Abstract

The response of the superconducting state and crystal structure of LiFeAs and NaFeAs to chemical substitutions on both the Li and the Fe sites has been probed using a combination of high-resolution X-ray and neutron diraction measurements, magnetometry, and muon-spin rotation spectroscopy. The superconductivity is extremely sensitive to composition: Li-deficient materials (Li1-yFe1+yAs with Fe substituting for Li) show a very rapid suppression of the superconducting state, while substitution of Fe by small amounts of Co or Ni results in monotonic lowering of the superconducting transition temperature Tc and the superfluid stiffness as the electron count increases. A similar effect is found in NaFeAs and it appears that electron count is the dominant factor, since Ni doping has double the effect of Co doping for the same doping level. The addition of 0.1 electrons per Fe atom is sufficient to traverse the superconducting domain, and that magnetic order coexists with superconductivity at doping levels less than 0.025 electrons per Fe atom. That the superfluid stiffness in the LiFeAs-derived compounds is higher than in all of the iron pnictide materials underlines the unique position that LiFeAs occupies in this class. We bring together the results of structural and muon measurements, as well as insights from the band structure, to understand the phase diagrams of these 111 pnictide superconductors.

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