Tunneling STM/STS and Break-Junction Spectroscopy of the Layered Nitro-Chloride Superconductors MNCl (M = Ti, Hf, Zr)

The layered superconductors beta-MNCl with the critical temperatures T-c = 14 K (M = Zr) - 25 K (M = Hf) were investigated by means of scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy. The STM/STS was used to investigate the surface electronic structures in nanometer length scale, while the BJTS was employed to precisely determine the gap characteristics. Both techniques consistently clarified the unusually large size of the superconducting gap. Wide gap distributions with large-scale maximum gap values were also revealed in alpha-KyTiNCl with a different crystal structure.