Preparation and Properties of Layer Structured High-Tc Superconductors

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1. The layer structured nitride chlorides, β-MNCl (M = Zr, Hf) can be electron-doped by the intercalation of lithium, and are changed into superconductors with the superconducting transition temperatures (Tc's) of 14 and 25.5 K for M = Zr and Hf, respectively. The interlayer spacing can be controlled by the co-intercalation of polar solvent molecules with lithium ions, and the anisotropy of the superconductivity is greatly enhanced. The NMR and muon spin relation measurements revealed that the electron-doped layer structured nitride superconductors have the following characteristics ; (i) highly anisotropic exotic superconductors, (ii) low carrier density near Fermi level, 〜10^<21>/cm^3, (iii) very small isotope effect on the replacement with ^<15>N, and (iv) low phonon excitation. The superconductivity is very unconventional.2. The layer structured nitride chlorides, β-MNCl (M = Zr, Hf) were so thin and so soft crystals that it was very difficult to use for X-ray structural analysis a … More nd physical property measurements. We have succeeded to develop a new high pressure synthesis method to obtain thick single crystals using ammonium chloride as flux. All kinds of β-MNX (M = Zr, Hf ; X = Cl, Br, I) were also obtained as single crystals, and the crystal structures were determined by X-ray single crystal analyses. It was found that the compounds were crystallized into the two different polytypes, the SmSI and the YOF structures.3. In an attempt to tailor new layer structured nitride compounds, thin artificial nitride films were prepared using electron-beam evaporation and lazer ablation methods with a nitrogen radical source under ultrahigh vacuum conditions. Titanium nitride, TiN, films were epitaxially grown on MgO (100) substrates, and the layer-by-layer growth was successfully monitored by RHEED. CrN could be epitaxially deposited on Si(100), and an expitaxial NbN film was subsequently grown on the CrN film. This finding suggests the possibility of the synthesis of artificial CrN-NbN films with superstructures. Less