The generation of electricity from heat, also known as thermoelectric energy conversion, is 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 reports the observation of a sizable nonlinear Nernst effect in an inversion symmetry-breaking form of trilayer graphene known as ABA trilayer graphene.
The team aimed to be the first to experimentally observe this effect and validate the recent theoretical predictions. To observe the NNE in non-magnetic ABA trilayer graphene, the scientists 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. They then used low-frequency electric harmonic measurements under an alternating thermal gradient to detect the nonlinear Nernst effect.
The team 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.
The team's work is reportedly the first experimental observation of a giant NNE in a non-magnetic material under zero magnetic field. The researchers 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.
Looking ahead, a major goal will be to extend the NNE from the low-temperature observations (below 12 K) to room temperature, which is essential for broader practical applications. The team plans to explore other time-reversal invariant and non-centrosymmetric materials that might exhibit this effect at higher temperatures and from other mechanisms. Ultimately, they aim to realize the NNE in three-dimensional bulk materials to overcome the limitations of 2D systems.
