CAREER: Electronic and Optical Properties in Generalized Moire Systems from First Principles

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NONTECHNICAL SUMMARYThis CAREER award supports theoretical and computational research, software development, and educational efforts on new classes of materials that can be created by combining a few layers (typically from two to four) of atomically thin materials.This family of layered materials has demonstrated great promise for use in electronic, optical, and quantum information applications. Their properties depend sensitively on the chemical composition of individual layers, the relative twist angle between each layer, and even the presence of nearby supporting materials. Hence, the vast combination of such layered systems that can be engineered with unique properties offers an untapped scientific and engineering opportunity.The PI will use a combination of new theoretical methods and large-scale atomistic simulation tools to chart this large phase space of layered systems. The methods that will be developed in this award will not only speed up simulations, but also give conceptual guidance on how to realize systems with desired quantum properties. The PI will study a few selected materials that can potentially host unconventional electronic and optical properties.The award will also synergistically involve community college students to broaden the participation of underrepresented minorities in science, technology, engineering, and math (STEM) fields. Finally, the research outcomes will also inform a proposed graduate-level course that introduces concepts of 2D and quantum materials to materials scientists and engineers. Such efforts will make the field of layered materials more accessible to a broader community, increase the pace of fundamental and applied materials discoveries, and translation of such discoveries to industry.TECHNICAL SUMMARYThis award supports theoretical and computational studies and educational efforts focused on vertically stacked, atomically thin layered materials. The award focuses on systems that display moiré patterns with a wavelength much larger than that of the crystal periodicity of the individual layers.Such systems received considerable interest with the discovery of superconductivity in twisted bilayer graphene in 2018. Indeed, there has been a growing interest in stacking insulating, semiconducting, ferroelectric, and magnetic monolayers, and engineering novel emergent excitations by carefully controlling the energy landscape at these longer wavelength scales.Utilizing large-scale first-principles computer calculations, the PI will develop methods to study such material combinations. The research activities are structured in three complementary objectives. First, the PI will develop methods to compute effective moiré potentials in complex, multilayer van-der-Waals structures using DFT and first-principles-parametrized force-field calculations. The outcome should allow the community to understand the electronic properties in a vast set of layered interfaces at low computational cost. Second, the award will support the development of first-principles approaches to compute excitons in complex van-der-Waals materials – which either include unusual materials combinations or a large number of layer stacking. These will be carried out by ab initio many-body perturbation theory calculations based on the GW and GW combined with Bethe-Salpeter equation (GW-BSE) approaches, together with new methods to incorporate the effect of the moiré potential without requiring large calculations on large supercells. The PI will systematically study the coherence and coupling of emergent excitonic states in such multilayer systems. Finally, this award will support investigations on how twisted 2D materials can realize unusual 1D physics, with applications in sensing and storage.In parallel, this award includes a multi-pronged educational component to reach out to students at different stages of their careers, with a particular emphasis on the local Hispanic/Latinx community which is underrepresented in STEM fields. These efforts involve, for instance, the mentoring of community college students through mini-internship opportunities, and designing and offering a graduate-level course on quantum and 2D materials, closing a common gap in the traditional educational curriculum of materials science and physics courses.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.