Co-NMR Knight Shift of NaxCoO2 \dot yH2O Studied in Both Superconducting Regions of the Tc-nuQ3 Phase Diagram Divided by the Nonsuperconducting Phase

TL;DR

This study investigates the superconducting properties of NaxCoO2·yH2O across different regions of its phase diagram using NMR Knight shift measurements, revealing spin-singlet pairing and charge instability effects.

Contribution

The paper provides the first NMR Knight shift measurements in the higher-nuQ3 superconducting phase of NaxCoO2·yH2O, confirming spin-singlet pairing across multiple regions.

Findings

Superconducting pairs are in the spin-singlet state in both phases.

Superconductivity exists over a wide nuQ3 range but is suppressed at the middle region.

Charge instability may suppress superconductivity in the narrow middle region.

Abstract

In the temperature (T)-nuQ3 phase diagram of NaxCoO2 \dot yH2O, there exist two superconducting regions of nuQ3 separated by the nonsuperconducting region, where nuQ3 is usually estimated from the peak position of the 59Co-NQR spectra of the 5/2-7/2 transition and well-approximated here as nuQ3~3nuQ,nuQ being the interaction energy between the nuclear quadrupole moment and the electric field gradient. We have carried out measurements of the 59Co-NMR Knight shift (K) for a single crystal in the higher-nuQ3 superconducting phase and found that K begins to decrease with decreasing T at Tc for both magnetic field directions parallel and perpendicular to CoO2-planes. The result indicates together with the previous ones that the superconducting pairs are in the spin-singlet state in both phases, excluding the possibility of the spin-triplet superconductivity in this phase diagram. The superconductivity of this system spreads over the wide nuQ3 regions, but is suppressed in the narrow region located at the middle point of the region possibly due to charge instability.