Superconducting energy gap versus pseudogap in hole-doped cuprates as revealed by infrared spectroscopy.

We present in-plane infrared reflectance measurement on two superconducting cuprates with relatively low Tc: nearly optimally doped Bi2Sr1.6La0.4CuO6+δ with Tc = 33 K and underdoped La1.89Sr0.11CuO4 with Tc = 30 K. The measurement clearly reveals that the superconducting energy gap is distinct from the pseudogap. They have different energy scales and appear at different temperatures. The results suggest that the pseudogap is not a precursor to the superconducting state. The data also challenge the longstanding and predominant viewpoint that the superconductivity within the ab plane is in the clean limit and the superconducting pairing energy gap could not be detected by in-plane infrared spectroscopy. [ABSTRACT FROM AUTHOR]