Investigations of electronic properties of novel anisotropic superconductors with light-mass ions by scanning tunneling microscopy/spectroscopy

本文用低温超高真空扫描隧道显微镜/光谱（STM/STS）研究了层状氮化物超导体。5 K下的STM形貌清楚地表明了α（FeOCl）和β（SmSI）层状多晶型物之间金属原子表面排列的差异。STS光谱在它们之间表现出很大的差异。对于α-K_xTiNCl，能隙2Δ在~20nm^2的区域内是非常不均匀的，而对于β-HfNCl_<0.7>，它们甚至在~100nm^2的区域内都是均匀的。α-K_xTiNCl的最大能隙比2Δ/kBT_c（k_B：Boltzmann常数，Tc：临界温度）~40和β-HfNCl的均匀能隙比2Δ/kBT_c ~10<0.7>均大于BCS值3.5，表明其配对机制不同于常规超导体。

We have investigated the layered nitride superconductors by means of the low-temperature ultra-high vacuum scanning tunneling microscopy/spectroscopy (STM/STS). The STM topographies at 5K clearly indicate the difference in the surface arrangements of metal atoms between α(FeOCl) and β(SmSI) layered polymorphs. The STS spectra exhibit a large difference between them. For α-K_xTiNCl, the energy gap 2Δ is very inhomogeneous down to the area of ～20nm^2, while for β-HfNCl_<0.7> they are homogeneous even up to ～100nm^2. The maximum gap ratio 2Δ/k_BT_c (k_B : Boltzmann constant, Tc : critical temperature) ～40 for α-K_xTiNCl and the uniform ratio 2Δ/k_BTc～10 for β-HfNCl_<0.7> are both larger than the BCS value 3.5, thereby indicating that the pairing mechanism is different from that of the conventional superconductors.