Superconductivity and a van Hove singularity con ned 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 con ned to the top and bottom surfaces within the crystal’s ab plane. As a remarkable consequence, we present the rst 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 CaAg_1-xPd_xP. 

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-con ned 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.