Optical investigation of nominally undoped Pr₂CuO₄ films

Superconducting Pr₂CuO₄ films with T′ structure and T_c beetween 25 and 27 K have been investigated by different optical methods in a wide frequency range (5 – 55000 cm⁻¹) and for temperatures from 2 to 300 K. From the infrared reflectivity spectra, a superconducting gap of 2Δ₀ = 17 meV = 7.4 κ_BT_c is estimated. Absolute values of the London penetration depth (Δ_L) have been calculated from phase-sensitive teraherz measurements. The zero-temperature limit of (Δ_L) is 1.6 µm. The overall temperature dependence of (Δ_L)shows a behavior typical for the cuprates. However, a closer look on the penetration depth at low temperatures reveals a flattening of the temperature dependence. We find (Δ_L)(T) ∝ Tⁿ with n = 2.8 ± 0.2.