Chirality induces huge charge rectification of superconductors

Recent studies have revealed that electrons passing through chiral molecules exhibit significant spin-polarized light, a phenomenon known as chirality-induced spin selectivity. This effect is attributed to the non-critical bond between electron motion and spins within chiral structures, but quantifies that it remains difficult.

To address this, researchers at the Institute of Molecular Science (IMS)/Sokendai investigated organic superconductors with chiral symmetry. They focused on the inphasics associated with spin-orbit coupling, observing very large inphasic transport in the superconducting state, far beyond theoretical predictions. Surprisingly, this was found in organic materials with essentially weak spin-orbital coupling. This suggests that chirality significantly enhances charge current spin coupling by inducing mixed spin triplet cooper pairs.

This work has been featured in the Journal Physical Review Research.

Non-trivial spin current coupling in chirality

In recent years, it has been revealed that chiral structures, such as spiral conformations, go beyond simple geometric motifs. They are not important for electronic transport. A prominent symptom of this phenomenon is the huge spin-polarized light known as chirality-induced spin selectivity (CIS), which has been widely reported in chiral organic molecules.

Traditionally, the efficiency of electron spin polarization during conduction is associated with the relative effects of spin-orbit coupling. This is a mechanism that makes it stronger with heavier elements. However, this traditional framework is lacking because the CISS effect is observed in organic materials composed of optical elements such as carbon and hydrogen, suggesting an unknown bond between electron motion and chirality-specific spins.

Despite numerous studies, this quantitative assessment of chiral spin current coupling remains a critical challenge, as it is difficult to detect CISS effects, select appropriate reference systems, and lack of comprehensive microscopic theoretical models.

In contrast to molecular systems, crystalline materials offer many interesting properties associated with spin-orbit coupling. In particular, in materials that lack spatial inversion symmetry, spin-orbit coupling causes inphasic transport.

So far, this phenomenon has been investigated primarily using polar structures, and recent studies have also observed incompatible transport of hyper-superconductors. To date, microscopic theory of incompatible superconductivity has been well established. The reported experiments are quantitatively reproduced through a model based on traditional spin-orbit coupling.

From another perspective, IMS researchers noticed that this progression places polar-type superconductors as a clear benchmark that allows comparisons of chiral-type superconductors. By examining the inphasicity of chiral superconductors and utilizing established microscopic theory, it is possible to quantitatively evaluate the spin current coupling induced by chirality.

Breakthroughs in research into chiral organic superconductors

Motivated by these insights, the team focused on the two-dimensional organic conductor κ-(bedt-ttf)₂cu(NCS)₂ (hereafter referred to as κ-NCS), which has a chiral structure and is superconducting. The superconducting stage of κ-NC has already confirmed the CISS effect, making it an ideal platform for examining the interaction between chirality and superconductivity.

In their study, they fabricated a thin film device of chiral superconductor κ-NC to investigate the presence of incompatible transport. Surprisingly, they observed huge incompatible signals that were significantly greater than reported sizes for inorganic hypersuperconductors. Given that inorganic pole superconductors generally utilize gravity to increase non-recurrence, achieving such a prominent effect with organic crystals composed solely of light elements is extraordinary.

Furthermore, their theoretical analysis revealed that the observed inphasics cannot be explained by traditional band parameters alone. Rather, along with the spin triplet components of the Cooper pair, effectively strengthened spin-orbit coupling is required, well beyond typical organic levels.

Even more impressive, another study of superconducting bulk rectification (superconducting diode effect) demonstrated a very high efficiency of up to 5%. This performance is unprecedented for organic materials and is comparable to the initial reported value (~6%) for inorganic hyperconductives.

These findings show that the non-critical coupling between spin and current driven by chirality acts as an effective spin-orbit coupling in the superconducting state, inducing huge inphasics in both electrical resistance and critical currents. Furthermore, the mixing of spin triplet cooper pairs appears to be driven by this enhanced, effective interaction.

Future perspective and social impact

The robust spin current coupling revealed in chiral superconductors address the long-standing challenges of quantitatively evaluating the interactions underlying the CISS effect. This breakthrough not only promises to have a major impact on both physics and chemistry, but also introduces a new perspective into the study of superconducting bulk rectification. This is an area that has previously focused on inorganic systems characterized by heavy elements and polar symmetry.

These new insights allow research into chirality and solid-state electronic properties to be poised to expand into a variety of material systems, paving the way for innovative superconducting devices and functional materials.