Peak effect in NdBa₂Cu₃O_7-δ superconductor and related Raman spectra

A peak effect of the critical current density was observed for all NdBa₂Cu₃O_7-δ (Nd123) samples prepared by the solid-state reaction method in atmospheres containing different oxygen concentrations. The peak position (field value) and its critical current density value measured at 30, 50, and 71 K depend strongly on the sintering time, and on the oxygen concentration in the sintering atmosphere. The synthesized samples have a high critical temperature T_c of about 96 K and a ΔT_c[ = T(0.9 ρ_onset) - T(0.1 ρ_onset), ρ_onset is the normal-state resistivity] lower than 2 K. It implies that the samples are of single superconducting phase without oxygen deficiency. Macro-Raman and micro-Raman spectra were recorded at room temperature for all samples. New modes, supposed to be related with disorder in chain layers, were observed. The disorder is assumed to be induced by a substitution of magnetic Nd³⁺ ions into the Ba²⁺ sites in the Nd123 lattice, and is considered to be a main cause of the obtained peak effect.