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Register a new userNodeless superconducting gap in NdFeAsO$_{0.9}$F$_{0.1}$ single crystals from anisotropic penetration depth studies
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This paper is no longer active and will NOT appear in print. For new data and analysis, please see: arXiv:0903.2220
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The superconducting penetration depth, $lambda(T)$, has been measured in RFeAsO$_{0.9}$F$_{0.1}$ (R=La,Nd) single crystals (R-1111). In Nd-1111, we find an upturn in $lambda(T)$ upon cooling and attribute it to the paramagnetism of the Nd ions, similar to the case of the electron-doped cuprate Nd-Ce-Cu-O. After the correction for paramagnetism, the London penetration depth variation is found to follow a power-law behavior, $Delta lambda_L(T)propto T^{2}$ at low temperatures. The same $T^2$ variation of $lambda(T)$ was found in non-magnetic La-1111 crystals. Analysis of the superfluid density and of penetration depth anisotropy over the full temperature range is consistent with two-gap superconductivity. Based on this and on our previous work, we conclude that both the RFeAsO (1111) and BaFe$_2$As$_2$ (122) families of pnictide superconductors exhibit unconventional two-gap superconductivity.
We have performed 75As Nuclear Magnetic Resonance (NMR) measurements on aligned powders of the new LaO0.9F0.1FeAs superconductor. In the normal state, we find a strong temperature dependence of the spin shift and Korringa behavior of the spin lattice relaxation rate. In the superconducting state, we find evidence for line nodes in the superconducting gap and spin-singlet pairing. Our measurements reveal a strong anisotropy of the spin lattice relaxation rate, which suggest that superconducting vortices contribute to the relaxation rate when the field is parallel to the c-axis but not for the perpendicular direction.
Magneto-optical imaging was used to study the local magnetization in polycrystalline NdFeAsO$_{0.9}$F$_{0.1}$ (NFAOF). Individual crystallites up to $sim200times100times30$ $mu m^{3}$ in size could be mapped at various temperatures. The in-grain, persistent current density is about $jsim10^{5}$ A/cm$^{2}$ and the magnetic relaxation rate in a remanent state peaks at about $T_{m}sim38$ K. By comparison with with the total magnetization measured in a bar-shaped, dense, polycrystalline sample, we suggest that NdFeAsO$_{0.9}$F$_{0.1}$ is similar to a layered high-$T_{c}$, compound such as Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ and exhibits a $3Dto2D$ crossover in the vortex structure. The 2D Ginzburg parameter is about $Gi^{2D}% simeq10^{-2}$ implying electromagnetic anisotropy as large as $epsilon sim1/30$. Below $T_{m}$, the static and dynamic behaviors are consistent with collective pinning and creep.
We use a magnetic force microscope (MFM) to investigate single vortex pinning and penetration depth in NdFeAsO$_{1-x}$F$_x$, one of the highest-$T_c$ iron-based superconductors. In fields up to 20 Gauss, we observe a disordered vortex arrangement, implying that the pinning forces are stronger than the vortex-vortex interactions. We measure the typical force to depin a single vortex, $F_{mathrm{depin}} simeq 4.5$ pN, corresponding to a critical current up to $J_c simeq 7 times 10^5$ A/cm$^2$. Furthermore, our MFM measurements allow the first local and absolute determination of the superconducting in-plane penetration depth in NdFeAsO$_{1-x}$F$_x$, $lambda_{ab}=320 pm 60$ nm, which is larger than previous bulk measurements.
Temperature and magnetic field dependent measurements of the microwave surface impedance of superconducting LaFeAsO$_{0.9}$F$_{0.1}$ (Tc $approx$ 26K) reveal a very large upper critical field ($B_{rm c2} approx 56$T) and a large value of the depinning frequency ($f_{0}approx 6$GHz); together with an upper limit for the effective London penetration depth, $lambda_{rm eff} le 200 rm nm$, our results indicate a strong similarity between this system and the high-$T_{rm c}$ superconducting cuprates.
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