Physicist identifies how electron crystals melt

Electronic states in TMD Moiré systems. Credit: Nature Communication (2022). DOI: 10.1038/s41467-022-34683-x

Mysterious changes in the phases of matter – from solid to liquid and back again – have fascinated Eun-Ah Kim since she was in elementary school in South Korea. Without cold drinking water readily available, on hot days children would bring bottles of frozen water to school.

Kim noticed that when the water melted, the volume changed.

“It told me there was something in there that I couldn’t see with my eyes,” said Kim, a physics professor at the College of Arts and Sciences. “The matter around me is made up of invisible entities that interact and do something together to change their state.”

Kim’s fascination with fusion lives on, but she now studies transitions in materials more exotic than water: electron crystals. In a new paper, Kim and first author Michael Matty, MS ’19, Ph.D. ’22, described an intermediate phase between liquid and solid for these electronic structures – a liquid crystal state.

Their paper, “Melting of Generalized Wigner Crystals in Transition Metal Dichalcogenide Heterobilayer Moiré Systems”, was published in Nature Communication.

Because electrons are all negatively charged, they repel each other; thus their preferred state is to be as far away from any other electrons in the material containing them as possible. The regular arrangement of electrons that results from this equal repulsion in all directions is called a Wigner crystal.

Kim and Matty wanted to know how electrons move from one regular arrangement in crystal form to another regular arrangement in crystal form, or how they “melt”.

To find the answer, researchers studied how electrons interact on an artificial grid, called a moire lattice, formed by placing two distinct atomically thin materials on top of each other. Because they are on a grid rather than a smooth surface, the electrons cannot choose arbitrary locations from each other, but must fill a point on the grid; the grid limits their arrangement.

“When the grid is partially filled, we see the impact of their repulsion and how strongly the electrons interact with each other,” Kim said. “Through their interaction, we see that they occupy a regular interval of sites on the network, not random intervals.”

The particular moiré network the researchers considered for their study was developed by Cornell experimentalists Kin Fai Mak, Professor of Physics (A&S) and Associate Professor of Physics at Cornell Engineering, and Jie Shan, Professor of Physics (A&S) and of applied physics and engineering (Engineering).

“Cornell experimenters are at the frontier of artificial moire materials research,” Kim said, “performing these amazing experiments with an astonishing degree of control, providing opportunities for theoretical ideas to manifest themselves in physical systems.” .

Shan and Mak had experimentally detected peculiar rigid structures that electrons formed in partially filled grids. Kim and Matty studied how one of these structures would transition to another. They found that when conditions changed, this very regular rigid network became more fluid.

The researchers identified an intermediate phase between solid and liquid in electrons that has some regularity but not as much as a solid, and not as much freedom as a liquid. They discovered that electrons in this state organize themselves into tiny bands that can move and orient themselves in structures.

“Electronic liquid crystals have been discussed theoretically, but we provide a visual picture of how they can form under the microscope: four or five electrons forming a lump that can be arranged,” Kim said. “What we have achieved is a microscopic understanding of what was only known in principle to be possible.”