Observation of periodic structures and pseudogaps in pristine compound -TiNCl by STM/STS and break junction methods

Scanning tunneling microscopy and spectroscopy (STM/STS) and break-junction tunnel spectroscopy (B.TTS) measurements are performed on the pristine layered alpha-TiNCl semiconductor (pri-TiNCl), being the precursor of superconducting nitride chlorides. The STM topography of pri-TiNCl shows basic crystal structures with the lattice periods of alpha(0) approximate to 0.38 nm and b(0) approximate to 0.31 nm, ensuring that a clean a-b surface of micro-single crystals is obtained. From the STS measurements, the averaged conductance versus voltage dependence dI/dV(V) reveals kink structures at relatively high bias voltages of |F| similar to -50 mV and -100 mV. The spatial (r-) conductance distributions dI/dV (V, r) (dI/dV maps) proportional to those of quasiparticle local densities of states (LDOSes) exhibit the bias-independent streak structures with the period of (4.8 +/- 0.2) b(0), which are predominantly observed within the range of |V| < 95 mV. The temperature dependence of the dI/dV B.TTS spectra shows the gap structure with the energy scale of 4 Sigma = 180 meV vanishing at T-s similar to 120 K. Thus, the ratio of the gap Sigma to the transition temperature T-s is 2 Sigma/k(B)T(s) similar to 10. Here, k(B) is the Boltzmann constant. Such a ratio is typical of the pseudogap features in cuprate superconductors and dielectric gap characteristics in layered chalcogenides with charge-density waves.