Probing penetration depth and residual loss anisotropy of high-T_c superconducting UTe₂

The recent discovery of unconventional superconductivity in UTe₂ has motivated careful studies of the electrodynamic properties of single crystals of this material [PRB 100, 220504 (2019); Nat. Comms 12:2644 (2021); arXiv:2105.13721]. The magnetic penetration depth shows a power-law temperature dependence at low temperatures in the superconducting state. The microwave losses show a large residual value, and its origin may be due to surface states of potentially intrinsic character. We extend the electrodynamics measurements to a new generation of UTe₂ crystals and extract the complex surface impedance Z_s and complex conductivity σ=σ₁-iσ₂ as a function of temperature from 100 mK to 20 K, over 5-12 GHz. Recent studies [PRB 106, L060505, (2022)] indicate that the normal state and superconducting properties of UTe₂ are anisotropic. We extend the cavity perturbation measurements to multiple resonant modes, each of which excites microwave currents in different crystallographic directions. This allows extraction of the anisotropic electrodynamic properties as a function of temperature, including the anisotropic magnitude of the screening length. We find that the penetration depth in new generation crystals (T_c=1.95 K) shows a robust power-law temperature dependence at low temperatures for currents in all directions, with exponents near 2, similar to the previous generation of UTe₂ crystals. Substantial residual resistance is seen in all UTe₂ crystals (old and new generation) and modes. This is expected for nodal superconductors, but it appears to be stronger than expected for point nodes alone. Our results are generally consistent with predictions of direction-independent power-law temperature dependence of the penetration depth in a generic topological superconductor having Weyl nodes and surface states [PRL 124, 067001 (2020)]. The results are also generally consistent with enhanced losses associated with the recently proposed topological anomalous skin effect [PRB 103, 104517 (2021)]. We will discuss the electrodynamic properties of the super- and normal-fluids in UTe₂ in light of this new data, and other measurements, as well as ongoing efforts to understand this unique material.