Recent studies have revealed that electrons passing through chiral molecules exhibit significant spin polarization--a phenomenon known as Chirality-Induced Spin Selectivity (CISS). This effect stems from a nontrivial coupling between electron motion and spin within chiral structures, yet quantifying it remains challenging.
To address this, researchers at the Institute for Molecular Science (IMS) /SOKENDAI investigated an organic superconductor with chiral symmetry. They focused on nonreciprocity related to spin-orbit coupling and observed an exceptionally a large nonreciprocal transport in the superconducting state, far exceeding theoretical predictions. Remarkably, this was found in an organic material with inherently weak spin-orbit coupling, suggesting that chirality significantly enhances charge current-spin coupling with inducing mixed spin-triplet Cooper pairs.
Nontrivial spin-current coupling in chirality
In recent years, chiral structures like helical conformations have been revealed to be beyond simple geometric motifs--they have a nontrivial influence on electron transport. A striking manifestation of this phenomenon is the enormous spin polarization termed Chirality-Induced Spin Selectivity (CISS), which has been reported extensively in chiral organic molecules. Conventionally, the efficiency of electron spin polarization during conduction is coupled to the relativistic effect of spin-orbit coupling, a mechanism that becomes stronger with heavier elements. However, since the CISS effect is observed in organic materials composed of light elements such as carbon and hydrogen, this conventional framework falls short, suggesting an unknown coupling between electron motion and spin inherent to chirality. Despite numerous studies, the quantitative evaluation of this chiral spin-current coupling has remained a significant challenge due to difficulties in detecting the CISS effect, selecting appropriate reference systems, and the absence of a comprehensive microscopic theoretical model.
In contrast to molecular systems, crystalline materials have offered a host of intriguing properties related to spin-orbit coupling. In particular, in materials lacking spatial inversion symmetry, spin-orbit coupling gives rise to nonreciprocal transport--a form of bulk charge rectification. So far, this phenomenon has been primarily investigated using polar-type structures, and recent studies have even observed nonreciprocal transport in polar superconductors. To date, the microscopic theory regarding the nonreciprocal superconductivity has been well established; reported experiments have been quantitatively reproduced through the model that is based on conventional spin-orbit coupling. Viewed from another perspective, the research team at IMS noticed that this progress positions polar-type superconductors as a clear benchmark against which chiral-type superconductors can be compared. By scrutinizing nonreciprocity in chiral superconductors and leveraging established microscopic theories, it becomes possible to quantitatively assess the spin-current coupling induced by chirality, which has been elusive thus far.
Research breakthroughs in a chiral organic superconductor
Motivated by these insights, the team focused on the two-dimensional organic conductor κ-(BEDT-TTF)₂Cu(NCS)₂ (hereafter κ-NCS), which possesses a chiral structure and exhibits superconductivity. In the superconducting phase of κ-NCS, the CISS effect has already been confirmed, making it an ideal platform for examining the interplay between chirality and superconductivity. In their study, thin-film devices of the chiral superconductor κ-NCS were fabricated to probe the presence of nonreciprocal transport. Remarkably, they observed a giant nonreciprocal signal whose magnitude significantly exceeds that reported for inorganic polar superconductors (Fig. 1). Given that inorganic polar superconductors typically exploit heavy elements to enhance nonreciprocity, achieving such pronounced effects in an organic crystal composed solely of light elements is extraordinary. Furthermore, our theoretical analysis revealed that the observed nonreciprocity cannot be explained by conventional band parameters alone; rather, it requires an effectively enhanced spin-orbit coupling far beyond the typical organic level, along with a spin-triplet component in the Cooper pairs.
Even more striking, investigations into another superconducting bulk rectification phenomenon--the superconducting diode effect--demonstrated an extremely high efficiency of up to 5%. This performance is unprecedented for organic materials and is comparable to the initially reported value (~6%) for inorganic polar superconductors. These findings indicate that the nontrivial coupling between spin and current, driven by chirality, acts as an effective spin-orbit coupling in the superconducting state, inducing a giant nonreciprocity in both electrical resistance and critical current. Moreover, the mixing of spin-triplet Cooper pairs appears to be driven by this enhanced effective interaction.
Future perspectives and societal impact
The robust spin-current coupling unveiled in chiral superconductors addresses a long-standing challenge in quantitatively evaluating the interactions underlying the CISS effect. This breakthrough not only promises to have a profound impact on both physics and chemistry but also introduces a novel perspective into the study of superconducting bulk rectification--a field that has hitherto largely focused on inorganic systems characterized by heavy elements and polar symmetry. With these new insights, research on chirality and solid-state electron properties is poised to expand into diverse material systems, paving the way for innovative superconducting devices and functional materials.
This work will be published online in the American journal Physical Review Research on April 15, 2025.
Information of the paper：
Authors: Takuro Sato, Hiroshi Goto and Hiroshi M. Yamamoto
Journal Name: Physical Review Research
Journal Title: "Sturdy spin-momentum locking in a chiral organic superconductor"
DOI: 10.1103/PhysRevResearch.7.023056
