Unconventional superconductivity in UTe₂

Abstract

The novel spin-triplet superconductor candidate UTe2 was discovered only recently at the end of 2018 and attracted enormous attention. We review key experimental and theoretical progress which has been achieved in different laboratories. UTe2is a heavy-fermion paramagnet, but right after the discovery of superconductivity it has been expected to be close to a ferromagnetic instability showing many similarities to the U-based ferromagnetic superconductors, URhGe and UCoGe. The competition between different types of magnetic interactions and the duality between the local and itinerant character of the 5f Uranium electrons, as well as the shift of the U valence appear as key parameters in the rich phase diagrams discovered recently under extreme conditions like low temperature, high magnetic field, and pressure. We discuss macroscopic and microscopic experiments at low temperature to clarify the normal phase properties at ambient pressure for field applied along the three axis of this orthorhombic structure. Special attention will be given to the occurrence of a metamagnetic transition at Hm = 35 T for a magnetic field applied along the hard magnetic axis b. Adding external pressure leads to strong changes in the magnetic and electronic properties with a direct feedback on superconductivity. Attention will be given on the possible evolution of the Fermi surface as a function of magnetic field and pressure.

Stability of a Nonunitary Triplet Pairing on the Border of Magnetism in UTe$_2$

“…Recent discovery of superconductivity in UTe 2 [1,2] has attracted much attention, inviting comparisons with ferromagnetic superconductors based on other uranium compounds UGe 2 , UCoGe, URhGe and UCoAl [3,4]. Indeed, a large upper critical field has been found to exceed the Pauli-Clogston limit, as expected from the spin-triplet superconductivity, also corroborated by the very weak temperature dependence of the nuclear magnetic resonance (NMR) Knight shift below T c [1,5].…”

Superconductivity and Local Inversion-Symmetry Breaking

“…For the sublattice symmetry, the relevant symmetry is in both cases D 2d , which lacks both C4 and inversion, but contains C2, together leading to intricate spin-orbit-coupling terms (52,53). Other examples include UTe2 (54,55), see Figure 2d, paramagnetic UCoGe (56), and UPt3 (44). In all these materials the U atoms do not sit on inversion centers, a feature that might be related to the appearance of odd-parity superconductivity (57).…”

Anisotropic signatures of the electronic correlations in the electrical resistivity of UTe$_2$

“…However, a difficulty is to reveal experimentally the intimate relationship between the magnetic and superconducting properties. The recent discovery of unconventional superconductivity in the paramagnetic compound UTe 2 [2,3,4] opened a new route to investigate the interplay between magnetism and unconventional superconductivity. A spin-triplet nature of superconducting pairing has been proposed for this compound initially presented as a nearly-ferromagnetic system.…”