Strong Suppression of Coherence Effect and Appearance of Pseudogap in Layered Nitride Superconductor Li_xZrNCl: ^91Zr- and ^15N- NMR Studies

TL;DR

This study uses NMR to investigate Li_xZrNCl, revealing suppressed coherence peaks and pseudogap phenomena that challenge conventional BCS theory, providing insights into its unconventional superconductivity.

Contribution

The paper reports the observation of suppressed coherence peaks and pseudogap behavior in Li_xZrNCl, highlighting unconventional superconducting properties distinct from traditional BCS superconductors.

Findings

Suppressed coherence peak in 1/T_1 measurements.

Presence of pseudogap near the insulating state.

Higher T_c associated with pseudogap coexistence.

Abstract

We present NMR measurements of the layered nitride superconductor Li_xZrNCl. The nuclear spin-lattice relaxation rate, 1/T_1, shows that the coherence peak is strongly suppressed in Li_xZrNCl in contrast to conventional BCS superconductors. In the lightly-doped region close to the insulating state, the system shows a gap-like behavior, i.e., pseudogap, that is characterized by a reduction in the magnitude of the Knight shift and 1/T_1T. 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 Li_xZrNCl.