Superconducting Energy Gap and $c$-Axis Plasma Frequency of $(\mathrm{Nd},\mathrm{Sm}){\mathrm{FeAsO}}_{0.82}{\mathrm{F}}_{0.18}$ Superconductors from Infrared Ellipsometry

We present far-infrared ellipsometric measurements of polycrystalline samples of the pnictide superconductor $R{\mathrm{FeAsO}}_{0.82}{\mathrm{F}}_{0.18}$ ($R=\mathrm{Nd}$ and Sm). We find evidence that the electronic properties are strongly anisotropic such that the optical spectra are dominated by the weakly conducting $c$-axis response similar to the cuprate high-temperature superconductors. We deduce an upper limit of the $c$-axis superconducting plasma frequency of ${\omega }_{\mathrm{pl},c}^{\mathrm{SC}}\le 260{\mathrm{cm}}^{-1}$ corresponding to a lower limit of the $c$-axis magnetic penetration depth of ${\lambda }_c\ge 6\mu \mathrm{m}$ and ${\lambda }_c/{\lambda }_{ab}\ge 30$ as compared to ${\lambda }_{ab}=185\mathrm{nm}$ from muon spin rotation [A. Drew et al., arXiv:0805.1042 [Phys. Rev. Lett. (to be published)]]. We also observe a gaplike suppression of the conductivity in the superconducting state with a shoulderlike feature at ${\omega }_{\mathrm{SC}}^{*}\approx 300{\mathrm{cm}}^{-1}$ and spectral shape which is consistent with an unconventional order parameter with $2\Delta \approx {\omega }_{\mathrm{SC}}^{*}\approx 37\mathrm{meV}$.

Figure 1

Temperature dependent resistivity of the polycrystalline ${\mathrm{NdFeAsO}}_{0.82}{\mathrm{F}}_{0.18}$ and ${\mathrm{SmFeAsO}}_{0.82}{\mathrm{F}}_{0.18}$ samples.

Figure 2

Temperature dependence of the infrared dielectric response of the ${\mathrm{NdFeAsO}}_{0.82}{\mathrm{F}}_{0.18}$ (Nd, $x=0.18$) and ${\mathrm{SmFeAsO}}_{0.82}{\mathrm{F}}_{0.18}$ (Sm, $x=0.18$) superconductors. (a) Representative spectra of the real part of the conductivity ${\sigma }_1(\omega )$ of the Nd, $x=0.18$ sample. The arrow marks the onset of the SC-induced gaplike suppression. The inset shows calculated reflectivity spectra. (b) Difference between the conductivity in the normal and the SC state in terms of ${\sigma }_1(10\mathrm{K})-{\sigma }_1(55\mathrm{K})$. The shaded area indicates the missing spectral weight due to the SC condensate. The inset details the temperature dependence of the integral of ${\sigma }_1(\omega )$ between 45 and $300{\mathrm{cm}}^{-1}$ (SW). (c) Corresponding spectra of the real part of the dielectric function ${ϵ}_1(\omega )$. The inset details the temperature evolution of ${ϵ}_1(50{\mathrm{cm}}^{-1})$. (d) Difference spectra ${ϵ}_1(10\mathrm{K})-{ϵ}_1(T_c)$ (solid lines) showing the SC-induced change of ${ϵ}_1(\omega )$. Dotted lines show fits with the function $-({\omega }_{\mathrm{pl},c}^{\mathrm{SC}}/\omega {)}^2$.