$ breaks the half-cycle symmetry of vibrational motion, leading to a net spin polarization $P$ that scales as

$$P \approx \sum_q \frac{\lambda |M^q|^2}{\hbar\omega_q}(2n_q+1)\frac{\Delta\omega_q}{\omega_q}\chi_\text{mol}$$

where $M^q$ is the electron–phonon coupling, $n_q$ is the Bose occupation factor, $\Delta\omega_q$ is the field-induced splitting of vibrational frequencies, and $\chi_\text{mol}$ is the molecular handedness. The effect is non-perturbative, robust to weak SOC, and reaches percent-level polarization even for light-atom systems.

Pseudo Jahn–Teller models (Kato et al., 2021) further show that vibrational coupling (pseudo Jahn–Teller distortion) between translational and rotational electronic states enhances or even supersedes the SOC dependence, providing large energy barriers for spin filtering and yielding polarization efficiencies $P \sim \tanh(\delta/2k_BT)$ that survive at room temperature and small $\lambda$.

2. Quantum, Correlated, and Non-Hermitian Approaches

Recent theoretical progress addresses equilibrium and many-body aspects. The pseudo-Hermitian quantum framework (Theiler et al., 28 Oct 2025) constructs a non-Hermitian Hamiltonian with a non-local metric $\eta=\exp[2(m\alpha/\hbar) x\cdot\sigma]$ coupling spin and spatial coordinates in a structurally chiral potential $V(x)$, where $\alpha$ quantifies the spin–drift velocity due to chirality. This framework yields cismagnetic order—a conserved spin–displacement parameter $\langle\sigma \cdot x\rangle_\eta$—with real spectra and thermodynamic consistency. Onsager reciprocity and detailed balance are maintained via a generalized antiunitary symmetry, even as $P$ and $T$ are individually broken, resolving the paradox of equilibrium spin polarization in CISS.

Many-body Keldysh–NEGF implementations (Fransson, 2019) reveal that local electron–electron interactions (on-site $U$, Hubbard terms) greatly magnify spin polarization, with normalized current-difference spin polarization reaching $5$–$10\%$ for short chains at room temperature. This amplification is absent in single-electron models, explaining quantitative discrepancies in earlier SOC-only theories.

The non-Hermitian exchange approach (Theiler et al., 9 May 2025) finds that breaking all mirror symmetries induces a non-Hermitian twin-pair exchange term $-i\alpha\,\sigma\cdot p$ in the electron Hamiltonian, leading to intrinsic spin–momentum locking and finite end spin accumulation (non-Hermitian skin effect), with the degree of selectivity scaling with the molecular twist angle and conjugation length.

3. Scattering Formalism, Group Theory, and Device Symmetry

A rigorous group-theoretic analysis (Dednam et al., 2022) establishes that the emergence and sign of CISS spin polarization are determined by the symmetries of the full junction, including electrodes. Any longitudinal mirror plane ($\sigma_l$) or $\pi$-rotation about the transport axis enforces $P_l=0$, precluding spin-selectivity unless disrupted. Spin polarization can be achieved even with achiral molecules or bare metallic contacts if the device lacks the full set of mirrors—a geometry-tuned, symmetry-driven effect. Electrode rotation or molecular connection geometry maps precisely onto sign switches and magnitudes of $P_l$ (longitudinal spin polarization), validated by DFT+SOC quantum transport.

Table: Symmetry Restrictions on CISS

Symmetry	Spin Polarization $P_l$
Longitudinal mirror ($\sigma_l$)	Forbidden ($P_l=0$)
$\pi$-rotation ($C_{2,l}$)	Forbidden ($P_l=0$)
No mirror, broken rotation	Allowed, magnitude depends on geometry

Device design thus requires deliberate breaking of spatial symmetries—either by choosing chiral molecules or by engineering electrode orientation—to maximize CISS spin polarization.

