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

Combining crystal structure search and first-principles calculations, we\nreport a series of two-dimensional (2D) metal borides including orthorhombic\n(ort-) MB6 (M=Mg, Ca) and hexagonal (hex-) MB6 (M=Mg, Ca, Sc, Ti, Sr, Y). Then,\nwe investigate their geometrical structures, bonding properties, electronic\nstructures, mechanical properties, phonon dispersions, thermal stability,\ndynamic stability, electron-phonon coupling (EPC), superconducting properties\nand so on. Our ab initio molecular dynamics simulation results show that these\nMB6 can maintain their original configurations up to 700/1000 K, indicating\ntheir excellent thermal stability. All their elastic constants satisfy the Born\nmechanically stable criteria and no visible imaginary frequencies are observed\nin their phonon dispersions. The EPC results show that these 2D MB6 are all\nintrinsic phonon-mediated superconductors with the superconducting transition\ntemperature (Tc??) in the range of 2.2-21.3 K. Among them, the highest Tc (21.3\nK) appears in hex-CaB6, whose EPC constant () is 0.94. By applying\ntensile/compressive strains on ort-/hex-CaB6, we find that the compressive\nstrain can obviously soften the acoustic phonon branch and enhance the EPC as\nwell as Tc. The Tc of the hex-CaB6 can be increased from 21.3 K to 28 K under\ncompressive strain of 3%. These findings enrich the database of 2D\nsuperconductors and should stimulate experimental synthesizing and\ncharacterizing of 2D superconducting metal borides.\n