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SONATA: synchrotron nanoscale imaging and spectroscopy of twisted graphene


Since the discovery of graphene in 2004, a large variety of two-dimensional (2D) crystals have been synthesized or isolated. They exhibit a large range of electronic, optical and mechanical properties, which can be combined through stacking in van der Waals (vdW) heterostructures, constituting new types of artificial materials. The first heterostructures were synthesized by encapsulating graphene inside hexagonal boron nitride (hBN), significantly improving the transport properties of graphene. This allowed important advances in the observation of many fundamental transport phenomena such as Klein tunneling or the quantum Hall effect in graphene. The latter phenomena were recently explored in the group of B. Hackens using scanning gate microscopy (SGM) a scanning probe technique yielding information on local electron transport dynamics.The properties of vdW heterostructures can in some cases be surprisingly radically different from the "simple" combination of the properties of their constituting 2D crystals: for example, stacking two monolayer graphene crystals with a slight misalignment between the crystal lattices (a twist angle of 1.1° in this case), superconductivity was observed at low temperature. Superconductivity in graphene, as well as several other 'strongly correlated phases' are still topics of intense activity in the community as they are not fully understood. For example, superconductivity is unconventional, i.e. in many ways similar to superconductivity in 'high-Tc' superconductors discovered in the 80's, but not yet understood. Open questions include, among others, the phase diagram of correlated phases with respect to the twist angle, and the phase stability with respect to local variations of twist angle. The overarching goal of the project is to explore the interplay between the moiré superstructure emerging in twisted graphene heterostructures, including its local reconstructions, and its local optical, electronic and transport properties. More specifically, through local spectroscopic measurements using hyperspectral s-SNOM imaging with synchrotron light, we first aim at reaching unprecedented local view both on the local optical properties of vdW heterostructures at different twist angles. (AU)

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