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

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 0.2-1.1eV in the chiral topological semimetal CoSi. We identify a large longitudinal photocurrent peaked at 0.4eV reaching similar to 550 mu A/V-2, 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 3.30.3 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.

Automated summary

The absence of mirror symmetry, or chirality, leads to striking natural phenomena in systems such as DNA and crystalline solids. Chiral semimetals with topologically protected band degeneracies exhibit a circular photo-galvanic effect that can be quantized under specific conditions. These conditions are non-universal and depend on material parameters and incident frequency. Terahertz emission spectroscopy on the chiral topological semimetal CoSi reveals a large longitudinal photocurrent, suggesting potential applications in mid-infrared detectors.