Superconductivity in the high-pressure tetragonal phase of UTe₂

Electrical transport and magnetic measurements have been made on UTe₂ under pressure P up to ~16 GPa to determine the superconducting transition temperature T_c vs. P phase diagram in the high-pressure tetragonal phase. Superconductivity emerges near ~5 GPa, coincident with the orthorhombic-tetragonal phase transition; in the tetragonal phase, T_c reaches a maximum value of ~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 H_c2(0) ≈ 1.2 T at 5.3 GPa, smaller than the Pauli paramagnetic limit, and is orbitally limited with a coherence length ξ_tetra ≈ 16.5 nm. This small value of H_c2(0) favors more conventional superconductivity; in contrast, the large values of H_c2(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" at 200 to 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.