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 , such as Bi 2 Sr 2 CaCu 2 O 8+δ , YBa 2 Cu 3 O 7−δ , Tl 2 Ba 2 CuO 6+δ and HgBa 2 CuO 4+δ , with particular emphasis on the Bi-compound which has been the most extensively studied with single-particle spectroscopies.