Spin and charge excitations in optimally dopedBi2Sr2CaCu2O8−δ

We present Raman spectra of low- and high-energy charge and spin excitations in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8\mathrm{-}\mathrm{\delta }\mathrm{}}$ single crystals with an optimized critical temperature of 95 K. The prominent feature of the high-energy background at around 250 meV is a rearrangement of spectral weight in $B_{1g}$ and $A_{1g}{+B}_{2g}$ symmetry below the critical temperature, similar to the observations in underdoped and optimally doped ${\mathrm{Y}}_{1\mathrm{-}\mathrm{x}}{\mathrm{Pr}}_{\mathrm{x}}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }\mathrm{}}$ compounds. The $B_{1g}$ spectra can be attributed to magnetic scattering and exhibit below the critical temperature an increase of scattering intensity indicating an enhanced magnetic coherence in the superconducting state. Scattering in $A_{1g}$ geometry reflects more incoherent electronic contributions. There, we observe a decrease of scattering intensity for energies around 120 meV similar to the observations made in superconductor-insulator-superconductor tunneling spectroscopy or at half that energy with superconductor-insulator-normal tunneling spectroscopy and angle-resolved photoemission spectroscopy. The resonance enhancement of the rearrangement is rather strong with a maximum above 2.71 eV laser-excitation energy. In the low-energy region, which is influenced by the effects at higher energies, a gap feature in $B_{1g}$ symmetry is observed yielding a value for the magnitude of the superconducting order parameter of $\Delta =34\mathrm{}$ meV. This gap feature is influenced by the orthorhombicity of the crystals and except for a small loss of spectral weight below 25 meV, no gap feature is visible in $A_{1g}$ scattering geometry. Our experimental results raise questions about the applicability of the conventional theories for the electronic Raman scattering of the cuprate superconductors, which neglect the interplay between low- and high-energy excitations.