Local structure of Sr₂CuO_3.3, a 95 K cuprate superconductor without CuO₂ planes

Area of Science:

• Materials Science
• Condensed Matter Physics
• Superconductivity

Background:

• Cuprate superconductors typically exhibit intact CuO2 planes and magnetization along the c-axis.
• Superconductivity in cuprates is thought to terminate when excess charge exceeds 0.27.
• The local structure of overdoped Sr2CuO3.3 has not been fully elucidated.

Purpose of the Study:

• To determine the local structure of the superconductor Sr2CuO3.3 using X-ray absorption fine structure.
• To investigate the relationship between structure, charge, and superconductivity in this overdoped cuprate.
• To challenge established models of cuprate superconductivity.

Main Methods:

• Cu K X-ray absorption fine structure (XAFS) spectroscopy.
• Magnetically oriented sample preparation.
• Analysis of local atomic arrangements and bonding.

Main Results:

• Magnetization is perpendicular to the c-axis, contrary to typical cuprates.
• Cu sublattice is tetragonal; oxygen sublattice features continuous and half-filled Cu-O chains.
• Significant apical oxygen vacancies and displaced Sr ions were observed, challenging standard CuO2 plane models.

Conclusions:

• Sr2CuO3.3 exhibits a unique structure and charge distribution, classifying it as a new type of Cu-O-based superconductor.
• Superconductivity in this material likely originates from a more complex structural unit than CuO2 planes.
• The findings necessitate a revision of the universally accepted features of cuprate superconductors.