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Classification of flat bands according to the band-crossing singularity of Bloch wave functions
摘要： We show that ?at bands can be categorized into two distinct classes, that is, singular and nonsingular ?at bands, by exploiting the singular behavior of their Bloch wave functions in momentum space. In the case of a singular ?at band, its Bloch wave function possesses immovable discontinuities generated by the band-crossing with other bands, and thus the vector bundle associated with the ?at band cannot be de?ned. This singularity precludes the compact localized states from forming a complete set spanning the ?at band. Once the degeneracy at the band crossing point is lifted, the singular ?at band becomes dispersive and can acquire a ?nite Chern number in general, suggesting a new route for obtaining a nearly ?at Chern band. On the other hand, the Bloch wave function of a nonsingular ?at band has no singularity, and thus forms a vector bundle. A nonsingular ?at band can be completely isolated from other bands while preserving the perfect ?atness. All one-dimensional ?at bands belong to the nonsingular class. We show that a singular ?at band displays a novel bulk-boundary correspondence such that the presence of the robust boundary mode is guaranteed by the singularity of the Bloch wave function. Moreover, we develop a general scheme to construct a ?at band model Hamiltonian in which one can freely design its singular or nonsingular nature. Finally, we propose a general formula for the compact localized state spanning the ?at band, which can be easily implemented in numerics and offer a basis set useful in analyzing correlation effects in ?at bands.
关键词： Chern number,singular band touching,flat bands,nonsingular band touching,bulk-boundary correspondence,compact localized states,Bloch wave functions
更新于2025-09-23 15:23:52
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Temporal Localized Structures in Mode-Locked Vertical External-Cavity Surface-Emitting Lasers
摘要： The possibility of using optical Localized States (LSs) in resonators to code and process information bits has attracted a great deal of attention in the last twenty years [1]. LSs are stable solutions of dissipative systems characterized by a correlation range much shorter than the size of the hosting system. Temporal LSs (TLSs) have been recently reported in passively mode-locked (PML) semiconductor lasers [2]. It was shown that for cavity round-trip (cid:87) larger than the gain recovery (cid:87)g and for large modulation of the losses induced by the absorber, several pulsating states with different number of pulses coexist for the same parameter values. In this multistable regime mode-locked pulses become localized, i.e. they can be addressed individually [3]. This was implemented in an electrically-pumped broad-area Vertical-Cavity Surface-Emitting Laser (VCSEL) coupled to a resonant semiconductor saturable absorber mirror (SESAM). VCSEL far-field transverse profile was imaged onto the SESAM in a self-collimating scheme for achieving stable mode-locking regime [4]. Though this peculiar scheme allowed efficient saturation of the SESAM, it also induced global coupling of the VCSEL’s transverse section.
关键词： Localized States,mode-locked,Temporal Localized Structures,Vertical External-Cavity Surface-Emitting Lasers
更新于2025-09-16 10:30:52
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Metal nano-composite assisted photons harvesting in thin film organic photovoltaic
摘要： Nickel oxide (NiO) nano-particles assisted photons harvesting is studied using P3HT:PCBM blend based bulk heterojunction thin film organic solar cell (TFOSC). A comparison between devices that were fabricated at different concentration of NiO and that of the pristine P3HT:PCBM active layer were drawn up. The experimental results suggest that the incorporation of NiO in the solar absorber medium was found to be favourable for solar energy harvesting. At optimum concentration of NiO, in P3HT:PCBM blend active layer, the power conversion efficiency has grown by over 140% compared to the pristine type of devices. Moreover, significant improvement were also recorded on the solar cell fill factor (FF) and short circuit current density (JSC), respectively. The newly fabricated solar cells are discussed in terms of the optical and electrical properties of the solar absorber film.
关键词： Localized states,Solar cell,Nickel Oxide,Nano-composites,Nano-particles
更新于2025-09-12 10:27:22