Revised superconducting phase diagram of hole doped Na$_{x}$(H$_{3}$O)$_{z}$CoO$_{2}\cdot y$H$_{2}$O

TL;DR

This study maps the superconducting phase diagram of hole-doped Na$_{x}$(H$_{3}$O)$_{z}$CoO$_{2}ullet y$H$_{2}$O, emphasizing the importance of Co valence and interlayer spacing in achieving optimal superconductivity.

Contribution

It provides a revised phase diagram showing the critical Co valence range for optimal Tc and highlights the role of interlayer spacing and doping mechanisms.

Findings

Optimal Tc occurs at Co valence 3.24 - 3.35.

Superconductivity is influenced by interlayer separation.

Na content alone does not determine electronic doping.

Abstract

We have studied the superconducting phase diagram of \NaH\space as a function of electronic doping, characterizing our samples both in terms of Na content $x$ and the Co valence state. Our findings are consistent with a recent report that intercalation of \oxp\space ions into Na$_{x}$CoO$_{2}$, together with water, act as an additional dopant indicating that Na sub-stochiometry alone does not control the electronic doping of these materials. We find a superconducting phase diagram where optimal \Tc\space is achieved through a Co valence range of 3.24 - 3.35, while \Tc\space decreases for materials with a higher Co valence. The critical role of dimensionality in achieving superconductivity is highlighted by similarly doped non-superconducting anhydrous samples, differing from the superconducting hydrate only in inter-layer spacing. The increase of the interlayer separation between CoO$_{2}$ sheets as Co valence is varied into the optimal \Tc\space region is further evidence for this criticality.