Researchers from the University of Salerno and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Italy have found that elemental bismuth thin films exhibit the so-called non-linear Hall effect.
This finding could have applications in technologies for the regulated use of terahertz high-frequency signals on electronic chips.
Among the many beneficial characteristics of bismuth that have not been discovered in other systems up to this point is the observation of the quantum effect at ambient temperature. Due to their ability to be put on plastic substrates, thin-layer films may find use in contemporary high-frequency technology applications.
The details of the team’s research were published in the journal Nature Electronics.
Potential for high-speed circuits
The creation of a transverse potential difference across an electrical conductor in response to an applied magnetic field perpendicular to the current and an electric current in the conductor is known as the Hall effect.
The team says that the Hall effect 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,” said Denys Makarov from the Institute of Ion Beam Physics and Materials Research at HZDR in a statement.
Most 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,” said Carmine Ortix, a professor at the Physics Department at the University of Salerno.
According to reseachers, this phenomenon is quite interesting because it opens the door to new kinds of components for high-speed circuits.
To further this, scientists from the two institutes collaborated to find appropriate materials and potential uses for this so-called non-linear Hall effect. The shared objective was to find a suitable material that is non-toxic, easy to handle and allows this quantum effect to arise in a regulated manner at room temperature.
Old material, fresh qualities
Various rounds of testing resulted in the team narrowing out an element that demonstrated these characteristics – bismuth. Since most of the substance has a significant classical Hall effect, bismuth is well-known for it. However, even at normal temperatures, the researchers found that quantum effects predominate and control surface current flow.
The approach’s main benefit is that the researchers can use their quantum-property thin films on various electronic substrates, including silicon wafers and even plastic. Through advanced microfabrication, the team is able to manage the effect, directly influencing the currents through the chip’s channel design.
Although other teams have already developed materials exhibiting the non-linear Hall effect, they do not possess all the desired characteristics. For instance, the team highlighted that graphene is harmless for the environment and has a well-controllable non-linear Hall effect, but only at temperatures lower than -70 degrees Celsius. This implies that the effect must be cooled using liquid nitrogen in order for the researchers to exploit it. They would need to employ much lower temperatures for additional chemicals.
According to the team, future wireless communication systems will need to extend the carrier frequency beyond 100 gigahertz into the terahertz range, which is unattainable with current technologies, in order to ensure noticeably higher data transfer speeds. “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”, said Ortix.
Study abstract
The nonlinear Hall effect with time-reversal symmetry is a second-order electronic transport phenomenon—seen as a quadratic voltage transverse to an applied electric field—that induces frequency doubling and occurs in non-centrosymmetric crystals with large Berry curvature. Optoelectronic devices based on this effect are limited because it typically appear at low temperatures and in complex compounds characterized by Dirac or Weyl electrons. Here we report a room-temperature nonlinear Hall effect in polycrystalline thin films of the centrosymmetric elemental material bismuth. The electrons at the (111) surface possess a Berry curvature triple that activates side jumps and skew scatterings, which generate nonlinear transverse currents. We show that the zero-field nonlinear transverse voltage can be boosted in arc-shaped bismuth stripes due to an extrinsic geometric classical counterpart of the nonlinear Hall effect. The electrical frequency doubling in curved geometries can be extended to optical second-harmonic generation in the terahertz spectral range. We also demonstrate efficient third-harmonic generation in polycrystalline bismuth films and bismuth-based heterostructures across a broad range of terahertz frequencies.
