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We studied the temperature dependences of the optical spectra for optimally and underdoped Bi₂Sr₂Ca₂Cu₃O₁₀+ₙ single crystals. Similarly to the other cuprates' cases, a gap-like conductivity suppression was observed with reducing the temperature from above T₂, creating a peak in the conductivity spectrum. The conductivity peak energy was insensitive to the doping level, namely T₂, which suggests that this gap is not a superconducting gap but is related to the pseudogap. Comparing the data of various mono-, double-, and triple-layer cuprates, we found a clear correlation between the optimal T₂ of each material and the pseudogap-related conductivity peak energy.
Tajima et al. (2024) studied this question.