Electronic Excitations Probed by Raman Spectroscoy in BiSr₂Ca₂Cu₂O_4+δ Cuprate Superconductors

The transformation from an antiferromagnetic (AF) Mott insulator to a metal superconductor as the holes number increases is a key element for understanding the physics of cuprates [1,2]. In this work, we tracked the doping dependence of the spin singlet excitation related to the AF lattice, the normal state quasiparticles excitation related to the mobile charge carriers and the Bogoliubov quasiparticles related to the superconducting gap in the BiSr₂Ca₂Cu₂O_4+δ (Bi-2212) cuprate by polarization resolved Raman spectroscopy measurements. The Pseudogap phase and the Mott insulator AF phase are intimately related such as the drop of temperature favors the reconstruction of the AF lattice, therefore the charge carriers are blocked. This result can be interpreted as a new signature of the Pseudogap at higher energy [3]. We also noticed that the maximum amplitude of the SC gap <!--[if gte msEquation 12]> style='font-family:"Cambria Math",serif;mso-ascii-font-family:"Cambria Math";
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EN;mso-fareast-language:EN-US;font-style:italic;mso-bidi-font-style:normal'> style='mso-bidi-font-style:normal'>? _{style='mso-bidi-font-style:normal'>SC} ^{style='mso-bidi-font-style:normal'>max} Δ^max_sc(p) ∝ <!--[if gte msEquation 12]> style='font-family:"Cambria Math",serif;mso-ascii-font-family:"Cambria Math";
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EN;mso-fareast-language:EN-US;font-style:italic;mso-bidi-font-style:normal'> style='mso-bidi-font-style:normal'>? _{style='mso-bidi-font-style:normal'>SSE} Δ_sse(p)Tc (p) where Δ_sse is the energy of the spin singlet excitations, this relation is similar to the one earlier got from angular resolved photo-emission spectroscopy measurements on under-doped Bi-2212 [4]. This proportionality suggests that the AF fluctuations play a key role in the SC pairing mechanism associated with the SC gap at its maximum value.

[1] Anderson, Science 235, 1196 (1987)

[2] Keimer et al., Nature 518, 179 (2015).

[3] Mezidi et al., arXiv: 2207.10166v1 (2022).

[4] Ding, et al. Phys. Rev. Lett. 87, 227001 (2001).