First of the and electronic 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 (Tc) and the K coverage (Kc). To elucidate the mechanism underlying the domelike dependence of Tc on Kc in K -coated FeSe (K/FeSe) films, we have studied the electronic and magnetic properties of the K/FeSe films at Kc = 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 Kc, 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 Kc's demonstrate that both the charge density redistributions around Se atoms and the spin density redistributions around Fe atoms have the domelike evolution with Kc, which coincide with the observed behavior of superconducting Tc. Furthermore, we find that the densities of states at the Fermi level [N(EF)] in these two AFM states also have a domelike dependence on Kc. 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.