Kagome metal’s quantum effects yield strong heat-to-power at zero field

Researchers have demonstrated a transverse thermopower of 18.4 μV K−1 in the kagome metal LuCo6Ge6 at room temperature and zero field, a performance level significantly exceeding the less than 6 μV K−1 typically observed in existing anomalous Nernst effect (ANE)-based magnetic systems. This enhanced thermoelectric response stems from a phenomenon called goniopolarity, achieved by leveraging flat-band- and van Hove singularity-driven electronic states within the material’s unique structure. The work also reports a transverse Peltier conductivity of 105 A m−1 K−1, values an order of magnitude greater than conventional ANE systems, establishing goniopolar kagome metals as promising candidates for efficient, field-free thermoelectrics.

Kagome Lattice Drives Emergent Quantum States

The kagome metal LuCo6Ge6 demonstrates a transverse thermopower reaching 18.4 μV K−1, altering the behavior of flat bands and van Hove singularities typically observed in two-dimensional systems. The structural geometry of the kagome lattice, a two-dimensional network of corner-sharing triangles, is central to these emergent quantum states and their potential for energy applications. Kagome materials naturally host Dirac fermions, flat bands, and van Hove singularities within their electronic structure, creating a platform for exploring correlated electron physics.

A key challenge in transverse thermoelectricity has been the electromagnetic interference caused by the magnetic fields required for operation in ANE systems; goniopolarity circumvents this limitation by enabling zero-field performance. The study notes that goniopolarity, also known as axis-dependent carrier polarity, enables opposite carrier polarities (p-type or n-type) in a single material along different crystallographic axes, highlighting the mechanism’s ability to deliver higher transverse thermopowers compared to ANE.

Theoretical models guided the experimental realization of these effects in LuCo6Ge6 and related materials, Lu(Ho)Cr6Ge6, establishing a conceptual route for engineering transverse thermoelectricity through precise electronic structure design. The researchers propose two distinct mechanisms driving goniopolar transport: a flat-band-enabled two-band mechanism and a van Hove singularity-driven single-band mechanism.

In three-dimensional flat bands, transport primarily occurs out-of-plane due to suppressed in-plane transport behaviors, with the addition of a dispersive band dominating in-plane conduction and inducing anisotropy. When these bands consist of electron-like and hole-like carriers, the system exhibits goniopolar transport, as evidenced by the observed in-plane and out-of-plane Seebeck responses.

Fermiology-Driven Goniopolar Transverse Thermoelectricity

The work establishes a new framework for designing transverse thermoelectric materials, moving beyond reliance on external magnetic fields and their associated electromagnetic interference. Extending the kagome structure from two to three dimensions fundamentally alters its electronic properties; the introduction of out-of-plane hopping disperses formerly flat bands and suppresses the divergent nature of van Hove singularities. This dispersion enables out-of-plane charge transport, while the modified VHSs contribute to asynchronous particle-hole asymmetry.

The presence of both electron-like and hole-like bands within the material is key to the goniopolar effect, as the system exhibits a directional dependence in its thermoelectric response when these bands coexist. A Fermi surface analysis demonstrated an in-plane flat band responsible for out-of-plane transport and an isotropic hole pocket driving in-plane conduction, resulting in the observed goniopolar Seebeck responses. The enhanced density of states associated with flat bands significantly contributes to the peak Seebeck response.

LuCo6Ge6 Exhibits High Zero-Field Thermopower

Researchers at the University of Tokyo and collaborating institutions are focusing on LuCo6Ge6, a kagome metal exhibiting a transverse thermopower of 18.4 μV K−1. The team’s findings establish LuCo6Ge6 and related kagome metals as promising candidates for developing high-efficiency, zero-field transverse thermoelectric devices, potentially offering a new pathway for waste heat recovery and thermal management.

Flat Bands and van Hove Singularities in Kagome Systems

Analysis of the three-dimensional kagome lattice reveals that extending the structure beyond two dimensions fundamentally alters the behavior of flat bands; out-of-plane hopping introduces dispersion, enabling charge transport perpendicular to the plane while suppressing in-plane movement. Theoretical modeling suggests two distinct mechanisms drive this goniopolar transport. The first involves flat bands enabling a two-band conduction system, while the second relies on van Hove singularities creating asynchronous particle-hole asymmetry.

