Finding high-temperature superconductors by metallizing the σ-bonding electrons

Raising superconducting transition temperature (T_c) is an important task of fundamental research on superconductivity. It is also a prerequisite for the large scale application of superconductors. Since the microscopic mechanism of high-T_c superconductivity is unknown, the conventional approach for increasing Tc is either to apply high pressure to a material which has the potential to become superconducting, or to push it close to an antiferromagnetic or some other quantum instability point by chemical doping. In this article, the authors point out that another general approach for raising T_c is to lift the σ-bonding bands to the Fermi level, or to metalize the σ-bonding elections. This approach can increase the probability of finding a novel high-Tc superconductor because the coupling of σ-bonding electrons with phonons is generally strong and the superconducting transition induced by this interaction can occur at relatively high temperatures.After elucidating the underlying mechanism, the authors discuss a number of schemes to metalize σ-bonding electrons, and present their recent prediction for the crystalline and electronic structures of two potential high-Tc superconductors, Li₂B₃C and Li₃B₄C₂, with T_c higher than 50 K.