Probing moire flat bands with optical spectroscopy

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Nontechnical Description: When a layer with a periodic pattern is laid on top of another similar pattern with minor difference(s) in pitch or orientation, moiré interference patterns appear. Such moiré patterns can occur even at an atomic scale by slightly misaligning the stacking of two 2D (two-dimensional) crystals, which can dramatically modify the properties of the atomic stacks. For example, on its own, graphene (a layer of carbon a single atom thick) conducts electricity like metal does, but with a moiré pattern it can behave either like plastics which don’t conduct electricity or like a superconductor which conducts electricity without generating any heat. This research investigates the unusual behaviors of electronic structures due to moiré patterns. Knowledge acquired through this study will reveal fundamental properties of moiré-patterned 2D systems and pave the way for developing devices that make use of these surprising phenomena. Beyond scientific and technological impacts, this project brings modern materials physics education to the broader Western Massachusetts community through workshops and lab visits. A new summer research internship will be established in collaboration with a Hispanic-serving regional community college, to provide promising underrepresented minority students with a science research experience in the principle investigator’s 2D materials lab. The objective of this internship is to provide educational enrichment opportunities and increase access to science careers to students who are traditionally underrepresented, so that they will be better prepared for matriculating into a four-year institution and later to graduate schools.Technical Description: Moiré superlattices with relatively long wavelength of ~10nm in 2D crystals can create almost dispersionless electronic bands, where the kinetic energies of the charges are strongly suppressed, giving way to Coulomb interaction that drives strongly correlated phases. In this project, the PI uses versatile optical spectroscopy techniques to investigate these moiré flat bands formed in stacked atomic layers of graphene and transition metal dichalcogenides. Quantum states such as Mott insulators, superconductivity, and Chern insulators are continuously tuned by the electric fields. Fundamental properties of these states are probed by measuring exciton polarons, magneto-phonon resonances and electronic Raman susceptibility. This project advances scientific knowledge of the quantum materials physics by focusing on three main questions regarding the nature of the moiré flat bands: 1) What is the size and nature of the spontaneously opened gaps in partially filled moiré bands? 2) Can the interaction break the spin and valley degeneracy in the absence of an external magnetic field, and what is the spin and valley polarization? 3) What kind of novel excitations can the correlated states host? Understanding and tuning these properties by the electric field effect are important not only for fundamental physics, but also for future device applications harnessing these unusual properties.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.