Stress analysis in melt processed RBa₂Cu₃O₇ (R = Y, Nd) by micro-raman spectroscopy

We have performed a detailed analysis of the micro-Raman spectra recorded around the non-superconducting R₂BaCuO₅ inclusions embedded in different melt grown RBa₂Cu₃O₇ (R = Y, Nd) samples. The Raman spectra were recorded every 0.4 or 0.5 μm along a line crossing one or more inclusions on two specific samples. Sample A, YBa₂Cu₃O₇ with 33 vol% of Y₂BaCuO₅ was prepared following a fast-melt-processing scheme. The cooling rate of the melt, crossing the peritectic temperature, was 180°C/h. We observed a downshift of the phonon modes of the superconducting phase around the inclusions. This softening can be related to the presence of tensile stresses around the inclusions. Sample B, NdBa₂Cu₃O₇ with 40% of Nd₄Ba₂Cu₂O₁₀ was prepared following an oxygen-controlled-melt-growth method. A cooling rate of 1°C/h was applied to produce this sample. In this case we found a hardening of the phonon modes of the superconducting phase around the inclusions towards the energy of the phonon modes obtained in single crystals. Again, this shift of the Raman peak position can be related to the variation of mechanical stress around the inclusions. The hardening can be caused by the relaxation of tensile stress in the superconducting matrix around the inclusions.