Anomalous normal fluid response in a chiral superconductor UTe₂

Recently reported evidence of two-component order parameters and broken time-reversal symmetry of UTe₂ suggests that UTe₂ could be an example of chiral superconductor [1]. The observation raises a question about if there exists a surface normal fluid which corresponds to the non-trivial topology of the system. In this talk, we report evidence of a significant surface normal fluid response from a chiral spin-triplet superconductor UTe₂ from a microwave surface impedance study [2]. The surface impedance of UTe₂ crystals is measured and converted to complex conductivity. The real part of conductivity in the zero temperature limit shows anomalous residual value supporting the presence of a significant normal fluid response in the ground state. The imaginary part of conductivity follows the low temperature behavior predicted for the spin-triplet state with point nodes, consistent with the scenario of the two-component chiral order parameter. Possible extrinsic origins of residual normal fluid such as high impurity scattering rate, pair-breaking rough surface, and microwave-excited quasiparticles are examined and excluded, putting more weight on intrinsic origin. Candidate intrinsic origin such as a surface Majorana normal fluid predicted for chiral triplet superconductor is discussed. A proposal experiment for further verifying the topological origin of the surface normal fluid is also discussed.

[1] I. M. Hayes et al., Science 373, 797 (2021)

[2] S. Bae et al., Nat. Commun. 12, 2644 (2021)