Entropy Per Carrier Universally Dictates Thermopower in Diverse Materials Systems.

Research demonstrates thermopower, a measure of thermoelectric efficiency, is universally determined by entropy per carrier, not specific heat. This principle applies to diverse systems including magnetic materials, superconductors, and single-molecule junctions, validated by experimental data showing consistent entropy-based scaling across magnetic and superconducting transitions.

The efficient conversion of heat into electrical energy, a cornerstone of thermoelectric technology, relies heavily on optimising a material’s thermopower – the magnitude of the voltage generated in response to a temperature difference. Despite extensive research, a unifying principle explaining thermopower across diverse materials – from conventional conductors to magnetic systems and nanoscale devices – has remained unclear. Researchers at the Institute for Advanced Studies in Basic Sciences and North Carolina State University now propose that entropy per charge carrier, rather than specific heat, is the fundamental determinant of thermopower in both closed and open systems. This framework, detailed in their paper ‘What Really Drives Thermopower: Specific Heat or Entropy as the Unifying Principle Across Magnetic, Superconducting, and Nanoscale Systems’, is supported by analysis of magnetic materials, superconducting niobium, and single-molecule junctions, and validated against existing experimental data. The work is led by Morteza Jazandari and Jahanfar Abouie, alongside Daryoosh Vashaeeb.

Entropy as the Primary Driver of Thermoelectric Performance

Recent research establishes a fundamental relationship between entropy per carrier and thermopower – a key parameter defining the efficiency of thermoelectric materials. This work demonstrates that, contrary to prevailing assumptions, specific heat does not universally govern thermopower. Instead, entropy per carrier emerges as the primary driver across diverse systems, both closed and open. The investigation leverages established thermodynamic relations, notably the Onsager-Kelvin relation, to derive a universal proportionality between thermopower and entropy per carrier, reframing understanding of thermoelectric behaviour and shifting focus from material heat capacity to the entropy associated with each charge carrier.

Apparent correlations between thermopower and specific heat arise only in materials exhibiting a continuous power-law temperature dependence of specific heat, challenging long-held assumptions within the field. Researchers developed a general expression for magnon-drag thermopower, applicable to both classical and relativistic magnon regimes, extending this principle to magnetic materials and providing a more robust theoretical foundation for understanding thermopower in magnetically ordered systems. A magnon is a quantum of spin wave excitation.

Validation of this unifying principle occurs through analysis of three distinct systems: magnetic materials, superconducting niobium, and single-molecule junctions, solidifying the research’s impact and broadening its applicability. Experimental data from superconducting niobium confirms the role of quasiparticle entropy near the critical temperature, while literature data from a range of ferromagnetic and antiferromagnetic materials demonstrates consistent entropy-based scaling across magnetic transitions.

The research begins by establishing a clear distinction between specific heat and entropy, highlighting entropy as the more fundamental property governing thermopower generation. By applying the Onsager-Kelvin relation – which describes the coupling between heat flow and electric field – they derive a universal proportionality between thermopower and entropy per carrier, providing a theoretical framework that transcends material-specific details and offers a unifying principle for understanding thermoelectric behaviour.

To extend this principle to magnetic materials, the team developed a theoretical model that accurately captures the behaviour of magnons in both classical and relativistic regimes, accounting for the complex interactions between spins and charge carriers. They addressed inconsistencies present in previous models by employing a relativistic energy-momentum tensor, avoiding reliance on ill-defined magnon masses in antiferromagnets.

Superconducting niobium serves as a crucial test case, providing a unique opportunity to investigate the role of quasiparticle entropy in determining thermopower. Researchers demonstrated that the anomalous thermopower originates from the entropy carried by Bogoliubov quasiparticles near the critical temperature, confirming the importance of entropy in this material. Bogoliubov quasiparticles are excitations in a superconductor, representing a superposition of electron and hole.

Single-molecule junctions represent a fundamentally different type of system, offering a unique opportunity to investigate the role of entropy in determining thermopower in nanoscale devices. Researchers demonstrated that thermopower in open systems is governed by entropy arising from fluctuations in the number of charge carriers, confirming the universality of the proposed framework.

The implications of this research extend far beyond fundamental science, offering exciting possibilities for developing new and improved thermoelectric materials and devices. By understanding the fundamental role of entropy in determining thermopower, researchers can design materials with enhanced thermoelectric performance, leading to more efficient energy conversion and reduced energy waste.

Looking ahead, the researchers plan to investigate the role of entropy in other thermoelectric materials and devices, exploring new materials and device architectures based on these principles. They also plan to develop more sophisticated theoretical models that capture the complex interplay between entropy, charge transport, and heat transfer in thermoelectric materials. This research promises to unlock new possibilities for energy conversion and storage, contributing to a more sustainable and energy-efficient future. The team is committed to collaborating with other researchers and industry partners to translate these fundamental discoveries into practical applications, addressing some of the most pressing energy challenges facing the world today.