First Principles Calculations of Superconducting Transition Temperature of ThCr_2Si_2-type Structure

摘要

High superconducting transition temperature (T_c) superconductors have a great potential in many industrial applications. However, discovering a compound having high T_c is still remaining a big challenge for experimental approach due to time-consuming and high cost. In this talk, we focus on ThCr_2Si_2-type compounds with a I4/mmm symmetry (Fig.1(a),(b)) and predict their T_c values by combining first-principles phonon simulations with models based on the Bardeen-Cooper-Schrieffer (BCS) theory [1]. Although thousand compounds belong to this structure, no comprehensive and systematic study has been done so far. Referring to the NIMS database [2], our T_c results showed a fairly good agreement with the experimental data (Fig.1(c)). Finally, our exploration within the ThCr_2Si_2 family revealed that ThCu_2Si_2 and ThAu_2Si_2 could be newly possible conventional superconductors having T_c around 3.88 K and 4.27 K, respectively [3]. Here our first principles phonon simulations confirmed that both of compounds have non-negative frequencies indicating the dynamical stability of the compounds at ambient temperature. In addition, we found that Fe or Co based compounds could not be explained just by electron-phonon interaction. This discrepancy between theory and experiment implies that those compounds are unconventional superconductors.