Raman spectroscopy of Bi2Sr2CaCu2O8+δ high-Tc superconductors

Single crystals of the Bi2Sr2CaCu2O8+δ high-Tc superconductor (Tc=92 K) have been investigated by means of Raman and and tunneling spectroscopy and Brillouin light scattering. Substitutions Bi→Pb, Ca→Y and Cu→Co have been performed in the Bi2Sr2CaCu2O8+δ compound and the relevant changes of the physical properties have been measured. In the normal conducting state the size of the charge-transfer gap has been determined to be ΔCT=2.6±0.1 eV using resonant Raman scattering, which als has revealed evidence for the existence of correlated states nearby the Fermi energy inside the charge-transfer gap. In the superconducting state, an anisotropic energy gap has been detected with different magnitudes parallel (2Δ parallel (0)/kBTc=5.7±0.2) and perpendicular (2Δ perpendicularto (O)/kBTc=3.4±0.2) to the CuO2-planes of the Bi2Sr2CaCu2O8+δ cuprate. The temperature dependence of both Δ parallel and Δ perpendicularto, measured by Raman spectroscopy, differs from the BCS-prediction and is explained within a theory of A.A. Abrikosov, which takes into account the layered structure of the Bi2Sr2CaCu2O8+δ superconductor. A complete symmetry-analysis of the Raman- active optical phonons has been performed and an assignment of the modes to respective atomic vibrations is given. Acoustic surface and bulk phonons have been measured by means of Brillouin spectroscopy, revealing anisotropic sound velocities parallel and perpendicular to the CuO2-planes. From those measurements, the complete set of elastic constants has been deduced. (orig.)