A group of researchers from The University of Oklahoma (OU) has recently predicted the presence of a new type of exciton, topological exciton, which could facilitate the development of future quantum devices.
Study: Theory of topological exciton insulators and condensates in flat Chern bands. Image Credit: KanawatTH/Shutterstock.com
Topological Excitons
Excitons are neutral quasiparticles that form when bound states are created between holes and electrons by Coulomb interactions. They can condense and display superfluidity at sufficiently low temperatures owing to their bosonic nature.
Excitons have been observed in materials powering modern computers, such as semiconductors. In a recent study published in the Proceedings of the National Academy of Sciences, the OU-led research team predicted the presence of a new exciton type with finite vorticity in momentum space/topological exciton, which exists in Chern insulators/the midgap of generic flat Chern bands.
These topological excitons can contribute to advances in quantum devices. Topology, a branch of mathematics, studies the properties of surfaces and shapes that remain unchanged even when they are bent, twisted, or stretched.
Scientists utilize topological ideas to describe materials whose electronic properties are not affected by imperfections. In topology, Chern primarily refers to a class where the shapes’ key characteristics are represented using whole numbers. Specifically, Chern insulators allow electrons to orbit a material’s edge but do not internally conduct electricity.
However, they do form one-way currents spontaneously that flow either counterclockwise or clockwise along a two-dimensional material’s edges. These unidirectional currents are measured accurately in basic units of current.
The Study
In this research, researchers theoretically showed the existence of topological excitons in the midgap of generic flat Chern bands when the valence and conduction bands are topologically distinct. Although excitons can form spontaneously in narrow gap insulators, they can be more easily observed with pumped light.
Monochromatic photons promote coherent population inversion between flat bands over states spanning the whole Brillouin zone. Thus, researchers developed the nonequilibrium theory for light-pumped excitons in topological flat bands and studied the quantum geometry’s manifestations on the exciton profile functions and the exciton center of mass motion.
The topological excitons’ envelope wavefunction possessed a finite vorticity that was mandated by gauge invariance. Researchers corroborated their analysis by solving the out-of-equilibrium equation of state explicitly for excitons on the lattice for the flattened Haldane model.
Exciton superfluidity can be detected through photoluminescence. At the beginning of superfluidity, the spectrally integrated photoluminescence intensity can be significantly enhanced, while the photon statistics displayed by the emission can strongly deviate from a Poissonian distribution, demonstrating a bunching transition.
Significance of the Research
The researchers predicted that excitons generated through Chern insulators by shining light under well-defined conditions would inherit the host material electrons and holes’ nontrivial topological properties. This prediction is robust as it was based on fundamental concepts in place of computer simulations.
Moire heterostructures of graphene and transition metal dichalcogenides could offer a possible platform for the observation of topological excitons. In insulators, the electrons were excited by the light from the valence band to the conduction band, where these electrons could move freely. The resulting excitons were also topological when the valence and conduction bands were topologically distinct.
Thus, those excitons were predicted to emit circularly polarized light spontaneously once they decay by releasing energy. These topological excitons can be utilized for designing a new class of optical devices.
Excitons at low temperatures formed a novel neutral superfluid type that can be utilized to create advanced photonic devices or robust polarized light emitters for quantum computing. Specifically, the excitons condensed below the Kostelitz-Thouless transition temperature and formed a novel type of topological neutral superfluid with profile wavefunctions in momentum space that carry a finite vorticity, indicating the Chern number of the state.
The prediction of this composite particle could facilitate the development of new topology-based optoelectronic devices. Moreover, this particle could play a crucial role in quantum communications and aid in engineering qubits with two entangled states, off and on, based on the polarization/vorticity of the emitted light.
In conclusion, the prediction of the existence of topological excitons has opened up new possibilities in quantum technology.
References
DeLozier, J. (2024) Quantum Researchers Publish ‘Exciting’ Particle Prediction [Online] Available at https://www.ou.edu/news/articles/2024/august/quantum-researchers-publish-exciting-particle-prediction (Accessed on 03 September 2024)
Xie, H., Ghaemi, P., Mitrano, M., Uchoa, B. (2024). Theory of topological exciton insulators and condensates in flat Chern bands. Proceedings of the National Academy of Sciences, 121(35), e2401644121. DOI: 10.1073/pnas.2401644121, https://www.pnas.org/doi/10.1073/pnas.2401644121
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