FERMI-SURFACE RIDGE AT 2ND AND 3RD UMKLAPP POSITRON ANNIHILATIONS IN YBA2CU3O7-DELTA

Results of statistical noise smoothing of the electron momentum distri bution got by two-dimensional angular correlation of the electron-posi tron annihilation radiation technique on untwinned YBa2Cu3O7-delta sin gle crystals are reported. Two distinct signatures of the sheet of Fer mi surface related to the CuO chains (the ridge) are resolved. The fir st occurs at second Umklapp processes, in agreement with previous evid ence. The second one, identified for the first time, occurs at third U mklapp processes. Comparison with FLAPW calculations confirms this res ult.