Scanning Tunneling Spectroscopy and Break Junction Spectroscopy on Iron-Oxypnictide Superconductor NdFeAs(O0.9 F0.1)
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Iron-oxypnictide superconductor NdFeAs(O0.9F0.1) 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 Vpp = 4 Δ/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)/kBTc is about 3~4, kB being the Boltzmann constant. This value is consistent with the conventional weak-coupling s-wave Bardeen-Cooper-Schrieffer theory.
This research was supported by Grant-in-Aid for Scientific Research (No. 19540370) from JSPS, Japan. AMG highly appreciates the 2009 Visitors Program of the Max Planck Institute for the Physics of Complex Systems (Dresden, Germany).
Physica C: Superconductivity and its Applications
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Graduate School of Integrated Arts and Sciences