Two-magnon Raman scattering in dielectric and superconducting YBa2Cu3O6+x crystals

Two-magnon Raman scattering in dielectric, as well as superconducting, YBa₂Cu₃O_{6 + x} single crystals with mobile oxygen content x = 0.2–0.7 and superconducting transition temperature T _c = 0–74 K is studied in detail. Doping with oxygen in the range of x = 0.2–0.5 leads to two-magnon scattering peak broadening and a shift in the spectral position of the peak towards lower energies. The most significant qualitative changes in two-magnon scattering in YBa₂Cu₃O_{6 + x} crystals are observed in a narrow oxygen concentration range near x = 0.7. This is explained by a considerable decrease in the correlation length ξ_AF of antiferromagnetic (AF) correlations upon an increase in the concentration of free carriers. For instance, doping is accompanied with a reduction of ξ_AF to values of several lattice constants a for x ≈ 0.7, a transition to the regime of short-range AF order, and local scattering of light from a small AF cluster with a size of 3 × 4 lattice constants. An increase in the free charge carrier concentration destroys the short-range AF order in a narrow range of the stoichiometry index near x = 0.7. Experimental data also indicate heterogeneity of cuprate planes at microscopic level, which leads to coexistence of superconducting and AF regions in YBa₂Cu₃O_{6 + x} super-conducting crystals.