$σ$ bands driven high-temperature superconductivity in hydrogenated hexagonal BC$_3$ monolayer

Abstract

Material with metallic $σ$-bonding bands is expected to be a high-temperature superconductor, due to the sensitivity of $σ$ electrons to lattice vibration. Based on the first-principles calculations, electronic structures of hydrogenated BC$_3$ monolayers (H$_n$-B$_2$C$_6$ with $n$=1-8) are systematically investigated. At high coverage of hydrogen, the monolayer stabilizes in chair-like $sp^3$-hybridized configurations, leading to the metallization of $σ$ bands, especially in H$_7$-B$_2$C$_6$ and H$_8$-B$_2$C$_6$. This metallicity originates from the electron deficiency of boron, compared with insulating graphane. Utilizing Wannier interpolation, the electron-phonon coupling strengths for metallic phases of H$_n$-B$_2$C$_6$ are determined. As expected, strong couplings are identified between the conducting $σ$ electrons and low-frequency phonon modes. By solving the anisotropic Eliashberg equations, we confirm that H$_7$-B$_2$C$_6$ and H$_8$-B$_2$C$_6$ are single-gap superconductors with critical temperature being 87 K, exceeding the boiling point of liquid nitrogen. Considering that monolayer BC$_3$ has been synthesized in experiment, our results demonstrate that hydrogenation of two-dimensional BC$_3$ provides a viable pathway to achieve high-temperature superconductivity at ambient pressure.

Figures (6)

• Figure 1: Crystal structures of H$_7$-B$_2$C$_6$ (a) and H$_8$-B$_2$C$_6$ (b). In the top views, solid red and dashed blue circles denote hydrogen atoms adsorbed above and below the plane, respectively. Boron, carbon, and hydrogen atoms are shown in green, brown, and pink, respectively. The black lines denote the unit cell. Inequivalent boron atoms in H$_7$-B$_2$C$_6$ are labelled as B$'$ and B$"$.
• Figure 2: Electronic structures of H$_7$-B$_2$C$_6$ and H$_8$-B$_2$C$_6$. (a) and (d) $\sigma$ orbital-resolved band structures, with red, green, blue, and purple colors representing the weights of B-C, C-C, B-H, and C-H $\sigma$ orbitals. Line width is proportional to the orbital weight. The Fermi level is set to zero. Fermi surfaces, weighted by the Fermi velocity are shown as insets. (b) and (e) $\sigma$-orbitals projected DOS. (c) and (f) Partial DOS with atomic resolution.
• Figure 3: Phonon spectra of H$_7$-B$_2$C$_6$ (a) and H$_8$-B$_2$C$_6$ (c). The color scale represents the magnitude of $\lambda_{\mathbf{q}\nu}$ at each phonon wavevector and mode. (b) and (d) Strongly coupled vibrational modes highlighted by black arrows in (a) and (c), with blue arrows indicating the direction and relative amplitude of atomic displacements.
• Figure 4: (a) and (b) Eliashberg function $\alpha^2F(\omega)$ and accumulated $\lambda(\omega)$ of H$_7$-BC$_3$ and H$_8$-BC$_3$, respectively. Black arrows indicate the contribution of phonons below the pseudo gap to $\lambda$. (c) and (d) Projected phonon DOS $F(\omega)$.
• Figure 5: Temperature dependence of the gap values $\Delta_{n\mathbf{k}}$ on the Fermi surfaces at different temperatures for H$_7$-B$_2$C$_6$ (a) and H$_8$-B$_2$C$_6$ (c). Insets show the distribution of superconducting gap $\Delta_{n\mathbf{k}}$ on the Fermi surfaces at 20 K. (b) and (d) Normalized quasiparticle DOS in the superconducting state at at 20 K and 80 K, respectively.