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  • Open Access

An ab Initio Answer to Long-Debated Questions about Superconducting Nb3Sn

Alessio Cucciari* and Lilia Boeri†

  • *Contact author: alessio.cucciari@uniroma1.it
  • †Contact author: lilia.boeri@uniroma1.it
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PRX Energy 5, 013006 – Published 4 February, 2026

DOI: https://doi.org/10.1103/dmwh-1sbf

Abstract

Nb3Sn is the workhorse superconductor for high-field applications; however, several fundamental aspects remain open 70 years after its discovery. Using anharmonic ab initio Migdal-Eliashberg theory, we resolve three key questions: (1) the cubic-to-tetragonal martensitic transition is weakly first order; (2) the superconducting gap is strongly anisotropic but continuous over the Fermi surface; and (3) the drop of Hc2 across the transition arises from both reduced electron-phonon coupling and a redistribution of Fermi velocities. The orbital origin of the gap anisotropy suggests targeted doping strategies: Sn-site substitution can increase both Tc and Hc2, while Nb-site substitution raises Hc2 at the cost of lowering Tc.

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Popular Summary

Nb3Sn is a low-Tc conventional superconductor, widely regarded as the workhorse material for high-field applications, including MRI scanners, particle accelerators, and fusion reactors. It can sustain very high upper critical fields (up to 29 T) and critical current densities. A long-standing obstacle to optimal performance is the cubic-to-tetragonal martensitic transition at low temperatures, which is accompanied by a more than 30% reduction in the upper critical field.

Here, the authors present a fully ab initio microscopic description of Nb3Sn, employing state-of-the-art anharmonic Migdal-Eliashberg theory. This approach addresses key open questions about the nature of the structural transition, the anisotropy of the superconducting gap, and the microscopic origin of the critical field drop, demonstrating that the latter is directly driven by the martensitic transition. These results provide a clear picture of how different doping strategies affect superconducting performance, enabling targeted approaches for optimization. This work marks an important step toward realistic ab initio modeling of technological superconductors.

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PRX Energy 5, 013006– Published 4 February, 2026
  • Received 18 September 2025
  • Accepted 9 January 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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