Field and Temperature Dependence of the Superfluid Density in ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{F}}_x$ Superconductors: A Muon Spin Relaxation Study

We present zero field and transverse field muon spin relaxation experiments on the recently discovered Fe-based superconductor ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{F}}_x$ ($x=0.075$ and $x=0.1$). The temperature dependence of the deduced superfluid density is consistent with a BCS $s$-wave or a dirty $d$-wave gap function, while the field dependence strongly evidences unconventional superconductivity. We obtain the in-plane penetration depth of ${\lambda }_{ab}(0)=254(2)\mathrm{nm}$ for $x=0.1$ and ${\lambda }_{ab}(0)=364(8)\mathrm{nm}$ for $x=0.075$. Further evidence for unconventional superconductivity is provided by the ratio of $T_c$ versus the superfluid density, which is close to the Uemura line of high-$T_c$ cuprates.

Figure 1

Right: Temperature dependence of the resistance of ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{F}}_x$ at $x=0.1$ and $x=0.075$ in the vicinity of $T_c$. Left: Field cooled and zero field cooled magnetic susceptibility for $x=0.1$ and $x=0.075$.

Figure 2

Zero field $\mu \mathrm{SR}$ spectra of ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ for 1.6, 10, and 30 K. The very small temperature dependence of the ZF spectra is modeled by a multiplication of the Gaussian Kubo-Toyabe function with an exponential relaxation (the rate is shown in the inset).

Figure 3

Field dependence ${\sigma }_{sc}$ at 1.6 K for ${\mathrm{LaFeAsO}}_{0.925}{\mathrm{F}}_{0.075}$ and ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$. The dashed line is the expected behavior for an $s$-wave BCS superconductor with ${\mu }_0{H}_{c2}=32\mathrm{T}$ according to Eq. (1). The solid line is a fit of the data with Eq. (2) indicative for nodes in the gap function.

Figure 4

Temperature dependence of ${\sigma }_{sc}$ measured in a field of ${\mu }_{0}H=0.07\mathrm{T}$ for ${\mathrm{LaFeAsO}}_{0.925}{\mathrm{F}}_{0.075}$ and ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$. The ${\sigma }_{sc}(T)$ were fitted using a standard BCS curve and a power law $1-(T/T_c{)}^n$. In case of ${\mathrm{LaFeAsO}}_{0.925}{\mathrm{F}}_{0.075}$ the points below 4 K were omitted in the fit.

Figure 5

Uemura plot for hole and electron doped high $T_c$ cuprates. Points for the cuprates are taken from 38. The stars show the data for ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{F}}_x$ obtained in this work.