Comment on “Superconductivity in electron-doped layered TiNCl with variable interlayer coupling”

Abstract

In their article, Zhang et al. [Phys. Rev. B 86, 024516 (2012)] present a remarkable result for Ax(S)yTiNCl compounds (α-phase TiNCl partially intercalated with alkali A and optionally co-intercalated molecular species S), finding the superconducting transition temperature Tc scales with d−1, where the spacing d between TiNCl-layered structures depends on intercalant thickness. Recognizing that this behavior indicates interlayer coupling, Zhang et al. cite, among other papers, the interlayer Coulombic pairing mechanism picture [Harshman et al., J. Phys.: Condens. Matter 23, 295701 (2011)]. This Comment shows that superconductivity occurs by interactions between the chlorine layers of the TiNCl structure and the layers containing Ax, wherein the transverse Ax−Cl separation distance ζ is smaller than d. In the absence of pair-breaking interactions, the optimal transition temperature is modeled by Tc0∝(σ/A)1/2ζ−1, where σ/A is the fractional charge per area per formula unit. Particularly noteworthy are the rather marginally metallic trends in resistivities of Ax(S)yTiNCl, indicating high scattering rates, which are expected to partially originate from remote Coulomb scattering (RCS) from the Ax ions. By modeling a small fraction of the RCS as inducing pair breaking, taken to cut off exponentially with ζ, observations of Tc<Tc0 are quantitatively described for compounds with ζ<4Å and Tc≈Tc0 for Na0.16(S)yTiNCl with propylene carbonate and butylene carbonate co-intercalants for which ζ>7Å. Since a spatially separated alkali-ion layer is not formed in Li0.13TiNCl, the observed Tc of 5.9 K is attributed to an intergrowth phase related to TiN (Tc=5.6K).

Source-reported materials — not catalogue approval

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