Scanning tunneling spectroscopy and break junction spectroscopy on iron-oxypnictide superconductor NdFeAs(O_0.9F_0.1)

Iron-oxypnictide superconductor NdFeAs(O_0.9F_0.1) was studied using both low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) and tunnel break junction (BJ) methods. STM topography showed granular and spot structures with a typical size of several nanometers, most probably governed by fluorine atom distribution. The majority of STS conductance, G, versus voltage, V, curves revealed V-shaped structures, whereas some of G(V) dependences possessed coherent gap peaks or kinks at gap energies. At the same time, G(V) dependences obtained by the BJ technique showed clear-cut coherence peaks with peak-to-peak distances V_pp = 4Δ/e ∼ 25 mV at 4.2 K, where Δ is the superconducting energy gap, e > 0 is the elementary charge. This yields Δ(0) = 6-7 meV, so that the ratio 2Δ(0)/k _BT_c is about 3-4, k_B being the Boltzmann constant. This value is consistent with the conventional weak-coupling s-wave Bardeen-Cooper-Schrieffer theory.