Sudden adiabaticity signals reentrant bulk superconductivity in UTe₂

There has been a recent surge of interest in UTe₂ due to its unconventional magnetic field-reinforced spin-triplet superconducting phases persisting at fields far above the simple Pauli limit for fields H || [010]. Magnetic fields in excess of 35 T then induce a field-polarized magnetic state via a first-order-like phase transition. More controversially, electrical resistivity measurements suggest that a further superconducting state may exist for field orientations close to H || [011] and above 40 T. However, no Meissner effect or thermodynamic evidence exists to date for this phase, making it difficult to exclude alternative scenarios. In this talk, we will describe an experimental study using thermal, electrical, and magnetic probes to study UTe₂ single crystals in magnetic fields of up to 55 T applied between the [010] (b-axis) and [001] (c-axis) directions [1]. Our MHz conductivity data reveal a field-induced state of low or vanishing electrical resistance, and simultaneous magnetocaloric effect measurements (i.e., changes in sample temperature due to changing magnetic field) show the first definitive evidence for the opening of an excitations gap and adiabaticity. Put together with evidence for a finite penetration depth; our results are the hallmark of bulk field-induced superconductivity.

[1] R. Schönemann, P.F.S. Rosa, S.M. Thomas, Y. Lai, D.N. Nguyen, J. Singleton, E.L. Brosha, R.D. McDonald, V. Zapf, B. Maiorov, and M. Jaime. arXiv:2206.06508