Kagome metals enable goniopolar transverse thermoelectric effects via Fermiology

In a development that could reshape how engineers harvest waste heat, a team of researchers has demonstrated a powerful new form of transverse thermoelectricity in kagome metals, showing that the exotic geometry of electron motion in these materials can be harnessed to convert heat flow directly into electrical voltage perpendicular to the heat current. The work, published in Nature Materials, establishes a direct link between a material’s Fermi surface — the map of allowed electron momenta in a crystal — and its ability to exhibit what the authors call goniopolar transverse thermoelectricity, a property in which the sign and magnitude of the thermoelectric response depend on the crystallographic direction in which the heat is driven.

Conventional thermoelectric devices rely on the Seebeck effect, in which a temperature gradient along a material generates a voltage along the same axis. This geometry imposes a long-standing trade-off: the figure of merit for such devices couples electrical and thermal conductivities in ways that make simultaneous optimization notoriously difficult, limiting the efficiency of heat-to-electricity conversion. Transverse thermoelectricity offers a way around this bottleneck. By producing a voltage at right angles to the heat flux, it decouples the two currents geometrically, allowing engineers to design generators as thin films or plates in which the electrical and thermal paths are independent. The challenge has always been finding materials in which the transverse response is large, tunable and, crucially, controllable by something other than chemical doping.

The new study reports that in kagome metals — layered intermetallic compounds whose atoms form a lattice of corner-sharing triangles and hexagons — the transverse thermoelectric response can be switched in sign and optimized in magnitude simply by rotating the direction of heat flow relative to the crystal axes. The researchers trace this “goniopolar” behavior to the intricate topology of the Fermi surface in these compounds. In a kagome metal, the electronic band structure contains flat bands, Dirac crossings and saddle points that produce sharp features in the density of available electron states. When a temperature gradient is applied, charge carriers with different velocities and different signs of Hall response are deflected by the magnetic field of the measurement geometry — or, in some regimes, by intrinsic Berry curvature — in ways that depend sensitively on which portions of the Fermi surface dominate transport.

The experimental strategy combined two powerful probes. The team performed detailed Fermiology measurements, mapping the shape of the Fermi surface through quantum oscillation experiments in which the material’s magnetoresistance oscillates periodically as a function of the inverse magnetic field. These oscillations act as a fingerprint of the extremal orbits that electrons trace around the Fermi surface, revealing their cross-sectional areas and effective masses with high precision. In parallel, the group measured the transverse thermoelectric voltage generated when heat was pushed through the crystal along different in-plane directions. By comparing these two datasets, the authors were able to show quantitatively that the angular dependence of the transverse thermopower mirrors the angular dependence of the Fermi surface itself.

This correspondence is the conceptual heart of the paper. In an ordinary metal, the transverse thermoelectric coefficient arises from a product of the electrical conductivity and the energy derivative of the Hall conductivity, both of which are governed by the distribution of carrier velocities on the Fermi surface. In a kagome metal, the velocity distribution is highly anisotropic: electrons moving along one crystal direction can have completely different masses, lifetimes and Berry curvatures than electrons moving along a perpendicular direction. When heat drives carriers at these different velocities, the transverse deflections they experience do not cancel symmetrically. Instead, the anisotropy produces a net transverse voltage whose sign flips as the heat current is rotated past certain symmetry directions — the defining signature of goniopolar behavior.

The practical implications of a sign-switchable, angle-dependent thermoelectric response are considerable. In a conventional transverse generator, reversing the desired polarity of the output voltage requires reversing the field geometry or the connection scheme. In a goniopolar material, the same crystal can serve as either a positive or negative transverse thermoelectric element depending on its in-plane orientation, which means a single material platform could, in principle, supply both the n-type and p-type legs of a thermoelectric circuit. Because the polarity is set by geometry rather than chemistry, device designers would no longer need to synthesize chemically distinct n- and p-type materials with matched performance — a persistent obstacle in assembling efficient thermoelectric modules from brittle intermetallic compounds.

