Discovery of topological chiral crystals with helicoid arc states (1812.04466v1)

Abstract: The quantum behaviour of electrons in materials lays the foundation for modern electronic and information technology. Quantum materials with novel electronic and optical properties have been proposed as the next frontier, but much remains to be discovered to actualize the promise. Here we report the first observation of topological quantum properties of chiral crystals in the RhSi family. We demonsrate that this material hosts novel phase of matter exhibiting nearly ideal topological surface properties that emerge as a consequence of the crystals' structural chirality or handedness. We also demonstrate that the electrons on the surface of this crystal show a highly unusual helicoid structure that spirals around two high-symmetry momenta signalling its topological electronic chirality. Such helicoid Fermi arcs on the surface experimentally characterize the topological charges of $\pm{2}$, which arise from the bulk chiral fermions. The existence of bulk high-fold degenerate fermions are guaranteed by the crystal symmetries, however, in order to determine the topological charge in the chiral crystals it is essential to identify and study the helical arc states. Remarkably, these topological conductors we discovered exhibit helical Fermi arcs which are of length $\pi$, stretching across the entire Brillouin zone and orders of magnitude larger than those found in all known Weyl semimetals. Our results demonstrate novel electronic topological state of matter on a structurally chiral crystal featuring helicoid Fermi arc surface states. The exotic electronic chiral fermion state realised in these materials can be used to detect a quantised photogalvanic optical response or the chiral magnetic effect and its optical version in future devices as described by G. Chang \textit{et.al.,} `Topological quantum properties of chiral crystals' Nature Mat. 17, 978-985 (2018).

• The paper demonstrates the first empirical observation of topological quantum properties in chiral crystals using high-resolution ARPES and ab initio calculations.
• It reveals anomalously large helicoid Fermi arc states spanning the entire Brillouin zone with robust energy windows of approximately 1.2–1.3 eV.
• The study highlights potential quantum device applications by establishing chiral fermions with minimal non-zero chiral charges and stable topological phases.

Topological Chiral Crystals with Helicoid Arc States

This paper delineates the first empirical observations of topological quantum properties in chiral crystals within the RhSi family. The paper identifies these materials' intrinsic topological characteristics, which arise from the structural chirality of the crystals. This research provides a meticulous examination of novel topological electronic phases, highlighting the significance of helicoid Fermi arc surface states realized in these chiral crystalline structures.

Core Insights

The research focuses on materials belonging to the $X$Si category, where $X$ stands for Co or Rh. The authors leverage high-resolution Angle-Resolved Photoemission Spectroscopy (ARPES) alongside advanced ab initio calculations to probe and elucidate the topological features of CoSi and RhSi chiral crystals. These materials are observed to exhibit emergent topological properties with large, helicoid Fermi arcs. Of note, the Fermi arcs in these materials are anomalously large, spanning across the entire Brillouin zone, unlike any previously documented within known Weyl semimetals.

Numerical Findings and Claims

• CoSi and RhSi showcase an energy window where the topological characteristics are preserved, approximately 1.2 eV wide for CoSi and 1.3 eV for RhSi, underscoring the materials' robust topological nature.
• These substances host the minimum non-zero chiral charge (Chern numbers $+2$ and $-2$), with the bulk Brillouin zone chiral fermions constrained to maximally separate TRIMs, enhancing the utility of these materials for theoretical explorations and potential applications.
• The paper makes bold claims about achieving spectroscopic visualization of topological chiral fermions manifest as helicoid Fermi arc surface states for the first time.

Implications and Future Directions

The implications of these findings are considerable, especially within quantum materials science and topological quantum computing. The presence of long topological Fermi arcs provides promising pathways for exploiting unique quantum phenomena, such as the quantized circular photogalvanic effect or the chiral magnetic effect, which can manifest in such systems due to their topological attributes.

Moreover, the pronounced topologically non-trivial energy windows render these materials resilient against perturbations, making them ideal candidates for stable quantum devices. The paper illuminates possible avenues for synthesizing new materials that integrate these topological properties, thereby prompting advancements in the field.

Conclusion

The methodological approach and analytical discourse presented in the paper offer significant developments in understanding and applying novel topological phases of matter in chiral crystals. By precisely characterizing these crystalline structures' topological nature, the research extends the current knowledge bank of topological insulators and semimetals, suggesting innovative frameworks for both fundamental theory and applied science in quantum materials. Future investigations can build on this foundation to explore and harness the intriguing and potentially transformative properties observed within topological chiral crystals.