4. Dynamical and Transport Phenomena

Time-dependent quantum transport (Stuermer et al., 31 Oct 2025, Zhang et al., 4 Sep 2025) and donor–bridge–acceptor Lindblad master equation approaches reveal that spin-dependent group velocities (caused by chirality and SOC) yield transient and steady-state spin imbalances in chiral molecules. In two-terminal biased setups, the faster-propagating spin accumulates in spatial regions and persists as net polarization, directly reproducing experimental magnetic signatures and matching observed Hall signals. Analytically, spin polarization is proportional to the velocity difference:

$$P_\text{ss} \sim \frac{v_\uparrow - v_\downarrow}{v_\uparrow + v_\downarrow + 2\Gamma_d L}$$

where $\Gamma_d$ is the dephasing rate and $L$ the molecule length. Dynamical CISS is a balance of coherent helical transport, SOC-induced spin splitting, and controlled dissipation.

Direct time-resolved EPR spectroscopy (Eckvahl et al., 2023) has measured radical-pair spin polarization in isolated D–Bχ–A molecules, demonstrating CISS efficiencies $p \sim 50\%$ in orientational ensembles and enabling controlled quantum information applications via singlet–triplet mixing.

5. Experimental Evidence and the Spin Polarizer Paradigm

Recent RMCD imaging (Lee et al., 25 Sep 2025) and advanced magnetoresistance measurements have directly visualized spatial spin distribution in chiral nanowires and interfaces. Notably, both transmitted and reflected electrons exhibit identical spin polarization, disproving the classic "spin filter" picture (where transmitted and reflected beams have opposite polarities) and confirming the "spin polarizer" model (2208.00043). Spin polarization scales linearly with current, switches sign with chirality or bias direction, and relaxes over micrometer scales in low-SOC electrodes (graphene), matching theoretical spin-diffusion predictions.

Table: Experimentally Observed CISS Signatures

Device Type	Observed Signature	Interpreted Mechanism
Chiral Te NW + Graphene	RMCD maps; equal P in wire/contacts	Spin polarizer, not filter
Electrochemical CSA/CoPt	Magnetoconductance (MC) $>$1%	Vibration-driven CISS via RKKY
Donor–bridge–acceptor	$p \sim 50\%$ via TREPR	Velocity/mixing-driven CISS

6. Extensions: Polarons, Chiral Crystals, and Twisted Materials

Strong electron-phonon interaction (polaron formation) dramatically extends the energy scales of CISS (Klein et al., 2022). Phonon sidebands and polaronic band narrowing amplify the ratio $\Delta_\text{SOC}/t$, resulting in spin polarization $P(E)$ spread over hundreds of meV and magnetoresistance reaching up to 10%. Polaron fluctuation-induced CISS bridges the gap between narrow bare SOC bandwidths and experiment.

Transition metal dichalcogenide homobilayer systems (Menichetti et al., 2023) and chiral crystals (Yang et al., 2023) offer "giant-molecule" platforms, exhibiting gate-, twist-, and field-tunable CISS spin polarization exceeding 50% in two-monolayer devices. Spin and orbital polarization saturate as the material thickness grows, scaling with intrinsic SOC (spin) and with chirality (orbital), enabling applications in tunable spin filters, spin catalysts, and chiral superlattices.

7. Broader Implications for Spintronics and Molecular Design

The vibrational paradigm (Miwa et al., 2024) necessitates the enhancement of molecular vibrational modes (large $|M_q|$ and $\lambda$), control of diffusion interfaces, and utilization of RKKY-mediated exchange for enantioselective spin adsorption. CISS phenomena extend to drug discovery—vibration-driven enantiojunctions can enable robust enantiopure binding—molecular biology (spin in DNA/protein signaling), and astrochemistry (amplification of chirality in cosmic environments).

A plausible implication is that future devices should integrate symmetry engineering, electron–vibration design, correlation-driven tuning, and polaronic effects to optimize spin selectivity and enable new functionalities such as high-temperature spin injection, optically switchable chirality-mediated control, and quantum spin-memory elements.

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References:

Key papers cited include (Miwa et al., 2024, Theiler et al., 28 Oct 2025, Fransson, 2019, Zhang et al., 4 Sep 2025, Theiler et al., 9 May 2025, 2208.00043, Dednam et al., 2022, Eckvahl et al., 2023, Lee et al., 25 Sep 2025, Menichetti et al., 2023, Yang et al., 2023, Klein et al., 2022, Stuermer et al., 31 Oct 2025, Kato et al., 2021, Mena et al., 2024, Behera et al., 2024, Evers et al., 2021, Utsumi et al., 2020). These represent the most current and complete research on the mechanisms, experimental validation, theoretical formalism, and device-level optimization of CISS.