[Paper Review] Temperature-Dependent Chiral-Induced Spin Selectivity Effect: Experiments and Theory

Tapan Kumar Das, Francesco Tassinari+2 more|arXiv (Cornell University)|Aug 18, 2021

Quantum and electron transport phenomenaPhysics and Astronomy49 references121 citations

TL;DR

This study demonstrates that the chiral-induced spin selectivity (CISS) effect increases with temperature due to vibrational enhancement of spin-orbit coupling, resolving long-standing discrepancies between theory and experiment. The proposed model, incorporating temperature-dependent electron-vibration interactions and dissipative processes, quantitatively reproduces experimental spin polarization and magnetoresistance data, linking the CISS effect to molecular optical activity via low-frequency vibrational modes.

ABSTRACT

The theoretical explanation for the chiral-induced spin selectivity effect, in which electrons’ passage through a chiral system depends on their spin and the handedness of the system, remains incomplete. Although most experimental work was performed at room temperature, most of the proposed theories did not include vibrations. Here, we present temperature-dependent experiments and a theoretical model that captures all observations and provides spin polarization values that are consistent with the experimental results. The model includes the vibrational contribution to the spin orbit coupling. It highlights the importance of dissipation and the relation between the effect and the optical activity. The model explains the main features related to the chiral-induced spin selectivity effect and provides a new framework for future calculations and experiments.

Motivation & Objective

• To resolve the long-standing discrepancy between theoretical predictions and experimental observations of the CISS effect, particularly the failure of zero-temperature models to reproduce observed spin polarization magnitudes.
• To investigate the role of temperature and molecular vibrations in enhancing spin-orbit coupling and enabling measurable spin selectivity in chiral systems.
• To establish a theoretical framework that quantitatively explains the temperature-dependent increase in spin polarization and asymmetric magnetoresistance observed in α-helices, oligopeptides, and ds-DNA.
• To link the CISS effect to molecular optical activity by connecting vibrational contributions to polarizability and spin selectivity.

Proposed method

• A theoretical model incorporating electron-vibration coupling via vibrational modes is developed, with spin-orbit coupling enhanced through vibrational assistance.
• The model uses a tight-binding Hamiltonian with spin-dependent hopping and spin-orbit coupling terms, including contributions from vibrational excitations via the Bose-Einstein distribution.
• The spin polarization is calculated as SP = (I_up - I_down)/(I_up + I_down) × 100%, derived from temperature-dependent I-V curves under varying magnetic fields.
• The model accounts for dissipation and inelastic scattering via electron-vibration interaction terms, with energy scales shifted by hopping integrals and SOC parameters.
• Theoretical predictions are fitted to experimental data using parameters derived from molecular polarizability and vibrational frequencies.
• The model explains the non-monotonic temperature dependence of spin polarization through competition between spin-selective and spin-symmetric dissipation processes.

Experimental results

Research questions

• RQ1 Why does the CISS effect increase with temperature, contrary to expectations from zero-temperature models?
• RQ2 How do molecular vibrations enhance spin-orbit coupling to produce measurable spin polarization in chiral molecules?
• RQ3 What is the origin of the observed asymmetric magnetoresistance in CISS systems, and how does it differ from conventional magnetoresistance?
• RQ4 Why do zero-temperature models fail to reproduce the magnitude and symmetry of experimental CISS results?
• RQ5 How is the CISS effect related to the optical activity of chiral molecules?

Key findings

• Spin polarization increases with temperature for both ds-DNA and oligopeptide systems, with values rising significantly above 50 K, indicating a thermally activated mechanism.
• The model reproduces experimental magnetoresistance data with excellent quantitative agreement, showing a linear increase in ΔMR with temperature.
• The peak in spin polarization occurs around 50 K, followed by a decrease and a minor recovery upon further cooling, indicating competing spin-selective and spin-symmetric dissipation processes.
• The enhancement of spin-orbit coupling is attributed to vibrationally assisted processes involving low-frequency modes (30–40 cm⁻¹, ~1 THz), which are cooperative and weakly dependent on molecular details.
• The model links spin polarization to molecular polarizability, which is proportional to optical activity, explaining the correlation between CISS and optical activity observed experimentally.
• Theoretical calculations confirm that the symmetry of I-V curves and magnetoresistance in CISS systems is distinct from that in conventional spintronic devices, validating the model's consistency with experimental observations.