https://doi.org/10.1140/epjp/s13360-023-04123-w
Regular Article
Probing superconducting granularity using nonlocal four-probe measurements
1
Department of Experimental Physics, Faculty of Mathematics, Physics and Informatics, Comenius University Bratislava, 84248, Bratislava, Slovak Republic
2
O.O. Galkin Donetsk Institute for Physics and Engineering, National Academy of Sciences of Ukraine, Nauki Ave. 46, 03028, Kiev, Ukraine
3
Kyiv Academic University, Academician Vernadsky Blvd. 36, 03142, Kiev, Ukraine
4
Institut für Festkörperphysik, Friedrich-Schiller-Universität Jena, Helmholtzweg 5, 07743, Jena, Germany
Received:
6
March
2023
Accepted:
19
May
2023
Published online:
2
June
2023
The four-point probe is a relatively simple method of measuring thin-layer surface resistance using separate pairs of current-carrying and voltage-sensing contacts. This approach makes it possible to measure resistivity regardless of the resistance of the contacts that is especially important when the latter are large and non-ohmic, which makes simpler two contact measurements inapplicable. Nevertheless, its results turn out to be significantly dependent on the sample geometry as well as the position of the probes, leading in some cases, as shown below, to qualitatively incorrect conclusions. We demonstrate this with the example of an anomalous resistance peak at temperatures near the critical one Tc, which is often observed in experiments on mesoscopic superconducting samples. Below, with an oversimplified model for nonlocal four-probe measurements, we argue that the anomalous resistance behavior near Tc observed in traditional four-probe measurements can arise due to noticeable variations in local Tc values (superconducting granularity) and attributed the near-Tc anomaly to the current redistribution effect. The model results well explain temperature-dependent resistance data for 50 nm thick NbN films, which exhibited not only the resistance peak but also fundamentally different resistance-vs-temperature behavior depending on the arrangement of the contacts. Nonlocal probing performed for in-plane and out-of-plane magnetic fields clearly demonstrates the interplay of orbital and Zeeman couplings different for the two field orientations. We believe that the anomalous temperature behavior of the four-point resistance near the transition from the normal to superconducting state is sometimes an artifact that, if misinterpreted, can lead to incorrect conclusions. On the other hand, we show that such a nonlocal contact arrangement strongly enhances sensitivity to inhomogeneity factors. This gives grounds for using the corresponding experiments as a method of choice for revealing the spatial distribution of the order parameter in superconducting films.
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© The Author(s), under exclusive licence to Società Italiana di Fisica and Springer-Verlag GmbH Germany, part of Springer Nature 2023. Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.
