The anisotropy is one of the fundamentally important parameters to understand the electoronic properties of high-temperature superconductors including iron-based superconductors. However, for the LnFeAsO (Ln = Nd, Sm,...) system, which exhibits the highest superconducting transition temperature Tc among the iron-based superconductors [1], detailed studies on the anisotropy are limited mainly due to the difficulty in crystal sinthesis with large size. Recently, our group fabricated thin films of NdFeAs(O,H) (hereafter Nd1111:H) [2] and investigated the anisotropy of the normal resistivity γρ and that of the upper critical field Hc2 (γHc2) as a function of the H content [3,4]. Our findings indicated that γρ varies dramatically with the H content, while γHc2 only weakly. However, the H content dependence of gHc2 was still not fully explored, especially in the low H content region. In this work, therefore, we prepared several additional samples with different H contents, and measured γHc2.
Thin films of the parent phase NdFeAsO were prepared by MBE, followed by a H-substitution process with topotactic reaction using CaH2. The H content was adjusted by the processing temperature/time of the topotactic reaction. Since the c-axis length of Nd1111:H systematically decreases with increasing H-substitution, the c-axis length can be used as a measure of the H content. We measured the field angle dependence of resistivity under various magnetic fields at temperatures below the zero field Tc and determined γHc2. The samples with c < 8.5 Å exhibited a γHc2 of ~ 4 at 0.95Tc, which is consistent with our previous report [3,4]. On the other hand, for specimens with c > 8.5 Å (lower H content specimens), γHc2 was about 8-9 at 0.95Tc, which is larger than that of other iron-based superconductors. In LaFeAs(O,H) [5] and SmFe(As,P)(O,H) [6], the Tc dome has a two-peak shape, which can be ascribed to the existence of two superconducting phases in the LnFeAsO system. The observed abrupt change in γHc2 as a function of H content may indicate that there are two superconducting phases in NdFeAs(O,H) thin films as well, although not manifested itself in the variation of Tc.
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[2] K. Kondo et al., Supercond. Sci. Technol. 33, 09LT01 (2020).
[3] M. Chen et al., Phys. Rev. Mater. 6, 054802 (2022).
[4] T. Hatano et al., PC8-4, ISS2022 (2022).
[5] S. Iimura et al., Nature Commun. 3, 943 (2012).
[6] S. Matsuishi et al., Phys. Rev. B 89, 094510 (2014).