Emergence of Orbital Nematicity in the Tetragonal Phase of BaFe₂(As₁₋ₓPₓ)₂

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We report on ⁷⁵As-NMR measurements in single crystalline BaFe₂(As₀.₉₆P₀.₀₄)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-4pₓ and 4py orbitals, which reaches |nₓ-ny|/|nₓ+ny| 15% at T→ T S and is a natural consequence of the orbital ordering of Fe-3dxz and dyz 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.