Absence of superconductivity in single-phase CaFe₂As₂ under hydrostatic pressure

Recent high-pressure studies found that structural/magnetic phase transitions are very pressure sensitive in CaFe₂As₂ and that superconductivity can be achieved under modest pressure, although details of the sharpness and temperature of transitions vary between liquid medium and gas medium measurements. To better understand this issue, we performed high-pressure susceptibility and transport studies on CaFe₂As₂, using helium as the pressure medium. The signatures of the transitions to the low-temperature orthorhombic and collapsed tetragonal phases remained exceptionally sharp, and no signature of bulk superconductivity was found under our hydrostatic conditions. Our results suggest that superconductivity in CaFe₂As₂ is associated with a low-temperature, multicrystallographic-phase sample that is the result of nonhydrostatic conditions associated with the combination of a first-order structural phase transition and frozen liquid media.