In twisted bilayer graphene (TBG) a moiré pattern forms that introduces a new length scale to the material. At the 'magic' twist angle of 1.1°, this causes a flat band to form, yielding emergent... Show moreIn twisted bilayer graphene (TBG) a moiré pattern forms that introduces a new length scale to the material. At the 'magic' twist angle of 1.1°, this causes a flat band to form, yielding emergent properties such as correlated insulator behavior and superconductivity [1-4]. In general, the moiré structure in TBG varies spatially, influencing the local electronic properties [5-9] and hence the outcome of macroscopic charge transport experiments. In particular, to understand the wide variety observed in the phase diagrams and critical temperatures, a more detailed understanding of the local moiré variation is needed [10]. Here, we study spatial and temporal variations of the moiré pattern in TBG using aberration-corrected Low Energy Electron Microscopy (AC-LEEM) [11,12]. The spatial variation we find is lower than reported previously. At 500°C, we observe thermal fluctuations of the moiré lattice, corresponding to collective atomic displacements of less than 70pm on a time scale of seconds [13], homogenizing the sample. Despite previous concerns, no untwisting of the layers is found, even at temperatures as high as 600°C [14,15]. From these observations, we conclude that thermal annealing can be used to decrease the local disorder in TBG samples. Finally, we report the existence of individual edge dislocations in the atomic and moiré lattice. These topological defects break translation symmetry and are anticipated to exhibit unique local electronic properties. Show less
Ding, D.; Driel, D. van; Pereira, L.M.C.; Bauters, J.F.; Heck, M.J.R.; Welker, G.; ... ; Bouwmeester, D. 2020
Surface plasmon propagation and scattering on structured metal–dielectric interfaces are important tools for field confinement and enhanced atom–field interaction. In this paper, we demonstrate a... Show moreSurface plasmon propagation and scattering on structured metal–dielectric interfaces are important tools for field confinement and enhanced atom–field interaction. In this paper, we demonstrate a new type of surface plasmon (SP) lasing that more closely resembles the usual mirror-based laser, in a geometry that comprises a central 50-μm-long flat region between two metal hole arrays that serve as reflecting mirrors. The lasing mode shows features of double-slit interference modulated by the radiation pattern of, and selection rules set by, the scattering of SPs on the 2D hole arrays. Our results also provide information on the group velocity of surface plasmons and their scattering and penetration in hole arrays. Show less
Tenner, V.T.; Dood, M.J.A. de; Exter, M.P. van 2018
