Time-dependent structural phenomena at room temperature in quenched YBa 2Cu 3O 6.41: Local oxygen ordering and superconductivity

Abstract

The superconducting transition temperature, T c, of YBa 2Cu 3O 6.41 quenched from 500°C into liquid nitrogen increases from 0 to 20 K while annealing at room temperature during the first few days following the quench. Using neutron powder diffraction and Rietveld refinement we show that this time-dependent increase of T c is accompanied by changes in the structural properties. The a-and c-axes contract by 0.04%, while the percentage shortening of the b-axis is three times smaller. We attribute this behavior to charge transfer between the chains and planes, with the smaller contraction along the b-axis arising from oxygen ordering in the chains. Clear evidence for this charge transfer is seen in the relevant Cu-O and Cu-Cu distances. No significant changes in the average occupancies of oxygen sites are observed. We attribute the increase in T c to local ordering of oxygen atoms around the Cu(1) atoms with no change in the average site occupancies.