MANY-BODY EFFECTS IN ANGLE-RESOLVED PHOTOEMISSION : QUASIPARTICLE ENERGY AND LIFETIME OF A MO(110) SURFACE STATE

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]: