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High-resolution Fourier transform scanning tunneling spectroscopy (FT-STS) is used to study many- body effects on the surface state of Ag(111). Our results reveal a kink in the otherwise parabolic band dispersion of the surface electrons and an increase in the quasiparticle lifetime near the Fermi energy Ef. The experimental data are accurately modeled with the T-matrix formalism for scattering from a single impurity, assuming that the surface electrons are dressed by the electron-electron and electron-phonon interactions. We confirm the latter as the interaction responsible for the deviations from bare dispersion. We further show how FT-STS can be used to simultaneously extract real and imaginary parts of the self-energy for both occupied and unoccupied states with a momentum and energy resolution competitive with angle-resolved photoemission spectroscopy. From our quantitative analysis of the data we extract a Debye energy of ħΩD = 14 ± 1 meV and an electron-phonon coupling strength of λ = 0.13 ± 0.02, consistent with previous results. This proof-of-principle measurement advances FT-STS as a method for probing many body effects, which give rise to a rich array of material properties. [ABSTRACT FROM AUTHOR]

Defects in LiFeAs are studied by scanning tunneling microscopy and spectroscopy (STS). Topographic images of the five predominant defects allow the identification of their positions within the lattice. The most commonly observed defect is associated with an Fe site and does not break the local lattice symmetry, exhibiting a bound state near the edge of the smaller gap in this multigap superconductor. Three other common defects, including one also on an Fe site, are observed to break local lattice symmetry and are pair breaking, indicated by clear in-gap bound states, in addition to states near the smaller gap edge. STS maps reveal complex, extended real-space bound-state patterns, including one with a chiral distribution of the local density of states. The multiple bound-state resonances observed within the gaps and at the inner gap edge are consistent with theoretical predictions for the s± gap symmetry proposed for LiFeAs and other iron pnictides. [ABSTRACT FROM AUTHOR]

The superconducting compound LiFeAs is studied by scanning tunneling microscopy and spectroscopy. A gap map of the unreconstructed surface indicates a high degree of homogeneity in this system. Spectra at 2 K show two nodeless superconducting gaps with &Dgr;1 = 5.3 ± 0.1 meV and &Dgr;2 = 2.5 ± 0.2 meV. The gaps close as the temperature is increased to the bulk Tc, indicating that the surface accurately represents the bulk. A dip-hump structure is observed below Tc with an energy scale consistent with a magnetic resonance recently reported by inelastic neutron scattering. [ABSTRACT FROM AUTHOR]

Quasiparticle interference (QPI), by means of scanning tunneling microscopy/spectroscopy (STM/STS), angle-resolved photoemission spectroscopy (ARPES), and multiorbital tight-binding calculations, is used to investigate the band structure and superconducting order parameter of LiFeAs. Using this combination of techniques we identify intraband and interband scattering vectors between the hole (h) and electron (e) bands in the QPI maps. Discrepancies in the band dispersions inferred from previous ARPES and STM/STS are reconciled by recognizing a difference in the kz sensitivity for the two probes. The observation of both h-h and e-h scattering is exploited using phase-sensitive scattering selection rules for Bogoliubov quasiparticles. From this we demonstrate an s± gap structure, where a sign change occurs in the superconducting order parameter between the e and h bands. [ABSTRACT FROM AUTHOR]