Symmetry protection and large Fermi arcs in double Weyl semimetal CoGe

Unlike the two-fold Weyl fermions that are of significant theoretical and experimental interest presently, higher-fold chiral fermionic states have very recently been theoretically predicted in crystalline systems using symmetry arguments and Density Functional calculations. Here, we focus on such excitations in a few binary/ternary alloys (CoGe being the titular example), that belongs to space group (SG) 198. We found distinct three-fold, four-fold and six-fold chiral fermions in the bulk. We provide symmetry arguments for the protection of these degeneracies at high symmetry points in the Brillouin zone (BZ), with special emphasis on the four-fold fermions for spinless (at R point in the BZ) and spinfull (at {\Gamma} point) cases. Our surface simulations show that the size of Fermi arcs resulting from these chiral fermions are large, robust and untouched from the bulk states due to the near absence of bulk Fermi pockets. Spin-momentum locking in the presence of spin-orbit interaction is observed on the surface Fermi arcs. All these rich topological features make CoGe a promising candidate for future photo-emission and transport measurements.