  We report the observation of an unusual behavior of highly extended 5d
electrons in Y2Ir2O7 belonging to pyrochlore family of great current interest
using high resolution photoemission spectroscopy. The experimental bulk spectra
reveal an intense lower Hubbard band in addition to weak intensities in the
vicinity of the Fermi level, e_F. This provides a direct evidence for strong
electron correlation among the 5d electrons, despite their highly extended
nature. The high resolution spectrum at room temperature exhibits a pseudogap
at e_F and |e - e_F|^2 dependence demonstrating the importance of electron
correlation in this system. Remarkably, in the magnetically ordered phase (T <
150 K), the spectral lineshape evolves to a |e - e_F|^1.5 dependence
emphasizing the dominant role of electron-magnon coupling.
