Observation of bands with dxy orbital character near the Fermi level in NdFeAs1−xPx

We studied the band structure of $\mathrm{Nd}\phantom{\rule{0.16em}{0ex}}\mathrm{Fe}\phantom{\rule{0.16em}{0ex}}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}{\mathrm{O}}_{0.9}{\mathrm{F}}_{0.1}$ ($x=0$, 0.2, 0.4, and 0.6) using angle-resolved photoemission spectroscopy (ARPES) measurements. Two of the hole bands, ${\ensuremath{\alpha}}_{1}$ $({d}_{xz})$ and ${\ensuremath{\alpha}}_{3}$ $({d}_{{z}^{2}})$, were observed at the Brillouin zone center in the $P$-polarized light configuration, while the other two hole bands, ${\ensuremath{\alpha}}_{2}$ $({d}_{yz})$ and \ensuremath{\gamma} $({d}_{xy})$, were observed in the $S$-polarized alternative. The observed \ensuremath{\gamma} 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 ${\ensuremath{\alpha}}_{1}$ band, thus indicating the band reconstruction of the originally degenerate ${\ensuremath{\alpha}}_{1}$ and ${\ensuremath{\alpha}}_{2}$ $({d}_{xz}/{d}_{yz})$ bands by the unoccupied ${d}_{xy}$ band. The change in the energy level of the ${\ensuremath{\alpha}}_{1}$ band top with ${d}_{xy}$ orbital character is accompanied by a ${T}_{\mathrm{c}}$ upturn at $0.2<x<0.4$. The ${T}_{\mathrm{c}}$ continues to increase as the ${\ensuremath{\alpha}}_{1}$ 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 ${T}_{\mathrm{c}}$ 1111-type iron-based superconductors.