The theoretical proposal of chiral fermions in topological semimetals has led to a significant effort towards their experimental realization. In particular, the Fermi surfaces of chiral semimetals carry quantized Chern numbers, making them an attractive platform for the observation of exotic transport and optical phenomena. While the simplest example of a chiral fermion in condensed matter is a conventional $|C|=1$ Weyl fermion, recent theoretical works have proposed a number of unconventional chiral fermions beyond the standard model which are protected by unique combinations of topology and crystalline symmetries. However, materials candidates for experimentally probing the transport and response signatures of these unconventional fermions have thus far remained elusive. In this Letter, we propose the RhSi family in space group No. 198 as the ideal platform for the experimental examination of unconventional chiral fermions. We find that RhSi is a filling-enforced semimetal that features near its Fermi surface a chiral double sixfold-degenerate spin-1 Weyl node at $R$ and a previously uncharacterized fourfold-degenerate chiral fermion at $\mathrm{\Gamma }$. Each unconventional fermion displays Chern number $\pm 4$ at the Fermi level. We also show that RhSi displays the largest possible momentum separation of compensative chiral fermions, the largest proposed topologically nontrivial energy window, and the longest possible Fermi arcs on its surface. We conclude by proposing signatures of an exotic bulk photogalvanic response in RhSi.

中文翻译：

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RhSi中的非常规手性费米子和大拓扑费米弧

拓扑半金属中的手性费米子的理论建议已导致其实验实现的巨大努力。特别是，手性半金属的费米表面带有量化的Chern数，使其成为观察奇异传输和光学现象的诱人平台。虽然在凝聚态物质中最简单的手性费米子例子是常规的$|C|=\mathrm{1个}$Weyl费米子，最近的理论工作提出了超出标准模型的许多非常规手性费米子，它们受到拓扑结构和晶体对称性的独特组合的保护。但是，到目前为止，仍无法通过实验探测这些非常规费米子的运移和响应特征的候选材料。在这封信中，我们建议将198号空间群中的RhSi家族作为非常规手性费米子实验研究的理想平台。我们发现RhSi是一种填充增强的半金属，在其Fermi表面附近具有一个手性的双六倍简并自旋1 Weyl节点。$\mathrm{[R}$ 以及先前未鉴定的四倍简并手性费米子 $\mathrm{\Gamma }$。每个非常规费米子都显示陈恩数$\pm 4$在费米一级。我们还显示，RhSi在补偿性手性费米子上显示出最大可能的动量分离，在拓扑上平凡的能量窗建议的最大，并且其表面上可能出现的最长的费米弧。最后，我们提出了RhSi中异乎寻常的大量光电反应的特征。