Realization of a Type‐II Nodal‐Line Semimetal in Mg3Bi2.

Nodal‐line semimetals (NLSs) represent a new type of topological semimetallic phase beyond Weyl and Dirac semimetals in the sense that they host closed loops or open curves of band degeneracies in the Brillouin zone. Parallel to the classification of type‐I and type‐II Weyl semimetals, there are two types of NLSs. The type‐I NLS phase has been proposed and realized in many compounds, whereas the exotic type‐II NLS phase that strongly violates Lorentz symmetry has remained elusive. First‐principles calculations show that Mg3Bi2 is a material candidate for the type‐II NLS. The band crossing is close to the Fermi level and exhibits the type‐II nature of the nodal line in this material. Spin–orbit coupling generates only a small energy gap (≈35 meV) at the nodal points and does not negate the band dispersion of Mg3Bi2 that yields the type‐II nodal line. Based on this prediction, Mg3Bi2 single crystals are synthesized and the presence of the type‐II nodal lines in the material is confirmed. The angle‐resolved photoemission spectroscopy measurements agree well with the first‐principles results below the Fermi level and thus strongly suggest Mg3Bi2 as an ideal material platform for studying the as‐yet unstudied properties of type‐II nodal‐line semimetals. The type‐II nodal‐line semimetal phase with a tilted nodal‐line band structure strongly violates Lorentz symmetry and hosts exotic electronic properties that are inaccessible in conventional nodal‐line semimetals (NLSs). In this work, a material realization of type‐II NLS is reported in Mg3Pb2 and the related physics is explored in this compound by using a combined method of density functional theory and angle‐resolved photoemission spectroscopy. [ABSTRACT FROM AUTHOR]