Unconventional superconductivity and magnetic-related states induced in a conventional superconductor by nonmagnetic chiral molecules

Oded Millo

Hybrid ferromagnetic/superconducting systems are well known for hosting intriguing phenomena such as emergent triplet superconductivity at their interfaces and the appearance of in-gap, spin-polarized Yu-Shiba-Rusinov (YSR) surface-states bound to magnetic impurities. In this work we demonstrate that similar phenomena can be induced on a surface of a conventional superconductor upon chemisorbing non-magnetic chiral molecules. By applying scanning tunneling spectroscopy, we show that the singlet-pairing s-wave order parameter of Nb, NbN and NbSe2 is significantly altered upon the adsorption of chiral polyalanine alpha-helix molecules on the surface. The tunneling spectra exhibit zero-bias conductance peaks embedded inside gaps or gap-like features, suggesting the emergence of a triplet-pairing component, corroborated by fits to theoretical spectra. Conductance spectra measured on devices comprising NbSe2 flakes over which these chiral molecules were adsorbed, exhibit, in some cases, in-gap states nearly symmetrically positioned around zero bias. These states shift apart with magnetic field, akin to YSR states, as corroborated by theoretical simulations. Other samples show evidence for a collective phenomenon of hybridized YSR-like states giving rise to unconventional, possibly triplet superconductivity, manifested in the conductance spectra by the appearance of a zero bias conductance peak that diminishes, but does not split, with magnetic field. The transition between these two scenarios appears to be governed by the density of adsorbed molecules. Chiral molecules were also found to have a unique signature on the TC of Nb and NbRe films when linking Au nanoparticles to them. Finally, low-energy muon spin rotation (LE-μSR) data demonstrate clear evidence for a strong modification of the screening supercurrent distribution deep inside a Nb film upon adsorption of chiral molecules, providing evidence for unconventional chiralinduced superconductivity. The adsorption-modified local magnetic field profile inside the (65 nm thick) Nb film monitored by LE-μSR, a measure of the screening modification, is well fitted to a model calculation where the chiral molecules layer is considered as an insulating spin-active interface that is proximity-coupled to the Nb film.

The work was done in collaboration with the following groups: Yossi Paltiel, Hen Alpern, Nir Sukenik, Tamar Shapira, Shira Yochelis (The Hebrew University of Jerusalem), Jason Robinson, Harry Bradshaw (University of Cambridge), Angelo Di Bernardo, Elke Scheer (Konstanz University), Jacob Linder (Norwegian University of Science and Technology).