Pressure induced superconductivity in CaFe$_2$As$_2$

TL;DR

This study demonstrates that applying hydrostatic pressure to CaFe$_2$As$_2$ induces superconductivity within a narrow pressure range by suppressing structural transitions, revealing key features of FeAs-based superconductors without disorder.

Contribution

It provides a detailed pressure-temperature phase diagram of CaFe$_2$As$_2$, showing pressure-induced superconductivity and phase transitions without introducing disorder.

Findings

Superconductivity appears at pressures around 5 kbar with T < 12 K.

A high-pressure transition exceeds 300 K at about 17 kbar.

Superconductivity exists within a specific pressure window between 2.3 and 8.6 kbar.

Abstract

CaFe$_2$As$_2$ has been found to be exceptionally sensitive to the application of hydrostatic pressure and superconductivity has been found to exist in a narrow pressure region that appears to be at the interface between two different phase transitions. The pressure - temperature ($P - T$) phase diagram of CaFe$_2$As$_2$ reveals that this stoichiometric, highly ordered, compound can be easily tuned to reveal all the salient features associated with FeAs-based superconductivity without introducing any disorder. Whereas at ambient pressure CaFe$_2$As$_2$ does not superconduct for $T > 1.8$ K and manifests a first order structural phase transition near $T \approx 170$ K, the application of $\sim 5$ kbar hydrostatic pressure fully suppresses the resistive signature of the structural phase transition and instead superconductivity is detected for $T < 12$ K. For $P \ge 5.5$ kbar a different transition is detected, one associated with a clear reduction in resistivity and for $P > 8.6$ kbar superconductivity is no longer detected. This higher pressure transition temperature increases rapidly with increasing pressure, exceeding 300 K by $P \sim 17$ kbar. The low temperature, superconducting dome is centered around 5 kbar, extending down to 2.3 kbar and up to 8.6 kbar. This superconducting phase appears to exist when the low pressure transition is suppressed sufficiently, but before the high pressure transition has reduced the resistivity, and possibly the associated fluctuations, too dramatically.