Superconductivity and a van Hove singularity confined to the surface of a topological semimetal

The interplay between topology and superconductivity generated great interest in condensed matter physics. Here, we unveil an unconventional two-dimensional superconducting state in the Dirac nodal line semimetal ZrAs2 which is exclusively confined to the top and bottom surfaces within the crystal’s ab plane. As a remarkable consequence, we present the first clear evidence of a Berezinskii–Kosterlitz–Thouless (BKT) transition occurring solely on a material’s surface—specifically, ZrAs₂—unlike the inconsistent reports on PtBi₂, CaAgP, and CaAg₁₋ₓPdₓP. Furthermore, we find that these same surfaces also host a two-dimensional van Hove singularity near the Fermi energy. This leads to enhanced electronic correlations that contribute to the stabilization of superconductivity at the surface of ZrAs2. The`surface-confined nature of the van Hove singularity and associated superconductivity, realized for the first time, allows exploring the interplay between low-dimensional quantum topology and superconductivity in a bulk material without resorting to the superconducting proximity effect. The interplay between electronic topology and superconductivity is of great current interest in condensed matter physics. Here, the authors unveil an unconventional two-dimensional superconducting state accompanied by a van Hove singularity in the recently discovered Dirac nodal line semimetal ZrAs2, which is exclusively confined to the top and bottom surfaces.