Spin fluctuation-mediated unconventional superconductivity in ThFeAsN from first-principles

Superconducting (SC) pairing mechanism, origin of high $T_c$ and symmetry of SC order parameter in Fe-based superconductors are among the important unsolved problems in condensed matter and materials physics. We study the SC properties of ThFeAsN, a Fe-based high $T_c$ superconductor, by {\it ab initio} superconducting density functional theory calculations with electron-phonon coupling, screened static and dynamic electron-electron Coulomb repulsion and spin fluctuation (SF) mediated pair-interaction fully taken into account. Our calculations reveal that ThFeAsN is a SF-mediated multiband superconductor with the calculated $T_c$ of 22.4 K and the $d_{xy}$-wave SC order parameter with different signs on different Fermi surface sheets, in consistent with experiments. We also present distinct SC properties such as quasiparticle density of states and ultrasonic attenuation coefficient which can be immediately verified by experiments.