Biaxial strain engineering on the superconducting properties of MgB2 monolayer

Abstract

The effect of biaxial strain on the superconducting properties of MgB2 monolayer was studied by first-principles calculations. The stability analyses by phonon dispersions show that a biaxial strain of as much as 7% can be applied onto MgB2 without inducing any imaginary frequency. The superconducting property calculations based on the frame of Migdal-Eliashberg theory successfully reproduce the two-gap superconductivity of MgB2. The results show that the tensile biaxial strain can increase the critical temperature of MgB2 by as much as ∼20% while the compressive biaxial strain would decrease the critical temperature by ∼29%. The detailed microscopic mechanism of the biaxial strain effect on the superconducting properties was studied by calculations of electronic structures and phonon dispersions. The increased Tc is a combining result of the increased electron density at the Fermi level and the in-plane boron phonon softening. By means of high-throughput screening of proper substrates, it is found that most of the substrates would result in tensile strain in MgB2 film, some of which can result in tensile strain of higher than 10%, consistent with many experimental works that the MgB2 thin film has increased Tc. The results in this work provide a detailed understanding of the biaxial strain engineering mechanism and demonstrate that biaxial strain engineering can be an effective way of tuning the superconducting properties of MgB2 and other similar materials.