Large Fermi Arcs in Unconventional Weyl Semimetal RhSi (1706.04600v3)

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.

• The paper identifies a six-fold and a four-fold degenerate chiral fermion with Chern numbers ±4, underlining RhSi's topological character.
• The paper demonstrates RhSi's unprecedented momentum separation and a 1.2 eV nontrivial energy window, key for robust Fermi arcs and chiral transport.
• The paper predicts exceptionally long, spin-polarized surface Fermi arcs and a quantized circular photogalvanic effect, opening paths for optical and spintronic applications.

Analysis of Unconventional Chiral Fermions and Large Topological Fermi Arcs in RhSi

The paper "Unconventional Chiral Fermions and Large Topological Fermi Arcs in RhSi" explores the theoretical and experimental prospects of unconventional chiral fermions in the RhSi materials family. The authors propose RhSi as a promising candidate for studying these fermions' unique transport and optical properties due to its distinct electronic structure and topological characteristics.

At the core of the paper are the chiral fermions, which are quasiparticles characterized by non-zero Chern numbers, indicating robust topological behavior. The work identifies a chiral six-fold-degenerate double spin-1 Weyl node at the $R$ point and a previously uncharacterized four-fold-degenerate chiral fermion at the $\Gamma$ point in the Brillouin zone of RhSi. Both nodes exhibit a Chern number of $\pm 4$, indicating their significant role in determining the electronic properties of this material.

The authors highlight three key features that make RhSi particularly interesting:

1. Momentum Separation and Energy Scale: The momentum separation between compensative chiral fermions in RhSi is the largest possible in any crystal, providing a wide topologically nontrivial energy window of approximately 1.2 eV. This extensive energy range, along with the large momentum separation, renders RhSi a potent material for exploring topological surface states and chiral transport phenomena.
2. Surface Fermi Arcs: The $(001)$ surface of RhSi exhibits exceptionally long Fermi arcs, which connect projections of the bulk chiral fermions. This connectivity spans the entire surface Brillouin zone, providing a robust platform for observing surface phenomena associated with topological materials. The surface states are predicted to have considerable spin polarization, enhancing their potential for spintronic applications.
3. Exotic Bulk Photogalvanic Response: RhSi presents an opportunity to observe a quantized circular photogalvanic effect due to the asymmetry in energy levels of the chiral fermions at $\Gamma$ and $R$. This effect could lead to quantized photocurrent responses when excited by circularly polarized light, opening frontiers in optical applications of topological materials.

The work utilizes both first-principles calculations and tight-binding models to substantiate these findings, providing a comprehensive framework for understanding the electronic structure and topological phenomena in RhSi. The extensive use of k·p theory and symmetry analysis elucidates the unconventional characteristics of the high-fold fermions and their associated topological charges.

The implications of these findings are considerable both experimentally and theoretically. From an experimental standpoint, RhSi emerges as a prime candidate for the directly observable consequences of chiral fermion topological nature, such as in ARPES studies. Theoretically, this work extends the exploration of materials hosting unconventional fermions beyond what has been achieved in previously known Weyl and Dirac semimetals.

In conclusion, this paper propels RhSi into the spotlight for future research in topological materials. The outlined properties not only broaden the understanding of chiral semimetals but also invite further exploration into potential applications in electronics and photonics, leveraging the unique properties of unconventional chiral fermions. Future developments may entail the synthesis and experimental validation of the predicted properties, alongside exploring other materials in the same space group to harness the multifaceted potential of topological chiral fermions.