Effects of pressure on CaFe$_{2}$As$_{2}$ and related materials

The discovery of CaFe$_{2}$As$_{2 }$[1] and its extreme pressure dependence [2] (even for pressures below 1 GPa) has lead to it being used as a model system for understanding the effects of pressure on the (AE)Fe$_{2}$As$_{2 }$(AE = Ba, Sr, Ca) compounds.[3-5] We have found that the combination of extreme pressure sensitivity with a first order structural phase transition that involves significant changes in the unit cell dimensions makes CaFe$_{2}$As$_{2}$ very sensitive to the pressure medium used. In liquid medium, self clamping cells the higher temperature transitions, while generally detectable and highly reproducible [2,6] are smeared and broadened, especially near the first order phase transition between the low temperature orthorhombic and collapsed tetragonal phases. In He-pressure cells these transitions remain extremely sharp.[3-5] Superconductivity is detected between $\sim $0.3 and $\sim $0.7 GPa in the liquid medium cells and is essentially absent in the He-pressure cell.[2,5,6] This superconducting region can be associated with a coexistence of low temperature phases brought on by non-hydrostatic components associated with the medium's inability to respond to the high-temperature structural phase transitions.[2-5] The origin of the superconductivity in this mixed region remains a topic of keen experimental and theoretical interest. \\[4pt] [1] N. Ni et al., Phys. Rev. B \textbf{78}, 014523 (2008), [2] M. Torikachvili et al., Phys. Rev. Lett. \textbf{101}, 057006 (2008), [3] A. Kreyssig et al., arXiv:0807.3032, [4] A. I. Goldman et al., arXiv:0811.2013, [5] W. Yu et al., arXiv:0811.2554, [6] H. Lee et al., arXiv:0809.3550.