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

Tamar Shapira, Hen Alpern, Shira Yochelis, Ting-Kuo Lee, Chao-Cheng Kaun, Yossi Paltiel, Gad Koren, and Oded Millo

Phys. Rev. B 98, 214513 – Published 20 December 2018

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 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.

Received 13 August 2018
Revised 28 November 2018

DOI:https://doi.org/10.1103/PhysRevB.98.214513

Physics Subject Headings (PhySH)

Chirality Proximity effect Spin-triplet pairing Superconductivity

Scanning tunneling spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Tamar Shapira¹, Hen Alpern^1,2, Shira Yochelis², Ting-Kuo Lee³, Chao-Cheng Kaun⁴, Yossi Paltiel², Gad Koren⁵, and Oded Millo^1,*

¹Racah Institute of Physics and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
²Applied Physics Department the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
³Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
⁴Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan
⁵Department of Physics, Technion - Israel Institute of Technology, Haifa 32000, Israel

^*Corresponding author: milode@mail.huji.ac.il

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Vol. 98, Iss. 21 — 1 December 2018

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Images

Figure 1
(a) and (d) Schemes of the STM-STS measurement on the bare and (alpha helix) molecule-covered Au film deposited on NbN, respectively. (b) and (e) Typical topographic images measured on the bare $(55 \times 55 {nm}^{2})$ and molecule-covered $(32 \times 32 {nm}^{2})$ Au film, respectively. (c) and (f) Line height profiles taken along the lines, and in the arrow directions, drawn in (b) and (e), respectively. The green curve in (f) was measured in a direction normal to a stack of parallel-aligned molecules, such as those encircled by the white ellipse, whereas the blue curve was measured along a single molecule within the stack. The green and blue lines in (d) present schematically the cuts (with respect to the molecules) along which these green and blue height line profiles were taken.
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Figure 2
(a) Typical tunneling spectra measured on the bare surface of a $10 nm$ thick Au film deposited on $70 nm$ thick NbN layer, exhibiting proximity gaps. (b) Typical tunneling spectra measured after polyalanine alpha-helix molecules adsorption, showing the emergence of a ZBCP inside the gap. (c) Same as (b) but after three weeks in ambient conditions; the inset portrays a $100 \times 100 {nm}^{2}$ topographic image of the area where the spectra were acquired. (d) Same as (b) but for a $12 nm$ Au/NbN bilayer. The inset depicts tunneling spectra measured on the corresponding molecules-free Au/NbN bilayer.
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Figure 3
Theoretical fits to selected experimental dI/dV-V tunneling spectra (black curves). The theoretical spectra were calculated using the extended BTK formalism as detailed in the text and Supplemental Material. (a) Before polyalanine alpha-helix molecules adsorption; the theoretical spectrum (blue dashed curve) was calculated using only an s-wave component. (b) and (c) After adsorption; here the theoretical spectra were calculated assuming a combination of s-wave and d-wave (blue dashed line) and s-wave and chiral p-wave (red dashed line), as detailed in the text.
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Figure 4
(a) Spectra measured on $(10 nm) Au$ / $(70 nm) NbN$ bilayer after chiral alpha-helix polyaniline molecules adsorption under different magnetic fields, as indicated. Note that the ZBCP is not splitted with magnetic field, making the d-wave scenario less probable. (b) Typical tunneling spectra measured on a $(10 nm) Au$ / $(70 nm) NbN$ bilayer after adsorption of the nonchiral 1-decanethiol, for which ZBCP does not appear. A $30 \times 30 {nm}^{2}$ topographic image of the area where the spectra were measured, showing the molecular layer, is shown in the inset.
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Figure 5
Tunneling spectra measured on $(10 nm) Au$ / $(70 nm) NbN$ bilayer after adsorbing nonhelical chiral cysteine molecules. The spectra exhibit conventional gaps and no ZBCPs were found. The left inset presents a topographic image of the sample and the right one a cross section taken along the blue line drawn in the image.
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