Electrodynamics of the Superconducting State in CeRu₂

We have studied the complex conductivity in superconducting and normal metallic states of CeRu₂ by the surface impedance measurements at millimeter-wave frequency of 45.4 GHz. The temperature dependence of the complex conductivity below T_c = 6.2 K exhibits the characteristic behaviors as seen in a conventional superconductor. The coherence peak observed in the real part σ₁ provides a strong evidence for a conventional s-wave superconductivity as well as the previous NQR measurements. The temperature dependence of the penetration depth evaluated from the surface reactance at low temperatures has revealed the ratio of 2Δ(0)/k_BT_c = 4.4. The strong-coupling theories suggest the electron-phonon coupling strength with λ_ep < 2. These results indicate that CeRu₂ is an intermediate strong-coupling superconductor in a clean limit.