Strong suppression of coherence effect and appearance of pseudogap in the layered nitride superconductor LixZrNCl: 91Zr- and 15N-NMR studies

We present NMR measurements of the layered nitride superconductor LixZrNCl. The nuclear spin- lattice relaxation rate 1/T-1 shows that the coherence peak is strongly suppressed in LixZrNCl 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/T1T. 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 LixZrNCl.