Specific Heat of Na0.35CoO2,1.3H2O: Effects of Sample Age; Non-Magnetic Pair Breaking, Two Energy Gaps, and Strong Fluctuations in the Superconducting State

TL;DR

This study investigates how aging affects the superconducting properties of Na0.35CoO2·1.3H2O, revealing sample-dependent changes, non-magnetic pair breaking, two energy gaps, and strong fluctuations in the superconducting state.

Contribution

It demonstrates the impact of sample aging on superconductivity, linking structural changes to pair-breaking and multi-gap phenomena in this material.

Findings

Superconductivity diminishes with sample age.

Sample aging correlates with structural changes and oxygen vacancies.

Strong fluctuations influence the vortex transition independent of magnetic field.

Abstract

The specific heats of three samples of Na0.35CoO2,1.3H2O show an evolution of the superconductivity, and its ultimate disappearance, with increasing sample age. An overall increase in pair-breaking action, which occurs preferentially in an electron band with a small energy gap, produces a shift in the relative contributions of two electron bands to the superconducting condensation. The similarity of the time scale for these changes to that recently reported for structural changes in the CoO2 layers and the formation of O vacancies suggests a relation between the two effects and an explanation for the strong sample dependence of the properties of this material more generally. The onset of the transition to the vortex state is independent of magnetic field, suggesting the presence of unusually strong fluctuation effects.