Upper critical field of aligned Na_{x}(H_{3}O)_{z}CoO_{2}\cdot yH_{2}O superconductor by magnetization measurement

TL;DR

This study measures the upper critical magnetic field of aligned Na_x(H_3O)_zCoO_2·yH_2O superconductors, revealing spin-singlet pairing and intrinsic upward curvature near T_c, with implications for understanding their superconducting properties.

Contribution

The paper provides the first detailed magnetization-based determination of H_{c2} in aligned Na_x(H_3O)_zCoO_2·yH_2O, confirming spin-singlet pairing and intrinsic phase diagram features.

Findings

H_{c2} just within Pauli limit supports spin-singlet pairing.

Upward curvature near T_c is intrinsic to the material.

Superconducting transition temperature T_c(H) was accurately determined.

Abstract

We present magnetization measurements for the upper critical field $H_{c2}$ on aligned Na$_{x}$(H$_{3}$O)$_{z}$CoO$_{2}\cdot y$H$_{2}$O superconductors with magnetic field parallel to CoO$_2$ layers. The temperature-dependent part of the normal-state magnetization $M(H,T)$ can be well scaled with Brillouin function, suggesting the existence of impurity moments. By subtracting the contribution of the magnetic impurity moments, a well defined onset of superconducting diamagnetism allows the in-field superconducting transition temperature $T_{c}$($H$) accurately determined. No superconducting transition was observed down to 1.9 K under an applied field of 8 Tesla. The result suggests that the $H^{\|ab}_{c2}(0)$ value is just within the Pauli paramagnetic limit, supporting spin-singlet superconductivity in the cobalt oxyhydrate superconductor. Additionally, the upward curvature near $T_{c}$ in the $H^{\|ab}_{c2}-T$ phase diagram was confirmed to be intrinsic. Possible origin of the anomaly was discussed.