  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 (2DTIs) and their stacked
three-dimensional (3D) 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
layered superconducting material CaSn as a WTI with nontrivial Z2 as well as
nodal line semimetal protected by crystalline non-symmorphic symmetry. Our
systematic angle-resolved photoemission spectroscopy (ARPES) investigation on
the electronic structure exhibits excellent agreement with the calculation.
Furthermore, scanning tunnelling 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 non-symmorphic nodal
line semimetal CaSn, which may become a versatile material platform to realize
multiple exotic electronic states as well as topological superconductivity.
