Nanometer scale mapping of the density of states in an inhomogeneous superconductor

Using high-speed scanning tunneling spectroscopy (STS), we perform a full mapping of the quasiparticle density of states (DOS) in single crystals of Bi2-xPbxSr2CaCu2O8+delta. The measurements carried out at 5 K showed a complex spatial pattern of important variations of the local DOS on the nanometer scale. Superconducting areas, characterized by a well-pronounced superconducting gap, are coexisting with regions of a peakless gap structure which we attribute to the pseudogap. Within the large superconducting regions, the spectra reveal strong peak/dip/hump signatures, identical to the pristine Bi2Sr2CaCu2O8+delta case. On the contrary, in very small superconducting regions this ne structure is attenuated. Such a behavior of the local DOS suggests that the peak/dip/hump ne structure is not only a consequence of the superconducting state, but is also directly related to the scale of the phase coherence. The role of Bi-Pb substitutional disorder is discussed.