59Co-NMR Knight Shift of the Superconducting NaxCoO2.yH2O

TL;DR

This study uses 59Co-NMR to investigate the superconducting properties of NaxCoO2.yH2O, revealing a temperature-independent Knight shift above Tc and a decrease below, suggesting s-wave singlet pairing.

Contribution

First NMR analysis of NaxCoO2.yH2O showing evidence for s-wave singlet superconductivity based on Knight shift and relaxation rate behaviors.

Findings

Knight shift is temperature-independent above Tc

Knight shift decreases below Tc

Evidence of s-wave pairing symmetry

Abstract

Layered Co oxide NaxCoO2.yH2O with the superconducting transition temperature Tc =4.5 K has been studied by 59Co-NMR. The Knight shift K estimated from the observed spectra for powder sample exhibits almost temperature(T)-independent behavior above Tc and decreases with decreasing T below Tc. This result and the existence of the coherence peak in the spin-lattice-relaxation-rate versus T curve reported by the present authors indicate, naively speaking, that the singlet order parameter with s-wave symmetry is realized in NaxCoO2.yH2O. Differences of the observed behaviors between the spectra of the non-aligned sample and the one aligned in epoxy adhesive by applying the external magnetic field are discussed.