Electronic correlations in the superconductor $\mathrm{La}\mathrm{Fe}\mathrm{As}{\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}$ with low carrier density

The crystal structure and numerous normal and superconducting state properties of layered tetragonal $(P4∕nmm)$ LaFeAsO, with F doping of $\approx 11\%$, are reported. Resistivity measurements give an onset transition temperature $T_c=28.2\mathrm{K}$, and low field magnetic susceptibility data indicate bulk superconductivity. In applied magnetic field, analysis of the resistive transition results in a critical field $H_{c2}\approx 30\mathrm{T}$ and a coherence length ${\xi }_{\mathrm{GL}}\approx 35\mathrm{\AA }$. An upper limit for the electron carrier concentration of $1\times {10}^{21}{\mathrm{cm}}^{-3}$ is inferred from the Hall data just above $T_c$. Strong electron-electron correlations are suggested from temperature-dependent resistivity, Seebeck coefficient, and thermal conductivity data. Anomalies near $T_c$ are observed in both Seebeck coefficient and thermal conductivity data.

Figure 1

(a) (Color online) Crystal structure of $\mathrm{La}\mathrm{Fe}\mathrm{As}{\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}$, with unit cell shown in dotted lines. The structure is made from layers of face-sharing distorted square antiprism of $\mathrm{La}{\mathrm{As}}_4{\mathrm{O}}_4$ and sheets of Fe, perpendicular to the $c$ direction. (b) The refined x-ray powder diffraction of nominal composition $\mathrm{La}\mathrm{Fe}\mathrm{As}{\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}$. The upper set of Bragg ticks correspond to reflections from this structure; the lower ticks locate reflections from $\approx 5\%$ impurity phase of ${\mathrm{La}}_{4.67}{\left(\mathrm{Si}{\mathrm{O}}_4\right)}_3\mathrm{O}$. (c) TEM images and FFT calculated diffraction patterns for two crystallographic orientations of $\mathrm{La}\mathrm{Fe}\mathrm{As}{\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}$. The view along the $c$ axis and that perpendicular to it are shown in the top and bottom panels, respectively. The lengths of $a$ and $c$ axes are labeled.

Figure 2

Temperature dependence of the ZFC (filled circles) and FC (open circles) magnetic susceptibility for $\mathrm{La}\mathrm{Fe}\mathrm{As}{\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}$ in 20 and $1\mathrm{kOe}$ (inset).

Figure 3

(Color online) Temperature dependence of electrical resistivity for $\mathrm{La}\mathrm{Fe}\mathrm{As}{\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}$, which is shown as (a) $\rho$ vs $T$ and (b) $\rho$ vs $T^2$ between 0 and $\sim 225\mathrm{K}$. The inset of (a) is the enlarged low-temperature data, with the indicated superconducting transition temperature. In (b), the red solid lines represent the linear fit between 35 and $200\mathrm{K}$ (shown by arrows) for data in 0 and $8\mathrm{T}$.

Figure 4

(Color online) For $\mathrm{La}\mathrm{Fe}\mathrm{As}{\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}$, the temperature dependence of (a) resistivity at various applied fields and (b) the upper critical field $H_{c2}$ found from 90%, 50%, and 10% of the normal-state value, ${\rho }_N$. In (b), the dotted lines represent the WHH approach and the solid lines are the linear fits to experimental $H_{c2}\left(T\right)$.

Figure 5

The inferred variation of the carrier density $n$ with temperature for $\mathrm{La}\mathrm{Fe}\mathrm{As}{\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}$. The inset is the temperature dependence of the Hall coefficient $R_H$ obtained in a magnetic field of $8\mathrm{T}$.

Figure 6

Temperature dependence of Seebeck coefficient for $\mathrm{La}\mathrm{Fe}\mathrm{As}{\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}$ in applied fields of 0 and $8\mathrm{T}$. The inset shows the temperature dependence up to room temperature. The dotted line is extrapolation from $\sim 35\mathrm{K}$ down to 0.

Figure 7

Temperature dependence of thermal conductivity data for $\mathrm{La}\mathrm{Fe}\mathrm{As}{\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}$ in applied fields of 0 and $8\mathrm{T}$.