# Large Fermi Arcs in Unconventional Weyl Semimetal RhSi

## TL;DR

This paper identifies RhSi as an ideal material for studying unconventional chiral fermions, revealing large Fermi arcs and unique topological features that enable exploration of exotic transport phenomena.

## Contribution

It proposes RhSi as a prime candidate for experimental investigation of unconventional chiral fermions with large topological features and novel surface states.

## Key findings

- RhSi hosts a chiral double six-fold-degenerate Weyl node at R.
- RhSi contains a four-fold-degenerate chiral fermion at Γ.
- RhSi exhibits the largest momentum separation and Fermi arcs among known materials.

## Abstract

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 paper, we propose the RhSi family in space group (SG) $\#$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 six-fold-degenerate spin-1 Weyl node at $R$ and a previously uncharacterized four-fold-degenerate chiral fermion at $\Gamma$. Each unconventional fermion displays Chern number $\pm4$ 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.