We present NMR measurements of the layered nitride superconductor $\mathrm{Li}{}_x$ZrNCl. The nuclear spin-lattice relaxation rate $1/T_1$ shows that the coherence peak is strongly suppressed in $\mathrm{Li}{}_x$ZrNCl in contrast to conventional BCS superconductors. In the lightly doped region close to the insulating state, the system shows a gaplike behavior, i.e., pseudogap, that is characterized by a reduction in the magnitude of the Knight shift and $1/T_{1}T$. A higher superconducting (SC) transition temperature $T_c$ is achieved by coexisting with the pseudogap state. These unusual behaviors, which deviate from the ordinary BCS framework, are the key ingredients to understanding the SC mechanism of $\mathrm{Li}{}_x$ZrNCl.

• Received 9 January 2014
• Revised 20 June 2014

DOI:https://doi.org/10.1103/PhysRevB.90.020503