Fourfold ab-plane torque symmetry in the superconducting state of BaFe1.82Ni0.18As2

We performed in-plane angular dependent torque measurements on an overdoped iron-based superconductor Ba2Fe2-xNixAs2 with x=0.18 both in the normal state and mixed state. The torque signal were measured in both clockwise and counter-clockwise rotating directions. The reversible torque is the average of the torque data measured in two rotating directions which allows the determination of thermodynamic properties. A fourfold symmetry is observed in the reversible part of the superconducting torque, which disappears when superconductivity vanishes. So we concluded that in the overdoped part of the phase diagram, torque measurements reveal signatures of a fourfold symmetry of the superconducting gap structure. In addition, the specific heat results support the existence of nodes in the gap structure. Hence, our results point to a possible d wave pairing symmetry.