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

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.

• The paper demonstrates that applying a pressure around 5 kbar induces superconductivity below 12 K within a dome ranging from 2.3 to 8.6 kbar.
• The study uses high-precision resistivity measurements on single crystals to construct a detailed pressure-temperature phase diagram, identifying a first-order structural transition and a high-pressure phase.
• The paper highlights that superconductivity can emerge without chemical doping, offering a purer model to explore intrinsic superconducting mechanisms in FeAs-based compounds.

Analyzing Pressure-Induced Superconductivity in CaFe$_2$As$_2$

The paper "Pressure induced superconductivity in CaFe$_2$As$_2$" investigates the emergence of superconductivity in the stoichiometric compound CaFe$_2$As$_2$ under varying hydrostatic pressures. By methodically manipulating the conditions, this study offers insights that bypass the complexities and disorder introduced by chemical doping, which has traditionally been relied upon in related FeAs-based superconductors.

The crux of the findings lies within the compound’s pressure-temperature ($P-T$) phase diagram, revealing distinct pressure ranges where superconductivity is achieved. Noticeably, at ambient pressure, CaFe$_2$As$_2$ exhibits no superconductivity above 1.8 K and undergoes a robust first-order structural phase transition around 170 K. However, a pressure of approximately 5 kbar is crucial, where it not only suppresses the structural transition but also enables superconductivity below 12 K. This superconductivity forms a dome centered on 5 kbar, extending between pressures as low as 2.3 kbar and as high as 8.6 kbar. Beyond 8.6 kbar, superconductivity subsides, aligning with the onset of another, distinct high-pressure transition characterized by a sharp drop in resistivity. Fascinatingly, this second transition exhibits rapid growth in temperature exceeding 300 K at about 17 kbar.

The experimental setup utilizes single crystals of CaFe$_2$As$_2$, methodically grown and characterized for resistive behavior under pressures up to 20 kbar. Notably, the use of Sn flux for crystal growth and precise resistivity measurements under controlled pressure and temperature conditions yielded high-fidelity data, critical for constructing an accurate $P-T$ diagram.

The paper’s numerical results highlight compelling interdependencies: the initial structural phase transition drops at roughly -12 K/kbar, while the higher-pressure transition increases at +17 K/kbar. Such findings imply that superconductivity manifests when the initial transition is sufficiently suppressed and fluctuations remain that are inherent to the pressure spectrum, yet the subsequent resistivity drop is not fully realized.

This work counters the prevailing notion that doping is essential for inducing superconductivity in iron arsenides, instead suggesting that CaFe$_2$As$_2$ can be a purer model to explore intrinsic superconducting mechanisms without extraneous disorder. The data propose that the superconducting state is sensitive to both the suppression of the initial structural phase and the emergence of the high-pressure phase. The existence of superconductivity amidst these transitions posits a potential quantum critical point scenario, though further investigation is warranted for verification.

Implications of this research extend to understanding the inherent electronic and magnetic interactions in FeAs-based compounds. Future exploration could consider other characterization methods such as NMR or neutron scattering under pressure to better comprehend the structural and electronic origins of the high-pressure transitions. The pressure-dependent behavior in CaFe$_2$As$_2$ thus serves as a promising avenue to unravel the superconducting phenomenon in iron-based superconductors, paving pathways for theoretical and practical advancements in material science and quantum mechanics.