  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) and hexagonal (hex-) MB6 (M=Mg, Ca, Sc, Ti, Sr, Y). Then,
we investigate their geometrical structures, bonding properties, electronic
structures, mechanical properties, phonon dispersions, thermal stability,
dynamic stability, 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 700/1000 K, indicating
their excellent thermal stability. All their elastic constants satisfy the Born
mechanically stable criteria and no visible imaginary frequencies are observed
in their phonon dispersions. The EPC results show that these 2D MB6 are all
intrinsic phonon-mediated superconductors with the superconducting transition
temperature (Tc??) in the range of 2.2-21.3 K. Among them, the highest Tc (21.3
K) appears in hex-CaB6, whose EPC constant () is 0.94. 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 21.3 K to 28 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.
