NSF-BSF: Quantum Electron States in van der Waals Platforms

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NONTECHNICAL SUMMARYThe National Science Foundation and the United States-Israel Binational Science Foundation (BSF) jointly support this collaboration between a US-based researcher and an Israel-based researcher. The NSF Division of Materials Research (DMR) funds this award, which supports research and educational activities focused on understanding the physics of materials composed by stacking together a small number of two-dimensional layers.The quantum physics of electronic systems becomes very rich when they are confined to low dimensions, resulting in systems with exotic properties. In this project, the research team will investigate systems composed of single or small number of sheets, layered together into composite two-dimensional electron platforms. The materials available for building these systems have become increasingly diverse, offering a great variety of possibilities. Individual layers may be of the same material, with a prominent example being a bilayer made from two graphene sheets twisted relative to each other. This has recently been demonstrated to support remarkable insulating and superconducting states, with only a small difference in electron density determining which of these behaviors manifests. In other cases, the system may comprise layers of completely different materials, adhering to one another only weakly, so that their basic individual structures remain intact. Combining such structures can result in systems with electronic properties completely different than those of their constituents. In this project the team will explore exotic quantum electron states in these material systems, and the measurable properties they exhibit, which could find technological applications.As an NSF-BSF collaboration, this project will provide valuable training for young physicists, in methods for analyzing electronic materials, as well as in learning to work and collaborate within an international setting. Young women science students will benefit from regular visits to Indiana University by the senior Israeli participant, who will engage with students in the Women in Science group in the Physics Department, as well as with undergraduates at IU's STEM Living and Learning Center where she will become a visiting faculty fellow. The PI will participate in outreach at the annual IU ScienceFest, for which physics-based interactive activities for school children will be introduced. The PI will also mentor students in the IU Physics Department's Bridge Program. Through these activities both participants will seek to involve students from under-represented groups in this project.TECHNICAL SUMMARYThe National Science Foundation and the United States-Israel Binational Science Foundation (BSF) jointly support this collaboration between a US-based researcher and an Israel-based researcher. The NSF Division of Materials Research (DMR) funds this award, which supports research and educational activities focused on quantum electron states in van der Waals platforms.The specific systems to be studied involve combinations of graphene, transition metal dichalcogenide (TMD) layers, layers of black phosphorus (BP), and surfaces of topological insulators. A graphene-TMD heterostructure, for example, provides a very heterogeneous platform for electrons and holes, with the former supporting gapless Dirac electron states and the latter supporting unusual spin-orbit coupling in its hole states. In these systems, the team will consider the possibility of interlayer particle-hole pairs --excitons-- forming a condensate with behavior unexpected in either material individually (e.g., a valley superfluid.) The highly anisotropic structure of BP offers the possibility of inducing strong anisotropy in a partnered material such as graphene, and the team will explore the effects of this on both the single-particle and collective behavior of the graphene electrons, including the possibility of inducing superconductivity or charge-density-wave states. Studies of aligned graphene on a TI surface, in which the microstructure forms a moire pattern with a large unit cell, will focus on quantum Hall states with spontaneously broken symmetry, where opposing surfaces of the TI, hosting very different electronic states, are topologically tied together. The project also involves studies of twisted bilayer graphene, focusing on observed magnetic field effects in the vicinity of the Mott insulating/superconducting states, where novel electronic ordering may be occurring.As an NSF-BSF collaboration, this project will provide valuable training for young physicists, in methods for analyzing electronic materials, as well as in learning to work and collaborate within an international setting. Young women science students will benefit from regular visits to Indiana University by the senior Israeli participant, who will engage with students in the Women in Science group in the Physics Department, as well as with undergraduates at IU's STEM Living and Learning Center where she will become a visiting faculty fellow. The PI will participate in outreach at the annual IU ScienceFest, for which physics-based interactive activities for school children will be introduced. The PI will also mentor students in the IU Physics Department's Bridge Program. Through these activities both participants will seek to involve students from under-represented groups in this project.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.