Valence and Na content dependences of superconductivity in NaxCoO2.yH2O

TL;DR

This study investigates how sodium and water content influence superconductivity in NaxCoO2.yH2O, revealing that Na+ ions significantly affect superconductivity and supporting spin-triplet pairing at high magnetic fields.

Contribution

It provides a detailed superconducting phase diagram for different Na content and demonstrates the impact of Na+/H3O+ exchange on superconductivity, suggesting multiple Fermi surface types.

Findings

Superconductivity varies with Na content and water intercalation.

Na+/H3O+ exchange significantly influences superconductivity.

High-field measurements support spin-triplet pairing mechanism.

Abstract

Various samples of sodium cobalt oxyhydrate with relatively large amounts of Na$^{+}$ ions were synthesized by a modified soft-chemical process in which a NaOH aqueous solution was added in the final step of the procedure. From these samples, a superconducting phase diagram was determined for a section of a cobalt valence of $\sim$+3.48, which was compared with a previously obtained one of $\sim$+3.40. The superconductivity was significantly affected by the isovalent exchanger of Na$^{+}$ and H$_{3}$O$^{+}$, rather than by variation of Co valence, suggesting the presence of multiple kinds of Fermi surface. Furthermore, the high-field magnetic susceptibility measurements for one sample up to 30 T indicated an upper critical field much higher than the Pauli limit supporting the validity of the spin-triplet pairing mechanism.