Electronic structures of hexagonal RMnO3 (R=Gd, Tb, Dy, and Ho) thin films:: Optical spectroscopy and first-principles calculations

We investigated the electronic structure of multiferroic hexagonal RMnO3 (R=Gd, Tb, Dy, and Ho) thin films using both optical spectroscopy and first-principles calculations. One of the difficulties in explaining the electronic structures of hexagonal RMnO3 is that they exist in nature with limited rare earth ions (i.e., R=Sc, Y, and Ho-Lu), so a systematic study in terms of the different R ions has been lacking. Recently, our group succeeded in fabricating hexagonal RMnO3 (R=Gd, Tb, and Dy) using the epitaxial stabilization technique [Adv. Mater. (Weinheim Ger.) 18, 3125 (2006)]. Using artificially stabilized hexagonal RMnO3, we extended the optical spectroscopic studies on the hexagonal multiferroic manganite system. We observed two optical transitions located near 1.7 and 2.3 eV, in addition to the predominant absorption above 5 eV. With the help of first-principles calculations, we attributed the low-lying optical absorption peaks to interband transitions from the oxygen states hybridized strongly with different Mn orbital symmetries to the Mn 3d(3z)(2)-r(2) state. As the ionic radius of the rare earth ion increased, we observed a systematic increase of the lowest peak position, which became more evident when compared with previously reported results. We explained this systematic change in terms of a flattening of the MnO5 triangular bipyramid.