Understanding how atoms move is fundamental to making and using materials. Atoms on the surface of some glasses move at nearly the same rate as atoms on the inside. But for other glasses, surfaces atoms move a million times faster. Researchers in the Wisconsin MRSEC IRG 1 have combined experiments, simulations, and data-centric methods to understand why some surfaces are so much faster than others. They found that atoms in glasses move by breaking out of a “cage” of nearby atoms. On the surface, that cage can be weaker than inside the glass, allowing for faster motion. They also discovered a relationship that predicts surface motion from more accessible data about bulk motion. Their results unify behavior for glasses of organic molecules, metals, and oxides and make creating glasses for applications like light-emitting diodes, quantum computers, and hard coatings easier.
Glasses have disordered arrangements of atoms without the repeating patterns that crystals have. However, there are small-scale patterns of atoms that touch each other that strongly affect the energy of the glass, how the atoms move when they get hot, and other properties like strength and response to an electric field. Unfortunately, there are many possible patterns and many slight variations of each one, so studying them is like sorting the grains of sand on a beach by size and color by hand–it’s an impossible task. Wisconsin MRSEC IRG 1 uses machine learning to sort the sand. They have developed algorithms to find small-scale atomic patterns in large simulations of glasses and link them to the glass’ energy. Ongoing studies have connected patterns to atomic motions, which provides a path to simulations of glasses over long times and low temperatures that are currently impossible.
Materials with a repetitive pattern the same size as the wavelength of a wave can be used to control the wave, causing it to bend, perfectly reflect or transmit, or even turn around corners. Where different patterns meet, even more exotic behavior occurs, including making highways for light or sound that only travel in one direction or where the waves cannot be dissipated. Synthesizing such materials is a major challenge, which Wisconsin MRSEC researchers have met by adapting a family of 3D printing techniques.
The Wisconsin MRSEC has shown that molecules inside in a type of topological photonic material called a Weyl crystal can exchange energy over much larger distances. The intricate twisting structure of the material uses light to connect one molecule to others much farther away. Developing photonic Weyl crystals may contribute to more efficient LEDs and solar cells and improve molecular sensors.
The Wisconsin MRSEC has developed a simple, inexpensive way for K-12 students to build a sustainable energy device using familiar, everyday materials including aluminum foil, a plastic egg, clear tape, and a bouncy ball. The device, called a triboelectric nanogenerator, converts the often-wasted energy of motion into useful electrical energy.
Engineers currently lack good substrate materials on which to grow thin films of materials like GaN with few defects. These layers are needed in applications like high-power transistors and solid-state lighting. Available bulk crystals have the wrong crystal structure or the wrong distance between the atoms. The Wisconsin MRSEC has developed a buffer layer material and related synthesis method that promises to alleviate the substrate problem.
The Wisconsin MRSEC Research Experience for Teachers (RET) program is a cross-cultural collaboration with the University of Puerto Rico-Mayagüez (UPRM). The RET program provides science teachers from Wisconsin and Puerto Rico an authentic research and cultural literacy professional development experience. The program culminates with an in-person capstone week where the RETs from both sites share the research-inspired classroom activities they have developed and learn from one another about teaching in a different cultural and geographic environment. The teachers learn about the area and culture that is unique to the site hosting the capstone.
The Wisconsin MRSEC has created thin films of a fascinating magnetic material, Pr2Ir2O7, in which the magnetic moments are frustrated: No matter how they are arranged, some of the moments are always fighting to change their direction, like two bar magnets with their north poles shoved together. Frustration creates a rich landscape for discovery and manipulation of new magnetic effects and of electronic phenomena linked to magnetism.
Using physical vapor deposition, researchers produced glassy films that are smooth and uniform, but which also have the molecules aligned with one another and organized in layers. This added structure could make the glass more efficient for conductors and expand the range of materials that can be used in future organic electronics. The colorful images in the figure show measurements using synchrotron x-rays that contrast the disordered starting material and the ordered glass.
IRG 1 showed how the atoms around the defects can restrict their ability to jump between configurations and how defects can talk to each other via sound waves. Both phenomena keep the defects from interfering with sound waves allowing the waves to travel long distances.