Scanning tunneling microscopy and spectroscopy (STM-STS) measurements have been carried out on the $\alpha$ (FeOCl)–type K${}_x$TiNCl ($x\sim 0.5$, $T_c=16$ K) and $\beta$ (SmSI)–type HfNCl${}_y$ ($y\sim 0.7$, $T_c=24$ K) layered nitride superconductors. The STM images at 5 K showed clear atomic arrangements for both the compounds, namely, the rectangular lattice on $\alpha$-K${}_x$TiNCl and the triangular lattice on $\beta$-HfNCl${}_y$. The tunneling spectra in the superconducting states at low temperatures demonstrate qualitatively different features between these superconductors. For $\alpha$-K${}_x$TiNCl, the spatial distributions of the density of states and the superconducting gap structures are very inhomogeneous, while those on $\beta$-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, $\overline{\Delta }\simeq 10.2$ meV and 7.5 meV for $\alpha$-K${}_x$TiNCl and $\beta$-HfNCl${}_y$, corresponding to the gap ratios $2\overline{\Delta }/k_B{T}_c\simeq$ 15 and 7.2, respectively, indicate the unusually strong coupling effects of the superconductivity.

• Received 13 May 2011

DOI:https://doi.org/10.1103/PhysRevB.85.144517