(TM = Cr, Mn, V and Cu): Why superconductivity cannot be achieved by these dopants

We have systematically studied nematic susceptibilities in nonsuperconducting Ba(Fe_1-xTM_x)₂As₂ (TM = Cr, Mn, V and Cu) single crystals by measuring the uniaxial pressure dependence of the resistivity along the Fe-As-Fe direction. The nematic susceptibilities in all samples show the Curie-Weiss-like behavior at high temperatures, where the nematic Curie constant A_n can be derived. In Cr, Mn and V doped samples, A_n decreases with the doping level, suggesting the suppression of nematic fluctuations by these dopants. In Ba(Fe_1-xCu_x)₂As₂, detailed neutron-diffraction measurements reveal that the collinear antiferromagnetic order persists up to x = 0.08 but becomes short-range above x > 0.04. Moreover, the nematic susceptibilities in the x >0.04 samples are significantly suppressed at low temperatures although A_n obtained by fitting the high-temperature data increases with increasing x. Compared with those systems that clearly exhibit superconductivity, such as Co, Ni, K or P doped samples, our results suggest that the reason that no superconductivity is found by doping Cr, Mn, V and Cu in BaFe₂As₂may be correlated with the suppression of nematic fluctuations.