Chirality-induced spin selectivity due to an emergent molecular orbital moment

summary

The gist

The gist: The mechanism for chirality-induced spin selectivity (CISS) in helical molecules is explained by an emergent current-induced molecular orbital moment (MOM) generated by a gauge field

In short

This research proposes an alternative mechanism for chirality-induced spin selectivity (CISS) that doesn't rely on traditional spin-orbit coupling. It suggests that a current-induced molecular orbital moment (MOM), generated by a gauge field from structural distortion, is the key driver. This MOM produces measurable CISS effects and explains observed I-V curve crossings, offering a new way to understand this phenomenon.

Key concepts

Chirality-induced spin selectivity (CISS): CISS is a phenomenon where chiral molecules connected to magnetic electrodes show large spin-dependent conductance asymmetry, even at room temperature. The paper seeks a microscopic explanation for this effect, moving beyond existing theories based on interfacial scattering or effective spin-orbit coupling.
Current-Induced Molecular Orbital Moment (MOM): The MOM is an emergent magnetic moment arising from the circulation of charge current within molecular loops. This moment is governed by a gauge field caused by the molecule's structural distortion and its handedness, providing a mechanism for chirality to influence transport properties.
Onsager-Casimir Reciprocity Relation: This is a fundamental principle in physics stating that certain physical quantities are symmetric under certain transformations. The paper demonstrates that its proposed MOM mechanism does not violate this relation, lending credibility to the theoretical framework.
Magnetochiral Conductance Asymmetry (MChA): MChA is a measurable transport property where the conductance asymmetry depends on both magnetic field and molecular chirality. The paper shows that the proposed MOM can produce finite MChA even at bias voltage, which is a key prediction of their theory.

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This episode discusses

The paper

Chirality-induced spin selectivity due to an emergent molecular orbital moment · Read on arXiv

Sumit Ghosh, Angela Wittmann, Frank Matthes, Daniel E. Bäurgler

Institute of Physics of the Czech Academy of Sciences · Institute of Physics, Johannes Gutenberg-University · Peter Grünberg Institut (PGI-6), Forschungszentrum Jülich GmbH

The microscopic origin of chirality-induced spin selectivity (CISS), a phenomena where chiral molecules connected to a magnetic electrode show large spin-dependent conductance asymmetry even in ambient temperature, still remains an open question. Most of the existent theories are based on some sort of effective spin-orbit coupling or interfacial scattering, which do not provide satisfactory explanations for experimental observations such as the magnitude of spin polarization or the length dependence of the effect. We propose an alternative theoretical framework rooted in an intrinsic molecular contribution that shows a robust CISS effect and scales with the length of the molecule. The mechanism is based on the emergent molecular magnetic moment (MOM), which can produce experimentally observed point-symmetric I-V curves without violating the Onsager-Casimir reciprocity relation. The MOM has an intrinsic component that is governed by the chirality of the molecule and leads to finite CISS magnetoresistance and magnetochiral conductance asymmetry (MChA) that are even in bias voltage. By decomposing the MOM into its Fermi sea and Fermi surface components, we show that under strong bias, the MOM can be dominated by the Fermi surface contribution, which causes a sign change in the MChA leading to a behavior similar to that of electric magnetochiral anisotropy (eMChA). Finally, we analyze the effect of a magnetic field within the same framework, which may provide better insight into the crossover between CISS and eMChA.

Transcript

Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: We're looking at the paper: "Chirality-induced spin selectivity due to an emergent molecular orbital moment".

Mira: Here's the gist of the paper. The mechanism for chirality-induced spin selectivity (CISS) in helical molecules is explained by an emergent current-induced molecular orbital moment (MOM) generated by a gauge field arising from structural distortion, which produces finite CISS magnetoresistance and magnetochiral conductance asymmetries without violating Onsager-Casimir reciprocity relations.

Kai: Alternative Mechanism for CISS The paper presents an alternative theory of CISS that addresses the molecular contribution to this effect, which is often disregarded in existing interfacial or scattering based theories.

Mira: The mechanism is based on the circulation of charge current in molecular loops that generates a molecular orbital moment (MOM).

Kai: The direction of the MOM is governed by the gauge field arising from the structural distortion of the molecule and is associated with the handedness of the molecule.

Paper summary: Kai: To summarize, this paper introduces an alternative theory for CISS by suggesting that it comes from an emergent current-induced molecular orbital moment.

Mira: The thesis is that this MOM isn't just some weak effect; it arises from the circulation of charge current in molecular loops, and its direction is governed by a gauge field resulting from structural distortion.

Lev: So, the key claim here is that this mechanism produces finite CISS magnetoresistance and magnetochiral conductance asymmetry even at bias voltage, without violating Onsager-Casimir reciprocity.

Kai: They model this using heptahelicene molecules as a prime example, where the bond lengths show systematic variation regardless of handedness. This variation creates a gauge field that can be incorporated as a Peierls phase in the Hamiltonian of the system.

Mira: They then calculate an effective molecular ring current by summing over all bonds in a ring using the lesser Green’s function, and this gives them this current-induced MOM, which they align along the plus or minus z direction.

Lev: That alignment along plus or minus z is important because it allows them to incorporate the interaction of this localized magnetic moment with a conduction electron's spin magnetic moment as a modification of the onsite energy with a double exchange-type interaction.

Kai: They then discuss how the variation of the MOM with bias voltage happens in two parts, one from the bias window and one from the rest of occupied energy levels.

Mira: The paper shows that under strong bias, this Fermi surface contribution can dominate and cause a sign change in the magnetochiral conductance asymmetry, leading to behavior similar to electric magnetochiral anisotropy.

Lev: They also found an antisymmetric component of the MOM, denoted as Mchi4, which can exist even without any gauge field, stemming from the absence of Fermi sea contribution.

Kai: This gauge-free MOM is smaller and antisymmetric with respect to bias voltage because it only originates from the bias window itself, following the sign of the bias voltage and resulting in non-crossing I-V curves.

Conclusion: Kai: So, looking at this paper, "Chirality-induced spin selectivity due to an emergent molecular orbital moment," the authors are basically arguing that we need a mechanism that happens inside the molecule itself for CISS.

Mira: They propose this current-induced MOM as that mechanism, driven by structural distortion creating a gauge field linked to molecular handedness, which then dictates the transport properties.

Lev: It's important because it shifts the focus from just looking at interfaces to understanding how internal structure and charge flow can produce these spin effects.

Kai: The implication is that we can predict specific transport behaviors like crossing of I-V curves based on the bias voltage, which is a signature they see in experiments.

Mira: They are suggesting this intrinsic molecular effect could be generic enough to explain why we see CISS even when there's no obvious intrinsic spin-orbit coupling present in the molecule.

Lev: For us, it means that if we want to build real hardware, we need to make sure our qubit designs account for these kinds of internal gauge fields and how they might affect the error rates.

Kai: It gives us a concrete physical picture for what's happening on the molecular scale when a current flows through chiral systems.

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