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Multiple topological transitions in twisted bilayer graphene near the first magic angle
摘要： Recent experiments have observed strongly correlated physics in twisted bilayer graphene (TBG) at very small angles, along with nearly flat electron bands at certain fillings. A good starting point in understanding the physics is a continuum model (CM) proposed by Lopes dos Santos et al. [Phys. Rev. Lett. 99, 256802 (2007)] and Bistritzer et al. [Proc. Natl. Acad. Sci. USA 108, 12233 (2011)] for TBG at small twist angles, which successfully predicts the bandwidth reduction of the middle two bands of TBG near the first magic angle θ0 = 1.05°. In this paper, we analyze the symmetries of the CM and investigate the low energy flat band structure in the entire moiré Brillouin zone near θ0. Instead of observing flat bands at only one 'magic' angle, we notice that the bands remain almost flat within a small range around θ0, where multiple topological transitions occur. The topological transitions are caused by the creation and annihilation of Dirac points at either K, K', or Γ points or along the high symmetry lines in the moiré Brillouin zone. We trace the evolution of the Dirac points, which are very sensitive to the twist angle, and find that there are several processes transporting Dirac points from Γ to K and K'. At the Γ point, the lowest energy levels of the CM are doubly degenerate for some range of twisting angle around θ0, suggesting that the physics is not described by any two-band model. Based on this observation, we propose an effective six-band model (up to second order in quasimomentum) near the Γ point with the full symmetries of the CM, which we argue is the minimal model that explains the motion of the Dirac points around Γ as the twist angle is varied. By fitting the coefficients from the numerical results, we show that this six-band model captures the important physics over a wide range of angles near the first 'magic' angle.
关键词： flat bands,twisted bilayer graphene,continuum model,magic angle,topological transitions,Dirac points
更新于2025-09-23 15:22:29
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Wafer Scale Graphene Field Effect Transistors on Thin Thermal Oxide
摘要： In this study, we present the feasibility to fabricate back-gated graphene field-effect transistors (GFETs) on 10 nm thermal SiO2 substrate. Here, we compare the mobility of graphene devices at different locations of the transferred CVD graphene. We observed that there is a n-type doping of the graphene devices with Dirac points within ± 0.5 V from an ideal value of 0 V. The downscaling of the back-gate dielectric thickness reduces the operating voltage range, commonly required for low power electronics, and the devices are stable during operation in air under ambient conditions. The extracted contact resistance is comparable to the earlier reports found in literature and this provides a feasibility to fabricate low power futuristic graphene based nanoelectronics.
关键词： CVD graphene,n-type doping,thermal SiO2,mobility,Dirac points,low power electronics,field-effect transistors,graphene,contact resistance
更新于2025-09-23 15:21:01
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Wave packet dynamics in slowly modulated photonic graphene
摘要： Mathematical analysis on electromagnetic waves in photonic graphene, a photonic topological material which has a honeycomb structure, is one of the most important current research topics. By modulating the honeycomb structure, numerous topological phenomena have been observed recently. The electromagnetic waves in such media are generally described by the 2-dimensional wave equation. It has been shown that the corresponding elliptic operator with a honeycomb material weight has Dirac points in its dispersion surfaces. In this paper, we study the time evolution of the wave packets spectrally concentrated at such Dirac points in a modulated honeycomb material weight. We prove that such wave packet dynamics is governed by the Dirac equation with a varying mass in a large but ?nite time. Our analysis provides mathematical insights to those topological phenomena in photonic graphene.
关键词： Photonic graphene,Dirac points,Bloch decomposition,Honeycomb structure
更新于2025-09-19 17:13:59