Topological nodal loop semimetals and insulators in alkaline earth triarsenides

The $X$As$_3$ class of semimetals has remained unexplored since their discovery in the 1980s by Bauhofer and von Schnering. We have discovered that this class, with $X$=Ca, Sr, Ba, Eu, are nodal loop semimetals (NLS), having a single loop of accidental degeneracies crossing the Fermi energy and a gap elsewhere in the zone: the nodal loop region dominates the transport Large single crystals of monoclinic SrAs$_3$ have been synthesized, allowing studies of its magnetotransport properties andits ARPES spectrum. Density functional calculations reveal that monoclinic CaAs$3$, the sole insulating member of this class, would be a NLS in the absence of spin-orbit coupling (SOC). The position and characters of the nodal Fermi surfaces and associated boundard states will be discussed. The stark low symmetry of monoclinic CaAs$_3$ (with only inversion symmetry) makes it the ``hydrogen atom'' of NLSs; unlike all other classes, space group symmetry does not provide any ``protection'' of its loop of accidental degeneracies.