Assessing the possible superconductivity in the doped perovskite hydride <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>KMgH</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math>: Effects of lattice anharmonicity and spin fluct

The superconducting properties of uniformly hole-doped perovskite hydride ${\mathrm{KMgH}}_{3}$ with varying doping concentration and lattice parameter corresponding to different pressures were investigated from first principles. The superconducting transition temperature (${T}_{\mathrm{c}}$) was predicted from the density functional theory for superconductors (SCDFT), where the effects of lattice anharmonicity and spin-fluctuation were considered and examined. Although lattice anharmonicity tends to suppress superconductivity around the edge of dynamical stability, where the phase is stabilized due to anharmonic effects, ${T}_{\mathrm{c}}$ is enhanced. In the hole-doped ${\mathrm{KMgH}}_{3}$, substantial spin-fluctuation (SF) effects were discovered, which counter the phonon-mediated pairing and decrease ${T}_{\mathrm{c}}$. Such anomalously strong SF is evaluated for similar hydrides, where the hydrogen $1s$ bands are isolated at the Fermi level, and its correlation with the electronics density of states was explored.