LAUSR

Superconductivity in BaFe2(As1−xPx)2 iron pnictides emerges when its in-plane two-dimensional (2D) orthorhombic lattice distortion associated with nematic phase at Ts and three-dimensional (3D) collinear antiferromagnetic order at TN (Ts = TN) are gradually suppressed with increasing x, reaching optimal superconductivity around x = 0.30 with Tc ≈ 30 K. Here we show that a moderate uniaxial pressure along the c-axis in BaFe2(As0.70P0.30)2 spontaneously induces a 3D collinear antiferromagnetic order with TN = Ts > 30 K, while only slightly suppresses Tc. 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 BaFe2(As0.70P0.30)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.Superconductivity: Structural instability competesA moderate uniaxial pressure in BaFe2(As0.70P0.30)2 induces a three dimensional collinear antiferromagnetic order together with nematic structural instability. An international team led by Pengcheng Dai from Beijing Normal University and Rice University applied uniaxial pressure in BaFe2(As0.70P0.30)2 to study the interplay between structural/magnetic orders and superconductivity. At a pressure below 280 MPa, they observe a suppressed superconducting transition temperature (Tc) as well as a deviation of resistivity away from linear temperature dependence. Upon further increasing uniaxial pressure, the reduction in Tc reduces but the resistivity starts to deviate from linear temperature dependence at a higher temperature. Neutron diffraction experiments reveal that such a deviation arises from a pressure-induced collinear antiferromagnetic order, which also breaks rotational symmetry in the underlying lattice due to strong magnetoelastic coupling.