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

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 ${\mathrm{CaFe}}_{0.88}{\mathrm{Co}}_{0.12}\mathrm{AsF}$ 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 ($\mathrm{\ensuremath{\Delta}}$) exhibits the BCS-like temperature dependence with $\mathrm{\ensuremath{\Delta}}\ensuremath{\approx}2.88$ meV at 0 K and $2\mathrm{\ensuremath{\Delta}}/{k}_{B}{T}_{c}\ensuremath{\approx}4.22$ in agreement with previous reports. By contrast, Ca-1111 has a multigap structure with ${\mathrm{\ensuremath{\Delta}}}_{1}\ensuremath{\approx}1.99$ meV and ${\mathrm{\ensuremath{\Delta}}}_{2}\ensuremath{\approx}5.01$ meV at 0 K, and the ratio between the superconducting gap and ${T}_{c}$ is $2{\mathrm{\ensuremath{\Delta}}}_{1}/{k}_{B}{T}_{c}=2.2$ and $2{\mathrm{\ensuremath{\Delta}}}_{2}/{k}_{B}{T}_{c}=5.5$, 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.