Ab initio lattice dynamics and electron-phonon coupling in LixZrNCl -: art. no. 134527

Transition metal chloronitrides represent a class of layered compounds, which after intercalation, become superconductors with T-c's up to 25.5 K. We present a theoretical investigation of the phonon dispersions and electron-phonon coupling for the undoped and Li-doped ZrNCl, which is one prominent representative of this class of materials. The calculations are based on density functional perturbation theory in the framework of the mixed-basis pseudopotential method. For the undoped parent compound beta-ZrNCl, we are able to identify all structures in the measured phonon-density of states. For the doped system, the partial occupation of the Li site is treated by an artificial doping approach. Doping partially fills a quasi-2D conduction band and gives rise to almost cylindrical Fermi surfaces around the zone boundary K point. The doping induced electron-phonon coupling is also very 2D-like and exhibits a strong momentum-dependent renormalization involving selective phonon modes of in-plane vibrations of Zr and N. Although there is a larger coupling to the higher-frequency N vibrations, the average coupling constant is only approximate to 0.5. Within the standard Eliashberg analysis in the dirty limit, the calculated electron-phonon coupling is too weak to explain the observed T-c. Our results point to the need for a more accurate treatment of the Coulomb screening effects in these highly anisotropic layered superconductors.