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

TL;DR

This study demonstrates a giant, topologically protected circular photogalvanic effect in the chiral semimetal CoSi, with potential applications in mid-infrared detection and insights into quantized photocurrents.

Contribution

It provides the first experimental observation of a large, topologically driven photocurrent in CoSi, with evidence supporting the possibility of quantized CPGE under certain conditions.

Findings

Peak photocurrent of ~550 μA/V² at 0.4 eV in CoSi

Photocurrent originates from topological band crossings

Quantization constant reached 3.3±0.3, close to theoretical predictions

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 photogalvanic 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 eV - 1.1 eV in the chiral topological semimetal CoSi. We identify a large longitudinal photocurrent peaked at 0.4 eV reaching $\sim$ 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 from topological band crossings, reaching 3.3$\pm$0.3 in units of the quantization constant. Our calculations indicate that the quantized CPGE 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.