Angular-dependent torque measurements have been performed on the electron doped iron-pnictide superconductors BaFe[Formula: see text]NixAs2 with series of Ni concentrations. In the superconducting state, an irreversibility, as the evidence for the presence of vortex, is observed between the torque measured with increasing and decreasing angle. Our results in underdoped samples [Formula: see text] show that the irreversible torque signal survives up to a temperature [Formula: see text] well above the superconducting transition temperature [Formula: see text], suggesting the existence of superconducting vortices above [Formula: see text], which is likely to originate from superconducting fluctuations.
Nematic superconducting state in iron pnictide superconductors
