The lifetime of quasi-particles is a key characteristic of excited states in physical systems, which is closely related to many physical phenomena and essential for practical applications. The lifetime of quasi-particles is usually considered an inherent property of a system that is difficult to adjust actively. Current related research is mostly focused on understanding the physical mechanisms of various types of excited state relaxation and attempting to passively adjust the lifetime of specific excited states through material selection. In recent research, we have explored a new approach to actively adjust the lifetime of quasi-particles, which may provide new design ideas for future functional devices.
Due to the rich tunability of the graphene system, plasmon excitations in graphene (a type of electromagnetic wave mode) are an ideal platform for studying the control of quasi-particle lifetimes. In this study, we used a double-layer graphene model system with a boron nitride spacer layer and demonstrated that the lifetime of plasmons in a coupled Dirac system can be actively adjusted by controlling their relaxation pathways through an external electric field. Essentially, we used one layer of graphene in the double-layer graphene as a damping amplifier, changing the carrier density in the graphene layer through an external electric field, which then affected the relaxation rate of the entire coupled system. Using this approach, the decay rate of coupled plasmons can be increased by up to 1.7 times compared to the single-layer case. In fact, we can also use this damping modulator to design a damping switch to control the propagation of plasmons. This approach may also be applicable to the lifetime control of other quasi-particles and collective excitations.
The related research, titled "Electric field-controlled damping switches of coupled Dirac plasmons," was published in the recent issue of Physical Review Letters 129, 237402 (2022) and was selected as an Editors' Suggestion article. Professor Xiaoguang Li is one of the corresponding authors of the paper, responsible for the related theoretical calculations of graphene plasmons. This paper was completed in collaboration with the research group of Professor Changgan Zeng at the University of Science and Technology of China. The two research groups have a long-standing cooperation foundation based on graphene plasmon systems. Previously published papers on this collaboration involved the anisotropic excitation of plasmons in graphene superlattices [Physical Review B 90, 245415 (2014)], the scattering properties of plasmons on potential barriers in graphene [Nano Letters 18, 1373 (2018)], and the prediction of a new graphene plasmon edge excitation mode through phenomenological model analysis in graphene-substrate systems with high dielectric constants [Nanoscale 10, 16314 (2018)].
Moreover, in a similar bilayer Dirac electron system, Professor Xiaoguang Li and Professor Zhenyu Zhang from the University of Science and Technology of China recently collaborated on two theoretical studies, focusing on the possible new plasmon excitations in coupled Dirac electron systems. The related research results were published in Physical Review B: the first one studied the plasmon excitations in a graphene and topological insulator coupled system [Physical Review B 105, 205408 (2022)]; the second one investigated the plasmon modes formed by the coupling of two different 2D electron gases on the surface of a topological insulator [Physical Review B 105, 195150 (2022)].
The links to the relevant articles are as follows:
https:// doi.org/10.1103/PhysRevLett.129.237402
https://doi.org/10.1103/PhysRevB.105.205408
https://doi.org/10.1103/PhysRevB.105.195150
The above works were supported by the National Natural Science Foundation.
