Superconductivity in CaFe$_{2}$As$_{2}$ under uniaxial pressure: new insights from neutron diffraction experiments

CaFe$_{2}$As$_{2}$, a member of the 122 iron arsenide family, is not superconducting at ambient pressure. It undergoes structural and antiferromagnetic transitions at $T_{TO}$ = 172 K that are strongly coupled [1]. In clamped cell pressure measurements using a liquid medium, superconductivity has been observed with $T_{C}$ as high as 12 K [2]. However, measurements using a He-gas pressure cell, where non-hydrostatic pressure components are minimized show no evidence of superconductivity [3]. We report on neutron diffraction experiments using CaFe$_{2}$As$_{2 }$single crystals under uniaxial pressure applied along the $c$ axis. We find that, above 0.05 GPa, several structural phases coexist at low temperature. Simultaneous diffraction/resistivity measurements strongly suggest that a pressure-stabilized tetragonal phase is responsible for the superconductivity in CaFe$_{2}$As$_{2}$. -- The work at Ames Laboratory was supported by US DOE (DE-AC02-07CH11358). [1] A.I. Goldman, et al., PRB \textbf{78} (2008) 100506. [2] M.S. Torikachvili, et al., PRL \textbf{101} (2008) 057006. [3] W.Yu, et al., PRB \textbf{79} (2009) 020511.