Probing the current-phase relation in Josephson point-contact junctions between [Formula: see text]In[Formula: see text] and [Formula: see text]K[Formula: see text](FeAs)[Formula: see text] superconductors

Valeriy A Stepanov; Chengtian Lin; Renato S Gonnelli; Mauro Tortello

doi:10.1038/s41598-021-00762-0

Probing the current-phase relation in Josephson point-contact junctions between [Formula: see text]In[Formula: see text] and [Formula: see text]K[Formula: see text](FeAs)[Formula: see text] superconductors

Sci Rep. 2021 Dec 14;11(1):23986. doi: 10.1038/s41598-021-00762-0.

Authors

Valeriy A Stepanov¹, Chengtian Lin², Renato S Gonnelli³, Mauro Tortello³

Affiliations

¹ P.N. Lebedev Physical Institute of the Russian Academy of Sciences, Moscow, Russia 119333.
² Max-Planck-Institut für Festkörperforschung, 70569 Stuttgart, Germany.
³ Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, Torino, 10129 Italy.

Abstract

The Josephson effect in point contacts between an "ordinary" superconductor [Formula: see text]In[Formula: see text] ([Formula: see text]) and single crystals of the Fe-based superconductor Ba[Formula: see text]K[Formula: see text](FeAs)[Formula: see text] ([Formula: see text]), was investigated. In order to shed light on the order parameter symmetry of Ba[Formula: see text]K[Formula: see text](FeAs)[Formula: see text], the dependence of the Josephson supercurrent [Formula: see text] on the temperature and on [Formula: see text] with [Formula: see text] was studied. The dependencies of the critical current on temperature [Formula: see text] and of the amplitudes of the first current steps of the current-voltage characteristic [Formula: see text] [Formula: see text] on the power of microwave radiation with frequency [Formula: see text] were measured. It is shown that the dependencies [Formula: see text] are close to the well-known Ambegaokar-Baratoff (AB) dependence for tunnel contacts between "ordinary" superconductors and to the dependence calculated by Burmistrova et al. (Phys Rev B 91, 214501 (2015)) for microshorts between an "ordinary" superconductor and a two-band superconductor with [Formula: see text] order parameter symmetry at certain values of the transparency of boundaries and thickness of the transition layer. It is found that the dependencies [Formula: see text] cannot be approximated within the resistively shunted model using the normalized microwave frequencies [Formula: see text] with characteristic voltages [Formula: see text], [Formula: see text]-normal resistance of the contact) found from the low-voltage parts of the current-voltage characteristics. The reasons for this failure are discussed and a method is proposed for accurately determining the value of [Formula: see text], which takes into account all the features of the point contact affecting the period of the dependence [Formula: see text]. An analysis of the [Formula: see text] and [Formula: see text] dependencies shows that the superconducting current of the Josephson contacts under investigation is proportional to the [Formula: see text] of the phase difference [Formula: see text], [Formula: see text]. The implications of these results on the symmetry of the order parameter are also discussed.