µ-PES Studies on TiNCl and Quasi-two-dimensional Superconductor Na-intercalated TiNCl

Abstract

Electron-doped TiNCl is a superconductor, for which exotic mechanisms of the superconductivity have been discussed. However, difficulty in preparing large single-phase samples has prevented the direct observation of its electronic structure and how that changes with electron doping. In this study, micro-photoemission spectroscopy (µ-PES) was used to reveal the electronic structures of TiNCl and Na-intercalated TiNCl (Na-TiNCl). Comparison of Ti 2p core-level spectra shows the enhancement of a spectral feature in Na-TiNCl that suggests an increase in its metallic character. This indicates the introduction of electron carriers to the TiNCl layers. Although the overall valence band electronic structure of parent TiNCl could be reproduced by first-principle calculations, that of Na-TiNCl showed a marked deviation from the rigid band model near the Fermi level (EF). The spectral shape observed near EF of Na-TiNCl was found to be similar to a result in early transition metal oxides that has been attributed to the effect of strong electron correlation. The present study also demonstrated the usefulness of µ-PES to obtain reliable electronic structure data for samples that are difficult to make at large sizes.

Soft x-ray irradiation induced metallization of layered TiNCl

“…The initial valence band spectrum of TiNCl had a peak at 6 eV with a shoulder structure at 4 eV, and there was almost no intensity in the near-E F region. The overall spectral shape is consistent with that of the previous study [25], and thus the states of approximately 6 and 4 eV can be ascribed to Cl 3p and N 2p orbitals, respectively.…”

“…Upon Na intercalation between TiNCl layers, TiNCl becomes a superconductor with a relatively high superconducting transition temperature of ∼16 K [7]. As electron-doped β-form HfNCl and ZrNCl [8][9][10][11][12][13][14][15][16][17][18][19][20], electron-doped TiNCl is considered a candidate for unconventional superconductors, where exotic mediation forces for Cooper pairing other than phonon are discussed [21][22][23][24][25]. * Author to whom any correspondence should be addressed.…”

Synthetic Route of Layered Titanium Nitride Chloride TiNCl Using Sodium Amide

“…Second, the Ti 2 p HAXPES spectrum consists of two peaks at around 457.5 and 463.5 eV, corresponding to the spin–orbit split of Ti 2 p 3/2 and Ti 2 p 1/2 , respectively, as shown in Figure a. The Ti 2 p 3/2 peak has a broad shoulder-like component in the lower binding energy side, similar to the valence splitting of previously reported electron-doped samples. , …”

“…The Ti 2p 3/2 peak has a broad shoulder-like component in the lower binding energy side, similar to the valence splitting of previously reported electron-doped samples. 33,34 Third, the prominent peaks of TiNCl_A shift to the higher binding energy side about 0.3 eV compared to TiNCl_C, even though the sample was not a Na-doped one. The same chemical shift is also observed in the core-level spectrum of the other elements N 1s and Cl 2p (Figure S4).…”

“…First, the top of the valence band of TiNCl_A lies at ∼1.5 eV, comparable to the previously reported value (Figure S3). 33 There is a tiny intensity at the E F , appearing as a localized state. This fact is compatible with the semiconductor-like properties of hopping conduction suggested by the resistivity measurement.…”