Unconventional order parameter induced by helical chiral molecules adsorbed on a metal proximity-coupled to a superconductor

TL;DR

This study demonstrates that adsorbing helical chiral molecules on a metal-superconductor bilayer induces an unconventional order parameter, evidenced by zero-bias conductance peaks suggestive of triplet p-wave pairing, unlike non-helical molecules.

Contribution

It reveals that helical chiral molecules can induce unconventional superconductivity in proximity-coupled metal layers, expanding understanding of chiral molecule effects on superconducting states.

Findings

Adsorption of helical chiral molecules induces zero-bias conductance peaks.

Non-helical chiral molecules do not alter the superconducting spectra.

The induced peaks are consistent with equal-spin triplet p-wave pairing.

Abstract

Following our previous results, which provide evidence for the emergence of a chiral p-wave triplet-pairing component in superconducting Nb upon the adsorption of chiral molecules, we turned to investigate whether such an effect can take place in a proximal superconductor consisting of metal on superconductor bilayer. Note that in such proximity systems, correlated electron-hole (Andreev) pairs exist in the normal metal rather than genuine Cooper pairs. To that end, we used scanning tunneling spectroscopy (STS) on thin Au films grown in-situ on NbN (a conventional s-wave superconductor) before and after adsorbing helical chiral, alpha-helix polyalanine molecules. The tunneling spectra measured on the pristine Au surface showed conventional (s-wave like) proximity gaps. However, upon molecules adsorption the spectra significantly changed, all exhibiting a zero-bias conductance peak embedded inside a gap, indicating unconventional superconductivity. The peak reduced with magnetic field but did not split, consistent with equal-spin triplet-pairing p-wave symmetry. In contrast, adsorption of non-helical chiral cysteine molecules did not yield any apparent change in the order parameter, and the tunneling spectra exhibited only gaps free of in-gap structure.