Superconductivity in the high-pressure tetragonal phase of UTe2

Electrical transport and magnetic measurements have been made on UTe2 under pressure P up to approximately 16 GPa to determine the superconducting transition temperature Tc vs P phase diagram in the high-pressure tetragonal phase. Superconductivity emerges near 5 GPa, coincident with the orthorhombic to tetragonal phase transition; in the tetragonal phase, Tc reaches a maximum value of approximately 4 K at 6 GPa and then decreases with P and appears to vanish near 18 GPa. Tetragonal UTe2 has a relatively small upper critical field Hc2(0) approx 1.2 T at 5.3 GPa, smaller than the Pauli paramagnetic limit, and is orbitally limited with a coherence length xitetra approx 16.5 nm. This small value of Hc2(0) favors more conventional superconductivity; in contrast, the large values of Hc2(T) for orthorhombic UTe2 exceed the Pauli paramagnetic limit in all three crystallographic directions and have been attributed to unconventional superconductivity, widely believed to involve spin-triplet pairing. The temperature-pressure phase diagram of UTe2 shows a striking dichotomy: a narrow, fragile, unconventional superconducting region in the orthorhombic phase vs a broad, robust, and more conventional superconducting dome in the tetragonal phase. This dichotomy is consistent with the proposal that U-dimers, present (absent) in the orthorhombic (tetragonal) phase, may play a role in spin-triplet superconductivity of orthorhombic UTe2. In the tetragonal phase, the normal-state electrical resistivity ρ(T) exhibits metallic behavior with a knee between 200 and 240 K that depends weakly on P and most likely marks the onset of a transition to a magnetically ordered phase that coexists with superconductivity.