For decades, modern electronics have depended on billions of electrons moving through semiconductor materials. But moving those electrons requires energy, driving materials scientists to find new ways to configure material systems that consume less power while storing, transferring and processing data.
In her new project, Sandhya Susarla studies materials at the mesoscale, roughly 100 to 200 nanometers, where she believes hidden topological defects could pave the way for faster, lower-power technologies.
“Today, if you copy 200 gigabytes of data to your hard drive, it might take like 30 minutes,” Susarla says. “If this technology is realized, it could only take like three minutes. These numbers are examples, but we’re talking about orders of magnitude of change.”
To grasp how these defects emerge, one needs to understand a material’s structure at different scales. Think of a material, like a stadium. From afar, it looks like one complete structure. Only when you get inside do you begin to see rows of seats with people in them.
Similarly, at a smaller scale, materials are made of atoms and electrons interacting with one another. Those interactions give rise to what materials scientists call functional properties — collective behaviors such as magnetism, conductivity or how light moves through a material.
Now, imagine fans inside the stadium doing a coordinated wave. The people can remain steady, but the pattern of the wave can change. Topological defects behave similarly. They are not due to physical damage or missing atoms, but organized disruptions in the collective electronic patterns inside a material.
One such pattern involves electron spins, which can be imagined as tiny compass needles pointing in different directions. The way those spins collectively align inside a material strongly influences its magnetic behavior. The same correlation applies to other electronic patterns that Susarla studies, including polarization, atomic displacement, excitonic states and more.
An assistant professor of materials science and engineering in the School for Engineering of Matter, Transport and Energy, part of the Ira A. Fulton Schools of Engineering at Arizona State University, Susarla has been awarded the Army Research Office Early Career Program Award, or ARO ECP Award, to advance that idea further.
Solving energy
There’s an increased need for energy and secure systems as artificial intelligence, or AI, adoption continues to accelerate. Technology companies have designed innovative, more efficient microchips, but the need for more energy persists. Sandhya boils it down to a single challenge.
“Many modern semiconductor technologies rely on creating structural defects in materials, which requires breaking atomic bonds and consumes a lot of energy,” she says. “The essence of my research is that by just changing how topological patterns are arranged, you can actually change the properties of the material without having to break or make any bonds.”
Susarla’s project focuses on moiré heterostructures, advanced quantum materials formed by stacking two atomically thin semiconductor layers with a slight twist between them. She says the small misalignment creates large-scale repeating patterns. In particular, her team plans to study systems made from tungsten diselenide and molybdenum diselenide, materials widely researched for their unusual electronic and light-matter properties.
While she’s not the first to study topological defects in these materials, Susarla will take a slightly different approach.
Traditionally, researchers studying moiré materials have focused either on the atomic scale, examining just a few patterns at atomic resolution, or on much larger scales that average the material’s behavior. Drawing from her postdoctoral research on real-space topological defects in oxides and leveraging ASU’s advanced electron microscopy facilities, Susarla identified the mesoscale as the scale where hidden real-space topological defects begin to emerge.
Susarla highlights what she calls the most exciting feature of the defects.
“These topological defects can be manipulated using an electric field,” she says. “These would allow information to move through a material without relying entirely on conventional electron movement.”
As these defects move through a material, they can locally change its chirality, which is a property that describes whether a structure is arranged in a left or right-handed pattern. Because chirality influences how the material behaves electronically, Susarla says manipulating topological defects could potentially change how information is encoded.
“You can’t really debug that device because it would have a different logic than other kinds of devices,” she says.
It’s this aspect of Susarla’s research that drew the Army Research Office to fund her project. The funding will support the expensive electron microscopy experiments needed to image these defects at the mesoscale and help train students working on the project.
“For each hour, you pay about $100 to run the microscope,” she says. “This grant is critical in not only helping us complete our research, but also in training the next set of students as well.”
Over the next three years, Susarla will further investigate how topological defects influence excitonic states and how they can be manipulated using electric fields.
While her findings may take years to turn into everyday technologies, Susarla is confident that within the next three years, her team will uncover how topological defects influence the behavior of semiconductor materials.
