Prediction of superconductivity in sandwich XB₄ (X = Li, Be, Zn and Ga) films

Borophenes and 2D boron allotropes are metallic and exhibit a BCS superconducting state, unlike graphene. In-plane stretching vibrational modes in bulk MgB₂ boron layers induce phonon-mediated superconductivity. However, the effect of stretching vibrational phonon modes on transition temperature (T_c) still requires further investigations. Here, we use first-principles calculations combined with conventional BCS theory to explore the superconducting properties in a series of dynamically stable boron-based sandwich films that have not been realized experimentally. The sandwich films of XB₄ (where X = Li, Be, Zn, Ga) are predicted to exhibit good phonon-mediated superconductivity with high T_c values of 25.1 K, 28.7 K, 38.7 K, and 36.2 K, respectively. The origin of the superconducting states is mainly caused by the high metallicity and strong electron-phonon coupling (EPC), which can be attributed to the presence of intercalated atoms within the borophene layers. It is further demonstrated in the XB₄ compounds (where X = Li, Be, Zn, Ga) that the pronounced EPC is not solely attributable to the in-plane vibrations of B atoms, but it is also influenced significantly by the out-of-plane vibrations of B atoms. Sandwich (Li,Be,Zn,Ga)B₄ films may be a great choice for nanoscale superconductors as the electron-phonon coupling parameter becomes greater than unity, thereby providing a powerful approach for investigating these systems with high critical temperatures.