Quantum Effects Boost Room-Temperature Thermoelectricity in Weyl Semimetals

Researchers at the Zernike Institute for Advanced Materials, University of Groningen report room-temperature thermoelectric responses in the low-symmetry type-II Weyl semimetals WTe₂ and TaIrTe₄ without requiring magnetic fields or magnetic materials. Exploiting harmonic detection with gradient-reversal techniques, the team resolved all symmetry-allowed components of the second-order thermoelectric tensor, including the nonlinear Seebeck, nonlinear Nernst, and nonlinear mixed-directional thermoelectric effects. The work demonstrates that both Berry-curvature-related and scattering-induced contributions govern these nonlinear thermoelectric responses, revealing a nuanced interplay of quantum mechanical phenomena. These results show that nonlinear thermoelectricity arises intrinsically from reduced crystal symmetry, offering a versatile platform for heat-to-charge current conversion beyond linear responses.

A surprising discovery reveals that certain materials exhibit robust thermoelectric responses at ambient temperatures without relying on external magnetic fields. Recent work demonstrates that these nonlinear thermoelectric responses can arise intrinsically within specific materials, circumventing the need for complex device fabrication. This method allowed for precise measurement of how heat gradients induce electrical voltages in a non-linear fashion. The investigation into low-symmetry type-II Weyl semimetals WTe₂ and TaIrTe₄ revealed that their reduced crystal symmetry is fundamental to the observed nonlinear thermoelectricity. Thin flakes of these materials, belonging to a specific space group, retain only a single mirror symmetry, minimizing symmetry constraints on the thermoelectric response. “Exfoliated WTe₂ and TaIrTe₄, owing to their low crystal symmetry, provide an ideal platform with minimal symmetry constraints for exploring nonlinear thermoelectric responses,” the researchers state. Engineering effects related to scattering in these materials provides a versatile platform for exploring higher-order heat-to-charge current conversion beyond the linear response.

The pursuit of efficient thermoelectric materials, those converting heat into electricity and vice versa, has long focused on maximizing linear responses, but a growing body of research now investigates nonlinear effects offering potential for advanced thermal-management and energy-conversion. Researchers at the Zernike Institute for Advanced Materials, University of Groningen report the observation of room-temperature thermoelectric responses and investigated the low-symmetry type-II Weyl semimetals WTe₂ and TaIrTe₄, materials possessing a unique low symmetry. The researchers write, highlighting the materials’ suitability for this investigation. Manipulating scattering processes, the team states, could provide a versatile pathway to enhance heat-to-charge current conversion.

Harmonic Detection Resolves Second-Order Thermoelectric Tensor

Researchers at the Zernike Institute for Advanced Materials, University of Groningen report employing a sophisticated technique to dissect the origins of thermoelectric behavior in materials with unusual symmetry. This approach allowed them to move beyond simply observing nonlinear responses and instead resolve all symmetry-allowed components of the second-order thermoelectric tensor, including the nonlinear Seebeck, nonlinear Nernst, and nonlinear mixed-directional thermoelectric effects, revealing the underlying mechanisms at play. The investigation focused on thin flakes of WTe₂ and TaIrTe₄, materials chosen specifically for their low-symmetry type-II Weyl semimetals. This reduced symmetry, the researchers found, is a fundamental driver of the observed thermoelectric effects. It is not a case of one effect dominating; rather, both the curvature of electron energy bands and the way electrons scatter within the material contribute significantly to the overall response. This nuanced understanding is crucial because it suggests avenues for material design beyond simply optimizing band structure.

Manipulating scattering processes could provide a versatile pathway to enhance heat-to-charge current conversion, exceeding the limitations of linear thermoelectric materials. The observation of these effects at room temperature, without the need for external magnetic fields, is particularly noteworthy, suggesting potential for applications in energy conversion. The researchers unambiguously demonstrate intrinsic nonlinear thermoelectric effects arising from low crystal symmetry that persist even up to room temperature.

