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

Abstract

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_x²−y² and d_z² 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.