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2-D Materials

Solid, gas, liquid . . . hexatic phase?

Researchers observe exotic state of matter involved in 2-D materials’ melting

by Sam Lemonick
January 30, 2021 | A version of this story appeared in Volume 99, Issue 4


Diagram showing the start of melting in a 2-D material.
Credit: Elena V. Vasilieva
Particles in a 2-D hexagonal crystal (black line) have six neighbors. When it melts, some particles have five (red) or seven (blue) neighbors, a breakdown in order that signals a transition to the hexatic phase.

Scientists have observed transitions into and out of an exotic state of matter found between liquid and solid in 2-D materials (Sci. Rep. 2021, DOI: 10.1038/s41598-020-80082-x). The hexatic phase represents a partial breakdown of crystalline order in a melting 2-D material, an area of research that was recognized by the 2016 Nobel Prize in Physics. Elena V. Vasilieva and colleagues at the Russian Academy of Sciences and Moscow Institute of Physics and Technology used charged particles in a vacuum—called a dusty plasma—to simulate 2-D melting and observe the two-step transition through the hexatic phase, which previously had been observed in 2-D materials. In an ideal hexagonal crystal, each particle touches six others. As the crystal melts, this order begins to fall apart. Some particles gain a neighbor and others lose one, although the large-scale order of the material remains unchanged. “These defects are actually the ‘seeds’ that create the hexatic phase,” Vasilieva says. Physicist David Nelson of Harvard University, who helped develop the theory of the hexatic phase, says dusty plasmas are important systems for studying this phase because they share properties with materials like superconductors and electrolytes. He says the researchers provide “convincing evidence” that melting in these materials involves the hexatic phase. Vasilieva says understanding the details of phase transitions in 2-D materials could help explain behavior like superconductivity.


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