Beyond Kelvin's Relation: A Giant Transverse Thomson Response for Efficient Cryogenic Cooling

Efficient solid-state cooling at cryogenic temperatures remains a longstanding challenge because most thermoelectric materials and device architectures are designed for power generation rather than refrigeration. Here, we demonstrate a giant Thomson and transverse Thomson response in the correlated semiconductor FeSb₂ below 60 K, enabled by strong phonon-drag enhanced thermoelectric transport. The pronounced temperature dependence of the Seebeck coefficient produces an unusually large longitudinal Thomson coefficient (∼800 μV K^-1), while the simultaneous enhancement of the transverse thermopower under a magnetic field gives rise to a giant transverse Thomson effect (∼29 μV K^-1). These results identify phonon drag as a general and scalable design principle for realizing large Thomson responses. Beyond FeSb₂, this framework suggests a pathway to discovering new cryogenic cooling materials by targeting systems with strong phonon-electron coupling, low carrier density, long phonon lifetime, and large thermopower derivatives. More broadly, this work establishes the Thomson effect as a powerful, yet largely unexplored, route for engineering efficient, stable and versatile solid-state cooling platforms for future cryogenic and quantum technologies.