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Physics Colloquium - "Moiré Materials" Presented by A.H. MacDonald - UT Texas, Austin



Lewis Lab 316

The advent of two-dimensional materials in recent years has opened a new opportunity to design quantum metamaterials in which many-particle matter waves waves exhibit strongly-correlated and topologically non-trivial properties that are rare in naturally occuring crystals. For example, two-dimensional van der Waals crystals that are overlaid with a difference in lattice constant or a relative twist form a moiré pattern.  In semiconductors and semimetals, the low-energy electronic properties of these systems are accurately described by Hamiltonians that have the periodicity of the moiré pattern – artificial crystals with lattice constants on the 10 nm scale. Over the past several years substantial progress has been made in the fabrication of these moire metamaterials, especially ones based on graphene, hexagonal boron nitride, and transitional metal dichalocogenides (TMDs). Since the miniband widths in both graphene and TMD moiré materials can be made small comparted to interaction energy scales (by mechanisms [1,2] that differ), these materials can be used both for quantum simulation and for quantum design. An important property of moiré materials is that their band filling factors can be tuned over large ranges without introducing chemical dopants, simply by using electrical gates. In addition to realizing Mott insulators, density waves, a variety of different types of magnets, and superconductors – states of matter that are familiar from the study of strongly correlated atomic scale cyrstals – moire materials have emerged as perhaps the best plaform uncovered to date for studies of topologically non-trivial matter, especially strongly interacting topologically non-trivial matter. The role of band topology is natural in graphene moires, where it derives from the interesting band topology of graphene monolayers, but has been an unexpected bonus [3] in the case of TMD moires where it derives from twists in the layer degree of freedom. I will discuss the latest developments in this evolving story. Allan MacDonald is a condensed matter theorist with a focus on new or ununderstood phenomena related to the quantum physics of interacting electrons in materials. He has worked at the National Research Council of Canada Laboratory in Ottawa, at Indiana University, and since 2000 at the University of Texas at Austin. He has made contributions to theories of the integer and fractional quantum Hall effects, spintronics in metals and semiconductors, topological Bloch bands, correlated electron-hole fluids, and two-dimensional materials. In 2010 MacDonald predicted that it would be possible to realize strong correlation physics in graphene bilayers twisted to a magic relative orientation angle, foreshadowing the rise of twistronics. His current work is focused on anticipating new moiré material physics, and on achieving a full understanding of the graphene and transition-metal dichalcogenide systems that are under active experimental study. He is a member of the American Academy of Arts and Sciences and the US National Academy of Sciences and has been awarded the Herzberg Medal (1987), the Buckley Prize (2007), the Ernst Mach Honorary Medal (2012), the Wolf Prize (2020), the NIMS medal (2021), and the Hill Prize (2024).