Figure 2:
(a) The “preformed pairs” picture of cuprate superconductivity. Here the pairing energy scale (T_{p}) falls but the phase coherence energy scale (T_{θ}) rises with increasing doping. The zero resistance T_{c} is set by the smaller of the two energy scales, so optimal T_{c} 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].)