Signatures of Topological Surface State and Unconventional Magnetotransport Properties in Elemental Ruthenium

In recent years, the pursuit of new topological materials has created a vast and ever growing catalogue of compounds. However, the example of elemental topological metals is still rather limited. So far, the non-trivial topological states have been probed in only a handful of elements and that too rarely in a transition metal. By combining the angle-resolved photoemission spectroscopy (ARPES) and magnetotransport measurements, here, ruthenium (Ru) is experimentally confirmed to be a topological metal, validating a recent theoretical prediction. The ARPES data for Ru(0001) provide evidence of the topological surface state related to the bulk Dirac node. Ru also exhibits a prominent planar Hall effect, thus further indicating its Dirac semimetallic nature. Large electron and hole mobilities are estimated, which are comparable to several topological semimetals. The non-trivial nature of electronic band of Ru is also supported by the large non-saturating transverse magnetoresistance.