Chirality Induced Spin Selectivity -- The Role of Electron Correlations (1910.12532v1)

Abstract: Chirality induced spin selectivity, discovered about two decades ago in helical molecules, is a non-equilibrium effect that emerges from the interplay between geometrical helicity and spin-orbit interactions. Several model Hamiltonians building on this interplay have been proposed and while these can yield spin-polarized transport properties that agrees with experimental observations, they simultaneously depend on unrealistic values of the spin-orbit interaction parameters. It is likely, however, that a common deficit originates from the fact that all these models are uncorrelated, or, single-electron theories. Therefore, chirality induced spin selectivity is, here, addressed using a many-body approach, which allows for non-equilibrium conditions and a systematic treatment of the correlated state. The intrinsic molecular spin-polarization increases by two orders of magnitudes, or more, compared to the corresponding result in the uncorrelated model. In addition, the electronic structure responds to varying external magnetic conditions which, therefore, enables comparisons of the currents provided for different spin-polarizations in one of the (or both) leads between which the molecule is mounted. Using experimentally feasible parameters and room temperature, the obtained normalized difference between such currents may be as large as 5 - 10 % for short molecular chains, clearly suggesting the vital importance of including electron correlations when searching for explanations of the phenomenon.

Analysis of Chirality Induced Spin Selectivity: The Role of Electron Correlations

The phenomenon of Chirality Induced Spin Selectivity (CISS) has garnered significant attention due to its implications in spintronics and quantum computing. The paper by J. Fransson presents a comprehensive analysis of the CISS effect through the incorporation of electron correlations within helical molecular structures. The paper diverges from traditional single-electron models, utilizing a many-body approach to address the interplay of chirality, spin-orbit interactions, and electron correlations, revealing significant findings relevant to the fundamental understanding of spin-selective electron transport in chiral systems.

Theoretical Framework

Fransson challenges conventional single-electron theories by highlighting their reliance on exaggerated spin-orbit interaction parameters to achieve alignment with experimental data. The paper proposes a many-body framework that includes non-equilibrium conditions and systematically handles electron correlation effects. By incorporating electron-electron Coulomb interactions and considering the spin-polarization phenomena under realistic experimental parameters, the research demonstrates a substantial increase in intrinsic molecular spin-polarization — by two orders of magnitude or more — compared to uncorrelated models.

Numerical and Experimental Implications

With reference to observable outcomes, the introduction of electron correlations results in a normalized current spin-polarization difference, estimated to be as much as 5-10% for short molecular chains, even at room temperature. This finding underlines the importance of considering electron correlations in theoretical models aiming to explain the high degrees of spin-polarization seen in CISS experiments, challenging the adequacy of single-particle approaches traditionally employed in these studies.

Model and Techniques

The model deployed uses Hamiltonians consisting of on-site Coulomb interactions paired with nearest-neighbor hopping and next-nearest-neighbor spin-orbit interactions. This construction allows for a more realistic depiction of spin-dependent processes in helical molecules, promoting an understanding of intrinsic spin-polarization mechanisms. The model's results indicate that the spin-polarization scales additively with both hopping and spin-orbit interaction, reinforcing the significance of intermolecular electronic interactions.

Future Developments and Challenges

The paper positions itself as a pivotal contribution to advancing the theoretical modeling of CISS but also acknowledges areas requiring further investigation. Future research endeavors are anticipated to refine the model by incorporating additional electron interaction mechanisms and studying longer molecular chains, potentially leading to even more substantial spin-polarization effects. This paper paves the way for finer control over molecular spintronic devices, aligning with theoretical developments necessary to harness CISS phenomena for practical applications.

The exploration of CISS through the lens of correlated electron theory not only bridges gaps between experimental observations and theoretical models but also sets a standard for future studies aiming to explore electron spin dynamics in chiral systems. The approach elucidated by Fransson offers a robust foundation for addressing long-standing questions pertaining to spin selectivity in molecular electronic systems, ultimately facilitating the design of more sophisticated and efficient spintronics applications.