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. UTe2 is 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. 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. Superconductivity in UTe2 is extremely rich exhibiting various unconventional behaviors which will be highlighted. It shows an exceptionally huge superconducting upper critical field with a re-entrant behavior under magnetic field and the occurrence of multiple superconducting phases in the temperature field, pressure phase diagram. There is evidence for spin-triplet pairing. The different theoretical approaches will be described. Notably we discuss that UTe2 is a possible example for the realization of a fascinating topological superconductor.

arXiv comment47 pages, 53 figures, submitted to J. Phys.: Condens. Matter as "Topical Review"

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• Emergent ferromagnetic ladder excitations in heavy fermion superconductor CeSb${}_2$
• Higher-order topological phases for time-reversal-symmetry breaking superconductivity in UTe${}_2$
• Topological superconductivity in a spin-orbit coupled Kondo lattice
• Observation of vortex stripes in UTe${}_2$
• Reciprocal and nonreciprocal paraconductivity in bilayer multiphase superconductors
• Magnetic edge fields in UTe${}_2$ near zero background fields
• Structure-Driven Prediction of Magnetic Order in Uranium Compounds
• Metamagnetism in the high-pressure tetragonal phase of UTe${}_2$
• Finite-momentum and field-induced pairings in orbital-singlet spin-triplet superconductors
• Slow magnetic quantum oscillations in the c-axis magnetoresistance of UTe${}_2$
• Spin susceptibility of nonunitary spin-triplet superconductors
• Theory of the Little-Parks effect in spin-triplet superconductors
• $b$-axis and $c$-axis Knight shift measurements in the superconducting state on ultraclean UTe${}_2$ with $T_c$ = 2.1 K
• Optical and Raman selection rules for odd-parity clean superconductors
• Magnetic fluctuations and anisotropy in UTe2: a multi-orbital study based on GGA+U and RPA
• Nature of quantum criticality in the Ising ferromagnet TbV${}_6$Sn${}_6$
• A unified theoretical framework for Kondo superconductors: Periodic Anderson impurities with attractive pairing and Rashba spin-orbit coupling
• Antiferromagnetic Order and Magnetic Frustration in the Honeycomb Heavy-Fermion System Ce(Pt${}_{1-x}$Pd${}_x$)${}_6$Al${}_3$: ${}^{27}$Al and ${}^{195}$Pt NMR Studies
• Visualizing the Odd-parity Superconducting Order Parameter and its Quasiparticle Surface Band in UTe2
• Particle-hole spectral asymmetry at the edge of multiorbital noncentrosymmetric superconductors
• Nodal Superconductivity of UTe${}_2$ Probed by Field-Angle-Resolved Specific Heat on a Crystal with $T_{\mathrm{c}}=2.1$ K
• Nuclear spin relaxation rate of nonunitary Dirac and Weyl superconductors
• Metamagnetism and tricriticalilty in heavy-fermion ferromagnet URhGe
• Electronic structure of UTe${}_2$ under pressure
• Pressure-enhanced $f$-electron orbital weighting in UTe2 mapped by quantum interferometry
• Spin-triplet pairing instability in a two-dimensional repulsive Hubbard model
• Theoretical Study of Impurity Effects on Superconductivity in UTe2
• Quantum geometric magnetic monopole and two-phase superconductivity in CeRh${}_2$As${}_2$
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• Quasiparticle Interference of Spin-Triplet Superconductors: Application to UTe${}_2$
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