Researchers develop a theoretical description of light-induced topological states

Topological materials that possess certain symmetries at the atomic level, including topological insulators and topological semimetals, have aroused the fascination of many condensed-matter scientists because of their complex electronic properties. Now, researchers in Japan have shown that a normal semiconductor can be transformed into a topological semimetal by irradiating it with light. They also showed how spin-dependent reactions can occur when illuminated with circularly polarized laser light. Published in Physical Check BThis work investigates the possibility of creating topological semimetals and manifesting new physical properties through light control, which can open a rich physical frontier for topological properties.

Most common substances are either electrical conductors like metals or insulators like plastic. In contrast, topological insulators can exhibit unusual behavior where electric currents flow along the surface of the sample but not inside. This characteristic behavior is strongly related to topological properties inherent in the electronic state. In addition, a novel phase dubbed the topological semimetal provides a new playground for exploring the role of topology in condensed matter. However, the underlying physics of these systems is still a matter of debate.

Researchers from Tsukuba University studied the dynamics of excitations in zinc arsenide (Zn₃As₂) when irradiated with a laser with circular polarization. Zinc arsenide is usually considered a narrow gap semiconductor, meaning that electrons are not free to move but can be easily powered by energy from an external light source. Under the right conditions, the material can exhibit a special topological state called the “Floquet-Weyl semimetal,” which is a light-coupled topological semimetal. In this case, the electric current can be transported in the form of quasiparticles, so-called Weyl fermions. Because these quasiparticles move as if they had no mass and resist scattering, Weyl fermions can easily move through the material.

“Floquet-Weyl semimetals exhibit a handful of rare properties that can be exploited in electronic devices, including high mobility, titanic reluctance, and spin-polarized currents,” says author Professor Ken-ichi Hino. In the current work, the researchers showed that when a left-handed circularly polarized CW laser is tuned at a frequency close to the energy gap in the material, the spin-down electrons and the spin-up electrons form different phases, a Weyl semi-metal and a narrow-gap insulator. The latter is close to another topological semimetal called the nodal line semimetal.

“Our exploration of the transient dynamics of excitations in zinc arsenide can deepen the understanding of the underlying physics of these materials,” says senior author Runnan Zhang. This fundamental research can also help accelerate the development of techniques for light-induced surface magnetization of nonmagnetic materials.