Electrodynamic properties and low energy excitations of UTe₂

We explore the microwave surface impedance of spin-triplet UTe₂ single crystals as a function of temperature using resonant cavity perturbation measurements. Properties of a superconductor, such as the super- and normal-fluid responses, pairing mechanism, Fermi surface, and topological properties, influence its surface impedance. We employ a novel multi-modal analysis to gain insight into these properties. We focus on results which require minimal assumptions and analysis. We determine a composite surface impedance of the crystal for each mode using resonance data combined with the independently measured normal state dc resistivity tensor. The normal state surface impedance reveals the weighting of current flow directions in the crystal of each resonant mode. For UTe₂, we find an isotropic △λ(T)∼T^α power-law temperature dependence for the magnetic penetration depth for T≤T_c/3 with α<2, which is inconsistent with a single pair of point nodes on the Fermi surface under weak scattering. Though our observed values of α are similar to those of other studies of UTe₂, we find no systematic variation in α for currents flowing in different directions in the crystal. We also find a similar power-law temperature dependence for the low-temperature surface resistance R_s(T). We do, however, observe an anisotropy of the residual microwave loss across these modes. We explore the possibility of topological Weyl superconductivity in the context of our observed isotropic power-law and anisotropy of the residual loss.