The recently discovered heavy-fermion superconductor UTe2 exhibits exotic superconducting properties, such as extremely high upper critical field [1], reentrant superconductivity [2], and only a small reduction of the Knight shift [3], indicating a spin-triplet superconducting state. Moreover, scanning tunneling spectroscopy [4] and optical Kerr effect measurements [5] suggest a chiral superconducting state with a topologically non-trivial surface state. However, the symmetry and nodal structure of the order parameter in the bulk, which determines the Majorana surface state, remains controversial.
In this study, to determine the gap symmetry in UTe2, we performed magnetic penetration depth measurements with the applied ac field along each crystallographic axis, from which we successfully evaluated the anisotropy of the low-energy excitations. We find that the change of the magnetic penetration depth Δλ follows a power-law temperature dependence Δλ∝Tn with n≤2 for all directions, which rules out one-component spin-triplet order parameters. Our results strongly suggest the presence of multiple point nodes near ky- and kz-axes, which is most consistent with the chiral B3u+iAu state.
[1] S. Ran et al., Science 365, 684 (2019).
[2] S. Ran et al., Nat. Phys. 15, 1250 (2019).
[3] G. Nakamine et al., Phys. Rev. B 103, L100503 (2021).
[4] L. Jiao et al., Nature 579, 523 (2020).
[5] I. M. Hayes et al., Science 373, 797 (2021).