This article discusses the discovery of a new class of “high-TC” superconductors in the NaxCoO2 system, which could potentially rival the well-known high-temperature superconductors discovered over 17 years ago. The superconductivity is observed in the hydrated form of the material, where water intercalates between the Na and CoO2 layers, expanding the lattice and forming ice-like layers.
The Co4+ ions in the CoO2 layers are in the low-spin state with S=1/2, similar to the cuprates, and the system exhibits strong anisotropy and type-II behavior. The superconducting transition temperature so far reaches 5 K, but the researchers suggest that further optimization could potentially lead to higher transition temperatures.
The crystal structure and lattice dynamics of the superconducting hydrate and the related non-superconducting compound Na0.3CoO2 have been investigated. The structure of the superconductor consists of alternate layers of CoO2, Na, and water, with the water forming ice-like layers between the Na and CoO2 layers. The CoO2 layer thickness is found to inversely correlate with the superconducting transition temperature.
The lattice dynamical scattering for the superconductor is dominated by the hydrogen modes, with librational and bending modes similar to ice, supporting the structural model of water intercalation and ice-like layer formation. The generalized phonon density-of-states for the parent compound, deuterated superconductor, and protonated superconductor shows additional strong scattering in the 50-120 meV range for the protonated superconductor, matching the librational modes of ice.
The researchers suggest that the water may play an active role in the superconducting pair formation, either through traditional electron-phonon interaction or through anharmonic motion of the hydrogen and oxygen ions, similar to MgB2. The nature and mechanism of superconducting pairing in this new class of materials are still in the early stages of being addressed, but the discovery of this new class of superconductors opens up exciting possibilities for further research and potential applications.
Keywords: Mott, Insulator, Fluctuations, Cooper, Pairs
