Electron doping in the honeycomb bilayer superconductors (Zr, Hf)NCl

Based on a virtual crystal treatment, we show that alkali doping introduces electron carriers into a single light mass (m* = 0.6), low density of states band in LixZrNCl and NaxHfNCl (which have superconducting T-c up to 25 K) that is a symmetric interlayer combination of d(xy),d(x2-y2) character. Doping leads to simple K-centered three-fold symmetrical cylindrical Fermi surfaces that concentrate low-energy scattering into distinct regions of momentum. The lack of any observed magnetic tendencies. together with our calculation of small electron-phonon strength, suggests a pairing mechanism that is distinct from known superconductors.