Local structure of Sr<sub>2</sub>CuO<sub>3.3</sub>, a 95 K cuprate superconductor without CuO<sub>2</sub> planes.
basic_science · Level V
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- Record sourced from PubMed, PMID 32060125.
- Also identified by DOI 10.1073/pnas.1918890117 and PMC identifier 7060713.
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Abstract
The local structure of the highly "overdoped" 95 K superconductor Sr<sub>2</sub>CuO<sub>3.3</sub> determined by Cu K X-ray absorption fine structure (XAFS) at 62 K in magnetically oriented samples shows that 1) the magnetization is perpendicular to the <i>c</i> axis; 2) at these levels of precision the Cu sublattice is tetragonal in agreement with the crystal structure; the O sublattice has 3) continuous -Cu-O- chains that orient perpendicular to an applied magnetic field; 4) approximately half-filled -Cu-O- chains that orient parallel to this field; 5) a substantial number of apical O vacancies; 6) O ions at some apical positions with expanded Cu-O distances; and 7) interstitial positions that imply highly displaced Sr ions. These results contradict the universally accepted features of cuprates that require intact CuO<sub>2</sub> planes, magnetization along the <i>c</i> axis, and a termination of the superconductivity when the excess charge on the CuO<sub>2</sub> Cu ions exceeds 0.27. These radical differences in charge and structure demonstrate that this compound constitutes a separate class of Cu-O-based superconductors in which the superconductivity originates in a different, more complicated structural unit than CuO<sub>2</sub> planes while retaining exceptionally high transition temperatures.