Figure 4: Superfluid density of CeCoIn₅.

(a) Frequency-dependent superfluid density, 1/δ²(ω,T)≡ωμ₀σ₂(ω,T), plotted as a function of temperature, for frequencies from 0.13 to 19.6 GHz. , the zero-frequency limit of 1/δ²(ω,T), is obtained from fits to complex conductivity spectra and lies on top of the 0.13-GHz data. λ_L(T→0)=1,960 Å. The inset shows a close-up of the low-temperature region, in which the temperature slope of 1/δ²(ω,T) changes sign with increasing frequency. (b) The temperature-dependent part of the total superfluid density, , follows a T^1.25 power law. The paramagnetic part of the superfluid density, , isolates the contribution from nodal quasiparticles and follows a linear temperature dependence. Its zero-temperature intercept indicates a residual, uncondensed spectral weight of 7%. Inset: the normalized superfluid density of an s-wave superconductor, calculated using Mattis–Bardeen theory³⁷ for the same set of reduced frequencies (and same colour scheme) as the CeCoIn₅ experiment. In the s-wave case, the isotropic energy gap leads to exponentially activated behaviour at low temperatures.