image: (a) Schematic illustration of the tunable injection of chiral polarized Weyl fermions. In the presence of an electric field, the chirality of circularly polarized light can couple with the chirality of magnetic Weyl cones, resulting in the injection of chiral polarized Weyl fermions, which manifests as the generation of IA .The injection of chiral polarized Weyl fermions can be flexibly controlled by the FM order or the external electric field. (b) Distribution of IA measured under opposite magnetization and electric field directions. All scale bars are 10 μm.
Credit: ©Science China Press
Weyl semimetals, a class of quantum materials defined by their unique band structure, host emergent chiral Weyl fermions that exhibit distinct physical properties. These materials hold great potential for developing novel information-processing schemes based on new quantum degrees of freedom. In Weyl semimetals, the spin of a Weyl fermion is locked parallel or antiparallel to its momentum, giving rise to two opposite chiralities. This chiral nature couples with circularly polarized light through optical transition selection rules, producing measurable photocurrent responses. If this coupling can be flexibly controlled by external fields, it offers a pathway to precisely manipulate the chiral degree of freedom, enabling the use of Weyl fermion chirality as a fundamental carrier for information encoding and storage.
In condensed matter systems, Weyl fermions appear when either spatial inversion symmetry or time-reversal symmetry is broken. In time-reversal-symmetry-breaking Weyl semimetals (magnetic Weyl semimetals), magnetism provides an additional means of control. For example, in the ferromagnetic Weyl semimetal Co₃Sn₂S₂, the chirality of the Weyl cones is directly tied to the direction of ferromagnetic magnetization. This order can be tuned in multiple ways: magnetically, via external magnetic fields; electrically, through spin-transfer or spin–orbit torques; or thermally, by inducing phase transitions. This tunability makes magnetic Weyl semimetals an attractive platform for exploring light–chirality interactions. However, their experimental exploration began relatively late. In inversion-symmetric magnetic Weyl semimetals, paired Weyl cones of opposite chirality are related by symmetry, often causing their contributions to cancel out—further complicating experimental detection. As a result, systematic studies on chiral optical coupling in these materials have remained limited.
Recently, Professor Dong Sun’s research group at the School of Physics, Peking University, made important progress in this direction. The team demonstrated selective injection of chiral Weyl fermions in Co₃Sn₂S₂ using circularly polarized mid-infrared light, enabled by a third-order nonlinear optical process under a static electric field. Crucially, they showed that this optical injection can be flexibly controlled by tuning both the external electric field and the ferromagnetic order, opening a new avenue for controlling topological quantum states.
The team systematically investigated Co₃Sn₂S₂ through photocurrent measurements sensitive to light handedness. In the mid-infrared range, they observed a pronounced helicity-dependent photocurrent response when an in-plane electric field was applied in the ferromagnetic phase. Wavelength-dependent studies revealed that this effect only arises under low-energy mid-infrared excitation, consistent with coupling to Weyl cones. Notably, they detected a sign reversal in the helicity-dependent photocurrent at different wavelengths, corresponding to imbalanced injection of chiral-polarized Weyl fermions on opposite sides of the Weyl cone. Further analysis confirmed that the observed effect originates primarily from photocurrents generated by chiral-polarized Weyl fermions injected by mid-infrared photons.
These findings highlight the exceptional versatility of magnetic Weyl semimetals in chiral control. Because the chirality of their Weyl cones is directly coupled to magnetic order, the chiral optical response can be modulated in real time and in a reversible manner by external magnetic fields, electric fields, or temperature. This property establishes magnetic Weyl semimetals as a more adaptable platform than inversion-symmetry-breaking counterparts for chiral regulation, laying a foundation for future quantum devices based on chiral degrees of freedom.
This work was recently published in National Science Review. The School of Physics at Peking University is the first affiliation. PhD student Zipu Fan (Peking University) is the first author, and Professors Dong Sun (Peking University), Jinluo Cheng (Changchun Institute of Optics, Fine Mechanics and Physics), and Enke Liu (Institute of Physics, Chinese Academy of Sciences) are the co-corresponding authors.
Journal
National Science Review