**Modeling Intercalated Group-4-Metal Nitride Halide Superconductivity with Interlayer Coulomb Coupling**, D. R. Harshman and A. T. Fiory [arXiv]

Behavior consistent with Coulomb-mediated high-T_{C} superconductivity is shown to be present in the intercalated group-4-metal nitride halides *A*_{x}(*S*)_{y}*M*N*X*, where the *M*N*X* host (*M* = Ti, Zr, Hf; *X* = Cl, Br) is partially intercalated with cations *A*_{x} and optionally molecular species (*S*)_{y} in the van der Waals gap between the halide *X* layers, expanding the basal-plane spacing *d*. The optimal transition temperature is modeled by T_{C0} ∝ ζ^{–1}(σ/*A*)^{1/2}, where the participating fractional charge per area per formula unit σ/*A* and the distance ζ, given by the transverse *A*_{x}–*X* separation (ζ < *d*), govern the interlayer Coulomb coupling. From experiment results for β-form compounds based on Zr and Hf, in which concentrations x of *A*_{x} are varied, it is shown that σ = γ[v(x_{opt} − x_{0})], where x_{opt} is the optimal doping, x_{0} is the onset of superconducting behavior, v is the *A*_{x} charge state, and γ = 1/8 is a factor determined by the model. Observations of T_{C} < T_{C0} in the comparatively more disordered α-A_{x}(*S*)_{y}TiN*X* compounds are modeled as pair-breaking by remote Coulomb scattering from the *A*_{x} cations, which attenuates exponentially with increasing ζ. The T_{C0} values calculated for nine *A*_{x}(*S*)_{y}*M*NCl compounds, shown to be optimal, agree with the measured T_{C} to within experimental error. The model for T_{C0} is also found to be consistent with the absence of high-T_{C} characteristics for *A*_{x}*M*N*X* compounds in which a spatially separated intercalation layer is not formed.

Dale R. Harshman and Anthony T. Fiory, J. Supercond. Nov. Mater. **28**, 2967 (2015).