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 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-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.. Authors: Hossain, Md Shafayat [Princeton University, NJ (United States)] (ORCID:0000000237445877); Islam, Rajibul [University of Alabama at Birmingham, AL (United States)]; Cheng, Zi-Jia [Princeton University, NJ (United States)]; Muhammad, Zahir [Beihang University, Hefei (China)] (ORCID:0000000323565760); Zhang, Qi [Princeton University, NJ (United States)]. DOE Contract: SC0002613. Subjects: Electronic properties and materials; Superconducting properties and materials; Topological matter