Condensate pair fluctuations in a two-dimensional [ital d]-wave superconductor and Raman scattering
- Department of Physics, University of Toronto, Toronto, Ontario, M5S 1A7 (Canada)
The condensate pair fluctuations in the frequency region [omega][approx lt]2[Delta] and the associated Raman-scattering intensity of a two-dimensional (2D) [ital d]-wave weak-coupling BCS superconductor are investigated. Our model includes a dominant [ital d][sub [ital x]][sup 2][minus][ital y][sup 2] ([ital L]=2) and a weaker [ital s]-wave ([ital L]=0) pairing interaction. All response functions involving density and pair operators are evaluated (in the low-[bold q] limit). For neutral [ital d]-wave superconductors (no Coulomb interaction), the expected phononlike phase mode is obtained in the [ital L]=0 channel (which couples to density fluctuations). In contrast, excitonlike modes corresponding to excited Cooper pair states are obtained in the [ital L]=2 channel. We find an amplitude fluctuation mode with frequency [radical]3 [Delta]. For charged [ital d]-wave superconductors, the [ital L]=0 phonon is renormalized into a 2D plasmon, but the [ital L]=2 excitonlike mode remains unaffected by the Coulomb interaction. At [ital T]=0, the latter is shown to be completely washed out in the Raman scattering spectrum ([bold q]=0) due to large [ital p]-[ital h] damping (which arises in the absence of a finite pair-breaking gap in [ital d]-wave superconductors). However, at finite temperatures, we find the energy of the excitonlike mode is drastically lowered [relative to 2[Delta]([ital T])] when the [ital s]-wave attraction is comparable to the [ital d]-wave pairing. This leads to a decrease in the damping and, as a result, the mode shows up as a low-frequency resonance in the Raman cross section. Due to the anisotropy of the [ital d]-wave order parameter, the quasiparticle excitation spectrum and the noninteracting two-particle spectrum are strongly dependent on the direction of [bold q]. We also find that the excitonlike mode frequency becomes anisotropic for wave vectors of the order of [Delta]/[ital v][sub [ital F]].
- OSTI ID:
- 6712573
- Journal Information:
- Physical Review, B: Condensed Matter; (United States), Vol. 51:2; ISSN 0163-1829
- Country of Publication:
- United States
- Language:
- English
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