Topological nodal line and superconductivity of highly thermally stable two-dimensional TiB4

By means of first-principles calculations, we study the electronic structures, lattice dynamics, electron-phonon coupling (EPC), and superconductivity of ${\mathrm{TiB}}_{4}$ monolayer (TBML) and ${\mathrm{TiB}}_{4}$ bilayer (TBBL). We find that both TBML and TBBL are nodal line semimetals, and the occurrences of their nodal lines are mainly due to the band inversions between B-${p}_{x}+{p}_{y}$ and B-${p}_{z}$ for TBML and between Ti-${d}_{xz}+{d}_{yz}$ and Ti-${d}_{{z}^{2}}$ for TBBL. The distortion of Ti atoms in the TBBL induces a horizontal glide mirror plane, which protects its nodal line against the spin-orbit coupling. The computed EPC constant $\ensuremath{\lambda}$ of TBML is 0.65, higher than that of the TBBL with $\ensuremath{\lambda}=0.35$. Both TBML and TBBL are identified to be phonon-mediated two-dimensional (2D) superconductors with the calculated ${T}_{c}=1.66\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and 0.82 K, respectively. The ${T}_{c}$ of the TBBL can be further enhanced to 6.43 K by applying a tensile strain of 11%. Moreover, they exhibit excellent thermal stability. The coexistence of the topological nodal-line states around the Fermi level and superconductivity in the square-lattice ${\mathrm{TiB}}_{4}$ monolayer may show more potential for realization of exotic physics.