Complete pressure-dependent phase diagrams for SrFe₂As₂ and BaFe₂As₂

The temperature-dependent electrical resistivity of single-crystalline SrFe₂As₂ and BaFe₂As₂ has been measured in a liquid-medium modified Bridgman anvil cell for pressures in excess of 75 kbar. These data allow for the determination of the pressure dependence of the higher-temperature structural/antiferromagnetic phase transitions as well as the lower-temperature superconducting phase transition. For both compounds the ambient-pressure higher-temperature structural/antiferromagnetic phase transition can be fully suppressed with a domelike region of zero resistivity found to be centered about its critical pressure. Indeed, qualitatively, the temperature dependence of the resistivity curves closest to the critical pressures is the closest to linear, suggesting the possibility of quantum criticality. For pressures significantly higher than the critical pressure the zero-resistivity state is suppressed and the low-temperature resistivity curves asymptotically approach a universal low-temperature manifold. These results are consistent with the hypothesis that correlations/fluctuations associated with the ambient-pressure, high-temperature, tetragonal phase have to be brought to low enough temperature to allow superconductivity, but if too fully suppressed it can lead to the loss of the superconducting state.