ÖPG-CMD Joint Meeting 2026

Thermodynamic Probes of Hidden Superconducting Phase Boundaries in UTe2

Sep 23, 2026, 5:30 PM

30m

HS 15.14 (University of Graz)

4) Invited talk M02 - Heavy quasiparticles in heavy fermion compounds Mini-Colloquium

Speaker

M. Valiska (Charles University, Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Ke Karlovu 5, 121 16 Prague 2, Czech Republic)

Description

UTe$_2$ is one of the most prominent candidate materials for spin-triplet and potentially topological superconductivity. Its superconducting phase diagram is exceptionally rich, with multiple field- and pressure-induced phases whose topology remains intensely debated. A particularly important unresolved issue concerns the high-field superconducting regime for magnetic field applied along the hard b axis. Previous ac-susceptibility and transport studies suggested an internal superconducting phase boundary near 14–15 T [2,3], but clear bulk thermodynamic evidence for this boundary had been missing.
This work presents high-field ultrasound measurements on ultraclean UTe$_2$ single crystals with Tc above 2 K [1]. By measuring several elastic modes in static magnetic fields up to 18 T and temperatures down to 0.33 K, we identify a distinct anomaly in the longitudinal C₃₃ mode near 14–15 T. A weaker response is observed in C₄₄, whereas no corresponding anomaly is resolved in C₅₅. This mode selectivity demonstrates that the high-field superconducting state couples anisotropically to lattice strain and provides symmetry-sensitive constraints on the superconducting order parameter.
The observed elastic anomaly supplies the missing bulk thermodynamic evidence for the internal superconducting phase line. This line terminates near 13.5 T and 1.25 K at a tetracritical point, completing the local four-boundary topology of the H–T phase diagram for H ∥ b [1]. The results support field-induced multicomponent superconductivity in UTe$_2$ and connect the ambient-pressure high-field phase diagram to the recently established pressure-induced tetracritical regime [4]. More broadly, they demonstrate the power of ultrasound to reveal hidden superconducting phase boundaries that may remain weak or unresolved in specific heat.

[1] M. Vališka et al., arXiv: 2604.25896 (2026).
[2] H. Sakai et al., Physical Review Letters 130, 196002 (2023).
[3] Y. Tokiwa et al., Physical Review B 108, 144502 (2023).
[4] S. Kamat et al., arXiv:2603.17905 (2026).