Giant topological longitudinal circular photo-galvanic effect in the chiral multifold semimetal CoSi

Abstract

The absence of mirror symmetry, or chirality, is behind striking natural phenomena found in systems as diverse as DNA and crystalline solids. A remarkable example occurs when chiral semimetals with topologically protected band degeneracies are illuminated with circularly polarized light. Under the right conditions, the part of the generated photocurrent that switches sign upon reversal of the light’s polarization, known as the circular photo-galvanic effect, is predicted to depend only on fundamental constants. The conditions to observe quantization are non-universal, and depend on material parameters and the incident frequency. In this work, we perform terahertz emission spectroscopy with tunable photon energy from 0.2 –1.1 eV in the chiral topological semimetal CoSi. We identify a large longitudinal photocurrent peaked at 0.4 eV reaching ~550 μ A/V2, which is much larger than the photocurrent in any chiral crystal reported in the literature. Using first-principles calculations we establish that the peak originates only from topological band crossings, reaching 3.3 ± 0.3 in units of the quantization constant. Our calculations indicate that the quantized circular photo-galvanic effect is within reach in CoSi upon doping and increase of the hot-carrier lifetime. The large photo-conductivity suggests that topological semimetals could potentially be used as novel mid-infrared detectors.

Optical signatures of multifold fermions in the chiral topological semimetal CoSi

“…Most notably, we provide evidence of the existence of the fourfold spin-3/2 quasiparticles. Our results not only shed light on the interpretation of circular photogalvanic measurement in this family of materials (17,31,43,45,46) but also pave the way to study optical signatures in other chiral topological semimetals in the future (16,34).…”

“…An accurate measurement of the optical conductivity is also essential to precisely extract nonlinear optical responses such as second harmonic generation (36,37) and photogalvanic effect precisely (38)(39)(40)(41)(42)(43)(44). Clarifying the nonlinear responses in these noncentrosymmetric topological semimetals acts to certify the presence and energy range where topological nodes are active (43,45,46). Determining the carrier lifetime and energy range at which topological crossings are activated is the key to observing the quantized circularly photogalvanic effect and to using them in the next generation of efficient topological optoelectronics (31).…”

Optical Response of Chiral Multifold Semimetal PdGa

“…The lack of inversion and mirror symmetries, in combination with SOC, leads to the splitting of bands around the high symmetry points Γ and R, giving rise to a non-collinear spin arrangement with Chern numbers χ = ±4 [29] with maximally extended surface states, ranging from the center to the edge of the Brillouin zone [25,27,30]. In such semimetals, the quantized circular photogalvanic effect has been predicted, which, by effect of topological states, constitutes a photocurrent, quantized in units of material-independent fundamental constants [31][32][33][34].…”

Terahertz detection based on nonlinear Hall effect without magnetic field

“…Unlike photogalvanic effects at optical frequencies due to interband transitions (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26), the photoresponse of metals and degenerate semiconductors in the terahertz frequency range is usually governed by an intraband process involving electrons near the Fermi surface only. Importantly, since the dispersion relation of the Bloch electron in nonmagnetic materials necessarily satisfies k = −k due to time-reversal symmetry, the second-order response of noncentrosymmetric materials is an inherently quantum-mechanical effect arising from the inversion asymmetry of the electron wavefunction within the unit cell.…”