Emerging two-dimensional superconductors TiB3C and Ti2B3C2 with monolayer kagome borophene

Abstract

Probing emerging two-dimensional (2D) superconductors, including 2D metal borocarbides, has recently garnered significant research interest. However, 2D superconducting Ti-B-C monolayers with unique boron motifs have yet to be discovered. In this study, we conducted a comprehensive investigation of the 2D Ti-B-C system through a first-principles structure search method and identified two stable metallic monolayers, TiB₃C and Ti₂B₃C₂, featuring unanticipated kagome boron layers. Electron–phonon coupling (EPC) simulations reveal that both monolayers exhibit phonon-mediated superconductivity, with superconducting transition temperatures (T_c) of 3.2 and 6.5 K, respectively. Furthermore, biaxial compressive strain enhances superconductivity in TiB₃C, increasing its T_c to 11.3 K under −2 % strain, while biaxial tensile train elevates the T_c of Ti₂B₃C₂ to 15.5 K under 7 % strain. This improvement in superconductivity is primarily attributed to the increase in EPC strength. Additionally, we designed a novel TiB₃CH₂ monolayer with metallic characteristics using a hydrogenation strategy. This monolayer exhibits a significantly higher T_c of 18.7 K, chiefly due to strong coupling between Ti 3d electron states near the Fermi level and mid-frequency B-, C-, and H-associated phonons. Our findings provide valuable insights into 2D boron–carbon-based superconductors containing metals and pave the way for designing new 2D superconducting materials.