No abstract

Graphene on silicon carbide (SiC) bears great potential for future graphene electronic applications 1-5 because it is available on the wafer-scale 6-8 and its properties can be custom-tailored by inserting various atoms into the graphene/SiC interface 9-15 . It remains unclear, however, how atoms can cross the impermeable graphene layer during this widely used intercalation process 9,16,17 .Here we demonstrate that, in contrast to the current consensus, graphene layers on SiC are not homogeneous, but instead composed of domains of different crystallographic stacking [18][19][20] . We show that these domains are intrinsically formed during growth and that dislocations between domains dominate the (de)intercalation dynamics. Tailoring these dislocation networks, e.g. through substrate engineering, will increase the control over the intercalation process and could open a playground for topological and correlated electron phenomena in two-dimensional superstructures 21-24 .

We introduce a new method to continuously map inhomogeneities of a moiré lattice and apply it to open-device twisted bilayer graphene (TBG). We show that the variation in the twist angle, which is frequently conjectured to be the reason for differences between devices with a supposed similar twist angle, is about 0.04° over areas of several hundred nm, comparable to devices encapsulated between hBN slabs. We distinguish between an effective twist angle and local anisotropy and relate the latter to heterostrain. Our results suggest that the lack of evidence for superconductivity in open devices is not a consequence of higher heterogeneity in the twist angle, but possibly due to the absence of interaction with a top hBN layer. Furthermore, our results imply that for our devices, twist angle heterogeneity has a roughly equal effect to the electronic structure as local strain. The method introduced here is applicable to results from different imaging techniques, and on different moiré materials.

Nuclear spin-lattice relaxation times are measured on copper using magnetic resonance force microscopy performed at temperatures down to 42 mK. The low temperature is verified by comparison with the Korringa relation. Measuring spin-lattice relaxation times locally at very low temperatures opens up the possibility to measure the magnetic properties of inhomogeneous electron systems realized in oxide interfaces, topological insulators and other strongly correlated electron systems such as high-Tc superconductors.Comment: We revised the manuscript by including the supplemental material. The manuscript is changed from a Letter to a Research Article after change of journa

In ‘magic angle’ twisted bilayer graphene (TBG) a flat band forms, yielding correlated insulator behavior and superconductivity. In general, the moiré structure in TBG varies spatially, influencing the overall conductance properties of devices. Hence, to understand the wide variety of phase diagrams observed, a detailed understanding of local variations is needed. Here, we study spatial and temporal variations of the moiré pattern in TBG using aberration-corrected Low Energy Electron Microscopy (AC-LEEM). We find a smaller spatial variation than reported previously. Furthermore, we observe thermal fluctuations corresponding to collective atomic displacements over 70 pm on a timescale of seconds. Remarkably, no untwisting is found up to 600 ∘C. We conclude that thermal annealing can be used to decrease local disorder. Finally, we observe edge dislocations in the underlying atomic lattice, the moiré structure acting as a magnifying glass. These topological defects are anticipated to exhibit unique local electronic properties.