Evidence for a Lifshitz transition in electron-doped iron arsenic superconductors at the onset of superconductivity

The iron arsenic high-temperature superconductors(1,2) exhibit particularly rich phase diagrams. In the AE(Fe1-xTx)(2)As-2 family (known as '122', with AE being Ca, Sr or Ba and T being a transition metal), the simultaneous structural/magnetic phase transition that occurs at elevated temperature in the undoped material splits and is suppressed by carrier doping(3,4). A superconducting region appears as likely in the orthorhombic/antiferromagnetic (AFM) state as in the tetragonal/paramagnetic state(3,5,6). An important question then is what determines the critical doping at which superconductivity emerges, as the AFM order is fully suppressed only close to optimal doping. Here we report evidence from angle-resolved photoemission spectroscopy that marked changes in the Fermi surface coincide with the onset of superconductivity in electron-doped Ba(Fe1-xCox)(2)As-2. The presence of the AFM order leads to a reconstruction of the electronic structure, most significantly the appearance of the petal-like hole pockets at the Fermi level. These hole pockets vanish-that is, undergo a Lifshitz transition(7)-as the cobalt concentration is increased sufficiently to support superconductivity. Superconductivity and magnetism are competing states in this system: when petal-like hole pockets are present, superconductivity is fully suppressed, whereas in their absence the two states can coexist.