Figure 1: A view of the crystal lattice of a topological insulator discovered by RIKEN researchers (blue spheres: tellurium atoms; reds spheres: tin, lead and indium atoms). © 2025 RIKEN Center for Emergent Matter Science
Thin films featuring a special combination of electrical and topological properties have been created by RIKEN physicists for the first time1. This demonstration could help to realize new forms of electronics that are highly energy efficient.
Intriguing effects can arise in materials that have electrons with unusual 'band structures'.
"Electrons behave as waves in solids, and the relationship between their energy and momentum is described by band structures," explains Ryutaro Yoshimi, a visiting scientist at the RIKEN Center for Emergent Matter Science. "In some materials, different electronic bands can cross and have the same energy."
Such band crossing fundamentally alters the properties of the electron waves, leading to all kinds of emergent phenomena. "For instance, electrons can experience a fictitious magnetic field that can be incredibly strong, sometimes up to 100 times larger than fields generated by conventional coils," says Yoshimi.
Harnessing such emergent phenomena could lead to practical applications. "If we can learn to control and engineer these powerful emergent effects, it could lead to the development of highly efficient electronics," says Yoshimi.
Unconventional band structures give rise to a special class of materials known as topological insulators. Their surfaces can conduct electricity, but their interiors are insulators. Topological insulators have been generating a lot of interest because of their unusual properties.
Now, Yoshimi and his co-workers have produced thin films that are topological insulators and exhibit ferroelectricity (Fig. 1), which means that they contain tiny electric dipoles with positive and negative ends.
"This achievement represents the realization of a new phase of matter that merges two key concepts in solid-state physics: topology and ferroelectricity," notes Yoshimi.
This combination is helpful from a practical perspective, since it gives engineers a convenient way to modify the material's topological properties.
"A material's topology is typically robust and difficult to alter with external stimuli, whereas ferroelectricity can be readily controlled by an external electric field," says Yoshimi. "By coupling these two properties, we have opened up a new avenue for controlling topological states on the material's surface."
That's what Yoshimi's team intends to explore next.
"Our next goal is to apply an electric field to the device and demonstrate that we can change the number of active Dirac states on the surface," says Yoshimi. "This would allow us to externally control the material's electrical conductivity and spin polarization, which would be a major step towards creating functional devices based on this new state of matter."
