Effect of Orbital Differentiation on Magnetism in the 111 iron-based superconductors

Though similar lattice and electronic structures were found in MgFeGe, LiFeAs and NaFeAs, distinct behaviors at low temperature were reported from experiments. While MgFeGe is nonmagnetic and non-superconducting down to $2$~K, LiFeAs is a good superconductor with T$_c=18$~K, which are in sharp contrast to NaFeAs where a magnetically driven structural phase transition above the superconducting transition is detected. By calculating orbitally resolved Pauli susceptibility, we conclude that the occurrence of magnetism and superconductivity in these materials can be well interpreted from the weak coupling limit provided orbital degrees of freedom are considered.The stronger magnetic instability appearing in the d$_{x^2-y^2}$ orbital is responsible for the occurrence of weak magnetism in NaFeAs, compared to the superconducting LiFeAs, while featureless q dependent magnetic instability is responsible for the nonmagnetism in MgFeGe.