Far-infrared conductivity in electron-doped cuprate La 2 − x Ce x CuO ₄

We performed far-infrared and submillimeter-wave conductivity experiments in the electron-doped cuprate La 2 − x Ce x CuO 4 with x = 0.081 (underdoped regime, T c = 25 K). The onset of the absorption in the superconducting state is gradual in frequency and is inconsistent with the isotropic s -wave gap. Instead, a narrow quasiparticle peak is observed at zero frequency and a second peak at finite frequencies, clear fingerprints of the conductivity in a d -wave superconductor. A far-infrared conductivity peak can be attributed to 4Δ 0 , or to 2Δ 0 + Δ spin , where Δ spin is the resonance frequency of the spin fluctuations. The infrared conductivity as well as the suppression of the quasiparticle scattering rate below T c are qualitatively similar to the results in the hole-doped cuprates.