  In layered polar semiconductor BiTeI, giant Rashba-type spin-split band
dispersions show up due to the crystal structure asymmetry and the strong
spin-orbit interaction. Here we investigate the 3-dimensional (3D) bulk band
structures of BiTeI using the bulk-sensitive $h\nu$-dependent soft x-ray angle
resolved photoemission spectroscopy (SX-ARPES). The obtained band structure is
shown to be well reproducible by the first-principles calculations, with huge
spin splittings of ${\sim}300$ meV at the conduction-band-minimum and
valence-band-maximum located in the $k_z=\pi/c$ plane. It provides the first
direct experimental evidence of the 3D Rashba-type spin splitting in a bulk
compound.
