Coexistence of Magnetic Fluctuations and Superconductivity in the Pnictide High Temperature Superconductor ${\mathrm{SmFeAsO}}_{1-x}{\mathrm{F}}_x$ Measured by Muon Spin Rotation

Muon spin rotation experiments were performed on the pnictide high temperature superconductor ${\mathrm{SmFeAsO}}_{1-x}{\mathrm{F}}_x$ with $x=0.18$ and 0.3. We observed an unusual enhancement of slow spin fluctuations in the vicinity of the superconducting transition which suggests that the spin fluctuations contribute to the formation of an unconventional superconducting state. An estimate of the in-plane penetration depth ${\lambda }_{ab}(0)=190(5)\mathrm{nm}$ was obtained, which confirms that the pnictide superconductors obey an Uemura-style relationship between $T_c$ and ${\lambda }_{ab}(0{)}^{-2}$.

Figure 1

(a) Example $\mathrm{ZF}\mathrm{\text{−}}\mu \mathrm{SR}$ spectra for $x=0.18$. Inset: log-log plot of the polarization function $G(t)$ [13]. (b) Temperature evolution of ${\lambda }^{\mathrm{ZF}}$ for $x=0.18$. Inset: Corresponding data for $x=0.3$. (c) Shape parameter $\beta$ for $x=0.18$. Inset: Corresponding data for $x=0.3$. In (b) the solid line is the sum of a temperature-independent component (horizontal dashed line) and an activated component with an activation energy of 13 meV. Dotted lines are a guide to the eye.

Figure 2

Field dependence of ${\lambda }^{\mathrm{LF}}$ at 60 K for the $x=0.18$ sample. The dashed line shows a simulation assuming decoupling from a simple static or dynamic local field distribution (see text). The dotted line is a guide to the eye and illustrates the more abrupt transition which is actually observed.

Figure 3

(a) The main component of ${\sigma }^{\mathrm{TF}}$ (ARGUS spectrometer). The inset shows additional data below 4 K (MuSR spectrometer), which reveals a steep increase of ${\sigma }^{\mathrm{TF}}$, likely due to the ordering of the Sm moments. The dashed lines illustrate the respective contributions from the SC vortex lattice and the Sm ordering. (b) The diamagnetic shift due to the development of the SC state. Inset: A detail of the behavior above $T_c$. Dotted lines are guides to the eye.

Figure 4

(a) The field dependence of the SC vortex contribution to the rms linewidth of our data. The fitted field dependence is given by a powder averaged version of the model in Ref. 23. Data for the La compound from Ref. 16 are shown for comparison [24, 25]. (b) The Uemura plot for the pnictide superconductors measured to date by $\mu \mathrm{SR}$. The trends observed for the cuprates are shown as dashed lines for comparison.