Monash Physicists Twisted Two Atom-Thin Sheets of Tungsten Diselenide by a Tenth of a Degree and Photographed Nanoscale 'Whirlpools' of Electric Charge Spinning in Opposite Directions.
Merons and antimerons had only ever been seen in thick oxide crystals. Now a semiconductor a few atoms thick hosts a whole network of them, and the team can tell which come from twist and which from strain.
Researchers led by Monash University have directly imaged swirling patterns of electrical polarization, known as merons and antimerons, inside a semiconductor just a few atoms thick. The structures, described in Science Advances, form when two single-layer sheets of tungsten diselenide are stacked and twisted by a mere 0.1 degrees, producing a repeating pattern called a moiré superlattice. Until now, these topological polar textures had been observed mainly in far thicker oxide crystals.
A meron is a kind of half-vortex: the direction of electric polarization inside the material rotates around a central point, like water circling a drain, but only sweeps out half of a full sphere of possible directions. An antimeron winds the opposite way. Because that winding is a topological property, it cannot be undone by small nudges, which is what makes such textures attractive for storing and moving information in future low-power electronics.
The team used angle-resolved, high-resolution vector piezoresponse force microscopy, a technique that senses how a surface deforms under an electric field, to map the polarization across the moiré pattern and build vector maps of where it points. "Here we use angle-resolved, high-resolution vector PFM to spatially resolve polarization components and topological polar nanostructures in marginally twisted bilayer WSe₂ and provide experimental evidence for the existence of topologically nontrivial meron/antimeron structures," said co-lead author Dr. Thi-Hai-Yen (Emily) Vu, who did the work as a Monash Ph.D. student and is now a postdoctoral researcher at Deakin University.
A key result is that the researchers could separate two effects that are easy to confuse in twisted materials. "By constructing vector maps we were able to differentiate between twist and strain and quantify the contributions of each in a moiré superlattice," Vu said. Strain from stacking imperfections can mimic the effects of a deliberate twist, and untangling the two is essential for anyone hoping to engineer the textures on purpose.
The measurements were backed by density functional theory calculations and a full moiré-scale molecular dynamics model, which reproduced the circulating polarization and the winding numbers expected of merons and antimerons. "With differing interpretations across recent studies, we needed experiment and theory together," said co-lead author Daniel Bennett, an assistant professor at Nanyang Technological University in Singapore who was previously a postdoc at Harvard. "Our PFM measurements across different samples and twist angles, backed by DFT and a full moiré-scale molecular dynamics model, reveal the same circulating polarization and winding. That's the clearest evidence yet of real merons and antimerons in a twisted semiconductor."
The authors say the meron-antimeron network could in principle be manipulated with electric fields, strain or engineered substrates, opening a route to ultrathin devices that switch with very little energy. The method also gives the wider two-dimensional materials community a tool to check whether polarization patterns in their own twisted samples come from the twist, from strain or from both.
Originally reported by Phys.org.