Observation of Chiral Fermions with a Large Topological Charge and Associated Fermi-Arc Surface States in CoSi

Takane, Daichi; Wang, Zhiwei; Souma, S.; Nakayama, K.; Nakamura, Takechika; Oinuma, Hikaru; Nakata, Yuki; Iwasawa, Hideaki; Cacho, Céphise; Kim, T. K.; Horiba, Koji; Kumigashira, Hiroshi; Takahashi, Takashi; Ando, Yoichi; Sato, Takafumi

doi:10.1103/physrevlett.122.076402

Cited by 300 publications

(257 citation statements)

References 41 publications

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“…These fermionic materials belong oscillations, large intrinsic anomalous Hall effect, and chiral anomaly. These results were recently experimentally verified in ARPES measurements by Takane et al [16] In refs. This effect vanishes thus for perpendicular magnetic field (I⊥B).…”

Section: Introductionsupporting

confidence: 72%

Signatures of a Charge Density Wave Phase and the Chiral Anomaly in the Fermionic Material Cobalt Monosilicide CoSi

Schnatmann

Geishendorf

Lammel

et al. 2019

Adv Elect Materials

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to the emerging class of materials with non-trivial electronic band structure like topological insulators, Dirac or Weyl materials, featuring different topologically protected electronic states. In Weyl materials, these topologically protected electronic states are highly stable quasiparticles in the bulk, known as Weyl fermions, that were first predicted by Hermann Weyl in 1929. [1] These Weyl fermions carry a non-zero chiral charge and always appear in pairs. In a common Weyl material, this chiral charge is ±1; hence, the total chiral charge is zero. In contrast, the fermionic materials host Weyl fermions that are degenerated and carry a higher chiral charge. Remarkably, recently long Fermi arcs were observed in the fermionic material CoSi by Rao et al. [2] Also, Sanchez et al. [3] discovered helicoid-arc quantum states in CoSi.A necessary condition for the existence of Weyl fermions is a broken space or time inversion symmetry. The chiral charge of the material strongly depends on the space group of the crystal structure. CoSi has a cubic B20 crystal structure (space group 198 P2 1 3) with broken space inversion symmetry. This space group was predicted to host sixfold degenerated Weyl fermions. [4] In CoSi, the Weyl fermions carry a chiral charge of up to ±4. In detail, spin 1 excitations with a chiral charge of ±2 and spin 3/2 excitations with a chiral charge of ±4 (Rarita-Schwinger-Weyl fermions) were predicted. [5,6] Electrical transport experiments are a powerful tool to probe the energy spectrum of a material and thereby the specific nature of Weyl fermions. Topological states are massless due to the linear dispersion relation, and thus have an extremely high mobility. While often superimposed by the normal band transport, contributions of the Weyl fermions increase the total electrical conductivity. Consequently, the emerging class of materials with non-trivial electronic band structure has a large overlap with other classes of functional materials like the class of thermoelectric materials. [7] This also holds for CoSi. [5,8,9] Interestingly, related silicides like MnSi, [10] Fe 0.75 Co 0.25 Si [11] and germanides like Fe 1 −x Co x Ge [12,13] also feature skyrmion states.In general, Weyl materials show characteristic quantum effects in the transport, resulting for example in quantum Materials with topological electronic states have emerged as one of the most exciting discoveries of condensed quantum matter, hosting quasiparticles with extremely low effective mass and high mobility. Weyl materials contain such topological states in the bulk and additionally have a non-trivial chiral charge. However, despite known quantum effects caused by these chiral states, the interplay between chiral states, and a charge density wave phase, an ordering of the electrons to a correlated phase is not experimentally explored. Indications for the formation of a charge density wave phase in the Weyl material cobalt monosilicide CoSi are observed. Furthermore, the typical signatures of the charge density wave phase together...

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Section: Introductionsupporting

confidence: 72%

Signatures of a Charge Density Wave Phase and the Chiral Anomaly in the Fermionic Material Cobalt Monosilicide CoSi

Schnatmann

Geishendorf

Lammel

et al. 2019

Adv Elect Materials

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“…1h shows a threefold-degenerate point at the Brillouin zone (BZ) centre It has been known that the surface projections of chiral fermions are surrounded by helical surface states [35]. The helical surface states for CoSi have been theoretically predicted [28,30] and experimentally confirmed on the (001) surface [31,33] It should be mentioned that the SOC effects exist in these materials. Due to the lack of inversion symmetry, the bands of both bulk and surface states split when SOC is included, as shown in our calculations with SOC in the Supplementary Figs.…”

Section: Introductionmentioning

confidence: 99%

“…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].…”

Section: Introductionmentioning

confidence: 99%

Chiral fermion reversal in chiral crystals

Li¹,

Xu²,

Rao³

et al. 2019

Nat Commun

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In materials chiral fermions such as Weyl fermions are characterized by nonzero chiral charges, which are singular points of Berry curvature in momentum space. Recently, new types of chiral fermions beyond Weyl fermions have been discovered in structurally chiral crystals CoSi, RhSi and PtAl. Here, we have synthesized RhSn single crystals, which have opposite structural chirality to the CoSi crystals we previously studied. Using angle-resolved photoemission spectroscopy, we show that the bulk electronic structures of RhSn are consistent with the band calculations and observe evident surface Fermi arcs and helical surface bands, confirming the existence of chiral fermions in RhSn. It is noteworthy that the helical surface bands of the RhSn and CoSi crystals have opposite handedness, meaning that the chiral fermions are reversed in the crystals of opposite structural chirality. Our discovery establishes a direct connection between chiral fermions in momentum space and chiral lattices in real space.

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“…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].…”

Section: Introductionmentioning

confidence: 99%

Optical conductivity of multifold fermions: The case of RhSi

Maulana

Manna²,

Uykur

et al. 2020

Phys. Rev. Research

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We measured the reflectivity of the multifold semimetal RhSi in a frequency range from 80 to 20 000 cm −1 (10 meV -2.5 eV) at temperatures down to 10 K. The optical conductivity, calculated from the reflectivity, is dominated by the free-carrier (Drude) contribution below 1000 cm −1 (120 meV) and by interband transitions at higher frequencies. The temperature-induced changes in the spectra are generally weak -only the Drude bands narrow upon cooling with an unscreened plasma frequency being constant with temperature at approximately 1.4 eV, in agreement with a weak temperature dependence of the free-carrier concentration determined by Hall measurements. The interband portion of conductivity exhibits two linear-in-frequency regions below 5000 cm −1 (∼ 600 meV), a broad flat maximum at around 6000 cm −1 (750 meV), and a further increase starting around 10 000 cm −1 (∼ 1.2 eV). We assign the linear behavior of the interband conductivity to transitions between the linear bands near the band crossing points. Our findings are in accord with the predictions for the low-energy conductivity behavior in multifold semimetals and with earlier computations based on band structure calculations for RhSi.

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Observation of Chiral Fermions with a Large Topological Charge and Associated Fermi-Arc Surface States in CoSi

Cited by 300 publications

References 41 publications

Signatures of a Charge Density Wave Phase and the Chiral Anomaly in the Fermionic Material Cobalt Monosilicide CoSi

Signatures of a Charge Density Wave Phase and the Chiral Anomaly in the Fermionic Material Cobalt Monosilicide CoSi

Chiral fermion reversal in chiral crystals

Optical conductivity of multifold fermions: The case of RhSi

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