Structural and Magnetic Phase Transitions near Optimal Superconductivity in BaFe2(As1-xPx)2

We use nuclear magnetic resonance (NMR), high-resolution x-ray, and neutron scattering studies to study structural and magnetic phase transitions in phosphorus-doped BaFe2(As1-xPx)(2). Previous transport, NMR, specific heat, and magnetic penetration depth measurements have provided compelling evidence for the presence of a quantum critical point (QCP) near optimal superconductivity at x = 0.3. However, we show that the tetragonal-to-orthorhombic structural (T-s) and paramagnetic to antiferromagnetic (AF, T-N) transitions in BaFe2(As1-xPx)(2) are always coupled and approach T-N approximate to T-s >= T-c (approximate to 29 K) for x = 0.29 before vanishing abruptly for x >= 0.3. These results suggest that AF order in BaFe2(As1-xPx)(2) disappears in a weakly first-order fashion near optimal superconductivity, much like the electron-doped iron pnictides with an avoided QCP.