  The band structures and optical properties of monoclinic HfO2 are
investigated by the local density approximation (LDA)+U approach. With the
on-site Coulomb interaction being introduced to 5d orbitals of Hf atom and 2p
orbitals of O atom, the experimental band gap is reproduced. The imaginary part
of the complex dielectric function shows a small shoulder at the edge of the
band gap, coinciding with the experiments. This intrinsic property of
crystallized monoclinic HfO2, which is absent in both the tetragonal phase and
cubic phase, can be understood as a consequence of the reconstruction of the
electronic states near the band edge following the adjustment of the crystal
structure. The existence of a similar shoulder-like-structure in the monoclinic
phase of ZrO2 is predicted.
