Superconducting phase diagram of NaxCoO2.yH2O

TL;DR

This study maps the superconducting and magnetic phases of Nax(H3O)zCoO2yH2O, revealing that superconductivity is highly sensitive to Na/H3O ratio and is closely linked to magnetic interactions, with magnetic fields inducing phase transitions.

Contribution

The paper provides the first detailed phase diagram of Nax(H3O)zCoO2yH2O at fixed Co valence, highlighting the relationship between Na/H3O ratio, superconductivity, and magnetic order.

Findings

Superconductivity appears in two separate regions separated by a magnetic phase.

Magnetic field can induce a transition from superconducting to magnetic order.

Upper-critical field exceeds the Pauli limit far from the magnetic phase.

Abstract

We synthesized Nax(H3O)zCoO2yH2O samples with various Na/H3O ratios but with the constant Co valence of s = +3.40, and measured their magnetic properties to draw phase diagrams of the system. The superconductivity is very sensitive to the Na/H3O ratio. With varying x under fixed s of +3.40, magnetically ordered phase appears in the intermediate range of x sandwiched by two separated superconducting phases, suggesting that the superconductivity is induced by moderately strong magnetic interactions. In the vicinity of the magnetic phase, transition from the superconducting state to the magnetically ordered state was induced by applying high magnetic field. This transition is of the second order, at least, above 1.8 K. The upper-critical field is expected to be much higher than the Pauli limit for a phase located far away from the magnetic phase regarding the Na/H3O ratio.