Using the linearized augmented-plane-wave (LAPW) -calculated electron and positron charge densities for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, the Brillouin-zone-reduced electron-positron momentum density is computed and the zone-reduced two-dimensional angular correlation of annihilation radiation (2D ACAR) spectrum is produced. The calculations show that the relative weights of the Fermi-surface discontinuities are substantially altered due to the positron preferentially sampling the Cu-O chain region. In addition, the reduced 2D ACAR spectrum contains large k-dependent wave-function effects. The theoretical zone-reduced 2D ACAR spectrum is compared to the several existing experimental spectra. It is concluded that, at present, positron-annihilation experiments do not provide consistent and clear evidence for the existence and shapes of Fermi surfaces in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$.

• Received 20 March 1990

DOI:https://doi.org/10.1103/PhysRevB.42.2696