Evolution of superconductivity in Ca$_{\mathrm{1-x}}$La$_{\mathrm{x}}$Fe$_2$As$_2$ under pressure*

The evolution of superconductivity in single crystals of the aliovalent La-doped CaFe$_2$As$_2$ is studied with both quasi-hydrostatic and hydrostatic applied pressures measuring transport, magnetic, and neutron scattering properties. The application of pressure to under doped samples of Ca$_{\mathrm{1-x}}$La$_{\mathrm{x}}$Fe$_2$As$_2$ suppresses the antiferromagnetic (AFM) transition and causes an abrupt appearance of superconductivity with $T_{\mathrm{c}}$ values similar to those (about 45 K) recently been reported at ambient pressure. This superconducting phase appears under both quasi-hydrostatic and hydrostatic pressures, indicating an intrinsic property of the observed superconducting state. Unlike transition metal-doped 122 iron-superconductors where superconductivity happily coexists with AFM, the little coexistence of SC and AFM appears to mimic that found in 1111 iron-superconductors, suggesting a similar phase diagram. The unusual dichotomy between lower-$T_{\mathrm{c}}$ systems that happily coexist with AFM and tendency for the highest-$ T_{\mathrm{c}}$ systems to show phase separation provides an important clue to the pairing mechanism in iron-based superconductors.