Researchers from the University of Salerno in Italy and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) have found that thin layers of pure bismuth display the non-linear Hall effect, offering potential applications in utilizing terahertz high-frequency signals on electronic chips.
Non-linear Hall effect in bismuth thin films can be controlled by the geometry of the microfabricated arc-shaped channels. Image Credit: B. Schröder/ Helmholtz-Zentrum Dresden-Rossendorf (HZDR)
According to the team's study, published in Nature Electronics, bismuth possesses a combination of advantageous properties previously unobserved in other systems. In particular, at room temperature, the quantum effect can be observed.
Due to their ability to be put on plastic substrates, thin-layer films may find use in contemporary high-frequency technology applications.
When we apply a current to certain materials, they can generate a voltage perpendicular to it. We physicists call this phenomenon the Hall effect, which is a unifying term for effects with the same impact, but which differ in the underlying mechanisms at the electron level. Typically, the Hall voltage registered is linearly dependent on the applied current.
Dr. Denys Makarov, Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf
The majority of these effects are caused by the material's inherent magnetism or magnetic fields. However, in 2015, researchers found that the Hall effect is not always dependent on magnetism.
We achieve this with materials whose crystalline arrangement enables Hall voltages that are no longer linearly related to the current.
Carmine Ortix, Professor, Department of Physics, University of Salerno
This effect is of significant interest as it opens up possibilities for the development of novel components for high-speed electronics.
The two scientists are working together to find appropriate materials and potential uses for this so-called non-linear Hall effect in real-world settings. Makarov contributes experimental experience and connections to other HZDR institutes that are heavily involved in the endeavor, while Ortix specializes in theoretical physics.
We got together with colleagues from the ELBE Center for High Power Radiation Sources, the High Magnetic Field Laboratory, and the Institute for Resource Ecology. The common goal: to identify a suitable material with which this quantum effect can appear in a controlled manner at room temperature and which is also easy to handle and non-toxic.
Dr Denys Makarov, Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf
Familiar Material, New Properties
The research team has identified bismuth as an elemental material that demonstrates these desirable properties.
Bismuth is recognized for its strong classical Hall effect, which is predominantly observed in the bulk of the material. The researchers identified that on surfaces, quantum effects dominate and dictate the flow of current, even at room temperature.
The method's main benefit is that the researchers can use their quantum-property thin films on a range of electronic substrates, including silicon wafers and plastic. Through advanced microfabrication, the team can manage the effect, directly influencing the currents through the chip's channel design.
New Quantum Materials With Technological Relevance
Although other materials exhibiting the non-linear Hall effect have already been developed by other researchers, they do not possess all of the desired characteristics. For instance, graphene is harmless for the environment and has a well-controllable non-linear Hall effect, but only at temperatures lower than about -70 ºC.
This implies that the effect must be cooled using liquid nitrogen for the researchers to exploit it. They would need to employ much lower temperatures for additional chemicals.
Currently, research is centered on identifying appropriate materials, yet scientists are already contemplating future prospects.
Ortix said, “We see technological potential above all in the conversion of terahertz electromagnetic waves into direct current using our thin-film materials. This will make new components for high-frequency communication possible.”
Future wireless communication systems will need to extend the carrier frequency beyond 100 gigahertz into the terahertz range, which is unattainable with current technologies, to ensure noticeably higher data transfer speeds.
Journal Reference:
Makushko, P., et al. (2024) A tunable room-temperature nonlinear Hall effect in elemental bismuth thin films. Nature Electronics. doi.org/10.1038/s41928-024-01118-y.