The H--T phase diagram of UTe2 for magnetic field along the hard b axis contains an unresolved internal boundary near μ0H∼14--15~T, previously inferred from ac susceptibility and transport experiments but lacking thermodynamic evidence. We report ultrasound results for several elastic modes in an ultraclean UTe2 single crystal with Tc>2~K for H∥b down to 0.33~K and up to 18~T. A pronounced anomaly in the longitudinal C33 mode, with a weaker response in C44 and no resolvable anomaly in C55, establishes this feature as a bulk thermodynamic phase boundary and reveals a symmetry-selective coupling to lattice strain. The phase line remains nearly constant in field near 14~T and terminates near 13.5~T and 1.25~K at a tetracritical point, providing the thermodynamic evidence for the fourth phase boundary in the H--T phase diagram. The results constrain the order-parameter structure of the high-field phase and support field-induced multicomponent superconductivity in UTe2.
@article{arxiv.2604.25896,
title = {Thermodynamic Identification of the Internal Superconducting Phase Boundary in UTe$_2$ for $H \parallel b$},
author = {Michal Vališka and Tetiana Haidamak and Andrej Cabala and Petr Proschek and Andreas Hausprug and Sergei Zherlitsyn and Vladimír Sechovský},
journal= {arXiv preprint arXiv:2604.25896},
year = {2026}
}