Strain-tunable Dirac semimetal phase transition and emergent superconductivity in a borophane

A two-dimensional (2D) Dirac semimetal with concomitant superconductivity has been long sought but rarely reported. It is believed that light-element materials have the potential to realize this goal owing to their intrinsic lightweight and metallicity. Here, based on the recently synthesized $\beta_{12}$ hydrogenated borophene [Science 371, 1143 (2021)], we investigate its counterpart named $\beta_{12}$-$\rm {B_5H_3}$. Our first-principles calculations suggest it has good stability. $\beta_{12}$-$\rm {B_5H_3}$ is a scarce Dirac semimetal demonstrating a strain-tunable phase transition from three Dirac cones to a single Dirac cone. Additionally, $\beta_{12}$-$\rm {B_5H_3}$ is also a superior phonon-mediated superconductor with a superconducting critical temperature of 32.4 K and can be further boosted to 42 K under external strain. The concurrence of Dirac fermions and superconductivity, supplemented with dual tunabilities, reveals $\beta_{12}$-$\rm {B_5H_3}$ is an attractive platform to study either quantum phase transition in 2D Dirac semimetal or the superconductivity or the exotic physics brought about by their interplay.