Figure 2: (a) The “preformed pairs” picture of cuprate superconductivity. Here the pairing energy scale ($Tp$) falls but the phase coherence energy scale ($Tθ$) rises with increasing doping. The zero resistance $Tc$ is set by the smaller of the two energy scales, so optimal $Tc$ is defined by the crossing of the two lines. (b) The “2-gap” picture of cuprate superconductivity is supported by the authors’ previous work showing that the $d$-wave gap saturates in underdoped cuprates, while a larger gap in a different region of $k$-space continues to grow with underdoping [15]. ( Panel (b) adapted from A. Pushp et al. [15].)