Specific heat has been investigated in electron-doped ${\text{Li}}_x{M}_y\text{ZrNCl}$ superconductors, in which interlayer spacing $d$ can be controlled by the intercalation of organic molecules $M$. It has been revealed that, upon enlargement of $d$ by molecule cointercalation, $T_c$ is enhanced concomitantly with the superconducting coupling strength whereas the density of states at the Fermi level is kept almost constant. Structural and spectroscopic analysis proved that the molecules are intercalated without affecting electronic nor vibrational states, simply increasing the interlayer spacing. These results lead us to conclude that the enhancement of $T_c$ is ascribed to the improvement of two dimensionality, which is consistent with a recent theory that discussed superconductivity in a band insulator on a honeycomb lattice with disconnected Fermi surfaces.

• Received 9 May 2010

DOI:https://doi.org/10.1103/PhysRevB.82.054504