Electronic structures of hexagonal RMnO_3 (R=Gd, Tb, Dy, and Ho) thin films: Optical spectroscopy and first-principles calculations

摘要

We investigated the electronic structure of multiferroic hexagonal RMnO_3 (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 RMnO_3 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 RMnO_3 (R=Gd, Tb, and Dy) using the epitaxial stabilization technique [Adv. Mater. (Weinheim Ger.) 18, 3125 (2006)]. Using artificially stabilized hexagonal RMnO_3, 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 MnO_5 triangular bipyramid.