The paper deals with the off-line processing of experimental data obtained by two-dimensional angular correlation of the electron-positron annihilation radiation (2D-ACAR) technique on high-temperature superconductors. A piecewise continuous window least squares (WLS) method devoted to the statistical noise smoothing of 2D-ACAR data, under close control of the crystal reciprocal lattice periodicity, is derived. Reliability evaluation of the constant local weight WLS smoothing formula (CW-WLSF) shows that consistent processing 2D-ACAR data by CW-WLSF is possible. CW-WLSF analysis of 2D-ACAR data collected on untwinned YBa2Cu3O7 − δ single crystals yields significantly improved signature of the Fermi surface ridge at second Umklapp processes and resolves, for the first time, the ridge signature at third Umklapp processes.

Results of statistical noise smoothing of the electron momentum distribution got by two-dimensional angular correlation of the electron-positron annihilation radiation technique on untwinned YBa 2 Cu 3 O 7− δ single crystals are reported. Two distinct signatures of the sheet of Fermi surface related to the CuO chains (the ridge) are resolved. The first occurs at second Umklapp processes, in agreement with previous evidence. The second one, identified for the first time, occurs at third Umklapp processes. Comparison with FLAPW calculations confirms this result.

We have observed, by 2D positron annihilation, the rigid-shaped Fermi surface related to the chains in the compound series (R)Ba2Cu3O7-delta for R = Y, Dy, Ho, and Pr. In the case of Y we have sighted the clear signature of the ridge in the fourth Brillouin zone and have observed the effect of substituting Cu with Ni, Zn, and Al. The presence of the ridge is also seen in PrBa2Cu3O7-delta which has an insulating character. This shows that chains have a Fermi surface but contribute little to conductivity. This observation is discussed in view of the above alloying experiments, and in view of the recent model of Fehren-bacher and Rice.