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

摘要

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