First-principles Study Of Possible High-temperature Superconductivity In Layered Insulators Through Hole Doping

Based on density functional first-principles calculations and Wannier interpolation technique,we have systematically investigated the electron-phonon coupling(EPC)in hole-doped monolayer hexagonal boron nitride(h-BN)and layered compound CaB2C₂.The crystal structure of monolayer h-BN is similar to graphene.Bulk h-BN is an insulator,with a direct energy band gap about 6 e V.We achieve hole doping by adjusting the number of electrons in monolayer h-BN.To align the Fermi level to a van Hove singularity,0.4 electrons must be removed from the system.It is the largest hole concentration that can be realized in monolayer h-BN.Beyond this concentration,imaginary phonon modes appear.Under this doping level,the EPC for free-standing monolayer h-BN is quite weak.However,biaxial tensile strain can enhance the EPC significantly.We predict that monolayer h-BN can become a phonon-mediated superconductor by the synergic effects of hole doping and biaxial tensile strain,with the largest transition temperature(Tc)exceeding 40 K.CaB2C₂ is a layered insulator with an energy gap about 0.8 e V.We obtain KCa B₄C₄ and Rb Ca B₄C₄ by substituting half of the Ca atoms with K or Rb atoms to realizing hole doping in CaB2C₂,respectively.By our calculations,KCa B₄C₄ and RbCaB₄C₄ can become phonon-mediated high-temperatures superconductors with Tc being 58.0 K and 56.4 K.The underlying physics is that hole doping by K or Rb substitution lifts the?-band to the Fermi level,resulting in strong coupling between?-band electrons and A_g phonon mode.