Müller Lecture in Physics set for March 6
Allan MacDonald, a theoretical condensed matter theorist and the Sid W. Richardson Foundation Regents Chair in Physics at the University of Texas at Austin, will present the 2025 Erwin Müller Lecture in Physics at 3:45 p.m. on Thursday, March 6, in 117 Osmond Laboratory on the Penn State University Park campus. The talk, titled “Moiré Materials,” is free and open to the public, with refreshments and conversation preceding the lecture at 3:15 p.m. on the Osmond/Davey Lab overpass.
“Moiré Materials”
Recent progress in fabricating two-dimensional material devices has enabled a new strategy to create designer quantum metamaterials in which electrons exhibit strongly correlated and topologically nontrivial properties that are rare in naturally occurring 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. Since the miniband widths in both graphene and TMD moiré materials can be made small compared to interaction energy scales (by mechanisms 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 crystals — moiré materials have emerged as perhaps the best platform uncovered to date for studies of topologically nontrivial matter, especially strongly interacting topologically nontrivial matter. The role of band topology is natural in graphene moirés, where it derives from the interesting band topology of graphene monolayers, but has been an unexpected bonus in the case of TMD moirés where it derives from the layer degree of freedom. MacDonald will discuss some of the latest developments in this evolving story.
About the speaker
Allan MacDonald is a theoretical condensed matter theorist whose work focuses on predicting the electronic properties of new materials and explaining poorly understood observations related to the quantum physics of interacting electrons in materials. Among other topics, he has made theoretical contributions to theories of the integer and fractional quantum Hall effects, spintronics in metals and semiconductors, topological Bloch bands, correlated electron-hole fluids and exciton and polariton condensation 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 recent work is focused on anticipating new physics in moiré superlattices and achieving a full understanding of magic-angle graphene and transition-metal dichalcogenide moiré systems.
MacDonald is a member of the American Academy of Arts and Sciences and the U.S. National Academy of Sciences and has been awarded the Herzberg Medal (1987), the Buckley Prize (2007), the Ernst Mach Honorary Medal (2012) and the Wolf Prize (2020).
About the Müller Lecture
The Erwin W. Müller Memorial Lectureship was established in 1978 in honor of the late Erwin Müller, to support lectures in surface and solid-state physics. From 1962 to 1977, Müller was the Evan Pugh Professor of Physics at Penn State, where he co-invented the atom-probe field ion microscope in 1967.