Strain dependent electronic and superconducting properties of MFeAs (M = Li/Na) thin films

Abstract

Using first-principles calculations based the density functional theory, we investigate the electronic structures and superconducting properties of iron-based superconducting films MFeAs (M = Li/Na) under the different strains. The calculations show that when the 1–5% compressive and tensile strains are applied, although the striped antiferromagnetic ground state of the MFeAs films remain unvaried, the Fermi surface nesting between electron-hole pockets, the electronic density of state at Fermi energy and magnetic moments of Fe ions all undergo the significant variations. The applications of compressive strains improve the Fermi surface nesting, decrease the localized magnetic moments of Fe ions, and increase the electronic density of states at the Fermi level of films. The variations suggest that the compressive strains favour the superconductivity by facilitating the spin fluctuations in films. However, the energy bands and electronic properties of MFeAs films under tensile strains show the opposite variations, and tend to suppress the superconductivity of films. Our results provide a simple solution and a comprehensive explanation for the improved superconductivity of MFeAs films by compressive strains.