Abstract
The in-plane magnetic penetration depth, λ(T), has been measured in single crystals of NdFe1−xCoxAsO, NdFeAsO1−xFx, and LaFeAsO1−xFx. In the Nd compounds we found an upturn in λ(T) below 4K and attribute it to the paramagnetic contribution from the Nd3+ ions. After the correction for this contribution, the in-plane temperature dependent London penetration depth, λL(T), in F-doped crystals was found to follow a power-law dependence, ΔλL(T)∝Tn, n≈2. Similar power-law behavior was found in direct measurements of non-magnetic LaFeAsO1−xFx with n≈2. In NdFe1−xCoxAsO, on the other hand, after the paramagnetic correction, we found n≈3. Our results indicate unconventional pairing in the pnictides, incompatible with a single isotropic gap or a gap with line nodes. The power-law behavior is observed down to T/Tc=0.02 indicating significant gap anisotropy.
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