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Helicity-dependent photocurrents in the chiral Weyl semimetal RhSi

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Science Advances  15 Jul 2020:
Vol. 6, no. 29, eaba0509
DOI: 10.1126/sciadv.aba0509
  • Fig. 1 Photocurrents from Weyl semimetals and experimental apparatus.

    (A) Helical radiation preferentially excites one side of a Weyl cone centered at the Fermi energy, generating a current parallel to the optical wave vector. (B) Schematic of the experimental geometry. Variable wavelength pump light is incident on the sample at either normal or 45° incidence. Terahertz (THz) radiation is collected and focused onto a ZnTe crystal for electro-optic sampling. PD, photodiode; WP, Wollaston prism; WGP wire grid polarizer. (C) Individual terahertz pulses measured from left and right circularly polarized 2000-nm pump light at 45° angle of incidence. Their difference is the photon helicity–dependent CPGE signal.

  • Fig. 2 Symmetry of CPGE and LPGE responses in RhSi.

    (A) Measurement of the terahertz polarization. Orange and green curves show the vertical and horizontal components of the pulse as a function of time. The reconstructed terahertz pulse (red curve) is then projected onto a plane, showing the direction of linear polarization, θ. (B) Dependence of the angle of LPGE terahertz polarization, θ, on angle of rotation of [111] face about the surface normal, ϕ, with pump at normal incidence. The relation θ = 3ϕ predicted by the space group P213 symmetry is confirmed. The CPGE signal is below measurement noise level in this geometry. (C) Same as (B) except for 45 incidence. LPGE polarization again varies as =3ϕ. CPGE is horizontally polarized independent of the crystal orientation, confirming that the CPGE current is parallel to the pump wave vector. (D) Schematic showing that the resulting in-plane CPGE current is fixed by the plane of incidence of the pump light. The CPGE current at normal incidence is normal to the surface of the sample and thus does not radiate into free space.

  • Fig. 3 CPGE spectrum.

    CPGE amplitude βτ in units of πe33h2× fs as a function of photon energy, showing abrupt quenching above 0.65 eV. The inset contains a schematic showing the surface 𝒮ω in k-space defined by the available optical transitions at photon energy ℏω. For ℏω < EC, 𝒮ω encloses a single node and has integrated Berry flux C = ±4. Above EC, it encloses two topological nodes of opposite chirality and C = 0. The blue shaded region in the main plot indicates the region where 𝒮ω encloses only a single node.

  • Fig. 4 Reflectivity and optical conductivity.

    (A) Measured reflectivity of RhSi. (B) Optical conductivity determined by reflectivity measurements and Kramers-Kronig analysis (blue curve). The Drude peak is used to infer that the scattering time has value τ = 8.6 fs. The orange curve represents the optical conductivity from the Γ and R nodes alone (34).

Supplementary Materials

  • Supplementary Materials

    Helicity-dependent photocurrents in the chiral Weyl semimetal RhSi

    Dylan Rees, Kaustuv Manna, Baozhu Lu, Takahiro Morimoto, Horst Borrmann, Claudia Felser, J. E. Moore, Darius H. Torchinsky, J. Orenstein

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