Twisting two layers creates electric vortices in a metal

When two crystalline sheets are stacked with a slight twist, their atoms do not line up in the same way everywhere. Yingzhuo Lun, Xinxin Hu, Qi Ren and colleagues used that geometry to produce and observe polarization vortices in metallic membranes of strontium ruthenate, whose chemical formula is SrRuO₃. Here polarization means a relative displacement of charge within the structure; a vortex is a spatial pattern in which those local directions turn around. The study appeared in Nature Materials.

The team stacked two thin membranes of the metal at different twist angles. The larger pattern formed when two slightly rotated periodic atomic lattices overlap is called a moiré pattern. Using electron microscopy, the researchers reconstructed atomic displacements and mapped strain. They also measured magnetic properties and electrical transport. Density functional theory calculations, a computational way to estimate electron behavior in materials, helped interpret the observations.

The maps revealed dipolar vortices, or turns in the local pattern of charge separation, associated with shear-strain gradients repeating with the moiré pattern. A change in strain that induces polarization is known as flexoelectricity. The magnitude of the polar pattern depended on the twist angle and on distance from the interface. Below the temperature at which the material develops persistent magnetic order, ferromagnetism coexisted with dipolar order. The two signals varied in opposite directions with twist angle, suggesting competition between them.

This is striking because mobile charges in metals generally make organized electric patterns difficult to sustain. The participating Catalan Institute of Nanoscience and Nanotechnology described the work in an institutional article by Chema Arcos. That report and the research paper describe observations and measurements; neither demonstrates a working electronic memory or reversible control in a device.

The evidence supports the conclusion that twisting can organize polarization vortices in a metal that is also ferromagnetic. It does not yet establish a unique microscopic cause or a technological application. Experiments that independently control angle and strain, then test the patterns’ stability and reversibility, would determine whether the effect can be used functionally.