Magnetic phase transitions in Sr-defective monolayer FeSe/SrTiO3 interfaces: A first-principles study

The significantly enhanced superconducting transition temperature (Tc) observed at the FeSe/SrTiO3 (FeSe/STO) interface has garnered substantial attention over the past decade. Numerous experiments have demonstrated that the complex TiOx-terminated STO interface structure plays a crucial role in mediating superconductivity. In this work, using first-principles calculations, we find that the magnetic ground state of the interface structures exhibits two distinct evolutionary trends: it shifts from paramagnetic (PM) to an (anti-)ferromagnetic state in the presence of Sr defects or charge doping, and transitions to a ferromagnetic state when the substrate thickness is less than five atomic layers. Calculated spin charge density differences reveal that the polarized spins are primarily distributed around the interface region. As the thickness of the STO substrate increases, electrons transfer from the interface to the substrate—this leads to a reduction in the polarized spin density near the interface and ultimately drives a transition to the PM phase. Our results reveal that the magnetic moments of Fe atoms in FeSe/STO with a TiOx interface structure increase with increasing atomic layer thickness, whereas those in FeSe/STO without a TiOx interface structure exhibit the opposite trend (i.e., decreasing). In addition, compared with the previously reported band structure, the additional Ti-derived bands crossing the Fermi energy are absent. More importantly, we can observe the replica band near the M point, which is in agreement with previously reported results. Our results indicate that the magnetic properties of the FeSe/STO interface are critical to achieving high-temperature superconductivity.