Theory of magnetic field enhanced triplet superconductivity in next generation ultraclean UTe₂

Since the discovery of spin-triplet superconductivity in UTe₂, multiple distinct superconducting (SC) phases have been observed at different applied magnetic fields and pressures, including field enhanced superconductivity (SC2) up to fields of about 40 T applied close to the hard magnetic direction that appears to be distinct from the low field phase (SC1), as well as reentrant superconductivity (SC3) above 40 T and up to at least 60 T. Many puzzles remain about the microscopic origins of these phases, whether common or distinct. To shed light on some of these origins, in this talk I will present a theoretical model of SC2 motivated by our high magnetic field measurements on a new generation of ultraclean UTe₂ crystals grown by a salt flux technique.

Remarkably, in these new samples SC2 is observed over a much broader range of field directions, suggesting disorder- and field direction-sensitive magnetic fluctuations as the pairing glue in the SC2 phase. We propose in particular that these are metamagnon fluctuations that occur about the polarised paramagnet local minimum of the magnetic free energy. This minimum becomes global above the metamagnetic phase transition, explaining why SC2 suddenly terminates at the metamagnetic phase transition when the magnetic field reaches the critical value H_m. As H_m decreases under pressure, consistent with Kondo lattice physics, our model also helps explain why the SC2 phase appears at zero field and with increasing critical temperature when pressure is applied.