Helicity-tunable spin Hall and spin Nernst effects in unconventional chiral fermion semimetals XY (X=Co, Rh; Y=Si, Ge)

TL;DR

This study investigates the intrinsic spin Hall and spin Nernst effects in unconventional chiral fermion semimetals CoSi, RhSi, and related compounds, revealing large effects and their correlation with structural chirality, with implications for spintronics.

Contribution

The paper provides the first ab initio analysis of SHE and SNE in the CoSi family, highlighting their large effects and connection to chiral structure, advancing understanding of spin transport in topological semimetals.

Findings

Large intrinsic SHE and SNE observed in CoSi family.

SHC and SNC tensors are correlated with structural helicity.

Potential for high spin Hall and Nernst angles in these materials.

Abstract

Transition metal monosilicides CoSi, CoGe, RhSi and RhGe in the chiral cubic B20 structure have recently been found to host unconventional chiral fermions beyond spin-1/2 WFs, and also exhibit exotic physical phenomena such as long Fermi arc surface states, GME and quantized CPGE. Thus, exploring novel spin-related transports in these unconventional chiral fermion semimetals may open a new door for spintronics and spin caloritronics. In this paper, we study the intrinsic SHE and SNE in the CoSi family based on ab initio relativistic band structure calculations. First, we find that unlike nonchiral cubic metals, the CoSi family have two independent nonzero SHC (SNC) tensor elements, namely, $\sigma_{xy}^z$ and $\sigma_{xz}^y$ ($\alpha_{xy}^z$ and $\alpha_{xz}^y$) instead of one element. Furthermore, the SHC ($\sigma_{xy}^z$ and $\sigma_{xz}^y$) 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. In particular, the calculated SHC of RhGe is as large as -140 ($\hbar$/e)(S/cm). The calculated SNC of CoGe is also large, being -1.3 ($\hbar$/e)(A/m K) at room temperature. 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 Pt metal. The SHC and SNC of these compounds can also be increased by raising or lowering $\mu$ to, e.g., the topological nodes, via either chemical doping or electrical gating. Our findings thus indicate that the CoSi family 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 and spin caloritronic devices.