Among the various iron-based superconductors, LnFeAs(O1-xFx) and LnFeAs(O1-xHx) (Ln = Nd and Sm) show the highest superconducting transition temperature Tc of 56 K [1-2]. Both fluorine and hydrogen substitution dope electrons to the system, but the remarkable difference is the solubility limit; it is x ~ 0.2 for the former, whereas x ~ 0.8 for the latter. Thus, the physical properties can be investigated in a broader range of carrier density with hydrogen doping. Recently we have grown NdFeAs(O1-xHx) thin films with various x [3] and studied the anisotropy by measuring the resistivity of the films grown on vicinal cut substrates [4]. The anisotropy of the normal state resistivity γρ (= ρc/ρab) showed a strong doping dependence and recorded 100–150 at 50 K with the optimally doped films (films whose Tc was higher than 40 K). With increasing the carrier concentration, γρ decreased significantly, but even the most H doped film had a high value of 9. On the other hand, the anisotropy of the upper critical field Hc2 measured in the superconducting state was much lower [4], although the H content dependence was yet not fully explored.
In the present work, we measured the anisotropy in the superconducting state in a wide range of H content x. The films were grown by the method reported in Ref. [3]. We measured the angle dependence of resistivity under various magnetic fields at temperatures below the zero field Tc and determined the anisotropy ratio of Hc2 (Γ). Our results indicate that Γ is only weakly dependent on doping and is much lower than γρ. In the standard GL theory, Γ is proportional to the square root of the mass anisotropy. As the mass ratio is also related to γρ, it is notable that the anisotropy parameters are quite different between the superconducting and normal states. We will discuss the possible origin of the large discrepancy between the two anisotropy parameters.
[1] Y. Kamihara et al., J. Am. Chem. Soc. 130, 3296 (2008).
[2] T. Hanna et al., Phys. Rev. B 84, 024521 (2011).
[3] K. Kondo et al., Supercond. Sci. Technol. 33, 09LT01 (2020).
[4] M. Chen et al., Phys. Rev. Mater. 6, 054802 (2022).