Self-Doping Effect in FeSe Superconductor by Pressure-Induced Charge Transfer

Several unambiguous experimental observations clearly showed that the critical temperature of FeSe superconductor depends significantly on the microstructure. Experiments also showed that the critical temperature can be greatly enhanced by the application of external pressure. The present paper deals with the effect of pressure on charge transfer and self-doping properties of the superconducting compound FeSe, based on the investigation of the pressure dependence of the Fermi surface by means of first-principles methods. From the numerically evaluated electronic and crystalline properties, the pressure-induced modifications of the FeSe Fermi surface's topology are determined. The Luttinger theorem was also used to evaluate the carrier concentration on the Fe atomic sites from the evolution of the Fermi surface. We have found that the electronic density at the Fe sites increases with the increase in external pressure, following the distortion of Fermi surface. Our simulations reveal that the pressure-induced charge transfer from the Se atoms to the Fe atoms in the FeSe superconductor can be interpreted as a direct correlation between the electron carrier concentration and the applied pressure. The pressure dependence of superconducting properties of FeSe can be reasonably ascribed to a self-doping effect. The predictions about the electronic density on the Fe sites (reaching a value of 0.1, where the corresponding pressure is about 8.6 GPa) are in good agreement with experimental data available in the literature. The interpretation of the pressure-induced T_c enhancement in FeSe caused by the electron transfer and self-doping effect is carried out, which can be the correct approach to explain the T_c behavior under a wide type of mechanical loading.