Domain-wall-to-domain interconversion in a polar conducting oxide

Inside many solid materials, atoms arrange themselves into vast, uniform regions called domains, much like neighborhoods in a city where everyone agrees on the same rules. In a special class of materials known as ferroics, these domains possess a specific internal order, such as a magnetic direction or an electrical polarization, that defines their behavior. For decades, scientists focused their attention on these large, three-dimensional neighborhoods, believing that the material's properties were determined solely by how these regions were arranged. However, a shift in perspective revealed that the boundaries between these neighborhoods, known as domain walls, are not merely empty fences. These walls are topological defects, unique two-dimensional zones where the atomic order shifts, often granting them strange and powerful abilities that the rest of the material cannot possess, such as conducting electricity when the bulk material is an insulator. The challenge has always been that these functional walls are incredibly thin, confined to a flat interface, which limits how much of the material can actually use these special powers.

Researchers at the University of Groningen and their collaborators have now demonstrated a way to break this confinement, effectively turning a thin, two-dimensional wall into a thick, three-dimensional region of new material. Working with a thin film of tungsten trioxide, a polar material that conducts electricity poorly in its normal state, the team discovered that they could use a tiny, electrically charged tip to heat a specific domain wall just enough to trigger a phase change. This process caused the narrow wall to expand outward, swallowing the surrounding material and transforming it into a highly conductive state. Remarkably, this transformation is reversible; once the electrical stimulus is removed, the expanded region slowly shrinks back to its original, confined state over the course of several hours. This ability to switch a material back and forth between a polar, insulating state and a non-polar, highly conducting state by manipulating the size of its internal boundaries offers a new way to build electronic devices that can be reconfigured on the fly.

The material at the heart of this discovery is tungsten trioxide, which naturally forms a crystal structure where the atoms are slightly tilted, creating a polar state that does not conduct electricity well. In the thin films used for this study, these polar regions are separated by domain walls. While the interior of the domains is insulating, these walls are naturally more conductive because the atomic tilts that define the bulk material disappear at the boundary, allowing electrons to move more freely. The researchers wanted to see if they could do more than just use these existing walls; they wanted to see if they could make the walls grow. By applying an alternating electric field through a scanning probe tip—a device that acts like a microscopic needle—they triggered a localized reaction. As the tip scanned across the surface, the domain walls began to widen, creating diamond-shaped patches of new material that spread across the film.

To understand what was happening inside these expanding patches, the team examined the atomic structure of the material using advanced electron microscopy. They found that the pristine, insulating material has a complex, tilted atomic arrangement. However, in the regions where the domain walls had expanded, the atoms rearranged themselves into a simpler, more symmetrical structure. This new structure, which resembles a tetragonal shape, is known to be highly conductive and typically only stable at very high temperatures, well above 700 degrees Celsius. The researchers realized that the alternating electric field from the tip was not just pushing the atoms around; it was generating intense, localized heat through electrical resistance. This heat was sufficient to raise the temperature of the domain wall to the point where it could transform into this high-temperature, conductive phase, even though the rest of the film remained cool.

The team confirmed this heating effect by running computer simulations and by comparing their results to experiments where they heated the entire sample in an oven. When they heated the whole film to high temperatures, the material underwent the same structural change from a tilted, polar state to a symmetrical, conductive state. The key difference in their experiment was that they achieved this change in a tiny, controlled spot at room temperature, using only the heat generated by the electric current flowing through the domain wall. They also ruled out other possibilities, such as the pressure from the tip or a direct current from the tip, finding that neither of these factors could induce the expansion. Only the specific type of alternating current that generates heat could trigger the transformation.

Once the electric field was turned off, the expanded conductive regions did not stay that way forever. Over a period of several hours, the material slowly relaxed, and the large conductive patches shrank back down until the domain walls returned to their original, narrow state. This behavior mimics the way a biological synapse, the connection between nerve cells, can strengthen or weaken its signal over time. The researchers tested this by measuring the electrical current flowing through the material as they repeatedly applied voltage pulses. They observed that the conductivity could be tuned up and down in a smooth, continuous manner, rather than just switching on or off like a standard light switch. This suggests that the material could be used to create electronic components that function like artificial synapses, capable of storing information in the form of electrical resistance and gradually forgetting it, a property essential for certain types of advanced computing.

The significance of this work lies in its ability to control the dimensionality of a material's properties. Previously, the unique conductivity of domain walls was locked into a two-dimensional plane, limiting its utility. By showing that these walls can be expanded into three-dimensional regions and then contracted back, the researchers have opened a new path for designing electronic devices. This approach allows for the creation of circuits where the very nature of the material—whether it is an insulator or a conductor—can be rewritten and erased. While the current experiments were performed on a microscopic scale, the underlying principle suggests a future where electronic components can be dynamically reconfigured, offering a new paradigm for flexible and adaptive technology. The study provides a clear, physical demonstration of how heat, electricity, and atomic structure can be harnessed to manipulate matter at the nanoscale, turning a static material into a dynamic, responsive system.