FSU scientists discover exotic states of matter in graphene, offering new possibilities for quantum computing

Florida State University Assistant Professor of Physics Zhengguang Lu and fellow researchers have discovered new states of matter in graphene — a form of carbon made from a single layer of atoms — with unusual electrical properties that could make them a valuable tool for building more powerful electronics and quantum computers.

In a study published in Nature, the researchers detailed how they designed structures made from five layers of graphene sandwiched between sheets of boron nitride and found that they exhibited unique electronic behavior at very low temperatures. In this configuration, electrons travel along the edges of the structure as fractions of a single charge without energy loss, a phenomenon protected by topology, meaning those properties are unchanged during bending, stretching or other deformations of the system.

“This is one of the special parts about physics — a tiny difference in a material’s structure can create a system that behaves completely differently,” said Lu, an FSU alumnus who was also a postdoctoral researcher on the team that first discovered this phenomenon in graphite systems at the Massachusetts Institute of Technology in late 2023.

The states of matter discovered by Lu and colleagues exhibit what are called quantum anomalous Hall states, meaning electric current can flow along the edges of the material with zero resistance and without needing a magnetic field.

More specifically, researchers found both an electron crystal state showing integer quantum anomalous Hall states, in which electrical conductance values are restricted to whole numbers, as well as fractional quantum anomalous Hall states, meaning they measured electrical conductance that reached fractional values instead of only integers. This finding is a sign of strongly correlated electron behavior.

“If the fractional quantum anomalous Hall effect is combined with a superconductor, the resulting quantum computer will be more efficient than current quantum computers and free of error. Even a weak magnetic field will eventually kill a superconductor, which is why uncovering these states at zero magnetic field is so important,” Lu said.

To investigate the graphene layers, the research team froze samples to below 40 millikelvin, or around -460 degrees Fahrenheit. At that temperature, the electrons arranged themselves into two new phases: fractional quantum anomalous Hall states at 5/9 and 5/11, in which electrons carried five-ninths and five-elevenths of a single charge, and an electron crystal state showing the integer quantum anomalous Hall effect in a wide range of electron density.

“Think of the fractional states as liquid, like flowing water, while the electron crystal state — what we call the extended quantum anomalous Hall state — resembles electron ice,” Lu said. “These liquid and solid phases exist similarly to a river flowing through glaciers. Remarkably, these two different electron phases can coexist in the system at ultra-low temperatures.”

Another key factor in these discoveries is the moiré pattern, a pattern that forms when the five-layer graphene interacts with nearby boron nitride. Moiré refers to the repeating spatial pattern created when overlaying sheets of atoms are slightly offset at a particular angle or are different sizes.

“The moiré potential is like a scissor that helps us cut out the most useful parts of a quantum material,” said Lu. “By engineering two-dimensional materials in this ‘twistronics’ fashion, we are unlocking new possibilities in quantum physics.”

For over two decades, graphene has been a key material in studying novel electron behaviors, but discovering new fractional states emphasizes how much remains unknown about even the simplest materials. This work highlights how rich quantum materials can be. Even something as common as pencil graphite can exhibit groundbreaking quantum properties.

“The kinds of multilayer graphene in which Zhengguang found the new quantum states are all present in natural graphite but were considered extremely difficult to identify and isolate,” said Peng Xiong, professor of physics and an expert in the field of mesoscopic electronic phenomena in quantum materials. “His ingenuity overcame this insurmountable obstacle and led to these breakthroughs — these fractional states are considered the holy grail of quantum computing.”

The multilayer rhombus-shaped graphene and hexagonal boron nitride system has become a highly versatile platform for exploring quantum phenomena, paving the way for future advances in quantum computing and materials science.

The particles that could make the bits needed for quantum computers possible are extremely sensitive to environmental disturbances, such as magnetic fields or temperature changes. Alternative methods, such as work developed by Lu and team, offer new possibilities for this emerging technology.

“Zhengguang brings FSU to the very forefront of one of the most exciting areas of research in physics today,” Xiong said. “In my view, he has been able to achieve all the successes he’s enjoyed in quantum materials research because he not only has a brilliant physics mind but is also able to make the impossible happen in the lab.”

Additional contributors to this research include scientists at MIT and researchers from the Research Center for Electronic and Optical Materials, part of the National Institute for Materials Science in Tsukuba, Japan.

To learn more about research conducted in the Department of Physics, visit physics.fsu.edu.

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