Two Energy Gaps
In the underdoped high-Tc cuprates, there exist two kinds of energy gap scales: the superconducting gap energy scale which is proportional to Tc, and the pseudogap energy scale which drops almost linearly with doping. This difference between the superconducting pairing energy scale and the pseudogap energy scale was first pointed out in the paper
C. Panagopoulos and T. Xiang, ＾The relationship between the superconducting energy gap and the superconducting transition temperature in high temperature superconductors￣, Physical Review Letters 81 (1998) 2336-2339.
based on a systematical analysis of low temperature penetration depth, ARPES and STM measurement data. This important observation has now been extensively verified by the Raman scattering, ARPES, STM and quite many other experiments.
This figure shows how the gap amplitude (or gap slope) at the gap nodes varies with Tc. It indicates that the superconducting gap is proportional to Tc, in contrast to the pseudogap energy which shows an opposite trend in the underdoped regime.