Revised Superconducting Phase Diagram of Hole-DopedNax(H3O)z

Christopher J. Milne

¹

,

D. N. Argyriou

²

,

A. Chemseddine

³

et al.

Abstract

We have studied the superconducting phase diagram of NaxCoO2 ·yH2O as a function of electronic doping, characterizing our samples both in terms of Na content x and the Co valence state. Our findings are consistent with a recent report that intercalation of H3O + ions into NaxCoO2, together with water, act as an additional dopant indicating that Na sub-stochiometry alone does not control the electronic doping of these materials. We find a superconducting phase diagram where optimal TC is achieved through a Co valence range of 3.24 -3.35, while TC decreases for materials with a higher Co valence. The critical role of dimensionality in achieving superconductivity is highlighted by similarly doped non-superconducting anhydrous samples, differing from the superconducting hydrate only in inter-layer spacing. The increase of the interlayer separation between CoO2 sheets as Co valence is varied into the optimal TC region is further evidence for this criticality.

Superconductivity in epitaxial thin films of NaxCoO2∙yD2O

“…It has fairly sharp maxima near x = 1/3 and 2/3 that are separated by a deep minimum near x = 1/2. It has indeed previously been noted that the range of x = 1/2 separates two metallic regions with high conductivity for x < 0.5 and x > 0.5 [2,3]. Notably, a correspondingly rapid variation in the FIR electronic response was not observed in previous optical measurements on single crystals [11,12].…”

“…Superconductivity in Na x CoO 2 · yH 2 O is a property of the cobalt oxide planes. Like in the case of the cuprates, the phase diagram of sodium cobaltate encompasses several competing electronic phases [2,3,4], and there are many indications that the superconducting state is unconventional [5,6]. While in high-temperature superconductors the two-dimensional character of the corresponding copper oxide planes is stabilized by the intrinsically strongly anisotropic crystal structure, in Na x CoO 2 a complicated water intercalation process is needed to amplify this anisotropy and induce superconductivity.…”

Tuning Co Valence State in Cobalt Oxyhydrate Superconductor by Postreduction

“…However, the SPD based on sodium content alone is not well reproducible. 3 Using redox titration Milne et al 4 proposed a revised SPD where the maximum T c was achieved at V Co ∼ +3.3, which means that the superconductor is hole-doped rather than electron-doped. Very recently, Barnes et al 5 suggested an asymmetric domelike SPD where the optimal V Co was about +3.5.…”

59Co, 23Na, and 1H NMR Studies of Double-Layer Hydrated Superconductors NaxCoO2 · yH2O

“…For anhydrated Na x CoO 2 , the Na concentration changes the carrier doping level of a CoO 2 plane, which develops the electronic state [10]. For doublelayer hydrated Na x CoO 2 ·yH 2 O, the Na concentration is not a unique parameter of T c [11,12,13]. Since no one has ever observed superconductivity of the anhydrated compounds, the carrier doping level is not a unique parameter of T c .…”

“…In the double-layer hydrated superconductors, not only the concentration of Na + ions [10] but also the H 3 O + ions [11,12,13] play key roles in the occurrence of superconductivity. The existence of the oxonium ions H 3 O + was evidenced by the observation of a bending mode and stretching modes of H 3 O + ions using Ramann spectroscopy [11].…”