Unconventional superconductivity of NdFeAsO(0.82)F(0.18) indicated by the low temperature dependence of the lower critical field H(c1).

We measured the initial M-H curves for a sample of the newly discovered superconductor NdFeAsO(0.82)Fe(0.18), which had a critical temperature, T(c), of 51 K and was fabricated at the high pressure of 6 GPa. The lower critical field, H(c1), was extracted from the deviation point of the Meissner linearity in the M-H curves, which show linear temperature dependence in the low temperature region down to 5 K. The H(c1)(T) indicates no s-wave superconductivity, but rather an unconventional superconductivity with a nodal gap structure. Furthermore, the linearity of H(c1) at low temperature does not hold at high temperature, but shows other characteristics, indicating that this superconductor might have multi-gap features. Based on the low temperature nodal gap structure, we estimate that the maximum gap magnitude Δ(0) = (1.6 ± 0.2)  k(B)T(c).