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  • Featured in Physics
  • Open Access

Vanishing Phase Stiffness and Fluctuation-Dominated Superconductivity in UTe2

Sahas Kamat1, Jared Dans2, Shanta Saha2, Daniel F. Agterberg3, Johnpierre Paglione2,4, and B. J. Ramshaw1,4,*

  • *Contact author: bradramshaw@cornell.edu
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Phys. Rev. X 16, 041006 – Published 6 October, 2026

DOI: https://doi.org/10.1103/qjt6-hj2k

Abstract

Superconductivity in three dimensions is almost universally governed by Ginzburg-Landau mean-field theory, with critical fluctuations typically confined to within a few percent of the transition temperature (Tc). We report that the heavy-fermion superconductor UTe2 exhibits superconducting fluctuations that extend over a temperature range as wide as Tc itself—the largest observed for any three-dimensional superconductor. Through ultrasound measurements of the elastic moduli and sound attenuation, we find that UTe2 transitions from a mean-field-like state at ambient pressure to a fluctuation-dominated state at higher pressures. This regime is marked by elastic softening and an increase in sound attenuation that onsets well above Tc, with the attenuation remaining anomalously high deep in the superconducting state. Our analysis suggests that these features stem from an extremely low superfluid phase stiffness. This results in a kinetic inductance as high as that of granular aluminum, but achieved in the clean limit. We propose a model where this exotic state is driven by dominant interband pairing mediated by ferromagnetic fluctuations, leading to “local” Cooper pairs with a coherence length of only a few lattice constants.

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synopsis

Elucidating Superconductivity with Ultrasound

Published 6 October, 2026

Researchers measured a crystal’s elastic modulus and sound attenuation to infer the properties of two superconducting states.

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Popular Summary

The transition from a normal metal to superconductor usually happens abruptly as a function of temperature. Using ultrasound measurements, we found that the heavy-fermion superconductor UTe2 behaves very differently: Signatures of superconductivity appear gradually and persist to temperatures more than twice the superconducting transition temperature. This unusually broad fluctuation regime is the largest yet observed in a three-dimensional superconductor.

Our results point to an unconventional form of superconductivity in which paired electrons remain tightly bound together over only a few atomic spacings. Such pairs produce an exceptionally large kinetic inductance, a property that is valuable for quantum technologies and is normally achieved by introducing disorder into a material. In contrast, UTe2 realizes this behavior in a pristine crystal, highlighting clean heavy-fermion superconductors as a promising platform for quantum superconducting circuits.

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Phys. Rev. X 16, 041006– Published 6 October, 2026
  • Received 13 January 2026
  • Revised 15 May 2026
  • Accepted 7 August 2026
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