Nanoscale electronic structure of the layered nitride superconductors α-KxTiNCl and β-HfNCly observed by scanning tunneling microscopy and spectroscopy

Sugimoto Akira

Shohara Kazuhiro

Ekino Toshikazu

Zheng Zhanfeng

Yamanaka Shoji

Physical Review B

Scanning tunneling microscopy and spectroscopy (STM-STS) measurements have been carried out on the α (FeOCl)–type KxTiNCl (x∼0.5, Tc=16 K) and β (SmSI)–type HfNCly (y∼0.7, Tc=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-KxTiNCl and the triangular lattice on β-HfNCly. The tunneling spectra in the superconducting states at low temperatures demonstrate qualitatively different features between these superconductors. For α-KxTiNCl, the spatial distributions of the density of states and the superconducting gap structures are very inhomogeneous, while those on β-HfNCly 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, Δ̅ ≃10.2 meV and 7.5 meV for α-KxTiNCl and β-HfNCly, corresponding to the gap ratios 2Δ̅ /kBTc≃ 15 and 7.2, respectively, indicate the unusually strong coupling effects of the superconductivity.

Physics [ 420 ]

eng

American Physical Society

(c) 2012 American Physical Society

[ISSN] 1098-0121

[DOI] 10.1103/PhysRevB.85.144517

[NCID] AA11187113

[DOI] http://dx.doi.org/10.1103/PhysRevB.85.144517