Spin and charge fluctuations inα-structure layered nitride superconductors

Abstract

To explore conditions underlying the superconductivity in electron-doped TiNCl where Tc = 16 K, we calculate the electronic structure, Wannier functions and spin and charge susceptibilities using first-principles density functional theory. TiNCl is the first high-temperature superconductor discovered in the α-structure of the layered transition-metal nitride family MNCl (M=Ti, Zr, Hf). We construct a tight-binding model based on Wannier functions derived from the band structure, and consider explicit electronic interactions in a multi-band Hubbard Hamiltonian, where the interactions are treated within the random phase approximation (RPA) to calculate spin and charge susceptibility. The results show that, consistent with TiNCl being a nonmagnetic material, spin fluctuations do not dominate over charge fluctuations and both may have comparable impact on the properties of the doped system.

Superconductivity in electron-doped layered TiNCl with variable interlayer coupling

“…22 As mentioned in the previous paragraph, the T c dependence to the d spacing of the TiNCl intercalation compounds is quite different from that of β-MNCl (M = Zr, Hf). The β-form compounds have a disconnected cylindrical Fermi surface favorable for nesting, 15,16 while the electron-doped α-structured TiNCl has a single oval Fermi surface centered at the point, 27 suggesting that the possible nesting of Fermi surfaces discussed for the β-form compounds should be excluded in the TiNCl compounds. The anisotropic properties on H c2 are quite different between the αand β-structured compounds.…”

“…Harshman et al 6 have discussed the importance of the Coulomb interlayer coupling and 2D carrier concentration in layered high-T c superconductors. Yin et al 27 also proposed a possible pairing assisted by charge fluctuations in nonmagnetic TiNCl superconductors. Bill et al 7 proposed the importance of electronic collective modes to explain the relatively high T c of β-HfNCl intercalation compounds and showed that even if the structure has strong anisotropy, it cannot be reduced to a purely 2D model, and interlayer Coulomb interaction should be taken into account for superconductivity.…”

Exploration of new superconductors and functional materials, and fabrication of superconducting tapes and wires of iron pnictides

“…The small anisotropy on H c2 and the Coulomb coupling between the superconducting layers should be the relevant nature of the superconductivity of the α -form layered nitrides [57]. For more discussion on the superconducting mechanisms and the anisotropy of the two different kinds of layered nitrides, a theoretical study including the electric band structure is required [328]. …”

Unconventional superconductivity in electron-doped layered metal nitride halides MNX (M= Ti, Zr, Hf; X= Cl, Br, I)

“…It will be interesting to compare their superconductivities. The β-structured compounds have disconnected cylindrical Fermi surfaces favorable for nesting [13,35,77], while the electron-doped α-structured TiNCl has a simple oval shaped Fermi surface around only the Γ point [78], suggesting that a similar possibility of the nesting is excluded. …”