The photoemission line shapes of the optimally doped cuprate Bi(2)Sr(2)CaCu(2)O(8+delta) were studied in the direction of a node in the superconducting order parameter by means of very high resolution photoemission spectroscopy. The peak width or inverse lifetime of the excitation displays a linear temperature dependence, independent of binding energy, for small energies, and a linear energy dependence, independent of temperature, for large binding energies. This behavior is unaffected by the superconducting transition, which is an indication that the nodal states play no role in the superconductivity. Temperature-dependent scaling suggests that the system displays quantum critical behavior.

High-resolution photoemission is used to study the electronic structure of the cuprate superconductor, Bi(2)Sr(2)CaCu(2)O(8+delta), as a function of hole doping and temperature. A kink observed in the band dispersion in the nodal line in the superconducting state is associated with coupling to a resonant mode observed in neutron scattering. From the measured real part of the self-energy it is possible to extract a coupling constant which is largest in the underdoped regime, then decreasing continuously into the overdoped regime.

In a high-resolution photoemission study of a Mo(110) surface state various contributions to the measured width and energy of the quasiparticle peak are investigated. Electron-phonon coupling, electron-electron interactions, and scattering from defects are all identified mechanisms responsible for the finite lifetime of a valence photohole. The electron-phonon induced mass enhancement and rapid change of the photohole lifetime near the Fermi level are observed for the first time. PACS numbers: 79.60.Bm, 73.20.At Recent investigations of strongly correlated electron systems have questioned the validity of one of the most fundamental paradigms in solid state physics-Fermi liquid theory. The latter picture is based on the existence of "quasiparticles," or single-particle-like low energy excitations which obey the exclusion principle and have lifetimes long enough to be considered as particles. Strictly speaking, the quasiparticle concept is restricted to zero temperature and a narrow region around the Fermi level [1], but its usefulness often continues to finite temperatures, and energies away from the Fermi level [2]. Indications for possible non-Fermi-liquid behavior have been found in some organic one-dimensional conductors [3] and in the normal state of high temperature superconductors [4]. A whole variety of experimental techniques have been employed in the search for such behavior, including resistivity measurements [5], infrared spectroscopy [6], scanning tunneling spectroscopy [7], and time-resolved two-photon photoemission [8]. Angle-resolved photoemission spectroscopy (ARPES) has an advantage, in that the energy and lifetime of the photohole are directly observable in the experiment. ARPES in principle measures the quasiparticle spectral function [9]:where´k represents the energy of the state in the Hartree potential, and S͑k, v͒ is the quasiparticle self-energy reflecting many-body interactions. Thus, momentum resolved self-energies are directly accessible in the experiment and, as such, ARPES represents a crucial experimental probe for the presence or absence of Fermi liquid behavior. Furthermore, complications connected to the lifetime of the photoelectron (in three-dimensional systems) may be overcome in quasi-low-dimensional systems. Indeed, there have already been several photoemission studies which quantitatively compare peak widths to calculated lifetimes for metallic surface states [10] and two-dimensional states in layered materials [11].When considering the lifetime of the valence hole, there are three main decay mechanisms: electron-electron scattering, electron-phonon scattering, and impurity (defect) scattering. In three-dimensional systems, the electronelectron interaction contributes to the total width or inverse lifetime with the term G e-e ͑v, T ͒ 2b͓͑pk B T ͒ 2 1 v 2 ͔. This scattering rate does not depend on the form of the interaction, but it may depend on the shape of the Fermi surface. If the scattering process is two dimensional, then the quadratic energy (temperature) dependence...

Superconductors are characterized by an energy gap that represents the energy needed to break the pairs of electrons (Cooper pairs) apart. At temperatures considerably above those associated with superconductivity, the high-transition-temperature copper oxides have an additional 'pseudogap'. It has been unclear whether this represents preformed pairs of electrons that have not achieved the coherence necessary for superconductivity, or whether it reflects some alternative ground state that competes with superconductivity. Paired electrons should display particle-hole symmetry with respect to the Fermi level (the energy of the highest occupied level in the electronic system), but competing states need not show such symmetry. Here we report a photoemission study of the underdoped copper oxide Bi(2)Sr(2)CaCu(2)O(8+delta) that shows the opening of a symmetric gap only in the anti-nodal region, contrary to the expectation that pairing would take place in the nodal region. It is therefore evident that the pseudogap does reflect the formation of preformed pairs of electrons and that the pairing occurs only in well-defined directions of the underlying lattice.

Photoemission studies show the presence of a high-energy anomaly in the observed band dispersion for two families of cuprate superconductors, Bi2Sr2CaCu2O8+delta and La 2-x BaxCuO4. The anomaly, which occurs at a binding energy of approximately 340 meV, is found to be anisotropic and relatively weakly doping dependent. Scattering from short range or nearest neighbor spin excitations is found to supply an adequate description of the observed phenomena.