Two gaps make a high-temperature superconductor?

One of the keys to the high-temperature superconductivity puzzle is the identification of the energy scales associated with the emergence of a coherent condensate of superconducting electron pairs. These might provide a measure of the pairing strength and of the coherence of the superfluid, and ultimately reveal the nature of the elusive pairing mechanism in the superconducting cuprates. To this end, a great deal of effort has been devoted to investigating the connection between the superconducting transition temperature T_c and the normal-state pseudogap crossover temperature T^*. Here we present a review of a large body of experimental data which suggests a coexisting two-gap scenario, i.e. superconducting gap and pseudogap, over the whole superconducting dome. We focus on spectroscopic data from cuprate systems characterized by T_c^max\sim 95\,K , such as Bi₂Sr₂CaCu₂O_8+δ, YBa₂Cu₃O_7-δ, Tl₂Ba₂CuO_6+δ and HgBa₂CuO_4+δ, with particular emphasis on the Bi-compound which has been the most extensively studied with single-particle spectroscopies.