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 TiB4 monolayer (TBML) and TiB4 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(z2) 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 lambda of TBML is 0.65, higher than that of the TBBL with lambda = 0.35. Both TBML and TBBL are identified to be phonon-mediated two-dimensional (2D) superconductors with the calculated T-c = 1.66 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 TiB4 monolayer may show more potential for realization of exotic physics.

Automated summary

First-principles calculations were used to study the electronic structures, lattice dynamics, electron-phonon coupling, and superconductivity of TBML and TBBL. Both materials were found to be nodal line semimetals, with varying EPC constants and critical temperatures. Applying strain can enhance the superconductivity of TBBL.