WTe₂ and TaIrTe₄ Demonstrate Nonlinear Seebeck, Nernst, and Mixed-Directional Effects

The pursuit of efficient thermal management and energy conversion has led researchers to explore nonlinear thermoelectric effects, responses that rectify heat flow into electrical current. A team investigated the low-symmetry type-II Weyl semimetals WTe₂ and TaIrTe₄, materials possessing a unique low symmetry. Manipulating scattering processes, the team demonstrates, could provide a versatile platform for exploring higher-order heat-to-charge current conversion beyond the linear response. The observation of these effects at room temperature, without the need for external magnetic fields, suggests potential for functional elements for nonreciprocal thermal sensing and energy conversion.

Each device incorporated platinum heaters and gold contacts patterned onto the semimetal flakes; crucially, “the heaters were made more than five times longer than the voltage detection channel, and the contact electrodes’ geometry is kept symmetric relative to the heaters.” These findings demonstrate that the observed nonlinear thermoelectricity arises intrinsically from reduced crystal symmetry and that engineering effects related to scattering in these materials provides a versatile platform for exploring higher-order heat-to-charge current conversion.

The expectation that robust thermoelectric effects require carefully engineered materials or low-temperature operation has been challenged by recent findings concerning the Weyl semimetals WTe₂ and TaIrTe₄. Researchers report intrinsic nonlinear thermoelectric responses in these materials extending to room temperature. This work, detailed in recent findings, resolves all symmetry-allowed components of the second-order thermoelectric tensor, including the nonlinear Seebeck, nonlinear Nernst, and nonlinear mixed-directional thermoelectric effects, through a sophisticated experimental approach. The team utilized a technique to achieve these results, and both the geometric properties of the material’s electronic band structure (Berry curvature) and the way electrons collide with imperfections (scattering) play essential roles. The observation of these effects in WTe₂ and TaIrTe₄, materials possessing low-symmetry type-II Weyl semimetals, provides an ideal platform for investigation, minimizing constraints on the observed responses. The findings establish intrinsic nonlinear thermoelectric transport as a symmetry-governed phenomenon.

Experimental Setup for NLS, NLMT, and NLN Effect Measurement

The ability to meticulously measure nonlinear thermoelectric effects at room temperature demanded a specialized experimental approach, detailed in recent findings. Researchers at the Zernike Institute for Advanced Materials, University of Groningen report the observation of devices fabricated from exfoliated WTe₂ and TaIrTe₄, materials chosen for their minimal symmetry constraints, to isolate intrinsic responses. Three distinct geometries were employed to probe specific thermoelectric phenomena: the nonlinear Seebeck effect (NLS), the nonlinear mixed-directional thermoelectric effect (NLMT), and the nonlinear Nernst effect (NLN). Each device incorporated platinum heaters and gold contacts patterned onto the semimetal flakes; “the heaters were made more than five times longer than the voltage detection channel, and the contact electrodes’ geometry is kept symmetric relative to the heaters.” This design minimized spurious temperature gradients that could obscure the intrinsic signals.

Data acquisition involved analyzing both second- and fourth-harmonic thermoelectric responses as a function of the applied temperature gradient. A symmetrization process was applied to the data “to isolate intrinsic nonlinear thermoelectric contributions.” This technique effectively removed any linear background, revealing the true nonlinear signals. The resulting measurements, as shown in accompanying figures, confirmed the presence of these effects even at room temperature, demonstrating a versatile platform for exploring higher-order heat-to-charge current conversion.

Establishing a clear distinction between inherent nonlinear thermoelectricity and the temperature-dependent behavior of linear effects proved critical for this research; without careful separation, the higher-order responses could be obscured or misinterpreted. Researchers addressed this challenge through a meticulous data analysis process, applying a symmetrization procedure designed to isolate the true intrinsic nonlinear contributions. This ability to isolate intrinsic effects is significant because it demonstrates that nonlinear thermoelectricity arises intrinsically from reduced crystal symmetry and that engineering effects related to scattering in these materials provides a versatile platform for exploring higher-order heat-to-charge current conversion beyond the linear response.

Their work, published this month, moves beyond simply observing these effects to understanding why they occur in certain materials, specifically the low-symmetry type-II Weyl semimetals WTe₂ and TaIrTe₄. The investigation centered on the second-order thermoelectric tensor, which dictates how a temperature gradient induces an electric field, and how that relationship is constrained by a material’s crystal symmetry. The findings demonstrate that both Berry-curvature-related and scattering-induced contributions govern the different nonlinear thermoelectric responses.