Li2C2中电声耦合及超导电性的第一性原理计算研究

通过第一性原理密度泛函和超导Eliashberg理论计算, 我们研究了Li₂C₂在Cmcm相的电子结构和电声耦合特性, 预言这种材料在常压和5GPa下是由电声耦合导致的转变温度分别为13.2 K 和9.8 K的超导体, 为实验上探索包含一维碳原子链的材料中是否可能存在超导电性、发现新的超导体提供了理论依据. 如果理论所预言的Li₂C₂超导电性得到实验的证实, 这将是锂碳化物中转变温度最高的超导体, 高于实验观测到的LiC₂的1.9 K和理论预言的单层LiC₆的8.1 K超导转变温度.

First-principles study of electron-phonon coupling and superconductivity in compound Li2C2

One-dimensional carbon chains are expected to show outstanding optical and mechanical properties. But synthesis of the compounds containing one-dimensional carbon chains is a challenging work, because of the difficulty in saturating the dangling bonds of carbon atoms. Recently, the transition from the Immm phase to the Cmcm one at a transition pressure 5 GPa has been predicted for Li₂C₂ by density-functional theory calculations. In Cmcm-Li₂C₂, there are one-dimensional zigzag carbon chains caged by lithium atoms. Under ambient pressure, the electronic structure of Cmcm-Li₂C₂ is as follows: The hybridization among 2s, 2p_y, and 2p_z orbitals of carbon atoms results in three sp²-hybridized orbitals that are coplanar with the zigzag chains of these carbon atoms, denoted as the y-z plane. The sp²-hybridized orbitals along y-axis (perpendicular to the zigzag chain) overlap with each other and form one πup-bonding band and one πup ^*-antibonding band. Likewise, the 2p_x orbitals of carbon atoms will provide also one πup-bonding band and one π^*-antibonding band. These two π^*-antibonding bands cross the Fermi level and contribute to the metallicity of Cmcm-Li₂C₂. The other two sp²-hybridized orbitals will give two σ-bonding bands, whose band tops are about 5 eV below the Fermi energy level. These two fully occupied σ bands are the framework of the zigzag carbon chains. The changes in electronic structure of Cmcm-Li₂C₂ under 5 GPa are negligible, compared with that in case of ambient pressure. To our best knowledge, there is no report upon the superconductivity for compounds containing one dimensional carbon chains. We choose Cmcm-Li₂C₂ as a model system to investigate its electron-phonon coupling and phonon-mediated superconductivity. To determine the phonon-mediated superconductivity, the electron-phonon coupling constant λ and logarithmic average frequency ω_log are calculated based on density functional perturbation theory and Eliashberg equations. We find that λ and ω_log are equal to 0.63 and 53.8 meV respectively at ambient pressure for Cmcm-Li₂C₂. In comparison, both the phonon density of states and the Eliashberg spectral function α²F(ω) are slightly blue-shifted at a pressure of 5 GPa. Correspondingly, λ and ω_log are calculated to be 0.56 and 58.2 meV at 5 GPa. Utilizing McMillian-Allen-Dynes formula, we find that the superconducting transition temperatures (T_c) for Cmcm-Li₂C₂ are 13.2 K and 9.8 K, respectively, at ambient pressure and 5 GPa. We also find that two phonon modes B_1g and A_g at Γ point have strong coupling with π^* electrons. Among lithium carbide compounds, the superconductivity is only observed in LiC₂ below 1.9 K. Besides LiC₂, theoretical calculations also predicted superconductivity in mono-layer LiC₆, with T_c being 8.1 K. So if the superconductivity of Cmcm-Li₂C₂ is confirmed by experiment, it will be the first superconducting compound containing one dimensional carbon chains and its T_c will be the highest one among lithium carbide compounds. Thus experimental research to explore the possible superconductivity in Cmcm-Li₂C₂ is called for.