Helicity-tunable spin Hall and spin Nernst effects in unconventional chiral fermion semimetals (CoSi, CoGe, RhSi, RhGe)

Transition metal monosilicides CoSi, CoGe, RhSi, and RhGe in the chiral cubic B20 structure (the CoSi family) have recently been found to host unconventional chiral fermions beyond spin-1/2 Weyl fermions, and also to exhibit exotic physical phenomena such as long Fermi arc surface states, gyrotropic magnetic effect, and quantized circular photogalvanic effect. Thus, exploring novel spin-related transports in these unconventional chiral fermion semimetals may open a new door for spintronics. In this work, we study the intrinsic spin Hall effect (SHE) and spin Nernst effect (SNE) in the CoSi family based on ab initio relativistic band structure calculations [Phys. Rev. B 106 (2022) 165102]. First, we find that unlike nonchiral cubic metals, the CoSi family have two independent nonzero spin Hall (Nernst) conductivity tensor elements instead of one element. Furthermore, the SHC and helicity of the chiral structure are found to be correlated, thus enabling SHE detection of structural helicity and also chiral fermion chirality. Second, the intrinsic SHE and SNE in some of the CoSi family are large. Due to their semimetallic nature with low electrical conductivity, these topological semimetals may have large spin Hall and spin Nernst angles, being comparable to that of platinum metal. The spin Hall conductivity and spin Nernst conductivity of these compounds can also be increased by raising or lowering the chemical potential to, e.g., the topological nodes, via either chemical doping or electrical gating. Our findings thus indicate that transition metal monosilicides not only would provide a material platform for exploring novel spin transports and exotic phenomena in unconventional chiral fermion semimetals, but also could be promising materials for developing better spintronic devices.