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The recently discovered 12442-type iron-based superconductors (IBSs), ACa 2 Fe 4 As 4 F 2  (A = K, Rb, Cs), are intrinsically self-hole doped stoichiometric compounds that exhibit superconductivity with  T c  = 30–33.5 K. In this paper, single crystals of Ni doped RbCa 2 (Fe 1− x Ni x ) 4 As 4 F 2  with 0 ⩽  x  ⩽ 0.1 have been successfully grown for the first time using a RbAs flux method and characterized by energy dispersive x-ray spectroscopy (EDS), x-ray diffraction (XRD), electrical resistivity, magnetic susceptibility, and Hall effect measurements. EDS and XRD measurements suggest that the Ni dopants are successfully doped into the crystal lattice. Based on the electrical resistivity and magnetization data, we construct the  T c – x  phase diagram. Furthermore, it is found that Ni dopants not only introduce extra electrons that modify the topology of Fermi surface, but also act as impurity scattering centers that contribute to the pair breaking effect, i.e., the superconducting transition temperature  T c  is suppressed with a rate of Δ T c /Ni-1% = −2.7 K. Intriguingly, such suppression of  T c  and those in other similar hole doped IBSs, such as Ba 0.6 K 0.4 Fe 2 As 2 , Ba 0.5 K 0.5 Fe 2 As 2 , and EuRbFe 4 As 4  with multiple nodeless gaps, can be well scaled together. Combining with relevant experimental data reported so far, we speculate that the pairing symmetry in 12442 system is very likely to be nodeless  s ±-wave. Finally, doping evolution of the upper critical field and its anisotropy are investigated and discussed in detail. Upon Ni doping, the coherence length  ξ c (0) is gradually increased and becomes larger than the FeAs interbilayer distance when  x  &gt; 0.07, indicating that the nature of superconductivity changes from quasi two-dimensional (2D) to three-dimensional (3D). The anisotropy of the upper critical field  γ H  close to  T c  shows a nonmonotonic dependence on doping, which first increases from 6.7 at the pristine sample to its maximum 8.1 at  x  = 0.03, and then decreases to 3.7 at  x  = 0.09.

new journal of physics

Single crystal growth and effects of Ni doping on the novel 12442-type iron-based superconductor RbCa2Fe4As4F2