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The Heusler System And How You Can Use It As A Lego Box To Build The States You Are Interested In
2:30 p.m. - 3:06 p.m.
The periodic table becomes one hundred years old just this year. The family of Heusler compounds uses nearly all the elements in the Periodic Table to allow for the design of materials with all sorts of properties. These include: hard and soft magnets, shape memory and magnetocaloric metals, thermoelectric semiconductors, topological insulators, and Weyl semimetals. These are just a few examples of more than 1000 known members of this remarkable class of materials that can display such a wide range of extraordinary multifunctional and tunable properties. Many more remain to be discovered! Just like a box of Lego bricks we can put together certain atoms (valence electrons), arranged in a particular symmetry, to achieve a desired electronic energy band structure. A necessary precondition for such a straightforward approach is a single particle picture: this allows for the prediction of many properties in this versatile class of materials, and equally enables “inverse design”. In my talk I will discuss the simple rules that we have learned to date and what the future might portend for further additions to the large and ever-growing Heusler family.
Stacking Atomic Layers One by One: Quest for New Materials and Physics
3:06 p.m. - 3:42 p.m.
Modern electronics has been heavily relied on the technology to confine electrons in the interface layers of semiconductors. In recent years, scientists discovered that various atomically thin materials including graphene, a single atomic carbon layer, can be isolated. In these atomically thin materials, quantum physics allows electrons to move only in an effective 2-dimensional (2D) space. By stacking these 2D quantum materials, one can also create atomic-scale heterostructures with a wide variety of electronic and optical properties. I will discuss the creation of new heterostructures based on atomically thin materials and emerging new physics with technological implications therein.
The Design And Growth Of Ultra-Stable Glasses
3:42 p.m. - 4:18 p.m.
Glasses are generally regarded as highly disordered and the idea of "controlling" molecular packing in glasses is reasonably met with skepticism. However, as glasses are non-equilibrium materials, a vast array of amorphous structures are possible in principle. Physical vapor deposition (PVD) allows a surprising amount of control over molecular packing in glasses and can be used to test the limits of amorphous packing in two ways. PVD can prepare glasses that approach the limits of the most dense and lowest energy amorphous packings that are possible. The activation barriers for rearrangements in these materials are very high, giving rise to high thermal and chemical stability. In addition, PVD allows control over anisotropic packing in glasses. For rod-shaped molecules, for example, glasses can be prepared in which the molecules have a substantial tendency to stand-up or lie-down relative to the substrate. As these materials have applications in organic electronics, an important question is: How much anisotropic order can be added to a glass without destroying key technological advantages such as macroscopic homogeneity? The high density and anisotropic packing of PVD glasses can be explained by a mechanism that is "anti-epitaxial" as structure is templated by the top surface rather than by the underlying substrate.
*Support from DOE (DE-SC0002161) and the UW-Madison MRSEC (NSF DMR-1720415) is gratefully acknowledged.
Colloidal Crystals, Quasicrystals and the Entropic Bond
4:18 p.m. - 4:54 p.m.
Entropy, information, and order are important concepts in many fields, relevant for materials to machines, for biology to economics. Entropy is typically associated with disorder; yet, the counterintuitive notion that particles with no interactions other than excluded volume might self-assemble from a fluid phase into an ordered crystal has been known since the mid-20th century. First predicted for rods, and then spheres, the ordering of hard shapes by nothing more than crowding is now well established. In recent years, surprising discoveries of entropically ordered colloidal crystals of extraordinary structural complexity have been predicted by computer simulation and observed in the laboratory. Colloidal quasicrystals, clathrate structures, and structures with large and complex unit cells typically associated with metal alloys, can all self-assemble from a disordered phase of identical particles due solely to entropy maximization. These findings demonstrate that entropy alone can produce order and complexity beyond that previously imagined. They also suggest that, in situations where other interactions are present, the role of entropy in producing order may be underestimated. We present the latest discoveries for entropic systems of identical particles, including a Bergman-like phase with a 432-particle unit cell, and fluid-fluid transitions preceding crystallization that are reminiscent of liquid-liquid phase separation in water, proteins, and even within cells. To understand these phenomena, and in loose analogy with traditional chemical bonds that produce order in atomic and molecular substances, we introduce the notion of the entropic bond.
Intracellular Liquid Condensates: New Approaches to Understand and Control Biomolecular Phase Transitions in Living Cells
4:54 p.m. - 5:30 p.m.
In this talk I will discuss our work to understand and engineer intracellular phase transitions, which play an important role in organizing the contents of living cells. Membrane-less RNA and protein rich condensates are found throughout the cell, and regulate the flow of genetic information. We've shown that liquid-liquid phase separation (LLPS) underlies the assembly of these structures. LLPS driven by intrinsically disordered protein regions (IDRs) explains many condensate features, for example the internal subcompartments of the nucleolus, which has important consequences for sequential ribosomal RNA processing. Our lab has developed a suite of new approaches, which use light to enable spatiotemporal control of intracellular phase transitions, allowing us to engineer the assembly and disassembly of these structures within defined subregions of the cytoplasm and nucleus. We are now using these tools to quantitatively map intracellular phase diagrams for the first time, providing unprecedented access to the biophysical principles underlying RNP condensate self-assembly. This approach has also begun to yield rich insights into the link between intracellular liquids, gels, and the onset of pathological protein aggregation, and still largely unexplored mechanical interactions between these structures and the genome.