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) similar to 95 K, such as Bi2Sr2CaCu2O8+delta, YBa2Cu3O7-delta, Tl2Ba2CuO6+delta and HgBa2CuO4+delta, with particular emphasis on the Bi- compound which has been the most extensively studied with single- particle spectroscopies.