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The phase diagram of a correlated material is the result of a complexinterplay between several degrees of freedom, providing a map of the material'sbehavior. One can understand (and ultimately control) the material's groundstate by associating features and regions of the phase diagram, with specificphysical events or underlying quantum mechanical properties. The phase diagramof the newly discovered iron arsenic high temperature superconductors isparticularly rich and interesting. In the AE(Fe1-xTx)2As2 class (AE being Ca,Sr, Ba, T being transition metals), the simultaneous structural/magnetic phasetransition that occurs at elevated temperature in the undoped material, splitsand is suppressed by carrier doping, the suppression being complete aroundoptimal doping. A dome of superconductivity exists with apparent equal ease inthe orthorhombic / antiferromagnetic (AFM) state as well as in the tetragonalstate with no long range magnetic order. The question then is what determinesthe critical doping at which superconductivity emerges, if the AFM order isfully suppressed only at higher doping values. Here we report evidence fromangle resolved photoemission spectroscopy (ARPES) that critical changes in theFermi surface (FS) occur at the doping level that marks the onset ofsuperconductivity. The presence of the AFM order leads to a reconstruction ofthe electronic structure, most significantly the appearance of the small holepockets at the Fermi level. These hole pockets vanish, i. e. undergo a Lifshitztransition, at the onset of superconductivity. Superconductivity and magnetismare competing states in the iron arsenic superconductors. In the presence ofthe hole pockets superconductivity is fully suppressed, while in their absencethe two states can coexist.

Critical change in the Fermi surface of iron arsenic superconductors at the onset of superconductivity