Nodal gap structure of iron-pnictide superconductors

Yuji Matsuda

 Knowledge of the superconducting (SC) gap structure is a major step toward identifying the interactions that produce pairing of Fe-pnictide superconductors. However, the SC gap structure remains one of the most controversial topics. Early measurements pointed to a fully gapped order parameter. On the other hand, more recent experiments revealed that many Fe-pnictide superconductors have line nodes in the gap. They include LaFePO, heavily-hole doped KBa2As2 and iso-valent doping system BaFe2(As1-xPx)2. The presence of line nodes in the latter material is especially intriguing because the transition temperature of this system in the optimally doped region (Tc=31 K at x=0.33), is comparably high as that of hole-doped Ba1-xKxFe2As2 (Tc=37 K) whose SC order parameter is argued to be fully gapped. Here we investigated the structure of the superconducting order parameter in BaFe2(As1-xPx)2 with line nodes by the angle-resolved thermal conductivity measurements in magnetic field rotating within the basal plane. We find that the thermal conductivity displays distinct fourfold oscillations with minima when the field is directed to the 45 deg with respect to the Fe-Fe bond direction. We discuss possible gap structures that can account for the data and conclude that the observed results are most consistent with the "closed nodal loops" located at the flat part of the electron Fermi surface with high Fermi velocity.