Scanning tunneling microscopy and spectroscopy (STM-STS) measurements have been carried out on the α (FeOCl)-type K x TiNCl (x ∼ 0.5, T c = 16 K) and β (SmSI)-type HfNCl y (y ∼ 0.7, T c = 24 K) layered nitride superconductors. The STM images at 5 K showed clear atomic arrangements for both the compounds, namely, the rectangular lattice on α-K x TiNCl and the triangular lattice on β-HfNCl y. The tunneling spectra in the superconducting states at low temperatures demonstrate qualitatively different features between these superconductors. For α-K x TiNCl, the spatial distributions of the density of states and the superconducting gap structures are very inhomogeneous, while those on β-HfNCl y are found to be almost homogeneous. The nanoscale electronic features between these compounds correlate with the different lattice structures of the M (=Ti or Hf) N conducting layers, which are caused by the lattice symmetry difference itself or induced by the difference in the local doping distributions in these chemically reactive compounds. The averaged gap magnitudes in the superconducting states,¯ 10.2 meV and 7.5 meV for α-K x TiNCl and β-HfNCl y , corresponding to the gap ratios 2¯ /k B T c 15 and 7.2, respectively, indicate the unusually strong coupling effects of the superconductivity.
Scanning tunneling microscopy/spectroscopy (STM/STS) measurements on multi-layered cuprate superconductor Ba 2 Ca 5 Cu 6 O 12 (O 1-x F x) 2 are carried out. STM topographies show randomly distributed bright spot structures with a typical spot size of ~ 0.8 nm. These bright spots are occupied about 28% per one unit cell of c-plane, which is comparable to the regular amount of apical oxygen of 20% obtained from element analysis. Tunneling spectra simultaneously show both the small and the large gap structures. These gap sizes at 4.9 K are about ∆ ~ 15 meV and ~ 90 meV, respectively. The small gap structure disappears at the temperature close to T C , while the large gap persists up to ~200 K. Therefore, these features correspond to the superconducting gap and pseudogap, respectively. These facts give evidence for some ordered state with large energy scale even in the superconducting state. For the superconducting gap, the ratio of 2∆ S /K B T C = 4.9 is obtained with T C = 70 K, which is determined from temperature dependence of the tunneling spectra.
a b s t r a c tIron-oxypnictide superconductor NdFeAs(O 0.9 F 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 Vshaped 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 = 4D/e $ 25 mV at 4.2 K, where D is the superconducting energy gap, e > 0 is the elementary charge. This yields D(0) = 6-7 meV, so that the ratio 2D(0)/k B T 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.
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