Unconventional Energetics of the Pseudogap State and of Superconducting State in High-T sub c Cuprates

A new insight into the nature of the charge carriers pairing in high-T sub c superconductors may be provided by a systematic investigation of the condensation energy. In this work we report on studies of the electronic kinetic energy across the complex phase diagram of these materials. The c-axis component of the electronic kinetic energy (determined from an analysis of the optical constants) is shown to be reduced below T sub c primarily in those compounds in which superconducting transition at T sub c is preceded by the formation of the partial gap in the density of states (pseudogap) at T sup * > T sub c. The examination of the doping dependence of the infrared conductivity in conjunction with the results of the photoemission spectroscopy suggests the lowering of the kinetic energy is a property of the electronic states close the intersection of the two-dimensional Fermi surface with the boundary of the Brillouin zone.