Controlling T_{c} through Band Structure and Correlation Engineering in Collapsed and Uncollapsed Phases of Iron Arsenides.

Recent observations of selective emergence (suppression) of superconductivity in the uncollapsed (collapsed) tetragonal phase of LaFe_{2}As_{2} has rekindled interest in understanding what features of the band structure control the superconducting T_{c}. We show that the proximity of the narrow Fe-d_{xy} state to the Fermi energy emerges as the primary factor. In the uncollapsed phase this state is at the Fermi energy, and is most strongly correlated and a source of enhanced scattering in both single and two particle channels. The resulting intense and broad low energy spin fluctuations suppress magnetic ordering and simultaneously provide glue for Cooper pair formation. In the collapsed tetragonal phase, the d_{xy} state is driven far below the Fermi energy, which suppresses the low-energy scattering and blocks superconductivity. A similar source of broad spin excitation appears in uncollapsed and collapsed phases of CaFe_{2}As_{2}. This suggests controlling coherence provides a way to engineer T_{c} in unconventional superconductors primarily mediated through spin fluctuations.