Generic thermodynamics of condensates with extremely small superfluid density ---Application to UTe 2 and hidden second phase transition---

Motivated by recent experimental findings on UTe 2 via NMR and specific heat experiments, we theoretically explore the thermodynamic signatures of condensates with small superfluid density and characterize their physical properties. We identify the hidden superconducting phase A 2 composed of spin-up Cooper pairs at lower temperature (T) and field (H) under ambient pressure of UTe 2 and show that A 2 reappears at higher H for all three principal field directions, a, b, and c-axis. The phase transition lines from the A 1 phase with spin down pairs to the A 2 phase in the H - T plane are all horizontal in common, implying a universal physical origin due to the originating from the magnetic-energy gain of the spin-up pairing state with increasing magnetic field. This multiple-phase diagram is extended to these under pressure (P), resulting in a simple picture based on non-unitary spin-triplet pairing symmetry 3B 3u, which enables us to consistently capture the evolution of the superconducting phases throughout the entire H - T - P phase space.