Abstract
Tunnel break-junction method has been adopted to study polycrystalline samples of iron-oxypnictide superconductor NdFeAs(O0.9F0.1) with Tc=48K. Measurements were carried out at 4.2K. Break-junction (BJ) conductance versus voltage curves showed gap-edge peaks with the peak-to-peak distances Vp-p=4Δ/e=28–40mV at 4.2K, where 2Δ(T) is the superconducting energy gap, e>0 is the elementary charge. This yields 2Δ(0)=14–20meV, so that the gap ratio 2Δ(0)/kBTc is about 4.1±0.7,kB being the Boltzmann constant. This ratio implies strong-coupling superconductivity in the framework of Bardeen–Cooper–Schrieffer theory, being, however, much smaller than that for high-Tc copper oxides. This suggests a significant difference in the pairing mechanism between those classes of materials.
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