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

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 withT(c)= 30-33.5 K. In this paper, single crystals of Ni doped RbCa2(Fe1-xNix)(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 theT(c)-xphase 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 temperatureT(c)is suppressed with a rate of Delta T-c/Ni-1% = -2.7 K. Intriguingly, such suppression ofT(c)and those in other similar hole doped IBSs, such as Ba0.6K0.4Fe2As2, Ba0.5K0.5Fe2As2, 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 nodelesss +/--wave. Finally, doping evolution of the upper critical field and its anisotropy are investigated and discussed in detail. Upon Ni doping, the coherence length xi(c)(0) is gradually increased and becomes larger than the FeAs interbilayer distance whenx> 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 gamma(H)close toT(c)shows a nonmonotonic dependence on doping, which first increases from 6.7 at the pristine sample to its maximum 8.1 atx= 0.03, and then decreases to 3.7 atx= 0.09.