Superconducting NbN and ${\mathrm{CaFe}}_{0.88}{\mathrm{Co}}_{0.12}\mathrm{AsF}$ studied by point-contact spectroscopy with a nanoparticle Au array

Superconducting NbN and ${CaFe}_{0.88} {Co}_{0.12} AsF$ studied by point-contact spectroscopy with a nanoparticle Au array

Y. F. Wu, A. B. Yu, L. B. Lei, C. Zhang, T. Wang, Y. H. Ma, Z. Huang, L. X. Chen, Y. S. Liu, C. M. Schneider, G. Mu, H. Xiao, and T. Hu

Phys. Rev. B 101, 174502 – Published 4 May 2020

Abstract

The point-contact-spectroscopy measurement is a powerful method to detect the superconducting gap and the spin polarization of materials. However, it is difficult to get a stable and clean point contact by conventional techniques. In this work, we fabricate multiple point contacts by depositing Au nanoparticle arrays on the surface of a superconductor through an anodic aluminum oxide patterned shadow mask. We obtained the superconducting gaps of niobium nitride thin film (NbN, $T_{c} = 16$ K) and iron superconductors ${CaFe}_{0.88} {Co}_{0.12} AsF$ single crystals (Ca-1111, $T_{c} = 21.3$ K) by fitting the point-contact spectroscopy with the Blonder-Tinkham-Klapwijk theory. We found that NbN's gap ( $Δ$ ) exhibits the BCS-like temperature dependence with $Δ \approx 2.88$ meV at 0 K and $2 Δ / k_{B} T_{c} \approx 4.22$ in agreement with previous reports. By contrast, Ca-1111 has a multigap structure with $Δ_{1} \approx 1.99$ meV and $Δ_{2} \approx 5.01$ meV at 0 K, and the ratio between the superconducting gap and $T_{c}$ is $2 Δ_{1} / k_{B} T_{c} = 2.2$ and $2 Δ_{2} / k_{B} T_{c} = 5.5$ , suggesting an unconventional paring mechanism of Ca-1111 also in agreement with previous reports on other Fe-based superconductors. Our multiple point-contacts method thus provides an alternative way to perform measurements of the superconducting gap.

Received 23 December 2019
Revised 9 April 2020
Accepted 13 April 2020

DOI:https://doi.org/10.1103/PhysRevB.101.174502

Physics Subject Headings (PhySH)

Andreev reflection Superconducting gap Superconductivity

Iron-based superconductors Nanostructures

Andreev point contact spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Y. F. Wu ^1,*, A. B. Yu^3,4,5,*, L. B. Lei^3,4,6, C. Zhang^3,4,5, T. Wang^3,4,6, Y. H. Ma^3,4,5, Z. Huang^3,4,6, L. X. Chen^3,4, Y. S. Liu^7,8, C. M. Schneider^7,8, G. Mu^3,4,5, H. Xiao², and T. Hu ^1,†

¹Beijing Academy of Quantum Information Sciences, Beijing 100193, China
²Center for High Pressure Science and Technology Advanced Research, Beijing 100094, China
³State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
⁴CAS Center for Excellence in Superconducting Electronics (CENSE), Shanghai 200050, China
⁵University of Chinese Academy of Sciences, Beijing 100049, China
⁶School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
⁷Peter Gruenberg Institute PGI-6, Forschungszentrum Juelich, D-52425 Juelich, Germany
⁸Fakultät für Physik, Universität Duisburg-Essen, D-47057 Duisburg, Germany

^*These authors contributed equally to this work.
^†hutao@baqis.ac.cn

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Vol. 101, Iss. 17 — 1 May 2020

