Quantitative assessment of pinning forces and the superconducting gap in NbN thin films from complementary magnetic force microscopy and transport measurements

TL;DR

This study combines magnetic force microscopy and transport measurements to quantitatively analyze vortex pinning forces and the superconducting gap in NbN thin films, confirming the effectiveness of MFM in probing local pinning landscapes.

Contribution

It introduces a combined approach using MFM and transport data to accurately determine pinning forces and superconducting gap in NbN thin films, validating MFM as a reliable local probe.

Findings

Superconducting gap amplitude Delta(0) = 2.9 meV consistent with previous reports.

Good agreement between MFM and transport measurements for pinning forces.

Monopole model effectively describes vortex profiles in thin films.

Abstract

Epitaxial niobium-nitride thin films with a critical temperature of Tc=16K and a thickness of 100nm were fabricated on MgO(100) substrates by pulsed laser deposition. Low-temperature magnetic force microscopy (MFM) images of the supercurrent vortices were measured after field cooling in a magnetic field of 3mT at various temperatures. Temperature dependence of the penetration depth has been evaluated by a two-dimensional fitting of the vortex profiles in the monopole-monopole model. Its subsequent fit to a single s-wave gap function results in the superconducting gap amplitude Delta(0) = 2.9 meV = 2.1*kB*Tc, in perfect agreement with previous reports. The pinning force has been independently estimated from local depinning of individual vortices by lateral forces exerted by the MFM tip and from transport measurements. A good quantitative agreement between the two techniques shows that for low fields, B << Hc2, MFM is a powerful and reliable technique to probe the local variations of the pinning landscape. We also demonstrate that the monopole model can be successfully applied even for thin films with a thickness comparable to the penetration depth.