Uniaxial Pressure Measurements on Single-Crystal CaFe$_2$As$_2$

Measurements of single crystal CaFe$_2$As$_2$ have uncovered a variety of phases spanning a large range of temperatures, which makes them an interesting and important material for sorting out competing influences on strongly correlated phases. Uniaxial pressure measurements give insight into the role of dimensionality and hybridization. We apply uniaxial c-axis pressure to a single-crystal sample of CaFe$_2$As$_2$, mounted between stainless steel pressure spacers within an ac susceptibility coil. This setup is located at the base of an Oxford dilution refrigerator modified specifically for uniaxial pressure measurements. We find that a very small applied pressure, less than 0.1 kbar, induces a superconducting phase near 7K, with a transition temperature that varies little as pressure is increased further. We also find a second, strongly pressure-dependent transition near 3K. This transition temperature reaches a maximum at a pressure slightly over 1 kbar. These measurements demonstrate the sensitivity of CaFe$_2$As$_2$ to uniaxial pressure, as well as mapping out parts of its unusual phase diagram.