Chiral Topological Semimetal with Multifold Band Crossings and Long Fermi arcs (1812.03310v2)

Abstract: Topological semimetals in crystals with a chiral structure (which possess a handedness due to a lack of mirror and inversion symmetries) are expected to display numerous exotic physical phenomena, including fermionic excitations with large topological charge [1], long Fermi arc surface states [2,3], unusual magnetotransport [4] and lattice dynamics [5], as well as a quantized response to circularly polarized light [6]. To date, all experimentally confirmed topological semimetals exist in crystals that contain mirror operations, meaning that these properties do not appear. Here, we show that AlPt is a structurally chiral topological semimetal that hosts new fourfold and sixfold fermions, which can be viewed as a higher spin eneralization of Weyl fermions without equivalence in elementary particle physics. These multifold fermions are located at high symmetry points and have Chern numbers larger than those in Weyl semimetals, thus resulting in multiple Fermi arcs that span the full diagonal of the surface Brillouin zone. By imaging these long Fermi arcs, we experimentally determine the magnitude and sign of their Chern number, allowing us to relate their dispersion to the handedness of their host crystal.

• The paper presents clear experimental evidence of fourfold and sixfold multifold fermions in AlPt using ARPES and ab-initio calculations.
• It shows how chiral symmetry leads to high Chern numbers and extensive Fermi arc networks across the surface Brillouin zone.
• The findings offer a promising platform for exploring exotic electromagnetic responses and potential applications in quantum devices.

Chiral Topological Semimetal AlPt: Multifold Fermions and Fermi Arcs

The paper of the structurally chiral topological semimetal AlPt presents compelling evidence of novel fourfold and sixfold fermions, advancing our understanding of topological semimetals. In the absence of mirror and inversion symmetry, these chiral structures are shown to host unique multifold band crossings and extensive Fermi arcs, properties not observed in their non-chiral counterparts. This paper delineates the synthesis, structural characterization, and electronic properties of AlPt, utilizing angle-resolved photoelectron spectroscopy (ARPES) and ab-initio band structure calculations.

The researchers articulate the significance of chiral crystals in the 65 Sohncke space groups, emphasizing their role in generating unusual optical and transport phenomena due to the lack of inversion symmetry. In condensed matter systems, chirality not only arises in the crystal structure but also manifests in the electronic wavefunctions of topological semimetals. This paper marks a shift from the previously known relativistic Weyl semimetals, expanding the category to encompass higher-spin generalizations, namely multifold fermions, with Chern numbers exceeding those in Weyl systems. The focus on chiral topological semimetals devoid of mirror planes allows for the experimental observation of their exotic properties, formerly restricted by symmetry constraints in non-chiral materials.

The synthesis of AlPt, crystallizing in the cubic chiral B20 structure, enables the realization of fourfold fermions at the Γ point and sixfold fermions at the R point. This structural chirality introduces a correspondingly high Chern number, |C|=4, verified through surface Fermi arcs distributed across the diagonal of the surface Brillouin zone, as evidenced in their ARPES experiments. These findings surpass typical expectations from non-chiral Weyl semimetals, where opposite Chern numbers cancel out, thus quenching similar phenomena.

Our ARPES data in conjunction with theoretical predictions establish that the nodal points entail higher band degeneracies, pivotal for the presence of the observed multifold fermions. The experimental methodology adeptly combines low-energy ARPES with soft X-ray techniques to elucidate both bulk and surface electronic structures, supplemented by first-principles calculations to validate these observations. Key facets of AlPt's electronic structure, including the fourfold crossing at Γ and sixfold crossing at R, are consistent across experimental and theoretical domains, with only minor deviations attributable to photoemission matrix element effects.

The implications of these findings are multifaceted. The presence of separated energy levels for the nodes with opposite Chern numbers offers a promising platform to paper phenomena like the gyrotropic magnetic effect and circular photogalvanic effect, which are linked to topological properties. Furthermore, this work suggests potential routes for experimental determination of chirality in topological semimetals by analyzing the Fermi arc dispersions in relation to the Berry curvature.

Looking forward, AlPt sets the stage for further exploration into other materials within the same family, potentially offering an expansive range of tunable properties through alloying or doping. Harnessing the chiral characteristics of multifold fermions not only broadens the horizons of quantum materials research but also paves the way for practical applications exploiting their unique electromagnetic responses in novel electronic devices.

Overall, the paper of AlPt culminates as a vital advancement in the recognition and utility of chiral topological semimetals. By substantiating the experimental existence of multifold fermions, it provides a cornerstone for future innovations and applications in condensed matter physics and material science.