Quantized circular photocurrents in chiral topological semimetals*

In materials that break inversion symmetry, light can generate a DC current via the photogalvanic effect. In this talk I will describe a surprising topological feature of the photogalvanic response to circularly polarized light: In chiral crystalline materials featuring point-like band crossings known as Weyl nodes, the circular photocurrent is exactly quantized in terms of fundamental constants only. The reason behind this is that the photocurrent directly measures the monopole charge of a Weyl node, a rare example of quantization in a gapless system at finite frequency. After presenting the basic theory behind the effect, I will discuss whether it can occur for multiband symmetry-protected crossings, such as those recently found in CoSi and related materials. I will also review the conditions to extract the quantized coefficient in different approaches, including DC photocurrents, THz spectroscopy and difference frequency generation, and discuss the recent experimental progress on this front.