This combination, detailed in the paper’s illustrations of Fermi surfaces, creates the conditions for goniopolarity. The behavior of van Hove singularities also plays a crucial role; unlike two-dimensional kagome lattices where VHSs exhibit divergent characteristics, the three-dimensional structure suppresses this divergence. Modeling shows that VHSs generate distinct particle-hole symmetry characteristics, influencing the direction of charge transport and further contributing to the goniopolar effect. Specifically, the researchers found that the VHS exhibits a unique three-dimensional shape, altering with slight energy deviations, which allows for fine-tuning of the thermoelectric properties for various applications.

Goniopolarity Enables Enhanced Transverse Thermoelectricity

LuCo6Ge6 presents a departure from conventional thermoelectric materials; instead of relying on magnetic fields to generate transverse thermopower, this kagome metal achieves substantial energy conversion at zero field. This efficiency stems from a phenomenon termed goniopolarity, where differing crystallographic axes exhibit opposite carrier polarities, effectively creating both p-type and n-type conduction within the same material. Researchers detailed two distinct mechanisms driving this goniopolar transport, both intrinsically linked to the unique electronic structure of the kagome lattice.

Theoretical modeling reveals that flat bands within LuCo6Ge6 enable a two-band conduction system, crucial for anisotropic charge transport. Specifically, the band structure exhibits prominent peaks in the density of states arising from these dispersive flat bands, contributing to a substantial Seebeck response. Complementing this, the material’s van Hove singularities (VHSs) also play a critical role, though their behavior differs markedly from that observed in two-dimensional kagome lattices.

While VHSs typically exhibit divergent characteristics in 2D systems, the three-dimensional structure of LuCo6Ge6 alters these divergences, resulting in more controlled particle-hole symmetry. The interplay between flat bands and VHSs isn’t merely additive; it’s synergistic. The researchers found that the flat bands facilitate out-of-plane transport, while the dispersive bands contribute to in-plane conduction, creating a distinct anisotropy. This configuration, illustrated in the paper’s depictions of Fermi surfaces, generates the conditions necessary for efficient goniopolar transport.

Two-Band and Single-Band Mechanisms for Goniopolar Transport

The kagome metal LuCo6Ge6 exhibits a unique electronic structure that facilitates two distinct mechanisms for goniopolar transverse thermoelectric transport, according to a new study published on July 31, 2026. The researchers propose a flat-band-enabled two-band mechanism where transport primarily occurs out-of-plane due to the vanishing group velocity or diverging effective mass within the flat bands. This is complemented by a dispersive band dominating in-plane transport, creating an anisotropic system with electron-like and hole-like carriers.

When these bands combine, the material demonstrates goniopolar transport, exhibiting opposite carrier polarities along different crystallographic axes. Analysis of the Fermi surface generates the conditions necessary for a substantial goniopolar Seebeck response. Beyond flat bands, VHSs also play a critical role in this process, influencing the direction of charge transport through particle-hole asymmetry. Unlike their divergent behavior in 2D systems, VHSs in the 3D kagome lattice exhibit suppressed electron instabilities.

The study demonstrates that VHSs can contribute to a single-band goniopolar mechanism, with the in-plane and out-of-plane Seebeck responses differing in sign. Examination of the three-dimensional Fermi surface around the VHS reveals a saddle point, further illustrating the anisotropic transport characteristics. The findings establish LuCo6Ge6 as a promising platform for goniopolar transport and provide a conceptual framework for designing materials with enhanced thermoelectric properties.

Kagome Materials as a Platform for Energy Conversion

Researchers detailed their findings in a paper published on July 31, 2026, outlining the material’s unique electronic structure as contributing to this improved performance. These mechanisms are linked to the material’s high electrical conductivity and the enhanced density of states (DOS) associated with both flat bands and VHSs, creating conditions highly conducive to goniopolarity. This ability to tune thermoelectric properties through carrier concentration and mobility presents a critical advantage for adapting the material to diverse application requirements, potentially paving the way for more efficient waste heat recovery and thermal management solutions.