The authors emphasize that the effect is genuinely Fermiology-driven, meaning it originates in the detailed shape of the electronic structure rather than in a coincidental balance of scattering rates. This distinction matters for materials search. If the transverse response were governed mainly by impurity scattering or grain boundaries, it would be fragile and difficult to predict. Because it is tied to the Fermi surface, it can be computed from band-structure calculations, screened systematically across candidate compounds, and tuned deliberately — for example by slight changes in carrier density that move the Fermi level across a saddle point, or by strain that distorts the kagome lattice and shifts the anisotropic features of the band structure.

Kagome metals themselves have been at the center of condensed-matter attention for several years. Their lattice geometry produces destructive interference that flattens portions of the electronic bands, concentrating electron states into narrow energy windows where interactions between electrons are dramatically enhanced. The same geometry also supports Dirac points — linear band crossings protected by symmetry — and, in certain compositions, exotic phases ranging from charge order to unconventional superconductivity. The new work adds thermoelectric functionality to this list, suggesting that the kagome platform is not merely a playground for fundamental physics but also a source of technologically useful transport phenomena.

The measurement of transverse thermoelectricity in these materials is experimentally demanding. The signals of interest are small voltages generated in response to modest temperature differences, and they must be separated from spurious effects such as contact thermopowers and ordinary longitudinal Seebeck contributions. The researchers addressed this by performing measurements with the heat current aligned along multiple well-defined crystallographic directions, exploiting the symmetry of the kagome lattice to distinguish the intrinsic transverse signal from parasitic mixing of longitudinal effects. The observed angular dependence — oscillating in sign with the rotation angle of the heat current relative to the lattice — is difficult to produce by any extrinsic mechanism and stands as strong evidence for the intrinsic, Fermi-surface-driven origin of the effect.

Beyond the immediate demonstration, the study opens a broader research direction sometimes described as Fermiology-guided thermoelectrics: the systematic search for materials in which the topology of the Fermi surface, rather than the magnitude of the band gap or the power factor alone, becomes the primary design variable. Traditional thermoelectric optimization has centered on semiconductors with carefully tuned carrier concentrations and reduced lattice thermal conductivity. The transverse approach shifts the emphasis to metals, which were long dismissed for thermoelectric applications because their high thermal conductivity wastes temperature gradients. In a transverse geometry, high thermal conductivity is far less detrimental, because the heat current and the electrical output are extracted along different axes.

The findings also connect to recent interest in anomalous transverse transport phenomena, in which the Berry curvature of electronic bands — a quantum-geometric property that acts like an intrinsic magnetic field in momentum space — deflects carriers without any external magnetic field. In magnetic kagome systems, such Berry-curvature effects can produce enormous anomalous Hall and Nernst signals. The present work demonstrates that even the detailed non-topological geometry of the Fermi surface, its corrugations, necks and saddle points, can be leveraged for transverse energy conversion, providing a complementary and potentially more tunable knob than magnetism alone.

For applications, the road ahead involves optimizing the magnitude of the transverse thermoelectric conductivity and engineering the materials into device-compatible forms. Thin films of kagome metals, patterned with controlled in-plane orientation, could be stacked into compact energy-harvesting modules that scavenge waste heat from electronics, vehicles or industrial processes. The ability to define the output polarity by lithographic orientation rather than material selection would simplify fabrication considerably. The researchers also point to the possibility of further tuning through carrier doping, pressure and strain, each of which reshapes the Fermi surface and therefore the angular pattern of the transverse response.

The study is a striking example of a theme that runs through modern condensed-matter research: quantum-mechanical features of the electronic structure, once considered esoteric curiosities, can be translated directly into device functionality. By showing that the very shape of the Fermi surface can dictate the polarity and strength of thermoelectric voltage generation, the work transforms a piece of fundamental Fermiology into a design principle. As researchers continue to map and manipulate the electronic geometry of kagome metals and related topological materials, the boundary between fundamental quantum matter and practical energy technology grows thinner — and the waste heat that flows invisibly through every electronic device becomes, in the right crystal at the right angle, a source of usable electricity.