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 nonhelical 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.
Bibliographical noteFunding Information:
We thank Hadar Steinberg for a helpful discussion. The research was supported in parts by the Israel Science Foundation F.I.R.S.T. program (Grant No. 687/16), the Niedersachsen Ministry of Science and Culture grant (O.M. and Y.P.), and a grant (AS-iMATE-107-95) from the joint Academia Sinica - Hebrew University Research Program (O.M., Y.P. and C.C.K). O.M. thanks support from the Harry de Jur Chair in Applied Science.
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