Data / Hora
Date(s) - 04/11/2021
17:00 - 18:00
Categorias
Scanning microwave impedance microscopy and its application to the study of moiré superlattice systems in twisted graphene bilayers
Douglas Ohlberg
Centro de Microscopia, Universidade Federal de Minas Gerais
04/11/2021 – 5a.f – 17h
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Scanning microwave impedance microscopy (sMIM) is a relatively recent addition to the techniques compatible with atomic force microscopy (AFM). In sMIM, a microwave signal is sent down a transmission line which terminates in a specially designed AFM probe as it rasters across a surface. Through measurement of the difference between the transmitted signal and reflected signal after it has interacted with the surface, it is possible to map local variation in the capacitance and conductivity of samples with nanoscale resolution. The technique has been applied to study the unique moiré superlattice patterns that arise when graphene monolayers are stacked with an intentional mismatch in the rotational (or twist) angle between succeeding layers. These systems became the subject of intense scrutiny after the observation of superconductivity in graphene bilayers prepared at a “magic” angle. A general overview of the theory behind sMIM will be presented as well as results collected over graphene bilayers prepared with a variety of twist angles that demonstrate the ability of sMIM to image moiré patterns with a resolution of 1nm. A model explaining this unexpected result will be presented and we conclude with a demonstration of how the iterative use of sMIM in tandem with Raman spectroscopy combines two complementary techniques to probe the vibrational and electronic structures of these bilayer systems, revealing possible clues to the mechanisms responsible for superconductivity in magic angle bilayers.