Abstract
We use nuclear magnetic resonance (NMR), high-resolution x-ray, and neutron scattering studies to study structural and magnetic phase transitions in phosphorus-doped . 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 . However, we show that the tetragonal-to-orthorhombic structural () and paramagnetic to antiferromagnetic (AF, ) transitions in are always coupled and approach () for before vanishing abruptly for . These results suggest that AF order in disappears in a weakly first-order fashion near optimal superconductivity, much like the electron-doped iron pnictides with an avoided QCP.
- Received 22 December 2014
DOI:https://doi.org/10.1103/PhysRevLett.114.157002
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