  The layered 5d transition metal oxides like Sr2IrO4 have attracted
significant interest recently due to a number of exotic and new phenomena
induced by the interplay between the spin-orbit coupling, bandwidth W and
on-site Coulomb correlation U. In contrast to a metallic behavior expected from
the Mott-Hubbard model due to more spatially extended 5d orbitals and moderate
U, an insulating ground state has been observed in Sr2IrO4. Such an insulating
behavior can be understood by an effective J_eff=1/2 Mott insulator model by
incorporating both electron correlation and strong spin-orbital coupling,
although its validity remains under debate at present. In particular, Sr2IrO4
exhibits a number of similarities to the high temperature cuprate
superconductors in the crystal structure, electronic structure, magnetic
structure, and even possible high temperature superconductivity. Here we report
a new observation of the anomalous high energy electronic structure in Sr2IrO4.
By taking high-resolution angle-resolved photoemission measurements on Sr2IrO4
over a wide energy range, we have revealed that the high energy electronic
structures show unusual nearly-vertical bands that extend over a large energy
range. Such anomalous high energy behaviors resemble the high energy waterfall
features observed in the cuprate superconductors, adding one more important
similarity between these two systems. While strong electron correlation plays
an important role in producing high energy waterfall features in the cuprate
superconductors, the revelation of the high energy anomalies in Sr2IrO4 points
to a novel route in generating exotic electronic excitations from the strong
spin-orbit coupling and a moderate electron correlation.
