Magnetic anomalies of hydrous cobaltate compound Na_x(H_3O)_zCoO_2 \cdot yH_2O detected by NMR and NQR measurements

TL;DR

This study uses NMR and NQR measurements to explore the magnetic and superconducting properties of bilayer-hydrate cobaltate Na_x(H_3O)_zCoO_2 yH_2O, revealing how different ground states emerge at low temperatures.

Contribution

It provides new insights into the relationship between magnetism and superconductivity in cobaltate compounds through detailed spectroscopic analysis of multiple samples.

Findings

Small internal magnetic fields appear below 6 K in one sample.

Superconductivity is observed in two samples under zero magnetic field.

Static internal magnetic fields are detected below 4 K in a specific sample.

Abstract

In order to investigate the relationship between superconductivity and magnetism in bilayer-hydrate cobaltate Na_x(H_3O)_zCoO_2 \cdot yH_2O, Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements were performed on three different samples, which demonstrate various ground states at low temperatures. The appearance of small internal fields is observed in the NQR spectra below approximately 6 K on one of the samples that possesses the largest c-axis length and the highest NQR frequency. The other two samples exhibit superconducting transition in zero magnetic field, while these two samples show different ground states in the magnetic fields greater than 5 T. The comparison of the NMR spectra of these two samples obtained in high magnetic fields reveals the appearance of static internal magnetic fields at the Co site below 4 K in the sample that possesses the intermediate c-axis length and the NQR frequency.