Unconventional Topological Fermions in Orthorhombic RhSi*

Topological semimetals with different types of band crossings, distinct from conventional Dirac and Weyl nodes, can give rise to novel fermionic excitations that may not have analogues in high-energy physics. Exotic multifold topological excitations have been predicted theoretically and observed in a family of transition metal silicides (e.g. β-RhSi). These compounds adopt the cubic FeSi structure type (space group No. 198), for which the calculations predict Chern numbers > 1 at specific high symmetry points within their Brillouin zone.
Through Density Functional Theory (DFT) calculations, we report that such multi degenerate fermions also exist in α-RhSi, which adopts an orthorhombic structure type (space group No. 62). Group symmetry analysis combined with DFT calculations indicate the coexistence of multiple types of multifold Dirac fermions at or near high symmetry points. Measurements of the angular dependence of the de Haas–van Alphen effect, indicate a Fermi surface whose topography is in reasonable agreement with the DFT calculations. We observe a pronounced anomaly in the magnetic torque of α-RhSi suggesting the possibility of topological phase-transitions.