Scanning Tunnelling Microscopy and Spectroscopy of the Layered Nitride Superconductor α-NaxTiNCl

Low-temperature scanning tunnelling microscopy and spectroscopy (STM/STS) measurements have been carried out on α-NaxTiNCl (x = 0.16) superconductors with Tc of 18 K. The STM image clearly reveals the streak patterns along b-axis with the spacing of period of ≈ 0.39 nm (= a0) and the circular spots that form a rectangular lattice inside streak patterns. The conductance dI/dV dependences on the voltage V show the typical superconducting gap value of Δ ≈ 9.5 meV, leading to a large gap to Tc ratio 2Δ/kBTc ≈ 12. The dI/dV maps show both a0 streak patterns and the long-periodic modulation with the period of ≈ 1.6 nm, which is about 5 times b0 length. Such a long period of the observed charge density wave correlates with the doping degree of Na ≈ 0.2 and is probably associated with the inverse wave vector of the Fermi surface nested section.