Gaplike Excitations in the Superconducting State of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ Studied by Resonant Raman Scattering

We report on the response of the electronic continuum from inelastic light-scattering experiments over an extended energy range between 1.970 and 4.504 eV in the superconducting state of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$. The formation of a substantial Raman feature at shifts below twice the superconducting gap as well as the additional weight above this energy are found to be strongly dependent on the incident photon energy. For excitation wavelengths observed in ultraviolet, we find an enhancement of the integrated spectral weight below $T_c$. The resulting composite feature shows three distinct resonances at 2.5, 3.3, and 3.8 eV. We strongly suggest that the superconductivity-induced changes are the result of both the opening of a superconducting gap and the appearance of a collective mode.

Figure 1

Resonance behavior of the spectral redistribution of electronic spectral weight in the superconducting state represented by the shaded regions in $ba$ scattering configuration. Incident photon energies range between 3.00 and 4.1 eV. The red and blue colors indicate the loss and the gain of spectral weight below $T_c$, respectively.

Figure 2

Experimental data (wiggled lines) and the model (solid line) of the electron and phonon responses as a function of the incident photon energy between 1.97 and 3.53 eV. In column (a) the bold solid line represents a fit to the experimental data. Also shown as thin solid and dashed lines are the electronic contribution due to the gap features at 320, 440, and $580{\mathrm{cm}}^{-1}$ as well as the marginal Fermi liquid, respectively. (b) Changes of the shape in the electronic response as a function of incident photon energy. Responses have been scaled to match.

Figure 3

Resonance profiles for the $440{\mathrm{cm}}^{-1}$ (gray), $320{\mathrm{cm}}^{-1}$ (light blue), and $580{\mathrm{cm}}^{-1}$ (violet) contributions to the gap of ${\mathrm{B}}_{1\mathrm{g}}$ symmetry as well as the single contribution in ${\mathrm{A}}_{1\mathrm{g}}$ (green) configuration. Also shown as a red line is the imaginary part of the dielectric function for light polarized along the $a$ axis of the crystal for comparison.