Abstract
Chiral topological semimetals are materials that break both inversion and mirror symmetries. They host interesting phenomena such as the quantized circular photogalvanic effect (CPGE) and the chiral magnetic effect. In this work, we report a comprehensive theoretical and experimental analysis of the linear and nonlinear optical responses of the chiral topological semimetal RhSi, which is known to host multifold fermions. We show that the characteristic features of the optical conductivity, which display two distinct quasi-linear regimes above and below 0.4 eV, can be linked to excitations of different kinds of multifold fermions. The characteristic features of the CPGE, which displays a sign change at 0.4 eV and a large non-quantized response peak of around 160 μA/V2 at 0.7 eV, are explained by assuming that the chemical potential crosses a flat hole band at the Brillouin zone center. Our theory predicts that, in order to observe a quantized CPGE in RhSi, it is necessary to increase the chemical potential as well as the quasiparticle lifetime. More broadly, our methodology, especially the development of the broadband terahertz emission spectroscopy, could be widely applied to study photogalvanic effects in noncentrosymmetric materials and in topological insulators in a contact-less way and accelerate the technological development of efficient infrared detectors based on topological semimetals.
Highlights
The robust and intrinsic electronic properties of topological metals —a class of quantum materials—can potentially protect or enhance useful electromagnetic responses[1,2,3,4]
In Dirac semimetals such as Cd3As2 and Na3Bi5,6 two doubly degenerate bands cross linearly at a single point, the Dirac point, and this crossing is protected by rotational symmetry[7,8,9]
A Dirac point can be understood as two coincident topological crossings with equal but opposite topological charge[4], and as a result, the topological contributions to the response to external probes cancel in this class of materials, rendering external probes insensitive to the topological charge
Summary
The robust and intrinsic electronic properties of topological metals —a class of quantum materials—can potentially protect or enhance useful electromagnetic responses[1,2,3,4]. Our optical conductivity and CPGE experiments are reasonably well reproduced by tight-binding and first-principle calculations when the chemical potential lies below the threefold node at the Γ point, crossing a relatively flat band, and when the hot-carrier lifetime is chosen to be ≈4–7 fs.
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