Ln FeAsO (Ln: lanthanide) exhibits superconductivity through partial substitution of F or H for O, achieving the highest critical temperature (Tc) among the iron-based superconductors. Specifically, substituting H offers the ability to adjust the carrier density across a wide range, and it is well established that Tc tends to plateau over a broad range of carrier concentration. However, for many other physical properties, it is not clear whether they are constant in this Tc plateau region or depend on carrier doping. We have been studying the growth of single-crystal thin films and have developed a technique that allows us to fabricate NdFeAs(O,H) thin films with different levels of H content [1]. Here, we report on the results of systematically measuring the anisotropy and the critical current density (Jc) over a wide range of carrier concentration using these single crystalline thin films.
To explore the resistivity anisotropy in the normal state, thin films were grown on vicinal-cut substrates, i.e., substrates whose [001] direction was inclined by 5º or 10º from the direction normal to the surface [2]. By measuring the resistance along and perpendicular to the projection direction of the tilted c-axis, we deduced the resistivity along the ab-plane and c-axis directions. Our results revealed that the anisotropy in the normal state strongly depended on the carrier density. The normal-state anisotropy at temperatures close to Tc first increased with carrier doping and reached a large value exceeding 100, and then decreased and finally reduced to about 10 for the sample with the highest carrier density.
In the superconducting state, we examined the anisotropy by measuring the angle dependence of resistance within the superconducting transition region. The isothermal angular dependence of resistivity followed well the scaling function derived from Tinkham’s model originally proposed for quasi two-dimensional superconductors, although we observed a tendency of deviation from the scaling with increasing the H content. Remarkably, the anisotropy in the superconducting state had a moderate value that is much smaller compared to the normal state anisotropy except in the strongly doped region, where the normal state anisotropy had significantly decreased. Additionally, the dependence on carrier doping was weak, and remained nearly constant within a large range of carrier density.
We also measured Jc of NdFeAs(O,H) thin films with various H contents. We found that Jc was notably high, surpassing 10 MA/cm2 at 4.2 K, and exhibited relatively weak dependence on doping over a certain range of the Tc plateau. Nevertheless, it started to decrease before reaching the upper bound of the Tc plateau. This reduction might be attributed to a decrease in the depairing current density, to which Jc is roughly proportional, as a result of a change in the coherence length and/or the penetration depth.
[1] K. Kondo et al., Supercond. Sci. Technol. 33, 09LT01 (2020).
[2] M. Y. Chen et al., Phys. Rev. Materials 6, 054802 (2022).