Temperature dependence of the London penetration depth in Pr2CuO4 from millimeter-wave optical experiments

We have investigated the in-plane penetration depth, λ(T), of nominally undoped superconducting Pr2CuO4 films (Tc = 27.5 K) by measuring the phase-sensitive millimeter-wave transmission at temperatures between 2 and 30 K. The overall temperature dependence of the superfluid density, ns = λ2(0 K)/λ2(T), resembles the behavior typical for the cuprate superconductors [ns(T) ∼1-(T/Tc)2]. However, a closer look on the penetration depth at low temperatures reveals flattening of its temperature dependence. We find λ(T)∼Tn with n = 2.8 ± 0.2.