Emergence of orbital nematicity in the tetragonal phase of BaFe₂(As₁-_xP_x)₂

We report on ⁷⁵As-NMR measurements in single crystalline BaFe₂(As_0.96P_0.04)₂ for magnetic fields parallel to the orthorhombic [110]_o and [100]_o directions above the structural transition temperature T_S ≃ 121 K. A large difference in the linewidth between the two field directions reveals in-plane anisotropy of the electric field gradient, even in the tetragonal phase. This provides microscopic evidence of population imbalance between As-4p_x and 4p_y orbitals, which reaches |n_x - n_y| /|n_x + n_y| ∼ 15% at T → TS and is a natural consequence of the orbital ordering of Fe-3d_xz and d_yz electrons. Surprisingly, this orbital polarization is found to be already static near room temperature, suggesting that it arises from the pinning of anisotropic orbital fluctuations by disorder. The effect is found to be stronger below ∼160 K, which coincides with the appearance of nematicity in previous torque and photoemission measurements. These results impose strong constraints on microscopic models of the nematic state.