  In this study, we systematically investigate 3D momentum($\hbar k$)-resolved
electronic structures of Ruddlesden-Popper-type iridium oxides
Sr$_{n+1}$Ir$_n$O$_{3n+1}$ using soft-x-ray (SX) angle-resolved photoemission
spectroscopy (ARPES). Our results provide direct evidence of an
insulator-to-metal transition that occurs upon increasing the dimensionality of
the IrO$_2$-plane structure. This transition occurs when the spin-orbit-coupled
$j_{\rm eff}$=1/2 band changes its behavior in the dispersion relation and
moves across the Fermi energy. In addition, an emerging band along the
$\Gamma$(0,0,0)-R($\pi$,$\pi$,$\pi$) direction is found to play a crucial role
in the metallic characteristics of SrIrO$_3$. By scanning the photon energy
over 350 eV, we reveal the 3D Fermi surface in SrIrO$_3$ and $k_z$-dependent
oscillations of photoelectron intensity in Sr$_3$Ir$_2$O$_7$. In contrast to
previously reported results obtained using low-energy photons, folded bands
derived from lattice distortions and/or magnetic ordering make significantly
weak (but finite) contributions to the $k$-resolved photoemission spectrum. At
the first glance, this leads to the ambiguous result that the observed
$k$-space topology is consistent with the unfolded Brillouin zone (BZ) picture
derived from a non-realistic simple square or cubic Ir lattice. Through careful
analysis, we determine that a superposition of the folded and unfolded band
structures has been observed in the ARPES spectra obtained using photons in
both ultraviolet and SX regions. To corroborate the physics deduced using
low-energy ARPES studies, we propose to utilize SX-ARPES as a powerful
complementary technique, as this method surveys more than one whole BZ and
provides a panoramic view of electronic structures.
