Observation of Bulk Superconductivity in NaxCoO2.yH2O and NaxCoO2.yD2O Powder and Single Crystals

TL;DR

This study confirms bulk superconductivity near 5 K in hydrated sodium cobaltate, with minimal impact from deuterium substitution, and reveals resistivity anisotropy and a notable upturn at 52 K, providing insights into its superconducting mechanism.

Contribution

It demonstrates bulk superconductivity in Na$_x$CoO$_2$ with H$_2$O and D$_2$O intercalation and investigates anisotropic resistivity and the effects of deuterium substitution.

Findings

Superconductivity observed at T_c ~ 5 K in hydrated samples.

Deuterium substitution affects T_c by less than 0.2 K.

Resistivity shows an upturn at T* ~ 52 K in both directions.

Abstract

Poly- and single-crystalline Na$_x$CoO$_2$ has been successfully intercalated with H$_2$O and D$_2$O as confirmed by X-ray diffraction and thermogravimetric analysis. Resistivity, magnetic susceptibility, and specific heat measurements show bulk superconductivity with T$_c$ close to 5 K in both cases. The substitution of deuterium for hydrogen has an effect on T$_c$ of less than 0.2 K. Investigation of the resistivity anisotropy of Na$_x$CoO$_2\cdot$yH$_2$O single crystals shows: (a) almost zero resistivity below T$_c$, and (b) an abrupt upturn at $T^* \sim$ 52 K in both the $\it {ab}$ plane and the $\it {c}$ direction. The implications of our results on the possible superconducting mechanism will be discussed.