Positron two-dimensional angular-correlation-of-annihilation-radiation study of untwinnedYBa2Cu3O6.9in thea-axis projection

Abstract

We present a two-dimensional (2D)-ACAR positron annihilation study of an untwinned ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.9}$ metallic sample in the a-axis projection; all existing data from untwinned samples have employed the c projection. The data show a clear presence of the Cu-O chain related ridge Fermi surface extending along the ΓZ direction and two associated umklapp images at higher momenta. These results are consistent with the previously investigated c projection of the momentum density in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.9}$ and do not appear to indicate a strong localization of these electron states along the c direction. We compare and contrast the structures in 2D-ACAR’s in the a and c projections, delineating similarities and differences between the results for the two projections.