JNCASR and IISc Scientists Shatter a Century-Old Limit on Turning Heat Into Electricity

A Textbook Limit, Broken

For more than a century, a basic rule of solid-state physics has capped how much voltage a temperature difference can squeeze out of a crystalline material. Known as the Boltzmann thermopower limit, it restricts the Seebeck coefficient — the voltage generated per degree of temperature difference — to a few millivolts per kelvin in conventional semiconductors and metals. Liquid electrolytes can blow past that ceiling, but solids, bound by band-transport physics, largely can't.

A team led by Bivas Saha at the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) in Bengaluru, working with collaborators at the Indian Institute of Science (IISc) and the University of Sydney, has now shown a fully crystalline solid can do it anyway. In a paper published in Science (Vol. 393, pp. 611–614, 2026), the group reports a Seebeck coefficient exceeding -124 mV/K near room temperature — one film reached -124.6 mV/K — in epitaxial thin films of magnesium-doped scandium nitride (ScN). That's roughly three orders of magnitude beyond what conventional transport theory allows in a solid.

How a Crystal Learns to Act Like a Liquid

The trick lies in a deliberately imperfect kind of doping. Scandium nitride is normally grown as an n-type semiconductor, its electrical behaviour set by donor impurities. Saha's team instead created what they call a "heavily doped, highly compensated" (HDHC) semiconductor: they added magnesium acceptors in near-equal measure to the material's existing donors, so the two dopant populations almost cancel each other's net charge contribution.

Instead of smoothing things out, that near-cancellation leaves behind large, random fluctuations in the local electrostatic potential across the crystal — pockets where charge carriers are trapped or free depending on their position, in a landscape that looks nothing like the clean, periodic potential of an ordinary semiconductor. Electrons and holes are forced to hop between these pockets in a percolative fashion, a mechanism much closer to how ions move through a liquid electrolyte than to how carriers move through a conventional crystalline band. It's that liquid-like percolative transport — achieved without ever leaving the solid, single-phase, epitaxial state — that lets the material's thermopower escape the Boltzmann ceiling.

The result isn't a lab curiosity confined to cryogenic conditions: the colossal thermopower shows up near room temperature, in a material system already of interest for its compatibility with silicon and III-nitride semiconductor processing.

Who Did the Work

The Science paper lists Renuka Karanje, Dheemahi Rao, Diksha Dadhich and Sourav Rudra of JNCASR; Ashalatha Indiradevi Kamalasanan Pillai and Magnus Garbrecht of the University of Sydney; and Subroto Mukerjee of IISc, alongside senior author Bivas Saha, an associate professor at JNCASR's International Centre for Materials Science and Chemistry and Physics of Materials Unit. Saha presented the work in a seminar at IISc's Department of Materials Engineering on August 20, 2026, titled "Colossal Thermopower Beyond the Boltzmann Limit," ahead of the paper's formal publication.

Why It Matters Beyond the Physics

A thermopower this large, in a solid that can be grown as an epitaxial thin film using standard nitride-semiconductor deposition tools, opens doors that liquid-electrolyte thermoelectrics never could. The team has already demonstrated an early prototype photon sensor built on the effect, and the result points toward potential use in ultrasensitive temperature sensors capable of resolving minute thermal fluctuations, and in quantum-precision metrology, where a solid-state device with electrolyte-like sensitivity but crystal-like stability and manufacturability would be a significant upgrade over existing options.

It also reframes a question materials scientists have mostly treated as settled: whether a single-phase crystalline solid can ever really behave like a disordered, ionic system. Saha's group's answer — yes, if the disorder is engineered carefully enough through compensation doping — could open a design space for a broader family of "percolative" thermoelectric and sensing materials beyond ScN itself, in III-nitride and other wide-bandgap semiconductor systems that Indian materials-science and semiconductor-fabrication groups are already working with under initiatives like India's Semiconductor Mission.

For now, the result stands as a rare instance of an Indian materials-science lab landing a fundamental, textbook-rewriting result in a flagship international journal — a marker of the kind of frontier condensed-matter and quantum-materials research JNCASR, IISc and their international collaborators have been building toward for years.

Sources

• https://materials.iisc.ac.in/seminar/bivas-20-aug-2026
• https://libjncir.jncasr.ac.in/jspui/handle/123456789/3440
• R. Karanje, D. Rao, D. Dadhich, S. Rudra, A. I. K. Pillai, M. Garbrecht, S. Mukerjee, B. Saha, "Heavily doped, highly compensated epitaxial ScN thin films exceed Boltzmann thermopower limits," Science 393, 611–614 (2026)