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

Zhuoliang Ni,Y Zhang,A G Grushin,C Felser,K Manna,X Han,K Wang,O Pozo,F De Juan,B Xu,Liang Wu,E J Mele,J Paglione

doi:10.1038/s41467-020-20408-5

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.

Highlights

The absence of mirror symmetry, or chirality, is behind striking natural phenomena found in systems as diverse as DNA and crystalline solids
We identify a large longitudinal Circular photo-galvanic effect (CPGE) peaked at 0.4 eV reaching ~ 550 μ A/V2
To stimulate second-harmonic generation (SHG), we focused light pulses centered at 800 nm under near-normal incidence to a 10-μm diameter spot on the sample and the second harmonic signal centered at 400 nm is measured

Summary

Introduction

The absence of mirror symmetry, or chirality, is behind striking natural phenomena found in systems as diverse as DNA and crystalline solids. The longitudinal CPGE is remarkable because it was recently predicted to be quantized[11,12,13,14] These materials feature protected nodal crossings near the Fermi level, and because all mirror symmetries are broken, nodes with opposite chirality generically appear at different energies[15] (see Fig. 1a, b), in contrast to mirror-symmetric Weyl metals, like TaAs with nodes at the same energy[16,17]. The existence of these nodes is protected by an integer topological charge C, which quantizes the longitudinal CPGE trace to Cβ0 where β0 = πe3/h211. We measure the CPGE by detecting radiated terahertz (THz) pulses emitted from the illuminated regions, a method with several advantages compared to DC current measurements 8,9,24–27

Methods

Results

Conclusion