First-principles study of the magnetic and electronic properties of K-coated FeSe films

Recent experiments have reported that depositing K atoms onto the surfaces of nonsuperconducting FeSe thin films can not only transform them into superconductors but also obtain a domelike relation between the superconducting transition temperature (${T}_{\text{c}}$) and the K coverage (${K}_{\text{c}}$). To elucidate the mechanism underlying the domelike dependence of ${T}_{\text{c}}$ on ${K}_{\text{c}}$ in K-coated FeSe (K/FeSe) films, we have studied the electronic and magnetic properties of the K/FeSe films at ${K}_{\text{c}}=0.125$, 0.25, and 0.5 ML [1 monolayer (ML) is defined as the areal density of the topmost Se site] based on the first-principles electronic structure calculations. Our calculations show that there are two antiferromagnetic (AFM) states being energetically quite close to each other regardless of the value of ${K}_{\text{c}}$, indicating that there may exist strong magnetic fluctuations in K/FeSe. On the other hand, it is worth noting that the differential charge and spin densities between the two AFM states at three ${K}_{\text{c}}$'s demonstrate that both the charge density redistributions around Se atoms and the spin density redistributions around Fe atoms have the domelike evolution with ${K}_{\text{c}}$, which coincide with the observed behavior of superconducting ${T}_{\text{c}}$. Furthermore, we find that the densities of states at the Fermi level $[N({E}_{\text{F}})]$ in these two AFM states also have a domelike dependence on ${K}_{\text{c}}$. Apart from helping us understand the superconducting mechanism in the K/FeSe system, these results also provide a different perspective for exploring the superconductivities in other FeSe-based superconductors.