Observation of bands with d(xy) orbital character near the Fermi level in NdFeAs1-xPxO0.9F0.1 using angle-resolved photoemission spectroscopy

We studied the band structure of Nd Fe As1-xPxO0.9F0.1 (x = 0, 0.2, 0.4, and 0.6) using angle-resolved photoemission spectroscopy (ARPES) measurements. Two of the hole bands, alpha(1) (d(xz)) and alpha(3) (d(z)2), were observed at the Brillouin zone center in the P-polarized light configuration, while the other two hole bands, alpha(2) (d(yz)) and gamma (d(xy)), were observed in the S-polarized alternative. The observed y band shifts downward as x increases, which is consistent with the theoretical prediction for the change in bond angle of As/P-Fe-As/P. Furthermore, a small amount of the d(xy) orbital component was observed at the same binding energy as that of the top of the alpha(1) band, thus indicating the band reconstruction of the originally degenerate alpha(1) and alpha(2) (d(xz)/d(yz)) bands by the unoccupied d(xy) band. The change in the energy level of the alpha(1) band top with d(xy) orbital character is accompanied by a T-c upturn at 0.2 < x < 0.4. The T-c continues to increase as the alpha(1) band shifts downward, crossing the Fermi level. The incipient band with the d(xy) orbital character on its top could be an important ingredient for high T-c 1111-type iron-based superconductors.