Former materials are indispensable for modern technology – they are in computers, solar cells, lasers and countless other applications. Now physicists have discovered a new quantum phenomenon with one of these materials. For the first time, they were able to demonstrate quasite particles – excitons – generated by stimulation with light on the surface of a layered antiferromagnetic semiconductor. So far, these quasi particles were only known from the inner such materials. The discovery of surface excitons is an important step for understanding these unusual quantum structures and the physics of 2D magnets, the team explains. At the same time, the newly discovered type of quasi particles could also enable new applications in photonics and with optical nanomaterials.
Exzitons are quasi -particles that arise, among other things, when radiation affects a half -conductive material. The photons of the light rain electrons in the material, which thus leave their original position. What remains is a positively charged “hole” in the crystal grille. This attracts the “escaping” electron and forms a kind of couple with it. Because these couples are from electron and hole like a new, independent particle, they are referred to as quasi particles. The peculiarity of the Exzitons is that they can store the energy of the originally stimulating light and can also transport them within the material due to their mobility, without a loading transport – because together, hole and electron are electrically neutral. When the Exzitons dissolve, release the energy as light. “Exzitons are extremely important for optical properties of nanomaterials,” explains co-orer Florian Dirnberger from the Technical University of Munich. Usually you can find excitons in non-magnetic semiconductors, because magnets are mostly metallic and therefore cannot train stable excitons.
Quasite particles in the layered semiconductor
But now physicists know some materials that form excitons despite magnetic properties. This is made possible, among other things, in antiferromagnets, which consist of layers with the opposite of the nuclear pins. “Only in the past four years has material physics discovered what potential the quasi particles have when they are generated in magnetic crystals,” says Dirnberger. “Then the excitons can also store and transport information in addition to the energy, but also release it as light. The exploration of the exotic quasi particles is still at the beginning, but could in future be the basis for new technologies that combine photonics and magnetism.” Antiferromagnetic semiconductor materials, which consist of many ultra-thin, only loose over van-der Waals forces with each other are particularly promising. “Such van der Waals materials crystallize in layer structures with only weak ties between the layers,” explain the physicists. “This shows remarkable physical properties.” Each individual layer almost behaves in such a material as if it were an isolated 2D material.
For their study, Dirnberger and his colleagues examined the anti-ferromagnetic quantum leader Chromium-Sulfide-Bromide (CRSBR). It was previously known from this that excitons can arise inside radiation. “These excitons in the CRSBR show large oscillator strengths,” report the physicists. This means that these quasi particles interact strongly with incident light and can absorb a lot of its energy. However, it was unknown whether the semiconductor material can also form excitons on its surface. Dirnberger and his team have now examined this. To do this, they breeded wafer-thin, but multi-layered chromium sulfide bromide crystals, cooled down to five Kelvin and analyzed the radiation with special spectrometers that emitted from the material after suggesting. “Because the surface excitons reflect and hand in the light with a slightly different color than the quasi particles inside the material, we were able to control them in a targeted manner,” explains senior author Alexey Chernikov from the Würzburg Dresden Excellence Cluster Ct.Qmat.
First detection of surface excitons in a semiconductor magnet
The physicists discovered something new in their experiment: “In the laboratory we saw Exzitons that not only exist deep in the material, but also on its surface,” reports Chernikov. These quasi particles are a previously not identified type of optical stimulation in such materials. Further analyzes showed that these newly discovered surface excitons differ from the excitons inside the semiconductor in some core characteristics. “Due to the different dielectric and magnetic environments, the exitz zones limited on the surface layers have different energies than the excitons in the inner layers,” explains the team. The oscillator strengths of the two Exziton species also change in different ways.
As the physicists explain, the new findings are important for both the basic understanding of magnetic materials and technological developments in this area. Because the production of such surface excitons in a magnetic semiconductor material provides new insights into the physics of 2D magnets, but also applications for optical nanomaterials and in photonics. “The physics of excitonic conditions presented here will be of interest in a broad science community that examines optical and magnetic phenomena in low -dimensional systems,” the team writes.
Source: Yinming Shao (Columbia University, New York) et al., Nature Materials, DOI: 10.1038/S41563-02122-Z
