Infrared signature of the superconducting state in Pr 2-x Ce x CuO 4

We measured the far infrared reflectivity of two superconducting ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ films above and below ${T}_{c}$. The reflectivity in the superconducting state increases and the optical conductivity drops at low energies, in agreement with the opening of a (possibly) anisotropic superconducting gap. The maximum energy of the gap scales roughly with ${T}_{c}$ as $2{\ensuremath{\Delta}}_{\mathrm{max}}∕{k}_{B}{T}_{c}\ensuremath{\approx}4.7$. We determined absolute values of the penetration depth at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ as ${\ensuremath{\lambda}}_{ab}=(3300\ifmmode\pm\else\textpm\fi{}700)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ for $x=0.15$ and ${\ensuremath{\lambda}}_{ab}=(2000\ifmmode\pm\else\textpm\fi{}300)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ for $x=0.17$. A spectral weight analysis shows that the Ferrell-Glover-Tinkham sum rule is satisfied at conventional low energy scales $\ensuremath{\sim}4{\ensuremath{\Delta}}_{\mathrm{max}}$.