Chiral topological semimetal with multifold band crossings and long Fermi arcs

Abstract

Topological semimetals (TSs) in structurally chiral crystals (which possess a handedness due to a lack of mirror and inversion symmetries) are expected to display numerous exotic physical phenomena, such as new 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, however, all experimentally confirmed TSs crystallize in space groups that contain mirror operations, which forces the aforementioned properties to vanish. Here, by employing angle-resolved photoelectron spectroscopy and ab-initio calculations, we show that AlPt is a structurally chiral TS that hosts new fourfold and sixfold fermions, which can be viewed as a higher spin generalization of Weyl fermions without equivalence in elementary particle physics. Remarkably, these multifold fermions are located at high symmetry points with Chern numbers larger than those in Weyl-semimetals, thus resulting in multiple Fermi-arcs that thread through the full diagonal of the surface Brillouin zone (BZ), spanning the largest portion of the BZ of any material. By imaging these long Fermi-arcs, we can experimentally determine the magnitude and sign of their Chern number, which allows us to relate their dispersion to the handedness of their host crystal.Revised manuscript 2 An object that cannot be superimposed with its mirror image is said to be chiral, a concept first proposed by Lord Kelvin 7 that has found widespread applications across the modern sciences, from high energy physics to biology 8,9 . In condensed matter physics (where the properties of crystalline materials are tightly constrained by spatial lattice symmetries), chiral crystals can only be found in the 65 Sohncke space groups, which contain only orientation-preserving operations, and can therefore be assigned a handedness. The structural chirality of these systems can endow them with fascinating properties, such as natural optical activity 10 , negative refraction in metamaterials 11 , non-reciprocal effects such as magnetochiral birefringence of light or electronic magnetochiral anisotropy 12,13 , chiral magnetic textures such as helices and skyrmions 14 , or unusual superconductivity 15 . In the recently discovered Weyl-semimetals, not just the crystal structure itself, but also the electronic wavefunctions can exhibit a chirality at point-like twoband crossings of the quasiparticle dispersion, which are known as Weyl fermions [16][17][18][19] . These crossings are topologically protected because they carry a topological charge, a quantized Berry flux through any surface enclosing them in momentum space. This charge has integer magnitude C= ±1, which is known as the Chern number and gives the electronic wavefunction a handedness. This topological property results in a plethora of exotic phenomena, such as Fermiarc surface states 20 , unconventional magnetoresistance 16,21,22 , nonlocal transport 23 , and many

Optical conductivity of multifold fermions: The case of RhSi

“…They can be realized in the multifold semimetals -the materials, which possess the characteristic electronic band crossings with degeneracies higher than two [1,2]. A number of such semimetals has recently been predicted and experimentally confirmed among the materials from the space group 198 (SG198), which symmetry is noncentrosymmetric and has no mirror planes, leading to a realization of "topological chiral crystals" [3][4][5][6][7][8][9]. In such semimetals, the quantized circular photogalvanic effect (QCPGE) has been forecasted in 2017 [10].…”

Chiral fermion reversal in chiral crystals

“…Recent angle-resolved photoemission spectroscopy (ARPES) experiments have confirmed new types of degenerate points that carry nonzero chiral charges in a series of crystals CoSi, RhSi, and PtAl [31][32][33][34], providing a new platform for exploring the physical properties of chiral fermions. It is worth noting that all these materials belong to space group P213 (#198) with structural chirality, making it possible to study the connection between the chiral lattices in real space and the chiral fermions in momentum space [34].…”

Topological Engineering of Pt‐Group‐Metal‐Based Chiral Crystals toward High‐Efficiency Hydrogen Evolution Catalysts

“…Thanks to the development of topological materials, a new type of topological semimetals of multifold fermions were theoretically predicted and experimentally verified in a class of chiral crystals 25–31. Owing to the chiral crystal symmetry, the band structures of this type of materials present as multifold degenerated points with a large topological charge of Chern number 4 (Figure 1B,C).…”