Orbital anisotropy underlying the superconducting dome in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ superconductors
We investigate the in-plane anisotropy of Fe 3d orbitals occurring in a wide temperature and composition range of BaFe2(As1-xPx)2 system. By employing the angle-resolved photoemission spectroscopy, the lifting of degeneracy in dxz and dyz orbitals at the Brillouin zone corners can be obtained as a measure of the orbital anisotropy. In the underdoped regime, it starts to evolve on cooling from high temperatures above both antiferromagnetic and orthorhombic transitions. With increasing x, it well survives into the superconducting regime, but gradually gets suppressed and finally disappears around the non-superconducting transition (x = 0.7). The observed spontaneous in-plane orbital anisotropy, possibly coupled with anisotropic lattice and magnetic fluctuations, implies the rotational-symmetry broken electronic state working as the stage for the superconductivity in BaFe2(As1-xPx)2.