Novel phase diagram of superconductor NaxCoO2-yH2O in a 75 % relative humidity

TL;DR

This study explores how the superconducting transition temperature and properties of NaxCoO2-yH2O are affected by humidity exposure duration, revealing a phase diagram linked to water intercalation time.

Contribution

It provides a systematic analysis of the relationship between water intercalation duration and superconductivity in NaxCoO2-yH2O, highlighting the evolution of Tc and local charge distribution.

Findings

Superconductivity appears after 2 days of water exposure.

Tc reaches up to 4.6 K after 7 days.

Local charge distribution changes with intercalation duration.

Abstract

We succeeded in synthesizing the powder samples of bilayer-hydrate sodium cobalt oxide superconductors NaxCoO2-yH2O with Tc = 0 ~ 4.6 K by systematically changing the keeping duration in a 75 % relative humidity atmosphere after intercalation of water molecules. From the magnetic measurements, we found that the one-day duration sample does not show any superconductivity down to 1.8 K, and that the samples kept for 2 ~ 7 days show superconductivity, in which Tc increases up to 4.6 K with increasing the duration. Tc and the superconducting volume fraction are almost invariant between 7 days and 1month duration. The 59Co NQR spectra indicate a systematic change in the local charge distribution on the CoO2 plane with change in duration.