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The Berry phase of wave functions is a key quantity to understand various low-energy properties of matter, among which electric polarisation, orbital magnetism, as well as topological and ultra-relativistic phenomena. Standard approaches to probe the Berry phase in solids rely on the electron dynamics in response to electromagnetic forces. In graphene, probing the Berry phase $\pi $ of the massless relativistic electrons requires an external magnetic field. Here, we show that the Berry phase also affects the static response of the electrons to a single atomic scatterer, through wavefront dislocations in the surrounding standing-wave interference. This provides a new experimental method to measure the graphene Berry phase in the absence of any magnetic field and demonstrates that local disorder can be exploited as probe of topological quantum matter in scanning tunnelling microscopy experiments.

Two-dimensional and van der Waals ferromagnets are ideal platform to study low-dimensional magnetism and proximity effects in van der Waals heterostructures. Their ultimate two-dimensional character also offers the opportunity to easily adjust their magnetic properties using strain or electric fields. Among 2D ferromagnets, the Cr 1 + x Te2 compounds with x = 0–1 are very promising because their magnetic properties depend on the amount of self-intercalated Cr atoms between pure CrTe2 layers and the Curie temperature ( T C ) can reach room temperature for certain compositions. Here, we investigate the evolution of the composition as well as the structural and magnetic properties of thin Cr1.33Te2 (Cr2Te3) films epitaxially grown on graphene upon annealing. We observe a transition above 450 °C from the Cr1.33Te2 phase with perpendicular magnetic anisotropy and a T C of 180 K to a composition close to Cr 1.39 Te2 with in-plane magnetic anisotropy and a T C of 240–250 K. This phase remains stable up to 650 °C above which a pure Cr film starts to form. This work demonstrates the complex interplay between intercalated Cr, lattice parameters, and magnetic properties in Cr 1 + x Te2 compounds. [ABSTRACT FROM AUTHOR]

Fractional charges are one of the wonders of the fractional quantum Hall effect. Such objects are also anticipated in two-dimensional hexagonal lattices under time reversal symmetry—emerging as bound states of a rotating bond texture called a Kekulé vortex. However, the physical mechanisms inducing such topological defects remain elusive, preventing experimental realization. Here, we report the observation of Kekulé vortices in the local density of states of graphene under time reversal symmetry. The vortices result from intervalley scattering on chemisorbed hydrogen adatoms. We uncover that their 2π winding is reminiscent of the Berry phase π of the massless Dirac electrons. We can also induce a Kekulé pattern without vortices by creating point scatterers such as divacancies, which break different point symmetries. Our local-probe study thus confirms point defects as versatile building blocks for Kekulé engineering of graphene's electronic structure. Kekulé vortices in hexagonal lattices can host fractionalized charges at zero magnetic field, but have remained out of experimental reach. Here, the authors report a Kekulé vortex in the local density states of graphene around a chemisorbed hydrogen adatom. [ABSTRACT FROM AUTHOR]

We demonstrate that stacking layered materials allows a strain engineering where each layer is strained independently, which we call heterostrain. We combine detailed structural and spectroscopic measurements with tight-binding calculations to show that small uniaxial heterostrain suppresses Dirac cones and leads to the emergence of flat bands in twisted graphene layers (TGLs). Moreover, we demonstrate that heterostrain reconstructs, much more severely, the energy spectrum of TGLs than homostrain for which both layers are strained identically, a result which should apply to virtually all van der Waals structures opening exciting possibilities for straintronics with 2D materials. [ABSTRACT FROM AUTHOR]