Electron-phonon coupling superconductivity in two-dimensional orthorhombic MB6 (M=Mg,Ca,Ti,Y) and hexagonal MB6 (M=Mg,Ca,Sc,Ti)

Abstract

Combining crystal structure search and first-principles calculations, we report a series of two-dimensional (2D) metal borides including orthorhombic (ort-)MB6 (M=Mg,Ca,Ti,Y) and hexagonal (hex-)MB6 (M=Mg,Ca,Sc,Ti). Then, we investigate their geometrical structures, bonding properties, electronic structures, mechanical properties, phonon dispersions, thermal stability, dynamic stability, charge density wave (CDW) phase transition, electron-phonon coupling (EPC), superconducting properties, and so on. Our ab initio molecular dynamics simulation results show that these MB6 can maintain their original configurations up to about 1000 or 700 K (only for hex-MgB6), indicating their excellent thermal stability. All their elastic constants satisfy the Born mechanically stable criteria and no imaginary frequencies are observed in their phonon dispersions. Interestingly, there may exist a CDW phase transition for ort-TiB6 from type-I to type-II 2×1 supercell structure and for ort-YB6 from type-I to type-III 2×1 supercell structure. Besides, these 2D MB6 are all predicted to be intrinsic phonon-mediated superconductors. By analytically solving the McMillan-Allen-Dynes formula derived from the microscopic theory of Bardeen, Cooper, and Schrieffer, we obtain the superconducting transition temperature (Tc) for these materials, which are in the range of 1.4–22.6 K. Among our studied MB6, the highest Tc (22.6 K) appears in hex-CaB6, whose EPC constant (λ) is 0.87. By applying tensile/compressive strains on ort-/hex-CaB6, we find that the compressive strain can obviously soften the acoustic-phonon branch and enhance the EPC as well as Tc. The Tc of the hex-CaB6 can be increased from 22.6 to 28.4 K under compressive strain of 3%. These findings enrich the database of 2D superconductors and should stimulate experimental synthesizing and characterizing of 2D superconducting metal borides.

Source-reported materials — not catalogue approval

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