Coexistence of topological node surface and Dirac fermions in phonon-mediated superconductor YB₂C₂

The interaction between nontrivial topology and superconductivity in condensed matter physics has attracted tremendous research interest as it could give rise to exotic phenomena. Herein, based on first-principles calculations, we investigate the electronic structures, mechanical properties, topological properties, dynamic stability, electron-phonon coupling (EPC), and superconducting properties of the synthesized real material YB₂C₂. It is a tetragonal structure with P4/mbm symmetry and exhibits excellent stability. The calculated electronic band structures reveal that a zero-dimension (0D) Dirac point and two-dimensional (2D) nodal surface coexist near the Fermi level. A spin-orbit coupling (SOC) Dirac point with the topological Fermi arc is observed on the (001) surface. These nodal surfaces are protected by a two-fold screw axis and time-reversal symmetry. Based on the Bardeen-Cooper-Schrieffer theory, the superconducting transition temperature (T_c) in the range 1.25-4.45 K with different Coulomb repulsion constant μ* for YB₂C₂ is estimated to be consistent with previous experimental results. In addition, the EPC is mainly from the coupling between the d and d orbitals of the Y atom and low-energy phonon modes. The presence of superconductivity and nontrivial topological surface state in YB₂C₂ suggests that it may be a candidate material for topological superconductors.