Nodal-line semimetals from Weyl superlattices

Abstract

The band crossings at the Fermi level of a Weyl semimetal are generally stable against disorder. The only way to open a gap in the spectrum is by annihilating pairs of Weyl nodes. Here, the authors show that coupling Weyl nodes by a superlattice does not always lead to gap opening, but does lead to a variety of different phases, including a nodal-line semimetal. The authors uncover a novel mechanism that protects the nodal line, a combination of a fractional lattice translation and charge-conjugation symmetry, and show that its in-gap surface states are not necessarily exponentially localized.