<title>Femtosecond spectroscopy and new possibilities for a correlated electron system study</title>

Abstract

subpicosecond time scale2. It was shown that the subpicosecond optical response of gold is characterized by delayedthermalization ofelectron gas2.The phenomenon of superconductivity in a non-Fermi liquid has engendered much interest recently as it has been argued thatthe normal state of high-temperature superconductors cannot be adequately described by Fermi-liquid theory (FL).9 Theproperties and the origin of a possible non-Fermi-liquid (NFL) ground state are, however, not yet known as the morefundamental question about the ground state of strongly correlated electrons in two and higher dimensions remainsunanswered. It is also not clear if superconductivity from such a NFL ground state would be any different from theconventional theory ofsuperconductivity which presumes the validity ofFL theory in normal state.The central question in the theory of high-temperature superconductors is related to the FL properties. Although theexistence of a Fermi surface (FS) seems to be establish,'0 pronounced deviations from FL behavior could be observed. Forstrongly overdoped compounds the experimental results seem to be consistent with a standard FL picture, whereas forsystem near optimal doping this is clearly not the case: indications of a linear energy-dependence of the inverse quasiparticle life-time reported in angle resolved photoemission measurements, the unusual temperature dependence of transportcoefficient'° can not be understood within the conventional FL picture. For underdoped systems the deviations from FLbehavior may be stronger due to the dominance of short range antiferromagnetic correlations.