Stability, electronic disruption, and anisotropic superconductivity of hydrogenated trilayer metal tetraborides (MB$_{4}$H; M=Be, Mg, Ca, Al)

TL;DR

This study demonstrates that two-dimensional hydrogenated trilayer borides MB$_4$H (M = Be, Mg, Ca, Al) are dynamically, thermally, and mechanically stable and retain boron-dominated metallicity. Hydrogenation significantly alters band dispersion and Fermi-surface topology, enabling strong electron–phonon coupling as quantified by $\lambda$ values up to $0.99$ and a dominant low-to-mid frequency phonon contribution, predicted via anisotropic Migdal–Eliashberg theory to yield multigap superconductivity. Notably, CaB$_4$H exhibits the strongest coupling and an intrinsic $T_c$ of $T_c = 64$ K, while AlB$_4$H shows $T_c = 22$ K, with BeB$_4$H and MgB$_4$H also supporting two-gap superconductivity; hydrogenation tunes the gap structure and $T_c$ across compositions. These results establish hydrogenated 2D borides as a tunable platform for high-$T_c$ superconductivity at ambient pressure, backed by comprehensive structural, electronic, phononic, and thermodynamic stability analyses.

Abstract

The discovery of superconductivity in MgB$_2$ (\(T_c = 39\) K) \cite{nagamatsu2001superconductivity} established metal diborides (MB$_2$) as a promising class of conventional superconductors. Recent advances in fabrication techniques have enabled the synthesis of 2D MgB$_2$ with a \(T_c\) of 36 K \cite{cheng2018fabrication}, reigniting interest in layered metal borides. This has led to predictions of superconductivity in various 2D metal borides, including MB$_4$ (M = Be, Mg, Ca, Al), with CaB$_4$ exhibiting the highest estimated \(T_c\) of 36.1 K. To explore the impact of hydrogenation on superconductivity, we systematically investigate two-dimensional hydrogenated trilayer metal borides (MB$_4$H; M = Be, Mg, Ca, Al). Our results reveal that these materials retain a metallic nature dominated by boron \(p\)-orbitals, while hydrogenation significantly alters their band dispersion and Fermi surface topology. Phonon calculations confirm their dynamical stability and reveal strong electron-phonon interactions, leading to multi-gap superconductivity. Among the studied compounds, MgB$_4$H, AlB$_4$H, and CaB$_4$H exhibit possible two superconducting gaps, with CaB$_4$H showing the strongest electron-phonon coupling, resulting in an intrinsic superconducting transition temperature of 64 K. In contrast, AlB$_4$H shows the weakest coupling, with \(T_c = 22\) K. The calculated electron-phonon coupling constants (\(λ\)) range from 0.62 to 0.99, demonstrating the tunability of superconducting properties through elemental substitution. These findings provide valuable insights into superconductivity in hydrogenated metal borides and highlight their potential for high-\(T_c\) applications.

Figures (8)

• Figure 1: Figures (a) and (b) show side and top views of the 2D MB$_{4}$H structure; M=Be, Mg, Ca, Al where metals (M), boron (B) and hydrogen (H) atoms are represented by blue, green and pink spheres, respectively.
• Figure 2: (a–d) Time evolution of the total energy fluctuation per atom, $E(t) - E_0$, during a 5 ps *ab initio* Born–Oppenheimer molecular dynamics (BOMD) simulation in the NVT ensemble at 300 K for a $2 \times 2 \times 1$ supercell (24 atoms) of the 2D MB$_{4}$H structure, where M = Be, Mg, Ca, and Al. The insets show top views of the final atomic configurations after 5 ps, demonstrating the dynamic structural stability of the systems.
• Figure 3: Figures show the electronic properties of MB$_{4}$ (left) and MB$_{4}$H compounds (right) near the Fermi level, (M = Be, Mg) illustrating the orbital-resolved electronic band structures, the electronic density of states, the orbital-projected density of states, and the Fermi surface.
• Figure 4: Figures shows electronic properties of MB$_{4}$ and MB$_{4}$H compounds (M = Ca, Al) close to the Fermi level illustrating the orbital-resolved electronic band structures, the electronic density of states, the orbital projected density of states, and the Fermi surface.
• Figure 5: Electron localization function (ELF) of MB$_4$H monolayer. High ELF values (red, ELF $>$ 0.8) between the lower B atoms indicate strong sp$^2$ covalent B--B bonds with delocalized $\pi$ electrons. Intermediate ELF values (0.5--0.7) between the upper B and metal atoms reveal mixed ionic--covalent M--B interactions due to partial electron donation from the metal to boron. The absence of an ELF attractor between B and H atoms suggests predominantly ionic B--H bonding, where H acts as an electron donor.