Evidence of a topological edge state in a superconducting nonsymmorphic nodal-line semimetal

Topological materials host fascinating low dimensional gapless states at the boundary. As a prominent example, helical topological edge states (TESs) of two-dimensional topological insulators and their stacked three-dimensional equivalent, weak topological insulators (WTIs), have sparked research enthusiasm due to their potential application in the next generation of electronics/spintronics with low dissipation. Here, we propose the layered superconducting material CaSn as a WTI with nontrivial ${Z}_{2}$ as well as nodal-line semimetal protected by crystalline nonsymmorphic symmetry. Our systematic angle-resolved photoemission spectroscopy (ARPES) investigation on the electronic structure exhibits excellent agreement with the calculation. Furthermore, scanning tunneling microscopy/spectroscopy (STM/STS) at the surface step edge shows signatures of the expected TES. These integrated evidences from ARPES, STM/STS measurement, and corresponding ab initio calculation strongly support the existence of TES in the nonsymmorphic nodal-line semimetal CaSn, which may become a versatile material platform to realize multiple exotic electronic states as well as topological superconductivity.