In contrast to the 7 meV binding energy of excitons in GaAs, in semiconducting 2D materials the binding energy is 100-1000 meV and excitons can be detected at room temperature (25 meV). In semiconducting carbon nanotubes and graphene nanoribbons the exciton binding energies are of about 400-700 meV. In quasi-metallic structures the reported theoretical values for high energy transitions (i.e. not across the curvature gap) are about 50-100 meV. The excitons across the intrinsic-strain-induced THz band gap are not well studied by the first
principles community and, in fact, very few reports based on k∙p models (that is a continuum approximation to semi-empirical tight-binding (TB) models) can be found. Indeed, there are numerical difficulties in treating such systems. It must be also noted that the specified above binding energies are comparable to the THz energy band gaps produced by the intrinsic strain in quasi-metallic nanotubes and nanoribbons. According to the theoretical predictions, when the energy gap is smaller than the binding energy of excitons, a novel strongly interacting phase of matter called exciton insulator may arise . In this phase, excitons form similar to superconducting Cooper pairs. Studies in carbon nanotubes and graphene nanoribbons with theoretical works but no conclusive experimental evidences have appeared so far. In contrast to the conventional 2D materials, the excitonic effects in quasi-metallic two-dimensional graphene superlattices with a quasi-1D nature have not been yet investigated. TeraExc action has studied two-dimensional superlattices with quasi-1D nature with first principles methods and reveal the basic physics of THz interband optical transitions. It obtained the some evidences of the possibility of excitonic insulators at the borderline between 2D [1] and 1D systems [2]. An independent check of these results is underway. Further work is needed in this direction. After verification with different program implementations and advanced theory levels, an experiment is needed to validate the expected effects. In principle such systems could be attempted to synthesize on (111) planes of CuAu that feature somewhat distorted trigonal arrays of Cu and Au atoms. The distortion coming from the tetragonal P4/mmm L1_0 structure of ordered CuAu, can be used as a template for the hydrogen adsorption that have been proven to be preferable for the curved surfaces by Prof. E. Perez group.
The potential impact may include development of optical devices utilizing novel physical principles in the area of THz emitters. This could facilitate development of the ultra-fast wireless communication in EU and beyond as well as ensure EU leadership in the telecom market.
1. Jia, Y. et al. Evidence for a monolayer excitonic insulator. Nature Physics (2021) doi:10.1038/s41567-021-01422-w.
2. Zhao, Y., Qu, H., Zhao, J., Kang, L. & Zhou, S. One-Dimensional Excitonic Insulator of M6 Te6 (M = Mo, W) Atomic Wires. Nano Lett. acs.nanolett.4c05448 (2025) doi:10.1021/acs.nanolett.4c05448.
