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In a new Nature Â鶹ÒùÔºics , researchers report the first experimental observation of the transverse Thomson effect, a key thermoelectric phenomenon that has eluded scientists since it was predicted over a century ago.

For over a century, thermoelectric effects have formed the foundation of how physicists understand the link between heat and electricity. Our knowledge of how heat and electricity interact within materials is rooted in the Seebeck, Peltier, and Thomson effects, all identified during the 1800s.

The Thomson effect causes volumetric heating or cooling when an electric current and a temperature gradient flow in the same direction through a conductor.

Scientists have long theorized that a transverse version of this effect should exist when an electric current, temperature gradient, and magnetic field are applied in orthogonal directions in a conductor.

Now, a research team led by Atsushi Takahagi from Nagoya University and Ken-ichi Uchida from the University of Tokyo has demonstrated that the transverse effect, one of the higher-order thermoelectric effects, exists.

"I have been deeply motivated by the thermoelectric effect, which has attracted attention as energy harvesting and thermal management technologies," Takahagi told Â鶹ÒùÔº.

The transverse Thomson effect has proven remarkably difficult to observe experimentally due to interference from competing thermal effects, like the Peltier and Ettingshausen effects.

Isolating the signal

To overcome the challenge of signal isolation, the research team proposed using advanced thermoelectric imaging techniques to observe and characterize the phenomenon. Their method is based on lock-in thermography.

"In our experiment, we used an infrared camera to observe the thermal response of the sample when a periodic electric current was applied," explained Uchida.

"By extracting the temperature modulation component that oscillates at the same frequency as the applied current from the taken thermal images, we were able to isolate the thermoelectric signals from the Joule heating."

The turning point came from recognizing that the spatial distribution of the transverse Thomson effect differs from other competing effects. The team performed measurements under two conditions, with and without a temperature gradient. Then they subtracted the results to isolate the pure transverse Thomson signal.

The first measurement with the temperature gradient captures the transverse Thomson effect and the Ettingshausen effects. The second measurement without the temperature gradient captures just the latter, yielding the isolated transverse Thomson effect when subtracted.

Choosing the right material

The researchers selected a bismuth antimony alloy (Bi₈₈Sb₁₂) for their experiments, a material known for its strong Nernst effect around room temperature.

When a temperature gradient and magnetic field are applied orthogonally to a material, it leads to the creation of an electric field perpendicular to both. This is known as the Nernst effect.

The Ettingshausen effect, mentioned earlier, is the reverse. When an electric current and magnetic field are applied orthogonally, a temperature gradient is created perpendicular to both.

"Our research revealed that the magnitude of the heat source responsible for the transverse Thomson effect is determined by both the temperature derivative of the Nernst coefficient and the Nernst coefficient itself," explained Takahagi.

"The Bi₈₈Sb₁₂ alloy has long been known as a material with a large Nernst coefficient, making it a particularly promising candidate for exhibiting the transverse Thomson effect."

This represents a fundamental difference from the conventional Thomson effect, which depends solely on the temperature derivative of the Seebeck coefficient.

Switching between hot and cold

One of the most surprising discoveries was the ability to switch between heating and cooling by simply changing the magnetic field direction. The effect showed complex field-dependent behavior, including a complete sign reversal at certain magnetic field strengths.

The researchers discovered that the transverse Thomson coefficient comprises two competing components: one related to the temperature derivative of the Nernst coefficient (generally causing heating) and another related to the Nernst coefficient magnitude (generally causing cooling).

The competition between these components creates the field-dependent sign reversal observed in their experiments.

Their numerical simulations accurately reproduced the experimental observations, confirming the theoretical understanding of the phenomenon and validating their measurement technique.

Potential applications

The discovery opens new possibilities for thermal management technologies, particularly in applications requiring precise, localized heat control.

"In recent years, it has been reported that the conventional Thomson effect can enhance the performance of Peltier cooling," noted Takahagi.

"Similarly, the transverse Thomson effect is expected to serve as a principle for improving the performance of transverse thermoelectric cooling devices."

The current work also points toward strategies for developing more efficient materials. In the Bi₈₈Sb₁₂ alloy studied, the two components of the transverse Thomson coefficient partially cancel each other out, limiting the overall magnitude of the effect.

"Identifying new materials in which these two components reinforce each other could lead to the discovery of high-performance materials for the transverse Thomson effect, representing an important avenue for future research," explained Uchida.

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