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The generation of electricity from heat, also known as thermoelectric energy conversion, has proved to be advantageous for various real-world applications. For instance, it proved useful for the generation of energy during space expeditions and military missions in difficult environments, as well as for the recovery of waste heat produced from industrial plants, power stations or even vehicles.

The successful conversion of heat into electricity relies on one of two distinct effects, known as the Seebeck effect and the Nernst effect. The Seebeck effect occurs when two dissimilar materials are joined at two junctions that are at different temperatures, which can generate an electric current flowing in the loop. The Nernst effect, on the other hand, entails the generation of a transverse voltage in a material with a temperature gradient.

So far, the Nernst effect has been primarily demonstrated in time-reversal symmetry-breaking systems, either by applying an external magnetic field or by using magnetic materials. Yet recent physics theories have introduced the idea that a nonlinear Nernst effect (NNE) could arise in non-magnetic materials, crucially, under zero external magnetic field.

Researchers at Fudan University and Peking University have now realized this idea in an experimental setting for the first time. Their paper, in Nature Nanotechnology, reports the observation of a sizable nonlinear Nernst effect in an inversion symmetry-breaking form of trilayer graphene known as ABA trilayer graphene.

"Our research was inspired by the unique challenges and opportunities in the field of thermoelectricity," Pan He, co-senior author of the paper, told Â鶹ÒùÔº. "The conventional Nernst effect, a thermoelectric phenomenon that generates a transverse voltage from a temperature gradient, typically requires breaking time-reversal symmetry, often with a magnetic field. This requirement poses a significant challenge for the miniaturization and integration of thermoelectric devices. Yet recently, theoretical predictions have suggested a new type of phenomenon called NNE."

The NNE has been hypothesized to occur in non-magnetic materials without the need to apply an external magnetic field. While this could be highly advantageous for the development of devices that can convert heat into electrical power, the effect had so far proved difficult to realize experimentally.

"Our primary objective was to be the first to experimentally observe this effect and validate the recent theoretical predictions," explained He. "To observe the NNE in non-magnetic ABA trilayer graphene, we had to first fabricate high-quality Hall bar devices with microfabricated heaters and thermometers to precisely control and measure a local temperature gradient across the sample. We then used low-frequency electric harmonic measurements under an alternating thermal gradient to detect the nonlinear Nernst effect."

He and his colleagues applied a sinusoidal current (i.e., a current that alternates direction smoothly and periodically following a specific pattern) to a heater. This induced a temperature gradient in the material that fluctuated at twice its regular frequency.

The researchers found that the NNE eventually manifested as a fourth harmonic transverse voltage in the material, representing a second-order response to the temperature gradient they produced, unlike the linear response of the conventional Nernst effect.

"Our work presents the first experimental observation of a giant NNE in a non-magnetic material under zero magnetic field," said He. "We measured a giant effective Nernst coefficient of up to 300 µVK^-1 at 2 K in ABA trilayer graphene, which is approximately two orders of magnitude larger than the highest values reported in magnetic materials at zero magnetic field."

The team's first observation of the NNE in trilayer graphene without the need for an external magnetic field could have various practical implications. Most notably, it offers an alternative solution for the harvesting of thermoelectric energy without the need for magnetic materials or external magnetic fields. In the future, the team's experimental methods could be leveraged to develop more compact thermoelectric devices that can be deployed in various real-world environments.

"Our work has opened several exciting directions for future research," said He. "A major goal is to extend the NNE from our low-temperature observations (below 12 K) to room temperature, which is essential for broader practical applications. We plan to explore other time-reversal invariant and non-centrosymmetric materials that might exhibit this effect at higher temperatures and from other mechanisms."

As part of their next studies, He and his colleagues plan to work on optimizing the NNE they observed. In addition, they would like to try to modulate the effect using external magnetic fields.

"Ultimately, we aim to realize the NNE in three-dimensional bulk materials to overcome the limitations of 2D systems," added He.

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