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 low
er 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.