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Figure 1
The fabrication process for multiple point-contact devices. (a) The nanoporous anodic aluminum oxide (AAO) mask is placed on top of NbN thin film. (b) The Au arrays are grown on NbN by using thermal vapor deposition via the AAO mask. (c) The obtained regular Au nanoarray structure.
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Figure 2
Characterization of the nano-Au array/NbN structure. (a) The SEM image of the Au array on NbN film. Scale bar is 400 nm. (b) The AFM image of a smaller range of the Au array. Scale bar is 100 nm. The white curve shows the measurements of the islands' height and width. (c) TEM image of the cross section of the Au/NbN heterojunction. Scale bar is 50 nm. From the lower right to the top left corner are the ${MgO}_{2}$ substrate, 50 nm NbN, Au array, and gold surface coating prior to focused ion beam processing, respectively.
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Figure 3
The temperature dependence of differentiate resistance at zero-bias voltage. The inset shows a schematic structure of the measurement configuration for electrical transport in the nano-Au array/NbN structure and the circle shows point contact from one Au island.
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Figure 4
Temperature ( $T$ ) and magnetic field ( $H$ ) dependence of tunneling spectroscopy of NbN film in the metal-array/ superconductor structure. (a) Temperature (from 2 to 21 K with the step of 1 K) evolution of the normalized conductance curve of 50-nm NbN film measured at 0 T. (b) magnetic field (from 0 to 1 T with the step of 0.2 T and from 1 to 9 T with the step of 1 T) evolution of the normalized conductance curve of 50-nm NbN film at 2 K. The inset is the three-dimensional view of tunneling conductance measured by the point-contact structure at different temperatures and magnetic fields.
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Figure 5
Superconducting energy gap. (a) The normalized PCAR spectra of 50-nm NbN film at 2 K (red open dots) with the best fitting result (red line) by the modified BTK model ( $Z = 2.13, Γ = 0.27$ meV, and $Δ = 2.89$ meV). (b) The temperature dependence of the superconducting energy gap (left axis, black dots) normalized by the value at $T = 0$ K and the broadening parameters ( $Γ$ ) (right axis, blue solid square) given by the BTK fit with a dashed line as a guide to the eye. The black dashed line shows the temperature dependence of the magnitude of the superconducting gap determined by BCS theory. (c) The magnetic field dependence of the superconducting gap (left axis) and the zero-bias normalized differential conductance (ZBC) (right axis) at 2 K. The black dashed line is the linear fitting curve to the $Δ$ and the blue dashed line is the linear fitting curve to the ZBC.
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Figure 6
The PCAR spectroscopy and superconducting gap of ${CaFe}_{0.88} {Co}_{0.12} AsF$ . (a) Normalized conductance curves (symbols) of Au- ${CaFe}_{0.88} {Co}_{0.12} AsF$ measured from 2 to 19 K with the step of 1 K under zero field. The solid lines are given by a two-gap BTK model. Inset: The temperature dependence of resistance. (b) and (c) show the temperature dependence of the broadening parameters ( $Γ_{1}$ and $Γ_{2}$ ) and gaps ( $Δ_{1}$ and $Δ_{2}$ ).
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Physical Review B

covering condensed matter and materials physics

Superconducting NbN and ${CaFe}_{0.88} {Co}_{0.12} AsF$ studied by point-contact spectroscopy with a nanoparticle Au array

Y. F. Wu, A. B. Yu, L. B. Lei, C. Zhang, T. Wang, Y. H. Ma, Z. Huang, L. X. Chen, Y. S. Liu, C. M. Schneider, G. Mu, H. Xiao, and T. Hu

Phys. Rev. B 101, 174502 – Published 4 May 2020

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Physical Review B

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Superconducting NbN and CaFe0.88Co0.12AsF studied by point-contact spectroscopy with a nanoparticle Au array

Y. F. Wu, A. B. Yu, L. B. Lei, C. Zhang, T. Wang, Y. H. Ma, Z. Huang, L. X. Chen, Y. S. Liu, C. M. Schneider, G. Mu, H. Xiao, and T. Hu

Phys. Rev. B 101, 174502 – Published 4 May 2020

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Superconducting NbN and ${CaFe}_{0.88} {Co}_{0.12} AsF$ studied by point-contact spectroscopy with a nanoparticle Au array