Prediction of nodal-line fermion and phonon-mediated superconductivity in bilayer α-borophene

Abstract

The electron deficiency of boron allows the formation of a variety of monolayer or few-layer two-dimensional structures (borophenes) with interesting physical properties. Recent experiments have also confirmed that interlayer covalent bonding makes the bilayer structure more stable than the monolayer. In this work based on α-borophene, we propose three free-stranding bilayer structures with dynamic stability. In these three metallic structures, the electronic band crossings around Fermi level form nodal lines. All these structures also exhibit strong electron-phonon couplings. The Bardeen–Cooper–Schrieffer superconducting critical temperature T c of the type-II structure went as high as 28.2 K, which was further improved to 32.0 K by the enhancement effect of Li adatom at the Debye frequency. However, no increase in critical temperature was observed in other Li-doping cases. Specifically, Li intercalation inside the bilayer causes a significant abrupt decrease in the critical temperature of type-I structure. Our results indicated that the bilayer borophene would be an ideal platform for the coexistence of topological electronic states and superconducting states.