Thermodynamic and Transport Measurements on Superconducting Na_xCoO_2 + yH_2O Single Crystals Prepared by Electrochemical De-intercalation

TL;DR

This study reports the synthesis of superconducting Na_xCoO_2 + yH_2O single crystals using an electrochemical method, and characterizes their thermodynamic and transport properties, revealing Fermi-liquid behavior and anisotropic upper critical fields.

Contribution

It introduces a precise electrochemical technique for preparing high-quality superconducting Na_xCoO_2 + yH_2O crystals, avoiding bromine de-intercalation and enabling detailed property measurements.

Findings

Bulk superconductivity confirmed by susceptibility and specific heat.

Normal state exhibits Fermi-liquid behavior with strong mass enhancement.

Significant anisotropy observed in upper critical field measurements.

Abstract

Superconducting single crystal samples of Na_xCoO_2 + yH_2O have been produced using an electrochemical technique which dispenses with the usual bromine chemical de-intercalation step and permits much more precise control of the Na content. After careful hydration, nearly single-phase crystals have been obtained in which over 90% of the sample's volume corresponds to the superconducting Na_{0.3}CoO_2 + 1.3H_2O structure. Susceptibility and specific heat measurements confirm that bulk superconductivity has been achieved. The extracted normal state density of states indicates Fermi-liquid behavior with strong mass enhancement and a modest Wilson ratio. Measurements of H_{c2} for H||c and H||ab reveal significant anisotropy. The estimated value of H_{c2} for H||c yields a coherence length of ~100 A, consistent with an extremely narrow bandwidth.