The layered nitride halides MNX(M=Ti, Zr, Hf; X=Cl, Br, I) have two different polymorphs, α-form and β-form. Both are insulators with a band gap, and when alkali metals are intercalated between the layers, they exhibit superconductivity because of electron doping. The β-form HfNCl is an interesting and well-known superconductor, exhibiting a maximum superconducting transition temperature (Tc) of ~26 K. It has been discussed as a candidate for unconventional superconductors based on experimental results, including very small isotope effects due to 15N substitution. On the other hand, TiNCl is known only in the α-form structure and exhibits a Tc of 16 K upon intercalation of alkali metals. Its superconducting properties are different from those of β-form, and the importance of interlayer coupling has been discussed. The possibility that superconductivity is induced by spin and charge fluctuations has also been discussed. Strong electron-electron correlations are often involved in such superconducting mechanisms.
Photoelectron spectroscopy is a powerful tool for studying the electronic structure of strongly correlated systems. However, because it is a surface-sensitive technique and therefore also observes surface electronic states that are different from the bulk electronic states, the bulk electronic states are not always easy to discuss. Therefore, it is important to experimentally investigate the bulk electronic structure of materials such as TiNCl that are sensitive to moist air and oxygen.
In this presentation, we report bulk-sensitive hard X-ray photoelectron spectroscopy (HAXPES) results for the core levels and valence band spectra of NaxTiNCl(x=0, 0.1, 0.25, 0.4). We observe a systematic change in the spectral shape of the core levels and valence band spectra as a function of Na concentration, and find that the electron doping dependence of the Ti 1s spectra is in qualitative agreement with the cluster model calculations. This result suggests that strong correlation effects caused by Ti 3d play an important role for the change in spectral shape.
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Keywords: Bulk electronic structure, Layered superconductor, strong correlation, HAXPES