Synthesis and transport properties of 112-type iron pnictide superconductors Ca1-xCexFe1-yCoyAs2

We studied the electronic transport properties in the 112-type Ca1-xCexFe1-yCoyAs2 (0.18 ≤ x ≤ 0.27, y = 0, 0.025) single crystals. Upon Ce doping on the Ca site, it is found that the antiferromagnetic(AFM)/structural phase transition is enhanced and a weak superconductivity, possibly of filamentary superconductivity in origin, emerges. However, only a small amount of Co co-doping in Ca0.8Ce0.2Fe0.975Co0.025As2 suppresses the AFM/structural transition completely and stabilizes a bulk superconductivity with Tc = 34.5 K. Intriguingly, Hall measurements reveal a sign change of RH only in Ca0.73Ce0.27FeAs2 around ∼80 K, which is presumably due to the band reconstruction driven by the AFM/structural transition. For other dopings, Hall coefficient (RH) is negative and shows nonmonotonic T-dependence with a broad minimum feature. Moreover, the magnetoresistance (MR) is found to be progressively suppressed with increasing Ce concentrations. While the size of MR tends to saturate at low T for Ca0.8Ce0.2Fe0.975Co0.025As2, it turns downward with decreasing temperatures below ∼80 K for all Co-free samples. Finally, the Kohler's rule is strongly violated only below a characteristic temperature TK, suggesting either a change of carrier number or exotic anisotropic scattering mechanism below TK in this system.