Direct observation of anomalous spintronic dark excitons on flat near band edges
Yuan-Ron Ma1,2*
1Department of Applied Informatics, Fo Guang University, Ilan, Taiwan
2Department of Physics, National Dong Hwa University, Hualien, Taiwan
* Presenter:Yuan-Ron Ma, email:ronma@gms.ndhu.edu.tw
We can directly observe anomalous spintronic dark excitons at zero magnetic field for bulk 2D van-der-Wall (vdW) VI3. The anomalous spintronic dark excitons corresponding to spin electron excitations are observed on flat near band edges (NBEs) without valley effect. Many 2D vdW materials have dark excitons [1-3], while they can be seen only at very high magnetic fields, indicating that dark excitons have a spintronic nature. Since bulk VI3 is a ferromagnetic semiconductor [4-6], each layer VI3 has parallel spin electrons on surface. The parallel spin electrons excited from the valence band (VB) to the conduction band (CB), while antiparallel spin holes remain on the highest occupied VB naturally. Spintronic dark excitons are formed with the spin electrons on the lowest unoccupied CB and the antiparallel spin holes on the highest occupied VB at the same reciprocal space, because of the attractive Coulomb force between spin electrons and holes. However, there is the other repulsive Coulomb force between the excited spin electrons and the antiparallel spin holes, so no photoluminescence (PL) light is released. In other words, when the excited spin electrons are bound to parallel spin holes, there are only the attractive Coulomb forces between spin electrons and spin holes. Then spin electron-hole recombination occurs to emit PL, so the formation of an excited spin electron bound to a parallel spin hole is called the bright exciton. Photomodulated reflectance (PR) spectroscopy is used to probe the anomalous spintronic excitons at a zero magnetic field for 2D ferromagnetic semiconductors. PL spectroscopy can observe only bright excitons, because of PL light. However, PR spectroscopy is superior to PL spectroscopy, because it can detect not only emission transitions but also absorption transitions [7].
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Keywords: Anomalous, Spintronic, Dark excitons, Photomodulated, Reflectance