c -axis pressure-induced antiferromagnetic order in optimally P-doped BaFe 2 (As 0.70 P 0.30 ) 2 superconductor

Superconductivity in BaFe 2 (As 1− x P x ) 2 iron pnictides emerges when its in-plane two-dimensional (2D) orthorhombic lattice distortion associated with nematic phase at T s and three-dimensional (3D) collinear antiferromagnetic order at T N ( T s = T N ) are gradually suppressed with increasing x , reaching optimal superconductivity around x = 0.30 with T c ≈ 30 K. Here we show that a moderate uniaxial pressure along the c -axis in BaFe 2 (As 0.70 P 0.30 ) 2 spontaneously induces a 3D collinear antiferromagnetic order with T N = T s > 30 K, while only slightly suppresses T c . Although a ~ 400 MPa pressure compresses the c -axis lattice while expanding the in-plane lattice and increasing the nearest-neighbor Fe–Fe distance, it barely changes the average iron-pnictogen height in BaFe 2 (As 0.70 P 0.30 ) 2 . Therefore, the pressure-induced antiferromagnetic order must arise from a strong in-plane magnetoelastic coupling, suggesting that the 2D nematic phase is a competing state with superconductivity.