Unconventional Upper- and Lower-Critical Fields and Normal-State Magnetic Susceptibility of the Novel Superconducting Compound $Na_{0.35} Co O_2 \cdot 1.3 H_2O$

TL;DR

This study investigates the magnetic properties and critical fields of the novel superconductor Na$_{0.35}$CoO$_{2}$$ullet$1.3H$_{2}$O, revealing unconventional superconducting parameters and magnetic behavior.

Contribution

The paper provides the first detailed measurements of critical fields and magnetic susceptibility, highlighting the unconventional nature of superconductivity in this compound.

Findings

Superconducting transition temperature T_C=4.6 K

Upper critical field H_C2(0)=61.0 T

Normal-state magnetic susceptibility upturn below 130 K

Abstract

Magnetic properties of the novel layered superconductor, Na$_{0.35}$CoO$_{2}$$\cdot$1.3H$_{2}$O have been investigated. From the temperature dependence and field dependence of the magnetization, the superconducting transition temperature, as well as upper- and lower-critical fields have been estimated to be $T_{C}=4.6$ K, $H_{C2}(0)=61.0$ T and $H_{C1}(0)=28.1$ Oe. These values give quite unusual phenomenological parameters, \textit{i.e.}, coherent length, penetration depth and Ginzburg-Landau parameter of $\xi=2.32$ nm, $\lambda=5.68\times 10^{3}$ \AA\ and $\kappa\equiv\lambda/\xi=244$, suggesting an unconventional nature of superconductivity. Normal-state magnetic susceptibility shows an upturn below 130 K, which is confirmed to be inherent by high-field magnetization data. The upturn may have relevance to the mechanism of the superconductivity.