A Theory from First-principles on Magnetism and Superconductivity in UTe2

We theoretically study magnetism and superconductivity in UTe2, a recently discovered strong candidate for an odd-parity spin-triplet superconductor [1,2]. Theoretical studies for this compound faced difficulty because first-principles calculations predicted an insulating electronic state, incompatible with superconducting instability. To overcome this problem, we consider electron correlation effects by a GGA+U method and show the insulator-metal transition by Coulomb interaction [1]. Using Fermi surfaces obtained as a function of U, we clarify the topological properties of possible superconducting states. Fermi surface formulas indicate topological superconductivity at an intermediate U for all the odd-parity pairing symmetry in the Immm space group. Symmetry and topology of superconducting gap nodes are analyzed, and the gap structure of UTe2 is predicted. A recent ARPES experiment is consistent with the topological superconductivity.

Next, we provide and analyze a periodic Anderson model for UTe2 [2]. The 24-band tight-binding model reproduces the band structure obtained from a GGA+U calculation. The Coulomb interaction of f-electrons enhances Ising ferromagnetic fluctuation along the a-axis consistent with experiments and stabilizes the spin-triplet superconductivity of either B3u or Au symmetry. When effects of pressure are considered in hopping integrals, the magnetic fluctuation changes to an antiferromagnetic one, and accordingly, the spin-singlet superconductivity of Ag symmetry is stabilized. Based on the results, we propose multiple superconducting phases under pressures and magnetic fields. Interestingly, a mixed-parity superconducting state with spontaneous parity violation is predicted.

[1] Jun Ishizuka, Shuntaro Sumita, Akito Daido, Youichi Yanase, Phys. Rev. Lett. 123, 217001 (2019).
[2] Jun Ishizuka and Youichi Yanase, arXiv:2008.01945.